An Examination of the Feasibility of

Ultrasonic Communications Links

by David Tofsted, Sean O’Brien, Sean D’Arcy, 

Edward Creegan, and Scott Elliott

ARL-TR-5200 June 2010

Approved for public release; distribution is unlimited.
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Army Research Laboratory

White Sands Missile Range, NM 88002-5513

ARL-TR-5200 June 2010

An Examination of the Feasibility of 

Ultrasonic Communications Links

David Tofsted, Sean O’Brien, Sean D’Arcy, 

Edward Creegan, and Scott Elliott

Computational and Information Sciences Directorate, ARL

Approved for public release; distribution is unlimited.
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An Examination of the Feasibility of Ultrasonic Communications Links

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David Tofsted, Sean D’Arcy, Edward Creegan, and Scott Elliott 

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7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

U.S. Army Research Laboratory

Computational and Information Sciences Directorate

Battlefield Environment Division (ATTN: RDRL-CIE-D)

White Sands Missile Range, NM 88002-5501

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14. ABSTRACT 

A study is performed to consider the feasibility of using the ultrasonic band just above the normal adult audio frequency range 

(20–25 kHz) for low data rate signaling. In this band several features of interest are explored and a potential application 

(Identification Friend-or-Foe [IFF]) is highlighted. A key finding is that the data transmission rate is restricted in the presence 

of multipathing effects. Data transmission rates in the 300-baud range are possible when Rician noise statistics dominate. For 

non-line-of-sight cases when Rayleigh statistics dominate, a data link may not be possible. However, in the presence of a 

dominant reflecting path data may be transmittable, but at greatly reduced rates, due to multipathing. To study the potential of 

this technology a series of measurements were performed to determine signal losses with range, angular emission properties of

a series of horns as a function of frequency, and Amplitude Modulation (AM) and Frequency Modulation (FM) data encoding 

methods.

15. SUBJECT TERMS

ultrasonic propagation, acoustics, multipathing, bit-error-rate

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UU

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62

19a. NAME OF RESPONSIBLE PERSON

David Tofsted

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(575) 678-8547

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iii

Contents

List of Figures v

List of Tables vi

Executive Summary vii

1. Introduction 1

1.1 Prior ARL Atmospheric Turbulence and Acoustics Efforts ...........................................1

1.2 Motivation for Current Ultrasonic Propagation Research...............................................2

1.3 Prior Art...........................................................................................................................3

1.4 Program of Investigation.................................................................................................4

2. Semi-Empirical Ultrasonic Absorption and Scattering Models and Early Experimental 

Findings 5

2.1 Acoustic Absorption and Scattering Cross Sections.......................................................5

2.2 Semi-Empirical Formulation of Absorption and Scattering Equations...........................8

2.2 Comparisons of Attenuation Data with Models............................................................12

3. Infrastructure Development 16

3.1 General Site Description ...............................................................................................16

3.2 Safety Concerns, Preparations, and Training ................................................................18

3.3 Angular Antenna Pattern Testing..................................................................................19

4. LabVIEW Software Developments 25

5. Propagation and Communications Characterization 30

5.1 Slant-Path Propagation Tests.........................................................................................30

5.1.1 Developmental Power versus Range Measurement Series at 19472 

(March/April 2009) ...........................................................................................30

5.1.2 Refined Power versus Range Measurement Series at 19472 

(October/November 2009) ................................................................................35

5.2 Ultrasonic Acoustic Communications Proof of Concept Tests (November 2009) .......37
iv

6. Feasibility Analysis Results 37

7. Conclusions 41

8. References 43

Appendix. Ultrasonic Measurements Safety Standard Operating Procedure 45

List of Symbols, Abbreviations, and Acronyms 51

Distribution List 52
v

List of Figures

Figure 1. Absorption coefficient versus relative humidity at f = 20 kHz (P = 880 mB, T 

= 13.3 °C). ..............................................................................................................................11

Figure 2. Total, temperature scattering, wind scattering, and total absorption coefficients as a 

function of frequency for the example case. ..........................................................................12

Figure 3. Comparison of SPL measurements with inverse square curves at 10-, 15-, and 20-kHz 

acoustic frequencies, 06 Nov 2008.........................................................................................13

Figure 4. Power law fit to measured SPL data, 10-, 15-, and 20-kHz acoustic frequencies, 06 

Nov 08. ...................................................................................................................................14

Figure 5. SPL field measurements with inverse square trend removed and fitted with a power 

law curve, 10, 15, and 20 kHz, 06 Nov 08.............................................................................14

Figure 6. Building 19472 compound on WSMR, NM, and its environs. .....................................17

Figure 7. Acoustic path receiver scaffold used in power vs. range testing at 19472 site. The top 

of building 19472 is seen in the background. Acoustic receiver is seen at the top of the 

scaffold’s second level. ..........................................................................................................18

Figure 8. SPL sensor microphone (bottom) and uncalibrated microphone configuration............20

Figure 9. “Trumpet”-style horn (narrow beam)............................................................................20

Figure 10. “Trumpet” horn antenna pattern. Sonic intensities at 5-, 10-, and 20-kHz frequencies 

(green, blue, and red lines, respectively)................................................................................21

Figure 11. Parabolic dish (narrow beam)......................................................................................21

Figure 12. Parabolic dish antenna pattern. Sonic intensities at 5-, 10-, and 20-kHz frequencies 

(green, blue, and red lines, respectively)................................................................................22

Figure 13. Round (wide mouth) horn. ..........................................................................................23

Figure 14. Round horn antenna pattern. Sonic intensities at 5-, 10-, and 20-kHz frequencies 

(green, blue, and red lines, respectively)................................................................................23

Figure 15. Rectangular (wide mouth) horn...................................................................................24

Figure 16. Rectangular horn antenna pattern. Sonic intensities at 5-, 10-, and 20-kHz 

frequencies (green, blue, and red lines, respectively). ...........................................................24

Figure 17. Front panel for signal generator used in dispersion experiments................................26

Figure 18. Front panel for SPL and wind measurements used in dispersion experiments. ..........27

Figure 19. Front panel for FSK data transmitter...........................................................................29

Figure 20. Block diagram of FSK data transmitter.......................................................................29

Figure 21. 5-m sound pressure distribution data for various frequencies.....................................31

Figure 22. 10-m sound pressure distribution data for various frequencies...................................32
vi

Figure 23. 15-m sound pressure distribution data for various frequencies...................................33

Figure 24. 20-m sound pressure distribution data for various frequencies...................................33

Figure 25. View down “test range” road leading to the 19472 site. .............................................35

Figure 26. Range versus propagated SPL power for varying wavelengths. .................................36

Figure 27. Multipathing bit error geometry. .................................................................................38

Figure 28. Illustration of strong direct signal in the presence of weaker reflected (multipath) 

signals.....................................................................................................................................39

Figure 29. Illustration of reflected (multipath) signal in the absence of a direct signal. ..............40

List of Tables

Table 1. Attenuation of sound in air (dB km−1). .............................................................................6

Table 2. Residual attenuation coefficients computed from data of figure 5.................................15
vii

Executive Summary

This report documents a feasibility study performed at White Sands Missile Range (WSMR), 

NM, in the fall of 2009. This study addressed the issue of the capabilities and limitations 

associated with ultrasonic acoustic data communications. There exist a number of relevant Army 

field problems that could potentially benefit from such systems, since many distributed systems 

such as intelligent minefields or arrays of meteorological sensors could be more easily emplaced 

if they did not require wires to connect to one another. 

The readiest alternative is, of course, the electromagnetic spectrum. However, this spectrum is 

already widely used and easily intercepted, possibly compromising the safety or security of a 

network using this technology. Infrared communications are likewise easily detected. 

The advantage of ultrasonic acoustic transmissions is that they operate at frequencies that are 

significantly attenuated by the atmosphere and therefore provide a stealth advantage. Ultrasonic 

signals are naturally undetectable by a human without instrumentation. Second, due to high 

attenuation rates, ultrasonic signals can only be detected within close proximity to an emitter. 

They are typically undetectable beyond approximately 100 m of a source. Also, because the air 

is the information carrier, signals can be broadcast to multiple sensors simultaneously, like radio. 

Low data rate signals can also be consolidated for rebroadcast from a single receive point via a 

single Radio Frequency (RF) uplink, avoiding bandwidth limitations. Hence, for a localized 

sensor network, as long as the needed bandwidth is not too high, an ultrasonic network could 

provide a useful option.

A series of hardware devices, software tools, and computer algorithms were developed to 

facilitate measurements and testing that allow an objective assessment of the feasibility for 

ultrasonic data communication in a field environment. The foundation for the basic ultrasonic 

communications system used in this investigation is based on the Laboratory Virtual Instrument 

Engineering Workbench (LabVIEW) software package and hardware interface modules. One of 

the metrics used in this study is the integrity of binary data that are ultrasonically broadcast from 

a transmitter to a downrange receiver. For this purpose, data strings were translated into a series 

of bits and transmitted using Amplitude Modulation (AM) and Frequency Modulation (FM) 

implementations. Both the transmitter modulation and receiver demodulation were controlled by 

the LabVIEW system implementation. As described in our report, early results focused our 

major interest on FM modulation using a binary Frequency Shift Keyed (FSK) system. 

A series of four emitting horns were employed to examine the role played by transmission 

patterns in the efficiency of data conveyance and energy propagation. A parabolic dish was 

found to concentrate the most energy in the forward direction, but for the purpose of 

broadcasting digital data signals, a wide-angle rectangular horn was deemed to be the most 
viii

versatile form factor, and was selected as the most appropriate for extended testing. On the 

receive side a high sensitivity uniform responsivity microphone was used. Using these 

components, it was found that for direct transmissions at relatively short ranges, i.e., 5–10 m, a 

bit-transmission-rate of 400 baud was obtainable.

However, significant reduction in fidelity is associated with multipathing geometries between a 

transmitter and receiver pair. We found that reflections from surfaces frequently confused our 

basic system, requiring the bit-transmission rate to be reduced considerably. 

We found that by reducing the baud rate to around five bits per second the effects of 

multipathing can be reversed from detrimental to a geometric advantage, since the addition of 

reflected trajectories permits signals to be transmitted over non-line-of-sight paths, effectively 

propagating around corners. Such data rates would, of course, not be useful for propagating 

significant amounts of information such as might be required for meteorological or smart mine 

applications. However, such signals could prove useful in an Identification Friend-or-Foe (IFF) 

application where relatively few bytes of data could yield information necessary to verify the 

identity of nearby friendly forces. As an example, this technology could be useful when ground 

assault forces are advancing into an urban setting from multiple access points. 

We have thus documented what we believe to be an initial capability to perform such low data 

rate tasks for an urban scenario involving building wall reflections, or longer range clear-path 

data propagation. Further refinements are expected to be necessary to properly exploit such a 

capability, to include a more extensive study of varying frequency transmission modes, 

refinement of the signal discrimination methodology, the handling of noise sources, and 

(particularly) wind variability effects that shift propagated frequencies.
1

1. Introduction

For over 40 years the U.S. Army Research Laboratory’s (ARL) Battlefield Environment Division 

(BED) (formerly the U.S. Army Atmospheric Sciences Laboratory) has provided models, 

measurements, and effects related models predicting the impacts of near-earth atmospheric 

characteristics on U.S. Army systems and soldiers. Development of models to characterize 

problems such as the propagation of electromagnetic energy through battlefield dust, explosive 

clouds, background aerosols, and other aerosol contaminants have been a critical element of the 

program. Another component of this program was an active effort to assess the impacts of 

optical turbulence on developmental laser and imaging systems. Indeed, the role of turbulence 

(both thermal and kinematic) has been crucial in many aspects of these studies. Optical 

turbulence can have a highly deleterious effect on the propagation of coherent energy. 

Turbulence also manifests itself in its influence on the transport and diffusion of smoke and other 

battlefield obscurants, generating fluctuations in aerosol density. Hence, the measurement of 

turbulence, both optical and mechanical, has been an ongoing interest. 

1.1 Prior ARL Atmospheric Turbulence and Acoustics Efforts

Optical devices known as scintillometers are among the sensors used to characterize turbulence 

levels. However, scintillometers only sense integrated path optical turbulence. For the past 

decade, ARL has increasingly relied on ultrasonic acoustic anemometers, such as the R. M.

Young series 81000 three-component sonic anemometers commonly used for quasi-point high 

frequency (20 Hz) wind speed and direction measurements. These devices measure each of the 

orthogonal (U, V, W) wind speed components based on the difference between the crossing 

times of two oppositely-directed ultrasonic pulses that are passed through a sampled air gap over 

a very short time span. At the same time, these devices use a calculation of the mean 

temperature to correct for temperature fluctuations during measurement. This temperature 

measurement also facilitates a rapid sensing of point thermal fluctuations. 

Also, historically, ARL has supported the general characterization of atmospheric conditions 

during field experiments. A common method used in the remote sensing of lower level winds 

has been Sonic Detection And Ranging (SODAR). SODAR equipment generates atmospheric 

profiles of wind and thermal fluctuations using Doppler scattering from density fluctuations. 

This SODAR instrumentation operates on the principle of signal processing and mapping of the 

acoustic energy backscattered from discrete atmospheric gradients of temperature and/or winds. 

In particular, SODAR energy will scatter at the level of thermal inversions in the atmosphere at 

night. As an example of an ARL product developed with this technology, the Meteorological 

Measuring Set (MMS) AN/TMQ-41 Atmospheric Profiler prototype developed by BED 

incorporated a Radio Acoustic Sounding System (RASS) for low-level virtual temperature

profile measurements. This system provides the U.S. Army field artillery with a remote lower-
2

echelon meteorological measurement capability which would take the meteorological balloon 

with its associated logistics off the battlefield. The implementation of the MMS RASS involved 

the incorporation of three different acoustic sounding sub-systems, all of which operated in the 

mid-audible range of a few kiloHertz (Cogan et al., 1997). (These instruments were large trailermounted items weighing several hundred pounds each.) 

The SODAR and RASS units that ARL evaluated emitted high amplitude acoustic energy so that 

the pulsed signals generated would be powerful enough to meet the requirements for 

measurements at maximized altitudes. By operating in the audible range they took advantage of 

the relatively low molecular attenuation rates of acoustic energy at low frequencies. This 

allowed for increased altitude performance capability. However, this required a tradeoff–greater 

range for greater weight.

1.2 Motivation for Current Ultrasonic Propagation Research

Revisiting some of these concepts, it is apparent that not all sound frequency domains have been 

fully exploited. In particular, various applications for the ultrasonic spectrum may be feasible. 

One example is a technique for studying raindrop size distributions and rain rates using a bistatic 

ultrasonic backscatter system based on the variable Doppler shift due to the dependence of 

terminal velocity on droplet size (Bradley and Webb, 2001). 

It may also be possible to sense wind profiles using ultrasonic devices that detect energy 

scattered from boundary layer turbulence. Though the ranges of such instruments would be 

necessarily shorter, in several applications short ranges may be advantageous. The shorter 

wavelength of ultrasonic radiation would also permit reductions in size and weight of such 

instrumentation and support greater granularity in any measurements made.

However, the application considered in the present study is the potential for ultrasonic 

communications capabilities. With the proliferation of radio signals at nearly every frequency, 

plus the ability to jam or remotely monitor radio equipment, an ultrasonic system that does not 

rely on radio could prove useful. While such a system would operate, necessarily, at short 

ranges, any attempt to jam it would also need to be performed at short range. For a system such 

as a smart minefield, any system that attempted to jam an ultrasonic communications system 

operated by the field would itself be detectable as an intrusion, alerting the field. 

Another possible outdoor application would be in a weather reporting network of interconnected 

meteorological (met) sensors. Such a net would not need to operate in a stealth mode, because it 

would primarily be used for information purposes. However, from the standpoint of RF 

bandwidth, the sensors of such a network could be set up with clear lines of sight connecting 

adjacent sensors, permitting high data transmission rates, yet avoiding the assignment of separate 

RF frequencies to each sensor and avoiding the interference possible between adjacent IR links. 
3

1.3 Prior Art

While the field of ultrasonic communications is not extensive, several research programs have 

studied various aspects of the problem. Haynes et al. (2000) considered various fundamental 

aspects of such systems. They developed both digitally controlled systems and digital/analog 

hybrid prototypes. Their initial efforts were conducted in 1993, focusing on frequency 

modulation methods designed to modulate voice and music signals to ultrasonic frequencies, 

propagate these signals through air, and demodulate these signals through receiver hardware. 

Initially this equipment consisted primarily of phase-lock-loop and voltage-to-frequency 

conversion hardware. This work was patented in 1996 (Akerman et al., 1996). A follow-up 

program was conducted in 1998–1999 that transitioned their hardware to largely computer 

controlled (Laboratory virtual Instrument Engineering Workbench [LabVIEW]) modulation 

methods. This project studied propagation of ultrasonic signals through water and air filled 

pipes. These efforts followed an approach similar to those reported here: digital methods were 

used to communicate symbols based on the American Standard Code for Information 

Interchange (ASCII) character codes. Frequency modulation methods were employed to avoid 

variations in signal amplitude. This technique suppresses audible detection of breaks between 

transmitted data bits. Their use of pipes to direct sound waves is an effective methodology for 

eliminating the deleterious impacts of multipathing on inter-symbol interference. 

Holm (2005) reported on his analysis of the channel capacity of an indoor ultrasonic positioning 

system. This system consisted of tagging hospital equipment with ultrasonic emitters that 

communicated with in-room receivers. Equipment transmitted its identity when moved. Holm 

analyzed the bit data transmission rate possible for this system in the presence of interfering 

noise sources, Doppler shift effects, and reverberations (presumably due to multipathing). His 

analysis relied on Shannon (1948) information theory and considered a Frequency Shift Keying 

(FSK) modulation scheme. For this case he found that the maximum channel efficiency should 

approach 0.025 bits per second per Hz. He observed, however, that this efficiency is less than is 

achieved by human speech and by certain systems of whistled languages (Busnel and Classe, 

1976). Hence he concluded that advanced modulation techniques should exceed the above 

channel efficiency. For ultrasonic signals in the 20–25-kHz range using the above result we

would expect to achieve 500–625-bits per second (baud) data rates using FSK modulation.

Li et al. (2008) approached the problem of indoor ultrasonic communications by considering a 

higher frequency regime (above 100 kHz). Presumably higher data rates could be achieved by 

increasing the bandwidth. However, from the perspective of our research, higher frequencies 

reduce range capability due to increased atmospheric attenuation. However, this paper contains 

a much more complete review of the research field. They cite Heard (1983), who appears to 

have been the earliest researcher in this area. He employed a high powered amplitude 

modulation to generate signals. They also cite a patent by Zurek et al. (2002) that used an 

ultrasonic system to operate a wireless computer mouse. 
4

One might wonder that there are numerous ultrasonic conferences, yet Li et al., cited so few 

relevant documents. The answer is that our application area and the ultrasound field are both 

designated under the umbrella term of ultrasonics. And though the technology of ultrasound is 

extensive, the frequency regime at which this equipment operates (200–300 kHz) limits its 

application to our research. Ultrasound propagates primarily over short ranges within variable 

body media. Atmospheric propagation applications involve different attenuation rates, scattering 

due to temperature and wind turbulence, and variable signal absorption dependent on 

atmospheric state.

Due to these factors, and especially due to turbulent scattering, most ultrasonic communications 

systems have been designed for indoor use only. The focus of our research is for both indoor 

and outdoor use, consistent with Army applications. Section 2 considers modeling of this 

propagation environment.

1.4 Program of Investigation

Two approaches were followed for determining the characteristics and limitations of ultrasonic 

data communication networks. As a basic first approach, we measured the levels of sound 

propagated to different ranges under varying propagation conditions for a simple ultrasonic 

emitter and receiver pair. In more sophisticated second approach, a simple communication 

system consisting of a single set of ultrasonic emitter and receiver devices was devised and 

exercised to test the capability for transmitting information over different communications link 

path geometries. 

Several test site venues were used for early ultrasonic propagation experiments prior to 

establishing a more permanent site for these measurements. The first tests were performed by 

propagating ultrasonic energy down a hallway path in the basement of building 1622 at White 

Sands Missile Range (WSMR). The goal of these first tests was to study the sound level range 

dependence of a battery powered ultrasonic sound generator. However, these tests revealed the 

extreme sensitivity of ultrasound to reflections from walls in enclosed spaces such as the tunnellike hallway. These indoor tests produced inconsistent results and were moved outdoors to 

eliminate spurious reflections from hard walls. 

Thus, a second venue for the ultrasonic propagation tests was in a flat, open, dirt-covered area 

behind building 1622. Results for ranges up to 16 m were measured. Here too, inconsistent 

variations in received acoustic intensity versus range (as measured by a Sound Pressure Level 

[SPL] meter) raised questions concerning the relative impacts of SPL losses due to molecular 

signal attenuation effects, atmospheric scattering due to turbulence, and the effects of 

multipathing. In these first outdoor experiments, the emitter used was not focused in any way, so 

that the close proximity of the ground to the acoustic path could well have enhanced multipath 

contamination of the direct-path signal.
5

The inconclusive nature of these first two indoor/outdoor tests near building 1622 led us to 

relocate to a more controllable environment at the building 19472 compound south of the main 

base area at WSMR. The advantages of this site are its remote location, where sound-based tests 

will not interfere with other range or ARL operations, as well as the ability to leave the test range 

in a particular configuration indefinitely. In particular, a scaffold was erected at a location on 

this site that permits the use of a focused emitter on a slant-path that substantially reduces the 

influences of ground reflections. The facility layout, equipment, safety procedures, and the 

testing schedule adopted are described in section 3.

With this new facility configuration, experiments were then designed to characterize the 

performance of ultrasonic communication proof-of-concept systems, with respect to acoustic 

propagation in general and to ultrasonic FSK coded propagation in particular. The observed 

effects of atmospheric attenuation, wind conditions, and multipath interference on coded signal 

propagation and fidelity are discussed below. The LabVIEW-based software developed to 

provide Amplitude Modulation (AM) and FSK modulation and demodulation of propagated SPL 

measurements are described in section 4. An analysis of the AM and Frequency Modulation 

(FM) binary-coded acoustic signals of ultrasonic, and audible, frequencies generated and 

received are evaluated in section 5. Section 6 also includes a discussion of the salient features 

and limitations of such systems under varying conditions. Section 7 then presents conclusions.

2. Semi-Empirical Ultrasonic Absorption and Scattering Models and Early 

Experimental Findings

The literature concerning models of ultrasonic atmospheric absorption and scattering from 

atmospheric turbulence has a history extending back several decades. The acoustic scattering 

and absorption model described by Shamanaeva and Burkatovskaya (2004) is a convenient 

distillation of various prior formulations. Acoustic energy absorption is divided primarily into 

classical and molecular absorption effects. Models of these influences derived from earlier 

publications by Neff (1975), Sutherland (1968), and even earlier works cited by Sutherland. 

Formulations for scattering of acoustic energy from temperature and wind turbulence used in this 

analysis were derived from Shamanaeva (1983) and Baikalova et al. (1988). In this section, we 

shall summarize these models and then compare these with some of data collected during our 

earliest outdoor experimental results. 

2.1 Acoustic Absorption and Scattering Cross Sections

We first consider a simplified theory for atmospheric absorption and scattering cross sections for 

ultrasonic energy. Of course, our interest in both these factors is not merely academic. We 

intend (ultimately, for some applications) to attempt to measure wind scattering of acoustic 
6

pulses. Likewise, molecular absorption of ultrasonic energy will strongly affect the level of path 

losses in virtually all potential applications. 

In this instance, we are only interested in direct propagation between a source and a receiver (in 

the absence of reflections). Therefore, the received to transmitted power ratio may be expressed 

as

2 2

0 / / P P Tr R R r t = , (1)

where the factor Tr is the path transmission, R is the range, and R0 is the baseline range 

(nominally 1 m) at which the initial power level is measured. The transmission factor 

encapsulates various loss mechanisms, and can be further expressed as 

Tr = −+ − exp ( )( ) [ α α a t R R0 ], (2)

where αa and αt are the absorption and turbulent scattering coefficients [m–1], respectively.

Kaye and Laby (2009), provide tabular results for the molecular attenuation coefficients, based 

on the ISO 9613-1 standard (1993). Representative values are given in table 1. These data 

reflect an interesting property of the absorption of acoustic energy which depends on the 

interactions between the acoustic energy, atmospheric density, and molecular absorption 

resonance frequencies.

Table 1. Attenuation of sound in air (dB km−1).

Frequency

(kHz)

Relative Humidity %

10 20 30 40 50 60 70 80 90

4 110 75 49 36 30 26 23 21 20

8 180 220 170 130 110 89 78 69 63

12.5 210 360 340 280 240 200 180 160 140

16 230 430 470 420 360 320 280 250 230

20 260 510 600 580 520 470 420 380 350

25 300 580 740 770 730 680 620 570 520

An algorithm modified by Desart (2004) calculates these coefficients using a relatively simple 

algorithm. A closer consideration of variations in humidity (which will be given in following
7

subsection) would be useful in future measurements, because model results indicate that even 

under extremely dry conditions, 2–3-dB losses would be experienced over 10 m propagation 

ranges. Our measurements have all been performed on relatively dry days that are characteristic 

of New Mexico climate, where relative humidity levels of 20% or less are the norm.

We now examine Ishimaru’s approach for predicting the impacts of turbulent scattering. He 

defines the acoustic scattering cross section per unit volume (Ishimaru, 1978, pg. 344, equations

[16–58]) as, 

4 2 ( ) 2 cos ( ) ( ) t n s αθ π θ κ = k Φ , (3)

where κ 2k sin(θ / 2) s = is a sampling wavenumber, θ is the scattering angle between the 

direction of the incident sound wave and the scattered wave, and k = 2π / λ is the radiation 

wavenumber. 

( ) Φn s κ is the turbulence spectral density given by 

2 22

0 0 ( ) cos ( / 2) ( ) / ( ) / 4 n s V s Ts Φ = Φ +Φ κ θκ κ c T , (4)

where ( ) ΦV s κ and ( ) ΦT s κ are the spectral densities of the velocity field and the temperature 

field, respectively, 0 c is the mean speed of sound [m/s], T0 is the mean temperature [K], and the 

spectra are given by,

2 11/3 ( ) 0.061 V s κ κ CV s

− Φ = and

2 11/ 3 ( ) 0.033 − ΦT κ s = CT κ s , (5)

where 2 CV and 2 CT are the structure parameters of velocity and temperature, respectively. 

Ishimaru lists typical ranges for these parameters as 4 2 10 1 CV

− < < [(m/s)2

/m2/3] and 

32 1 10 10 CT

− − < < [deg2

/m2/3]. 

These spectral forms, however are only valid for spatial frequencies occupying the inertial 

subrange: Lo s o 1/ < κ < 1/  , where Lo and lo are the inner and outer scales of turbulence, 

respectively. 

For a somewhat more quantitative range estimate for 2 CT , its relationship to the refractive index 

structure parameter, 2 Cn , can be considered. It is well known that typical values for 2 Cn at 

heights within the first 20–40 m of the surface generally range between 15 13 10 10 − − − m–2/3. One 

can estimate the value of 2 CT based on the expression, 2 24 22

0 / C CT AP T n = , where 6 79 10− A = ×

K/mbar, and P is the atmospheric pressure in millibars. Using standard pressure and 

temperature values (1013-mbar, 288-K), 2 CT ranges between 0.00107 and 0.107. So the low 

value obtained is under weak turbulence, and the high value used is under typical daytime 

conditions at WSMR. We also note that the maximum scattering due to wind fluctuations occurs 

in the forward direction, while there is zero contribution due to wind in the backscattering 
8

direction. Also note that Ishimaru’s (1978) form cannot be correct in the exact forward 

scattering direction since the sin( / 2) θ dependence in κ s when introduced into either spectrum 

produces an approximate 11/3 θ− divergent behavior at zero scattering angle. We attempted to 

observe results in the forward scattering direction, so the failure of the Ishimaru (1978) 

functional form in this domain is a matter of some concern. Fortunately, other models of a semiempirical nature (e.g., Shamanaeva, 2004) are available to estimate the forward scattering effects 

at ultrasonic frequencies. Shamanaeva’s model (which is based on a series of earlier results) will 

now be presented in the next subsection. Unfortunately, Shamanaeva used a different pair of 

governing variables: acoustic frequency, f, and wavelength, λ.

2.2 Semi-Empirical Formulation of Absorption and Scattering Equations

Shamanaeva’s approach to modeling the total attenuation coefficient is similar to that presented 

in equation (2), where the total attenuation coefficient can be divided into its molecular and 

turbulent scattering elements. 

The classical component of the acoustic absorption coefficient (αcl , in units of m–1) represents 

the combined impact of viscosity, heat conduction, diffusion, and energy transfer from the 

acoustic wave to molecular rotational modes. It is given by the empirical relation

11 2 4.02 10 cl α f − = × . (6)

In addition to the classical absorption coefficient, there is an even larger (at ultrasonic 

frequencies) molecular absorption coefficient (also in units of m–1). To formulate this term, one 

first evaluates the frequency of maximum molecular absorption due to energy transferring from 

acoustic waves to the lowest excited vibrational molecular states of diatomic oxygen and 

nitrogen. This frequency maximum is given by the semi-empirical expression 

2 0.8

0 0 11 ( ) 10 6600 ( ) 44400 ( ) ( ) / ( ) , Max i i i i i f z =+ +   h z h z Pz T z   (7)

where

1

1

1

0 1

( )

1

1

( ) ( ) /1014 ,

( ) (1.8 ( ) 492) / 519 ,

( ) 100 ( ) / ( ) ,

( ) 0.06107 10 ( ) ,

( ) 7.63 ( ) / [241.9 ( )].

i

i i

i i

i ii

M z

i i

ii i

Pz Pz

Tz Tz

h z e z Pz

e z U z

M z Tz Tz

=

= +

=

= ×

= +

(8)

Here, zi represents the height at which the calculation is performed, since most atmospheric 

properties vary primarily with height above the surface. In the analysis to follow height is set at 

1-m above ground level (AGL) for consistency with our early measurements. Other 

meteorological parameters are the relative humidity, U(zi), in percent; atmospheric pressure, P, 
9

in millibars (mB); temperature, T, in degrees Centigrade (°C). The ubiquitous appearance of the 

i subscript allows for a layered atmosphere within the computer model.

The first term in equation (7) represents the relaxation frequency for oxygen and nitrogen 

collisional absorption in dry air. The second term is associated with oxygen/nitrogen collisional 

interactions with water vapor and the third term addresses the effects of collisional interactions 

between water vapor molecules. 

At the peak frequency, fMax, the maximum molecular absorption coefficient is given by

{ } 2.5 1

1 1 ( ) 0.0018 ( ) ( ) exp 7.77[1 ( )] . Max i Max i i i α z f zT z T z − − = − (9)

These two parameters can then be used in modeling the general molecular absorption coefficient 

(expressed in m–1 units) as,

[ ]

1/2 2 2

2

2

( ) 2 () ( ) 0.18 ( ) , 304.8 1 ( )

Max i k i

mol i k i

k i

z f z

z f z f z

α

α

    =   +     +  

(10)

where another dimensionless parameter is introduced:

( ) / ( ) . k i Max i f z = f f z (11)

The total absorption coefficient is obtained by summing equations (6) and (10), i.e., 

ααα a cl mol = + . 

For turbulent scattering effects, Shamanaeva (2004) used the von Karman turbulence spectrum to 

evaluate volume scattering functions for temperature and wind coefficients, ααα t TV = + , given 

in the form,

7/6 7/3

1/3 2 2 7/3 2 5/6 5/3

0

1/6 1/3

0.07143 ( ) ( )

( ) 0.9 ( ) ( ) ( ) ( ) 0.1 ( ) ( ) ( )

() () ()

i i

T i i Ti i i i i i

ii i

Bz z

z zCzT zL z Az B z z

Az B z z

λ

α λ λ

λ

− − − − −

    −    

=   − −       − −      

, (12)

[ ]

( )[ ]

2/3 1/3 2 13/3

0

7/6 7/3

13/ 6 13/3

1/6 1/3

2 5/6 5/3

( ) 1.569 ( ) ( ) ( ) ( )

0.1429 ( ) 2 ( ) ( ) ( )

0.0769 ( ) ( )

() 2() () ()

0.2 ( ) ( ) ( ) ( )

V i i i i i

ii i i

i i

ii i i i

ii i i

z z zc zL z

Bz Az B z z

Bz z

A z Az Bz B z z

A z Bz B z z

α ελ

λ

λ

λ

λ

− −−

− −

= ×

 + −     

 − −     

 −+ −      − −     













  

, (13)

introducing parameters,
10

( ) 4 ( ) ( )

( ) 2 ( ) ( )

2 2

0

2 2

0

i i i

i i i

B z L z z

A z L z z

λ

λ

= +

= + . (14)

Here, λ(zi) is the acoustic wavelength at height zi , c(zi) is the speed of sound, L0(zi) is the outer 

scale of turbulence, CT

2

(zi) is the structure parameter of the turbulent temperature field, and ε(zi) 

is the dissipation rate for turbulent kinetic energy. The temperature structure parameter may in 

turn be derived from the more familiar refractive index structure parameter Cn

2

(zi) via the 

expression,

2

5

2

2 2

7.95 10

( 273.15) ( ) ( ) 















×

+ = − P

T C z C z T i n i . (15)

As an initial study, meteorological parameters were set to values consistent with our earliest 

outdoor WSMR-measurements of acoustic extinction conducted on the afternoon of 6 Nov 2008. 

The ambient conditions present were: relative humidity, U(zi) = 12%; pressure, P = 880 mB; 

temperature, T = 13.3 °C (56 °F). The WSMR-area value for the refractive index structure 

constant was estimated for the appropriate time of day (approximately 1600) using a model 

derived from the High Energy Laser Propagation Handbook (Hummel, 1984); this value was 

estimated as Cn

2

(zi) = 8.1x10–15 m–2/3. The outer scale was set equal to the height of the 

transmitter/receiver setup: zi = 1 m. A nominal daytime dissipation rate of ε(zi) = 5x10–3 m

2

s

–3

was selected based on Stull (1988) for use in computing the wind scattering coefficient of 

equation (13). 

However, relative humidity data were not taken directly. Instead, humidity data were obtained 

from a nearby surface station. The sensitivity of equations (7) through (11) to humidity 

variations is highlighted by figure 1, which displays the total absorption coefficient as a function 

of relative humidity at an acoustic frequency of 20 kHz for the temperature and pressure 

conditions indicated. The total absorption curve is the sum of the classical (equation 6) and 

molecular (equation 10) results. For all but the lowest relative humidities, the molecular 

absorption effect is dominant. The nominal relative humidity of 12% for our example case 

yields a total absorption coefficient of 0.054 m–1 (or about 0.24 dB m–1

). Note from equation (7) 

that the relaxation frequency fMax is essentially constant at very low relative humidities and 

increases very rapidly at high relative humidities. Thus, for the fixed ultrasonic acoustic 

frequency f, the molecular absorption coefficient versus relative humidity curve will show a peak 

for those relative humidities where the relaxation frequency fMax approaches f. This peaked 

behavior is also seen at some the higher acoustic frequencies displayed in table 1.
11

Figure 1. Absorption coefficient versus relative humidity at f = 20 kHz (P = 880 mB, T = 13.3 °C).

The scattering coefficients for wind and temperature turbulence given by equations (12) through 

(15) are compared with the total absorption coefficient as a function of acoustic frequency in 

figure 2.
12

Figure 2. Total, temperature scattering, wind scattering, and total absorption coefficients as a function of 

frequency for the example case.

As noted by Baikalova et al. (1988), both the temperature and wind turbulence scattering 

coefficients exhibit a quadratic increase with frequency, a trend that is evident in figure 2. This 

behavior is readily seen in equations (12) and (13) by noting that for most practical observation 

heights zi, L0 >> λ at ultrasonic frequencies, since L0 is on the order of meters, while λ is on the 

order of centimeters or less. Thus parameters A and B (equation 14) are practically constant with 

frequency, and the dominant terms in the quantities in brackets in equations (12) and (13) are 

those containing the λ

–5/3 factor. Baikalova et al., also note that these scattering coefficients scale 

as L0

5/3, again due to the dominant terms containing the λ

–5/3 factors in equations (12) and (13).

2.2 Comparisons of Attenuation Data with Models

Our initial outdoor experiments were conducted using a very simple ultrasonic transmitter and a 

bare microphone SPL meter receiver. The SPL instrument’s output used a simple analog needle 

panel meter that had to be manually read and recorded. Figure 3 compares the observed SPL 

data with inverse square curves chosen to intersect with the 1-m range data. 
13

Figure 3. Comparison of SPL measurements with inverse square curves at 10-, 15-, and 20-kHz acoustic 

frequencies, 06 Nov 2008.

Figure 4 shows least squares power law fits to these data. The inverse square trend was removed 

from the data in figure 5. Note the increased scatter in these detrended results, which is possibly 

indicative of reflected energy contributions.
14

Figure 4. Power law fit to measured SPL data, 10-, 15-, and 20-kHz acoustic frequencies, 06 Nov 08.

Figure 5. SPL field measurements with inverse square trend removed and fitted with a power law 

curve, 10, 15, and 20 kHz, 06 Nov 08.
15

The SPL data plots indicate that most of the attenuation observed was due to inverse square 

spreading from the quasi-point source. The residual plot of figure 5 provided values for 

attenuation coefficients at the measured frequencies in table 2.

Table 2. Residual attenuation coefficients computed from data of figure 5.

Frequency (kHz) Attenuation (dB/m) Attenuation (m-1

)

10 0.08 0.018

15 0.19 0.044

20 0.28 0.064

Comparing the results of table 2 with the model predictions of figure 2, it is apparent that the 

observed attenuation coefficients are considerably smaller than the modeled total attenuation. 

However, these early observations did not have any direct measurement of turbulence from 

which CT

2 could be estimated. There is also the strong possibility that ground surface reflections 

could have significantly contributed to the received SPL signal. It is true that the observed 

residual values of table 2 are comparable to the individual total absorption or wind turbulence 

scattered signals. 

Another factor affecting scattering behavior in this spectral range is the nominal ultrasonic probe 

wavelength. For a typical speed of sound of 0 c ~340m/s, a 20-kHz probe frequency yields a 

propagation wavelength on the order of ~1.5 cm. This equates to a wavenumber 

2 4 / 831 s κ πλ = = = k m–1

. These probe wavelength and wavenumber values are on the order of 

those that are characteristic of the inner scale of boundary layer turbulence, which has winddependent values that range from roughly 2–8 mm for length lo or 125–500 m–1 for the 1/ κ s o ≈ 

wavenumber. We might thus be able to avoid some deleterious path related scattering by 

increasing the probe frequency. This will, however, have little effect on Doppler shifting of 

signals due to along-path wind speed variations and will not be able to avoid increased 

attenuation (see table 1) due to molecular absorption effects. 

Microphone noise due to atmospheric density fluctuations advecting past a sonic receiver was 

also studied as a potential problem during our measurements. After some consideration, it was 

decided that such noise would not present a significant obstacle. Since ambient density 

fluctuations will have size scales comparable to the inner scale (a few millimeters) or greater,

and average wind speeds will be typically less than 8 m/s (less than 15 mph), it is expected that

the bandwidth of acoustic fluctuations should be on the order of 8 kHz (8m/s/1mm = 8 kHz). 

Testing at short range or with no external signal source present did not produce noticeable noise 

signals at frequencies over 10 kHz (though typically measurements have been taken on calm 

days so far). Unless high frequency noise sources are located within a short range of the 

receiver, it is also expected that strong atmospheric attenuation will suppress such noise 
16

signatures. Hence, we do not expect to encounter strong atmospheric high frequency noise 

sources within range of a receiver.

We may summarize our assessment of atmospheric effects upon ultrasonic propagation as the 

following: For propagation paths out to 30 m the losses incurred in the signals due to radial 

velocity, molecular, and scattering effects do not appear to significantly impede useful signal 

transmission. Even 30-dB losses are well within the limits for many typical acoustic detection 

systems to handle. Of more critical interest is the actual dynamics of testing an acoustic 

communications system as described next.

3. Infrastructure Development

As discussed in section 1, a program was developed to consider feasibility of ultrasonic 

communications in a field environment. The set of experiments that evolved from this concept 

took place primarily during the late fall of 2009, a time of year that was fortuitous for milder 

weather conditions that generally prevail throughout the October through November period in 

southern New Mexico.

3.1 General Site Description

The site chosen for the setup of the main testing of the prototype communications system 

developed was the building 19472 compound (figure 6) to the south of the main cantonment area 

of WSMR. The region surrounding this compound is relatively flat and treeless terrain 

characteristic of the high Chihuahuan desert. The main vegetation found are the yucca and 

creosote bushes, both standing approximately a meter high, interspersed with wide grasses and 

redroot pigweed (amaranthus retroflexa). The dirt is alluvial, derived from sand eroded from the 

nearby Organ Mountains. The proximity of the mountains creates a slight tilt in the general 

terrain due to the alluvial skirt, affecting the local vertical for wind flow considerations. The 

GPS location for building 19472 is: N 32° 21’ 15.4’’, W 106° 27’ 57.7’’. 
17

Figure 6. Building 19472 compound on WSMR, NM, and its environs.

Although the site is nearly flat, there are a few nearby perturbing features to optimal smoothterrain wind flow. These are located primarily to the east-southeast and the southwest of the 

compound involving a minor gully (arroyo). The 19472 compound itself, as shown in figure 6, 

consists of the single story main building surrounded by a number of parked trailers, small 

storage shelters, vehicles, a Morgan storage building, and a chain-link fence enclosure. For scale 

perspective, building 19472 measures 44 ft×32 ft (the building’s shadow is visible within the 

compound), the southeast fence line measures 201 ft, and the NNE fence line measures 136 ft, 

and the southwest corner of building 19472 is 73 ft from the WNW corner of the fence line. 

In the figure 6 satellite image, the dark dots seen scattered about the outside of the fence line are 

relatively intermittent but significantly-sized mesquite bush patches that are approximately 2 m 

high and up to several meters wide. A thicket of these bushes is contained within a shallow 

arroyo to the west and south of the site. 

A series of improvements were made to the 19472 site for the ultrasonic communications tests. 

Among these was a general clean up of equipment left over from previous projects and the 

installation of a scaffold near the main gate of the compound (figure 7). This scaffold permits 

the taking of acoustic data from a height of approximately 4-m AGL. By placing the receiver at 

this height, the influence of ground reflections (multipathing) can be minimized. Removing this 

effect from the received signals was believed to be a significant step in decoupling the impacts of 

molecular attenuation, multipathing, and atmospheric turbulent scattering from the resulting 

propagated signal strengths in testing.
18

Figure 7. Acoustic path receiver scaffold used in power vs. range testing at 

19472 site. The top of building 19472 is seen in the background. 

Acoustic receiver is seen at the top of the scaffold’s second level.

Additionally, the road leading to the 19472 site was recently paved, allowing us to apply 

permanent painted markers to the roadway so that specific propagation slant paths can be 

repeated based on distances measured to a receiver mounted on the scaffold’s upper level. The 

site therefore provides repeatable conditions for future prototype testing. 

Unfortunately, while these site preparations were underway, a serious electrical safety accident 

occurred at an ARL testing facility at Aberdeen Proving Ground that significantly impacted our 

test planning. As a result of that fatal accident, ARL field testing operations were suspended 

until the safety procedures for all testing facilities within ARL could be reviewed and recertified 

and all personnel retrained in electrical safety. In response to this review, a series of precautions 

and training actions were taken as indicated in the following section.

3.2 Safety Concerns, Preparations, and Training

In order for any field experimentation to be successfully completed, an evaluation of the risks 

involved must be conducted prior to implementation of the study. As the test plan is being 

drafted, and the risk analysis has been performed and reviewed, the personnel involved must be 

briefed on the safety concerns. Before undertaking the test, it must be confirmed that all 

involved personnel have current safety training tailored for the particular tasks of the field test. 

For this test, areas of concern were scaffolding use, electrical safety, and ultrasonic effects on 

human hearing at the frequencies and power levels of interest. To ensure that the best possible 

working environment was in place, ARL safety was involved in the risk analysis and in 
19

reviewing the safety standard operating procedure (SOP) developed for the test (see the 

appendix). 

To address the scaffolding safety issue, all personnel involved were required to complete the 

OSHA training regarding scaffolding prior to operations with the tower at building 19472. The 

tower had already been emplaced some time before the attendant heightened safety concerns had 

been raised, so during the subsequent safety stand-down period and before the final ARL 

approval of the new safety SOP, no use of the scaffold was made. 

The electrical safety issues were addressed by having the involved personnel take a series of 

several ARL electrical safety training sequences including laboratory and field operations and 

evaluating the electrical risk for the test. The electrical risk for this test was low, as the power 

levels involved are relatively small. Also, the systems in use are all UL approved commercial 

off-the-shelf (COTS) components.

The effect of ultrasonic radiation on human hearing was also researched and the results of those 

findings were included in the safety SOP for review and approval. As analyzed, such effects 

would only be significant if the sound was being propagated under water or at sound intensity 

levels approximately one hundred times more powerful than those used in these tests. Therefore,

the energy levels for this testing were sufficiently low that the safety concerns were minimal. 

This is also a valid operational conclusion as well. It is not expected that the use of such devices 

would be harmful to troops under operational circumstances. 

The required safety controls for site access during testing, however, did affect physical security 

at the site. To ensure that only authorized or permitted personnel are on site during testing, a 

checks system was initiated to ensure that the system was not energized while personnel were in 

locations that would subject them to the highest levels of energy.

Summarizing, all test personnel completed the mandatory training for this type of work, which 

included OSHA scaffolding use and/or installation training as well as ARL’s electrical safety for 

field experimentation sequence.

3.3 Angular Antenna Pattern Testing

As a preliminary development, a set of acoustic horns representing different form factor designs 

was purchased to assess the impact of form factor upon ultrasonic emission patterns in different 

acoustic propagation regimes. For instance, when focused energy needs to be propagated, a 

narrow horn would provide a better concentration of acoustic energy than a wide horn. On the 

other hand, when propagating a signal for detection purposes when wind effects might tend to 

deflect a tight beam off a point receiver, a narrow horn might not be the best option. In this case, 

a wider pattern source beam is desirable.

To characterize this series of horns (plus an emitter mounted to a parabolic dish), a series of 

measurements were performed on each. This testing was conducted on an angular layout 
20

positioned between building 19472 and the “Morgan” storage building of the 19472 compound. 

The distance from the emitter to the microphone was 8 ft 8 inches, and the microphones were 

then moved off axis in +/–15 degree steps all the way to 90 degree off axis point. The complete 

scan went from 0 degrees, to +90 and –90 degrees off axis for each antenna tested. The main 

detection instrument used for these measurements was the SPL meter. A continuous tone was 

broadcast for 30 seconds for each angular setting. The mean SPL value was then post-processed 

using a spreadsheet program in Linux. For these experiments an additional uncalibrated 

microphone with very flat response and good ultrasonic sensitivity was also used for future 

comparison purposes as seen in figure 8. 

Figure 8. SPL sensor microphone (bottom) and

uncalibrated microphone configuration.

A photograph of each horn (along with the parabolic dish) and its measured antenna pattern are 

depicted in figures 9–16.

Figure 9. “Trumpet”-style horn (narrow beam).
21

Figure 10. “Trumpet” horn antenna pattern. Sonic intensities at 5-, 10-, and 20-kHz frequencies 

(green, blue, and red lines, respectively).

Figure 11. Parabolic dish (narrow beam).
22

Figure 12. Parabolic dish antenna pattern. Sonic intensities at 5-, 10-, and 20-kHz frequencies 

(green, blue, and red lines, respectively).

We note that the antenna patterns of the trumpet horn and the parabolic dish provide the two 

narrowest patterns at small angle. However, while the parabolic dish supplies a very rapid drop 

at small angles, it does not continue to drop. Rather, it plateaus at approximately a –25 dB loss, 

while the trumpet horn pattern, particularly at the highest frequency range (20 kHz) exhibits a 

very high attenuation at large off-axis angles approaching –50 dB.
23

Figure 13. Round (wide mouth) horn. 

Figure 14. Round horn antenna pattern. Sonic intensities at 5-, 10-, and 20-kHz frequencies 

(green, blue, and red lines, respectively).
24

Figure 15. Rectangular (wide mouth) horn.

Figure 16. Rectangular horn antenna pattern. Sonic intensities at 5-, 10-, and 20-kHz frequencies 

(green, blue, and red lines, respectively).
25

We note that the pattern specified for the rectangular horn was measured in the plane that 

contained the widest aperture dimension. This plane is tilted slightly from horizontal in figure 

15. 

In comparing the patterns of the two wide-angle horns, it is interesting to note the differences. 

While the rectangular horn appears to exhibit similar behavior for both the 10- and 20-kHz 

results, the 20-kHz case for the round horn is significantly more focused than the results of either 

5- or 10-kHz signals. Based on this behavior, the rectangular horn appears to be more frequency 

independent, which should be more useful in testing cases involving broadcast signals that we 

intend to disperse uniformly in angle.

4. LabVIEW Software Developments

In order to conduct the necessary experiments for our research, a variety of tools were assembled 

to characterize and implement the communication system process. These include hardware for 

transmission, reception and measurement of acoustic signals, meteorological sensors to measure 

atmospheric conditions, as well as software codes to generate signals, support the data 

acquisition of these signals, and analysis codes to assess the quality of transmission channels 

under varying meteorological conditions. These functions were integrated into laptop computer 

systems that hosted software control of hardware, data acquisition, and data analysis.

The signal generation and data collection for propagation studies were accomplished using 

control software based on the National Instruments (NI) LabVIEW program. LabVIEW is a 

general instrument control program that utilizes a visual programming language suitable for 

receiving, filtering, and manipulating data streams in user-configured applications termed 

“virtual instruments” (VI). Each VI simulates a system through a graphically correct “Front 

Panel” that includes computer simulated control devices (slider bars, control knobs, switches, 

etc.) that implement the functionality of the measurement and control interface. This Front Panel 

is then connected to a virtual circuit board (termed the “Block Diagram”) that emulates the 

functional interconnections behind an actual instrument panel. LabVIEW provides a series of 

user designated processing elements on the block diagram that effectively simulate the logic and 

control operations of a traditional programming language. The principal difference between 

LabVIEW and an abstract language is that LabVIEW connects to real instrumentation through 

NI hardware interface modules that are controlled by the LabVIEW software.

Two VIs were developed for testing purposes. One VI functioned as a waveform generator to 

create tones of a specified frequency, amplitude and duration. This tone formed the test signal, 

and was sent through an analog output to an emitter amplifier and then to a speaker with attached 

focusing horn. A second VI acquired and logged the measured data from a SPL meter. Data 

from a sonic anemometer was also logged by this VI for future analysis, allowing the sound data 
26

to be correlated to variations in wind speed, direction and/or turbulence. Measurements from 

both the SPL meter and sonic anemometer were synchronized and time stamped, then saved to 

disk at a sample rate of 20 Hz. 

The front panel of the emitter controller is shown in figure 17. The figure illustrates the control 

widgets that select the signal duration, frequency, amplitude, and sampling rate. In figure 18 the 

front panel of the SPL data collector is shown. The upper portion of the front panel controls the 

input data ingest from the SPL meter, while the lower portion monitors the port interface, 

windowed data, and storage for the sonic anemometer data. The “base spl” control indicates the 

minimum reading level allowed by the meter (set through a hardware control knob on the meter 

itself) plus a file name in which the data is stored. L2W sets and indicates the sampling rate for 

data collection. In the lower portion of the SPL collector VI panel, the critical elements are the 

data baud rate produced by the sonic anemometer sensor output interface and the input port 

address. 

Figure 17. Front panel for signal generator used in dispersion experiments.
27

Figure 18. Front panel for SPL and wind measurements used in dispersion

experiments.

The ultrasonic emitter hardware consisted of a Motorola Powerline piezoelectric horn driver 

mounted to an appropriate horn (usually the rectangular aperture horn since it has the widest 

dispersion pattern and lowest frequency sensitivity). The driver/horn system was fed by a small 

computer speaker amplifier in this series of tests. The amplifier/driver system can produce 

acoustic energy at frequencies from 1.8 to 30 kHz, and was effective for most of the signal 

generation needs of our program. The sound pressure levels were measured by a Quest model 

155 sound level meter equipped with an OB-133 1/3 octave filter. Both the signal output to the 

amplifier and the input from the sound level meter were routed through an NI USB-6281 

multifunction data acquisition (DAQ) module. This data acquisition system allows multichannel 

collection of up to 500-k samples per second at 18-bits accuracy, with transmission of up to 1-M 

samples per second at 16-bits accuracy, and can fulfill data acquisition needs for many 

applications. The sonic anemometer used was an R. M. Young model 81000 device. RS-232 

formatted data transmitted by this device was also recorded from the COM1 serial port of the 

computer. 

After the preliminary measurements that characterized the sound intensity patterns emerging 

from the three horn devices and the parabolic reflecting dish were made, propagation 

measurements were then performed using LabVIEW transmitter/receiver control and analysis 

codes that we developed. These codes characterized the general influences of range attenuation 
28

due to 1/r2 losses, atmospheric absorption of sonic energy, and acoustic scattering of sound as a 

function of range. These results are described in section 2. 

A subsequent LabVIEW code development phase involved the building of a rudimentary 

communications system to allow the functional testing of the feasibility of ultrasonic modulation 

of digital signals. Here, a LabVIEW VI was designed to modulate and demodulate ultrasonic 

waveforms such that ASCII character data would be transmitted. This software was initially 

developed using AM pulsing, but was then adapted to perform FSK modulation. 

The transmitting software decomposed a string of ASCII characters into equivalent byte strings 

and added start and stop framing bits to the end of each byte, as in the RS-232 protocol. The 

software then encoded each bit as a pulse of ultrasonic sound at the appropriate frequency. In the 

AM mode, only “1” bits would be transmitted at the carrier frequency. In the FSK 

implementation, the “0” and “1” bits were each assigned unique carrier frequencies. 

The hardware used for data transmission in this study was the same as used for signal emission 

during the emission pattern propagation tests. However, on the receiver side of the data 

transmission tests, the SPL meter was replaced by a high fidelity Earthworks M50 measurement 

microphone, used in conjunction with a Grace Design M101 preamplifier. The M50 microphone 

was particularly suited to our measurement program due to its very flat frequency response 

between 5 Hz and 50 kHz, amounting to a ripple of less than 3 dB over this frequency range. 

The preamplifier used in this phase of our investigation allowed a very wide range of 

measurement with 65 dB adjustability in signal gain. The signal from the preamplifier could be 

acquired either by the USB-6281 used by the transmitter, or an NI Compact DAQ modular 

chassis with an NI 9205 module. The Compact DAQ chassis is more affordable, but is limited to 

250-kilo samples per second with 16-bit accuracy for input measurements using the NI 9205 

module.

The receiving software VI module was then designed to listen continuously on a microphone 

input for a signal on the “1” carrier frequency, used by the start bit of each byte. Upon 

identification of a clear tone of this frequency, the receiver software was then programmed to 

collect a 10-bit signal string appropriate for one ASCII byte. At the end of each bit, the data 

received is analyzed for frequency and assigned a 1 or 0 value depending on an assessment of 

which frequency it most strongly matched. Upon collecting all 10 bits, the bits were 

reassembled into an ASCII byte value and output to a string updated continuously in the “String” 

box of the software front panel as shown in figure 19.
29

Figure 19. Front panel for FSK data transmitter.

Figure 19 also indicates that the user can dynamically select the identities of the frequencies used 

for encoding the 0 and 1 bits. As an example of the wiring diagram methodology used to 

implement the LabVIEW block diagram construct, we depict the structure of the FSK transmitter 

code in figure 20. The main features of interest in this code diagram are the large outline boxes 

on the right side of the figure. These represent a double looping structure associated with the 

process of decomposing each ASCII character (outer box) into its individual bits and encoding 

these (inner box) onto a general data stream that is then output to the string of small boxes to the 

right of the main boxes. 

Figure 20. Block diagram of FSK data transmitter.
30

These smaller boxes represent the transfer of the main data buffer to the NI DAQ hardware. The 

boxes at the left of the main boxes represent control and data buffer initiation steps needed to 

generate the data buffer and determine the nature of the subsequent encoding of the data bits.

To determine the fidelity of the data transmission process a series of tests were performed using 

the above equipment. The results of these tests are described in the following section.

5. Propagation and Communications Characterization

Following the development of safety standards and prototype software models of transmitted 

power and communication systems, the tools described in the previous section were tested at the 

building 19472 site to determine the feasibility of an ultrasonic communication system. The first 

stage of this testing was a rather simple task, designed to benchmark the capabilities of the 

system to detect single-frequency sound levels as a function of range. The primary objective was 

to determine the validity of a model predicting molecular absorption rates of ultrasonic signals. 

To test this model we performed a series of tests along the road leading into the 19472 

compound. 

5.1 Slant-Path Propagation Tests

The purpose of these measurements was to achieve a more comprehensive and workable 

understanding of the general loss of signal with range at different frequencies across the audible 

and ultrasonic ranges (up to 24-kHz frequency). The primary loss mechanism would be the 1/r2

loss rate associated with the near-spherical wave expansion of the initial beam. The secondary 

loss mechanism was originally believed to be the molecular attenuation effect. However, it 

became apparent that turbulent scattering losses are also significant, even at short range.

The power transmitted was measured for a set path at different frequencies (4, 8, 12, 15, 16, 20, 

23, and 24 kHz). Readings were collected using the SPL meter and logged by the LabVIEW 

software controller. Ranges were varied from 5- to 30-m distant from the receiver placed atop 

the second level of the receiver scaffold. The range increment was 5 m. Samples were collected 

from the SPL at the rate of 20 Hz. 

5.1.1 Developmental Power versus Range Measurement Series at 19472 (March/April 2009)

An initial series of range vs. power measurements were performed in the spring of 2009 involved

the setup of an ultrasonic source emitter and a receiver plus associated computer control devices

using parabolic dishes on both the emitter and receiver sides. The object was to propagate 

ultrasonic signals under different atmospheric conditions and varying the frequency to study the 

effects of the atmosphere on the propagated signal. In addition to the ultrasonic units, a sonic 

anemometer on a 2-m mast was positioned alongside the acoustic pathway to characterize the 

wind conditions present. This unit is also connected to the LabView laptop, but data were 
31

collected using a stand-alone data acquisition program written for LabVIEW. Typical 

measurements consist of 1-min collections of data from the sonic anemometer and the SPL meter 

for a specified frequency of the emitter, and a specific range between emitter and receiver.

Unfortunately, this first series experienced further difficulties, even though the multipathing 

issue was believed resolved by positioning the receiver device atop the 19472 compound’s 

scaffold. Following data collection it was believed that variations in the detected sonic levels

could be averaged out over the complete time record of each data set. However, upon 

performing that averaging and analyzing the results, in attempting to correlate results obtained at 

different frequencies and at different ranges an inconsistent pattern of signal losses arose. 

Figures 21–24 intercompare sound data collected at 5-, 10-, 15-, and 20-m downrange, for 

various frequencies, on March 19, 2009. 

Figure 21. 5-m sound pressure distribution data for various frequencies.
32

Figure 22. 10-m sound pressure distribution data for various frequencies.

As an example of the difficulties noticed, while both the 4- and 8-kHz signals appeared to shift 

downward in energy by approximately the same amount (roughly 4 dB between 5 and 10 m), the 

amount of the shift expected should have been 6 dB associated with the energy loss with a path 

length doubling (the 1/r2 effect). In contrast, the remaining signals all appear to increase in 

strength from 5 to 10 m! Yet, perhaps (it was argued) either the 5- or 10-m readings were 

somehow anomalous. It was hoped that the 15- and 20-m readings would make sense of the 

previous ambiguities. Based on this premise, we considered the remaining plots.
33

Figure 23. 15-m sound pressure distribution data for various frequencies. 

Figure 24. 20-m sound pressure distribution data for various frequencies.

In considering these remaining plots, one constant appeared to be the relative shift between the 

23- and 24-kHz distribution patterns. They appear shifted by approximately 12 dB in each case. 

However, being in close proximity spectrally, we expect these two to behave similarly. 
34

For a less consistent pair, the 16- and 20-kHz distributions are roughly identical at 5 and 20 m, 

yet the 20-kHz signal exhibited a stronger peak response at 10 and 15 m. One possible 

explanation was that perhaps atmospheric scattering could account for these variations. Yet, if 

atmospheric variations were cause the means to drop anomalously, then there should be some 

corresponding change in the width of the distribution functions, and these did not appear to be 

correlated. 

The alternative possibility postulated was that during these measurements rather strong cross 

winds were present. It was hypothesized that the wind effects could be causing the beam 

generated by the parabolic dish to be pushed off the center of the receiver aperture or tilted, 

resulting in a varying focusing point of the sonic energy in the receiver dish. An alternative 

explanation was that possibly the parabolic dish was producing a lobe structure of the beam at 

different frequencies emerging from the emitter that would produce varying results depending on 

small angular changes in the directions of either the emitter or receiver dishes. 

By either reckoning it was believed that the parabolic dishes were a liability to the 

measurements. Therefore, it was decided that the dishes would be replaced on both ends of the 

acoustic path. On the transmitter side the dish was replaced by the rectangular horn. This 

combination featured the widest angle beam, producing (it was hoped) the least possible lobe 

structure or possibility for the beam to be blown off the receiver. On the receiver side, it was 

decided that the receiver would be used in an unaugmented fashion. Estimates of the decibel 

loss associated with this step were believed to be approximately 14 dB at most, and since the 

power of the initial signal was at greater than 90 dB in most cases, this loss was considered 

acceptable. 

However, prior to the restart of testing, our experimental operations were shut down due to an 

ARL safety stand-down. Prior to restarting, safety procedures in place had to be documented 

and approved, as discussed in section 3. While this process was occurring, funding arrived, and 

the set of rectangular, round, and trumpet horns was purchased that would permit a wider array 

of beam propagation options. Also, the testing software was augmented to permit the acoustic 

and sonic anemometer data sets to be collected in a time-tagged manner and merged into a single 

output file, and a data communications package (transmit and receive) was developed in 

LabVIEW. The road leading to the site was also paved (see figure 25).
35

Figure 25. View down “test range” road leading to the 19472 site.

5.1.2 Refined Power versus Range Measurement Series at 19472 (October/November 2009)

Once safety concerns had been addressed, new software had been written, and new equipment 

integrated into our data acquisition methodology, a follow-on series of measurements were 

scheduled in mid-October to again study the range versus power problem considered in the 

spring. The main propagation path lay down the road leading out of the site (figures 6 and 25). 

The receiver SPL meter was placed at the 12-ft level of the scaffold (figure 7) and the 

rectangular horn was used to propagate the beam so that a wide angle beam would be presented 

at the receiver, but the narrow vertical dispersion would avoid ground reflections of the beam.

For these measurements the 15- and 23-kHz measurement frequencies were dropped, but the 

range was extended to 30 m, again in 5-m increments. An initial series of measurements were 

taken in October and analyzed. These results are shown in figure 26 as colored balls. The 

results plotted have been normalized such that each trace of sound level (propagated power) 

versus range reads 100 at 5-m range. 
36

Figure 26. Range versus propagated SPL power for varying wavelengths.

This series of measurements appears to be somewhat improved over the prior results, yet there 

does not appear to be a significant difference in propagated power as a function of range for 

frequencies between 8 and 20 kHz. Nevertheless, there do appear to be the same type (though 

perhaps not to the same degree) of cross-overs between different frequency results. 

The one trend that does perhaps indicate a frequency shift effect can be seen in the 8-kHz line. It 

does appear to separate somewhat (by 3–4 dB) from the higher frequency results, particularly at 

30 m. Similarly, the 4-kHz results also appeared to diverge, but only at ranges through 25 m. 

The 30-m result appeared as an outlier. On the other hand, the 24-kHz results appeared to 

behave approximately similar to lower frequency results through 20 m, but then seemed to 

diverge low for the 25- and 30-m readings. 

Because of these anomalous behaviors, we felt that further measurements of the 4- and 24-kHz 

frequencies might be in order. In the original sets taken in October the emitter was repositioned 

at a given range from the receiver scaffold and all six frequencies were tested in turn. It was 

thought that perhaps due to the presence of varying wind conditions it would be more consistent 

to attempt a series of measurements involving repositioning of the emitter to different ranges 

while only testing one frequency at a time. In this way the wind conditions would presumably 

remain relatively constant for each frequency. It was decided to only test the two outlier 

frequencies. These results are shown in the figure as bowtie markings. 
37

Unfortunately, these seemed to exhibit as many odd behaviors in the second series of 

measurements as during the first. The 15-m data sets were particularly intriguing in that the 

resulting power levels increased for both frequencies by nearly the same amount. 

We conclude from these results that we were detecting significant amounts of acoustic scattering, 

even at short range. We may also note that the 1/r2 effect is clearly present. According to this 

rule we would expect the doubling in path length between 5 and 10 m to produce a 6-dB loss 

from 100 to 94 dB. This is observed. Similarly, this rule would indicate a drop to 88 dB at 

20 m. This also was observed, however we must view the two November results at 15 m as 

anomalous. Likewise, we would expect at least another 3.5-dB drop between 20 and 30 m, and 

there again the November 4-kHz result appears anomalous, but the remaining results appear 

consistent. In most cases, however, the drop between 20 and 30 m appears to be on the order of 

5 dB or greater. This would be consistent with increased signal losses due to molecular 

attenuation and atmospheric turbulence scattering losses. 

5.2 Ultrasonic Acoustic Communications Proof of Concept Tests (November 2009)

A repeat of acoustic propagation experiments at 4 and 24 kHz this week at the Wideband 

Acoustic Technology and CHaracterization Experimental Range (WATCHER) at building 19472 

confirmed that variations in the propagated power levels are likely due to atmospheric turbulence 

scattering the acoustic energy out of the main propagation pathway. Various statistical analysis 

methods are being considered to characterize the distribution of these effects of turbulence on 

sound intensities at different frequencies with range.

6. Feasibility Analysis Results

Once a pair of functional ultrasonic communications systems had been constructed (an AM 

version and an FM version), these systems were tested at the building 19472 compound. The 

first tests occurred indoors and consisted of propagating a series of bit-encoded characters across 

a single room. Both modulation schemes employed a transmitter with an attached rectangular 

horn that sent data to a receiver consisting of a single high sensitivity flat-spectral-response 

microphone. 

The first approach to be tested was the AM communications scheme. This approach was 

rejected almost immediately because its modulation frequency could be audibly detected, a 

property that is at odds with the intended reduction in observability for the intended ultrasonic 

application. 

A FSK modulated FM system was then tested. Although it did not generate as high an audible 

signature level as the AM modulation approach, the performance of this initial configuration for 

FM modulation was still disappointing in that respect. Frequently, the transitions between the 
38

“0” signal frequency to the “1” signal frequency did produce audible clicks in our first tests. 

This phenomenon was determined to be due to a combination of the frequencies chosen, the 

duration of each bit-state signal, and a lack of suitable tapering of the single-frequency signal 

envelope.

Despite these limitations, indoor tests showed that the FSK system was capable of 

communicating at a rate of 300 baud (bits per second) or more, as long as multipathing 

influences were not present. However, when the orientation of the emitter was changed so that it 

did not directly face the microphone, significant multipathing effects were observed due to wall 

reflections. When multiple wall reflection path signals were added to the transmitted signal an 

obvious degradation in data transmission reliability ensued. The explanation for observed 

multipathing problems is easily demonstrated with the aid of the schematic illustration of figure 

27.

Figure 27. Multipathing bit error geometry.

The impact of multipath distortion upon data transfer for our proof of concept system may be 

illustrated as follows. The emitter generates a single frequency signal pulse representing either a 

zero (20 kHz) or a one (25 kHz) tone. At the nominal speed of sound (approximately 340 m/s) 

and a 300-baud data transmission rate, each signal ‘bit’ pulse will have a physical length of 

approximately 1 m. This effect is illustrated in figure 27 by the red “Data Element” diamond. 

Since the direct path will also be the shortest path, this data packet will travel fastest to the 

receiver by the direct route. The alternative “Reflected Path” will take longer to traverse, but 

eventually the reflected signal will also reach the receiver aperture. Thus, if there is a sequence 

of data packages travelling along both the direct and reflected routes, and if the lengths of the 

two respective paths differ by more than the length of a single data packet, then the reflected 

signal of a previous packet may be received during reception the direct signal for a given packet. 

In this case, the system might need to invoke specialized techniques to be able to discriminate 

between the most direct path result and that of the time shifted reflected path result. 
39

It might still be possible to discriminate between the direct and reflected signals, particularly if 

the direct signal is significantly stronger than the reflected signal. This would be an application 

of “Rician” statistics, in which the main signal is much stronger than multiply reflected signals. 

This is illustrated by figure 28, which depicts a 300-baud, 3-bit FSK “101” signal combined with 

two delayed and attenuated reflections of itself. It is apparent that the direct signal has some 

margin above the multipath “noise”, but the margin is much reduced when reflections coincide in 

time and sum to a larger power level. On the other hand, in an extreme case there may not be 

any clear direct signal. In this instance, the received signal might consist of only a summation of 

multiple reflected signals. This case is characterized by “Rayleigh” statistics similar to that of a 

random walk experiment. Figure 29 shows the same scenario as that of figure 28, but with the 

direct path signal absent. The integrity of the 3-bit signal is clearly in doubt, as the noise margin 

is practically non-existent. In such cases advanced methods would need to be employed to avoid 

inter-symbol interference.

Figure 28. Illustration of strong direct signal in the presence of weaker reflected (multipath) signals.
40

Figure 29. Illustration of reflected (multipath) signal in the absence of a direct signal.

However, not all ultrasonic applications would need to pass high bit-rate data. There are some 

data messages that are significant with even just a few bytes or even a few bits of data. In such 

cases, the baud rate necessary is less important because the significance of individual bits is 

higher. A prime example for such an application of this technology is the implementation of an 

Identification Friend-or-Foe (IFF) signal system for dismounted infantry in an urban setting. For 

this case the baud rate could be lowered to the level where only a single frequency tone pulse 

would be detectable at any time within the sphere reachable by a given emitter. The size of this 

sphere (and thus the maximum allowable bit rate) would be defined by the receiver sensitivity 

and the strong range dependent signal losses inherent to ultrasonic sources. 

Assuming that an ultrasonic signal cannot be detected beyond approximately 70 m in any one 

direction, the baud rate could be lowered to 5 bits per second, allowing approximately enough 

data to transmit to identify one letter per second. 

We modified the initial FM system to reduce the baud rate to eight samples per second and found 

that the system could propagate around corners fairly easily given a suitable reflecting surface. 

The system also appeared to be relatively immune to multipathing effects over short ranges, 

though further testing is necessary to determine the exact extent of the modified system’s 

capabilities.

In addition to the reduction in baud rate, the emitter system was also modified to add a relatively 

simple sine-wave signal envelope designed such that for each data bit at a single frequency, the 
41

sine envelope varied over the range zero to pi. This taper of the signal envelope made the 

emitter inaudible to human hearing.

7. Conclusions

This report provides an initial overview of the potential for short range ultrasonic acoustic 

communications within the atmosphere. The applications examined here are of direct interest to 

the Army because they have potential to benefit the operational safety and combat effectiveness 

of the Soldier. In order to realize these benefits for the Soldier, one must attempt to unify 

various disparate elements based on available theory, experimental data, and past engineering 

results into specific engineering concepts. We have accomplished several stages of this process 

by reviewing past experimental and theoretical investigations on boundary layer atmospheric 

ultrasonic propagation, establishing an infrastructure for outdoor testing, formulating software 

control for several applications, and performing model analysis of ultrasonic proof-of-concept 

systems. Specifically, we have achieved the following milestones:

1. A literature survey of prior art in the application of ultrasonic systems in the atmospheric 

environment has been made (and is continuing).

2. A semi-empirical model for the ultrasonic propagation environment has been implemented 

from information available in the literature.

3. Preliminary experiments have produced results that are in reasonable agreement with these 

models.

4. A physical infrastructure and framework (in the form of the WATCHER facility) has been 

constructed.

5. A software virtual instrument framework based on the LabVIEW package has been 

developed and tested for ultrasonic sources, detectors, and analytical devices.

6. Using this infrastructure, more detailed experiments have characterized angular emission 

properties of sources and their propagation performance.

7. The LabVIEW framework has been used to create and test a low data rate proof-of-concept 

communication system.

While our initial experimental efforts have produced a few anomalies in the observed 

propagation data, they do correspond reasonably well with model results in general. It is clear 

from our model results that a comprehensive suite of meteorological support sensors will be 

necessary in order to better characterize the ultrasonic propagation environment. In particular, 

better characterization of the turbulence levels (through analysis of time series of sonic 

anemometer data or optical scintillometer data), outer scales of turbulence (through application 
42

of similarity techniques), and humidity (from local hygrometer measurements) should provide a 

better fit with model results and higher confidence in the characterization of the propagation 

environment.

Our experimental results for low speed data transmission using FSK modulation of an ultrasonic 

carrier indicate considerable promise for urban warfare IFF applications at the squad level. It has 

been suggested that spread-spectrum modulation methods that are commonly employed for 

acoustic communication in the submarine environment may also improve the data rate to the 

point that other atmospheric ultrasonic communications concepts could become feasible. 

Spread-spectrum techniques may also prove useful for short range, near-surface atmospheric 

sounding applications for ultrasonic systems.

We hope to extend our activity and results to help us develop and explore other ultrasonic 

applications that might be of interest and utility to the Army.
43

8. References

Akerman, A. M.; Ayers, C. W.; Haynes, H. D. Ultrasonic Speech Translator and 

Communication System. U.S. Patent 5,539,705, July 23, 1996. 

Baikalova, R. A.; Krekov, G. M.; Shamanaeva, L. C. Theoretical Estimates of Sound 

Scattering by Atmospheric Turbulence. J. Acoust. Soc. Am. 1988, 83, 1332–1335. 

Bradley, S.; Webb, T. Use of an Ultrasonic Sodar to Sense Raindrop Size Distribution, Physics 

Department, University of Auckland, Auckland, New Zealand (2001). 

Busnel, R.G.; Classe, A. Whistled Languages, Springer-Verlag: New York; 1976.

Cogan, J.; Measure, E.; Wolfe, D. Atmospheric Soundings in Near-Real Time from Combined 

Satellite and Ground-Based Remotely Sensed Data; ARL-TR-1481; U.S. Army Research 

Laboratory: White Sands Missile Range, NM, 1997.

Desart, E. Sound absorption by air as per ISO 9613-1, Mar 11, 2004. Acoustic Forum. 

http://forum.studiotips.com/viewtopic.php?t=158 (accessed April 2010).

Haynes, H. D.; Akerman, M. A.; Baylor, V. M. Final Report: Ultrasonic Communication 

Project, Phase 1, FY 1999, Report Y/NSP-252, National Security Program Office, Oak 

Ridge Y-12 Plant: Oak Ridge, TN, 2000. 

Heard, M. C. Short range communications using modulated ultrasound. IEE Colloquium

(Digest) 1983, 2.1–2.3.

Holm, S. Airborne Ultrasound Data Communications: The Core of an Indoor Positioning 

System. Proc. IEEE Ultrasonics Symposium; Rotterdam, the Netherlands, 2005; Vol 3, pp 

1801–1804. 

Hummel, J. R., Ed. High Energy Laser Propagation Handbook, Volume II, HEL System Test 

Facility Atmospheric Characterization; ASL-TR-0148; U.S. Army Atmospheric Sciences 

Laboratory: WSMR, NM, 1984.

Ishimaru, A. Wave Propagation and Scattering in Random Media, Volume 2, Multiple 

Scattering, Turbulence, Rough Surfaces, and Remote Sensing; Academic Press: New York, 

1978.

ISO 9612-1:1993, Acoustics–Attenuation of Sound during Propagation Outdoors.

Kaye and Laby. Tables of Physical and Chemical Constants, National Physical Laboratory: 

UK, 2009. http://www.kayelaby.npl.co.uk/general_physics/2_4/2_4_1.html (accessed 

April 2010).
44

Li, C.; Hutchins, D. A.; Green, R. J. Short-Range Ultrasonic Digital Communications in Air. 

IEEE Trans Ultrason Ferroelectr Freq Control 2008, 55, 908–918.

Neff, W. D. Quantitative Evaluation of Acoustic Echoes from the Planetary Boundary Layer. 

NOAA Technical Report ERL322-WPL: Boulder, CO, 1975. 

Shamanaeva, L. G. The Dependence of Sound Extinction on the Parameters of Thermal 

Turbulence in the Atmospheric Boundary Layer. J. Acoust. Soc. Am. 1983, 73, 780–784.

Shamanaeva, L. G.; Burkatovskaya, Yu. B. Statistical Estimates of the Contribution of 

Multiply Scattered Radiation to the Intensity of Acoustic Radiation Transmitted Through a 

Lower 500-m Atmospheric Layer. Russ Phys J. 2004, 47 (12).

Shannon, C. E. A Mathematical Theory of Communication. The Bell System Technical 

Journal 1948, 27, 379–423 and 623–656.

Stull, R. B. An Introduction to Boundary Layer Meteorology, Kluwer Academic Publishers:

Dordrecht, The Netherlands, 1988.

Sutherland, L. C., Ed. Sonic and Vibration Environments for Ground Facilities–A Design 

Manual; Contractor Report WR 68-2; Wyle Laboratories: Huntsville, AL, 1968.

Zurek, R. A.; Charlier, M. L.; Dietrich, A. Omnidirectional Ultrasonic Communication 

System. U.S. Patent 6,363,139, March 26, 2002.
45

Appendix. Ultrasonic Measurements Safety Standard Operating Procedure

The following attachment was drafted and approved by ARL.

Standard Operating Procedure for 

Ultrasonic Sensing Propagation Measurements at Bldg. 19472, WSMR, NM

(Last Revision: August 17th, 2009)

1. Purpose: This standard operating procedure (SOP) documents potential hazards during ultrasonic 

propagation testing at WSMR bldg. 19472, and addresses engineering and administrative controls to 

be implemented to mitigate these potential hazards.

2. Scope: This SOP applies to personnel engaged in ultrasonic measurements at bldg. 19472, as well as 

other personnel visiting the site during testing operations. 

3. General: Ultrasonic measurements at building 19472 consist of a laptop computer connected via 

National Instruments software and modules to an amplifier that powers an ultrasonic emitter. The 

19472 building compound is located in a desert area approximately 3 miles south of the main post 

area. The map below shows a satellite image of the 19472 building compound (center) and its 

environs including the nearest adjacent WSMR building compound, the 500K site, nearly 400 meters 

away.

 Building 19472 Compound.

 Signals generated by the ultrasonic emitter are propagated through air via either a direct emission 

(basically omni-directional) or through a focused propagation using a 12-inch parabolic dish. Signals 

generated are detected using a passive receiver device connected to a pre-amplifier that sends its 
46

signals back to a laptop computer through National Instruments software and modules. Emitter and/or 

receiver devices may at times (or may not) be set up on the second level of a 12 ft. stairway scaffold 

located near the gate of the compound, depending on conditions being tested.

 

 No deviations from this Standing Operating Procedure (SOP) will be permitted. Whenever the 

approved methods in this SOP must be changed, the SOP must first be revised and approved in writing 

by the ARL Safety Office.

4. Responsibilities:

 a. Training Requirements: All test personnel shall receive electrical safety training prior to operating 

test equipment or participating in setup activities. Only test personnel who have received proper 

scaffold training, that includes fall arrest training, may mount, erect, assemble, or disassemble 

scaffold equipment on site. 

 b. The test director (senior test person on site) shall ensure that test personnel are operating in 

accordance with this SOP, and that site visitors are properly equipped with hearing protection during 

testing. The test director shall be cognizant of all personnel on site during testing and ensure that all 

are aware of when active sound generation events are occurring.

 c. Supervisory personnel shall ensure that test personnel have been properly trained and equipped for 

testing operations at the site, as well as being properly instructed in the provisions of this SOP.

5. Personnel Limits: Only authorized and trained personnel shall operate electronic, emitter, and receiver 

test equipment. Only authorized and trained personnel shall mount scaffolding equipment. Visitors to 

the site shall be escorted during testing operations and shall be supplied with appropriate hearing 

protection by test personnel. 

6. Material Limits: All electronic devices will be operated within their design specifications. Scaffolding 

equipment will not be loaded beyond its design limitations.

7. Safety Requirements:

 a. Engineering controls: The scaffolding available at the bldg. 19472 compound will consist of an 

Upright 12 ft. framed stairway scaffold. This scaffold is mounted with toe-guards at both the 6 ft. 

landing and the 12 ft. walkway. The width of this scaffold is 4 ft., such that the height-to-width 

ratio is 3-to-1, which exceeds the OSHA 4-to-1 standard for free standing scaffolding. The 

scaffold is secured to the bldg. 19472 perimeter fencing to provide additional stability. The 

scaffold has also been equipped with additional hand railings and cross bars at appropriate heights 

to provide additional support points and to assure that accidental fall hazard risk from the scaffold 

is minimized. The scaffold has two horizontal platform levels at 6 feet and 12 feet above ground 

with sections of stairs connecting each level. Cross railings surround each platform level at 

appropriate heights to protect personnel from accidental falls off the scaffold. While 

instrumentation will be securely emplaced on the 12 foot platform from below, at no time will 

personnel climb completely up and place their feet to crouch or stand on the 12 foot platform 

level.

 All electrical equipment used for testing is UL approved.

 

 b. Administrative controls:

A copy of this SOP will be posted at the 19472 building site at all times.
47

Test personnel must read and understand this SOP, as well as sign the SOP signature page, prior 

to participating in testing activities.

Site Activity: Ultrasonic testing operations at the bldg. 19472 site can be performed Monday 

through Friday from 6:00am to 6:00 pm. Approval must be obtained from the safety office to 

work outside normal working hours. All testing will be attended by trained users and a test 

director. At least two personnel must be on site whenever testing is underway. No unattended 

testing will be permitted.

User Training: All test personnel must have completed appropriate ARL R&D Electrical safety 

training, including electrical safety in field testing environments, prior to operating or setting up 

electrical equipment. Personnel erecting, disassembling, and mounting the stairway scaffold on 

site must have received prior OSHA scaffold training that includes fall arrest training.

Desert Operations: All personnel must be familiarized with operations in desert environments. 

Hats, sunscreen, and shade will be used during hot conditions to avoid heat injuries. Water will 

be maintained in the 19472 refrigerator. Operations at the site are only authorized if 

environmental conditions within the building are suitable to mitigate weather conditions present 

during the measurement operations.

Inclement Weather: The ultrasonic measurement equipment will not be operated under 

inclement weather conditions, to include storms, lightning, or high wind events in the vicinity of 

the 19472 building compound. 

Electrical: In addition to other safety requirements, the following procedures shall be observed.

Analysis: Active test measurements may use (at times) a power amplifier to boost the ultrasonic 

acoustic emission. The power amplifier has the following specifications. Its maximum power 

output is 200 W. It can be operated in one of two modes, either bridged or un-bridged. In unbridged mode the output is 0-40 V. In bridged mode the output is -40 -- +40 V for an RMS 

output voltage of 28 V. The maximum current is 15 Amps, but is further restricted such that the 

maximum output power is no greater than 200 W. When in use, the most likely mode to be used 

will be the bridged configuration to ensure that any signal modulation will carry a zero amplitude 

low frequency component. This will avoid low frequency noise components in the sound output.

Electrical Hazard Mitigation Procedures:

1. Laboratory measurement devices must be properly grounded and equipped with a 

primary on/off electrical power switch to enable operation. This power on/off switch should be 

placed in the off configuration when the devices are not intended to be used, and anytime during 

setup or breakdown of the system.

2. Inspect power cables for cuts or gaps in the covering over conductive wires. All 

equipment used should be UL listed. Verify UL listing. If a cable is not properly covered do not 

attempt to use. 

3. Covers should never be removed from amplifiers or pre-amplifiers. 

4. Outdoor facility power may only be used if a proper UL approved GFI (Ground Fault 

Interruption) device is in place.
48

5. All electrical equipment will be operated within its design parameters.

Noise: Noise will be controlled and/or hearing protection will be used to avoid noise hazards in 

accordance with OSHA guidance on ultrasonic sound sources found at 

http://www.osha.gov/dts/osta/otm/noise/health_effects/ultrasonics.html . According to the OSHA 

noise standard 1910.95, table G-16 (copied here) provides data on general noise Threshold Level 

Values in two categories: Ceiling Values, and 8-hour Total Weighted Average (TWA) exposures. 

No experimentation will exceed these thresholds. 

Analysis: To provide more specific information, the conditions of testing are considered here in 

more detail. The emitter device being used in this testing generates 92 dB when driven by 1 Watt 

of power. This emitter is electrically equivalent to a 0.13 μF (micro-Farad) capacitor. Emitter 

equipment: Our primary emitter is electrically equivalent to a 0.13 μF capacitor. While the power 

supply is rated up to 200 W, the voltage output will be restricted to 50 V RMS based on the 

training of the operators. Frequencies emitted will also be limited to less than 25 kHz. At 25 kHz 

the emitter impedance reaches a minimum value of 49 Ω. This will mean the amplifier is 

supplying ~50 Watts of RMS power, a 17 dB power increase over the 1 W power level. This 

represents a maximum power level. At frequencies less than 25 kHz the power supplied is less. 

 

The gain on the parabolic reflector used has been estimated based on a beam measured as 15 

meters in radius at 75 meters in range. Assuming an otherwise hemispheric Lambertian sound 

source without the reflector, the reflector gain factor is 14 dB. Combining the initial 1 W noise 

power level plus the amplifier gain factor, and the reflector gain, the maximum sound level at 1 

meter from the emitter’s reflector exit aperture is 92+17+14 = 123 dB. For each doubling of 

range from the reflector beyond 1 m, since sound levels decrease using a power formula, the 
49

sound level drops 6 dB. At 10 meters the maximum sound level is reduced to 105 dB, the ceiling 

level indicated in the above table. Therefore, for active tests, assuming the highest output power 

level as a worst case for a safety envelope calculation, we will employ barricades at 10 meters 

from the emitter and employ sound protection within this 10 meter zone. We will otherwise keep 

test and visitor personnel safely clear of emitting sources during testing. Hearing protectors rated 

at 29 dB have been procured and should be sufficient to reduce the maximum 123 dB to 94 dB to 

mitigate noise hazards under all conditions.

Conversely, when operating outside the cone of the reflector-focused sound energy, one might 

expect the maximum sound level to be 92+17-14 = 95 dB, a level within tolerance for short 

exposures, and a level of only 77 dB for ranges beyond 10 meters, below the hazard threshold. 

The primary hazard zone is thus in the forward direction when using the parabolic reflector. 

When operating without the parabolic reflector the hazard is reduced to a maximum of 109 dB, 

but is omni-directional. For distances beyond 10 meters, this reduces to 91 dB, which can accept 

an exposure up to 3 hours per day, A-weighted. 

Noise Mitigation Procedures: 

1. All personnel on site will be equipped with hearing protection equipment during 

operations. All personnel will wear hearing protection when an emitter is propagating if working 

within 10 meters of the emitter.

2. No hearing protection will be necessary when operating in a passive (listening) mode.

Personal Protective Equipment: 

1. Personal protective equipment shall include appropriate shoes for mounting of 

scaffolding equipment and the use of a hard hat in the vicinity of the scaffold whenever 

equipment is being mounted onto the scaffold or is currently mounted on the scaffolding. 

2. A head covering is required for outdoor testing when temperatures exceed 95 degrees 

Fahrenheit. 

3. Hearing protection shall be used when sustained audible sound levels exceed 90 dB or 

when instantaneous sound levels exceed a 105 dB sound level.

Scaffolding: Only test personnel who have been trained in scaffold operations may use the 

scaffold. This testing is available on-line at:

http://www.oshacampusonline.com/construction-english/scaffolding.html

Personnel moving emitter and receiver equipment to the mounting platform at 12 ft. shall work in 

tandem by passing the equipment up the tower rather than attempt to carry it up the stairs 

singlehanded. Proper footwear will be worn when working on the scaffold to provide non-slip 

footing. Handholds will also be used whenever possible. When moving about the scaffold when 

instruments are mounted on the scaffold, hard hats must be worn. When mounting equipment on 

the top platform of the tower, equipment should be secured to the scaffold. The scaffold will not 

be used under wind conditions exceeding 20 MPH. Personnel will not climb to, stand or crouch 

on the top level of the scaffold.

The scaffold will not be moved during testing. When positioned for mounting, the scaffold 
50

wheels will always be locked.

1. Hazards to Avoid

a. Equipment will be operated for testing purposes. No other uses are authorized. 

b. At no time will the covers be removed from system amplifier equipment. 

c. At no time will electrical equipment be operated by untrained personnel.

d. At no time will the scaffold be mounted by untrained personnel. 

e. At no time will visitors be permitted on site when testing is under way except with the approval 

of test personnel and supplied with appropriate hearing protection.

f. Personnel shall not climb to, stand or crouch on the top platform of the scaffold. 
51

List of Symbols, Abbreviations, and Acronyms 

°C degrees Centigrade

AGL above ground level

AM Amplitude Modulation

ARL U.S. Army Research Laboratory

ASCII American Standard Code for Information Interchange

BED Battlefield Environment Division

COTS commercial off-the-shelf

DAQ data acquisition

FM Frequency Modulation

FSK Frequency Shift Keyed or Frequency Shift Keying

IFF Identification Friend-or-Foe

LabView Laboratory Virtual Instrument Engineering Workbench

mB millibars

met meteorological

MMS Meteorological Measuring Set

NI National Instruments

RASS Radio Acoustic Sounding System

RF Radio Frequency

SODAR Sonic Detection And Ranging

SOP standard operating procedure

SPL Sound Pressure Level

VI virtual instruments

WATCHER Wideband Acoustic Technology and CHaracterization Experimental Range

WSMR White Sands Missile Range
52

No. of

Copies Organization

1 PDF ADMNSTR

DEFNS TECHL INFO CTR

DTIC OCP 

8725 JOHN J KINGMAN RD 

STE 0944

FT BELVOIR VA 22060-6218

3 HCs US ARMY RSRCH LAB 

ATTN RDRL CIM P

TECHL PUB

ATTN RDRL CIM L

TECHL LIB

ATTN IMNE ALC HRR

MAIL & RECORDS MGMT

2800 POWDER MILL ROAD

ADELPHI MD 20783-1197

1 CD US ARMY RSRCH LAB

ATTN RDRL CIM G

TECHL LIB

APG MD 21005-5066

1 CD US ARMY RSRCH LAB

ATTN RDRL ROE N 

W BACH

PO BOX 12211

RESEARCH TRIANGLE PARK

NC 27009

1 CD US ARMY RSRCH LAB

ATTN RDRL CI 

J GOWENS

2800 POWDER MILL ROAD 

ADELPHI MD 20783-1197

1 CD US ARMY RSRCH LAB

ATTN RDRL CIE 

P CLARK

2800 POWDER MILL ROAD 

ADELPHI MD 20783-1197

1 CD US ARMY RSRCH LAB

ATTN RDRL CI 

R NAMBURU

2800 POWDER MILL ROAD 

ADELPHI MD 20783-1197

1 CD US ARMY RSRCH LAB

ATTN RDRL CI 

B SADLER

2800 POWDER MILL ROAD 

ADELPHI MD 20783-1197

 No. of

Copies Organization

1 CD US ARMY RSRCH LAB

ATTN RDRL CIE D 

D W HOOCK

WSMR NM 88002-5501

2 CDS US ARMY RSRCH LAB

ATTN RDRL CIE D 

D TOFSTED

WSMR NM 88002-5501

2 CDS US ARMY RSRCH LAB

ATTN RDRL CIE D

S OBRIEN

WSMR NM 88002-5501

2 CDS US ARMY RSRCH LAB

ATTN RDRL CIE D

M S DARCY

WSMR NM 88002-5501

1 CD US ARMY RSRCH LAB

ATTN RDRL CIE D

D S ELLIOTT

WSMR NM 88002-5501

1 CD US ARMY RSRCH LAB

ATTN RDRL CIE D

E CREEGAN

WSMR NM 88002-5501

2 CDS UNITED STATES MILITARY 

ACADEMY 

COMBAT SIMULATION LAB 

P WEST

WEST POINT NY 10996 

Total: 21 (1 PDF, 3 HCs, 17 CDs)