PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Quantitative characterization of high-intensity focused airborneultrasonic fields
Marko LIEBLER 1; Christoph KLING 2; Benjamin BEST1; André GERLACH1; Christian KOCH2
1

Robert Bosch GmbH, Robert-Bosch-Campus 1, 71272 Renningen, Germany
2

Physik.-Techn. Bundesanstalt (PTB), Germany

ABSTRACT
High-intensity focused airborne ultrasonic fields are increasingly applied in various technical fields, forexample, to generate haptic feedback during gesture interaction. The sound fields used pose specialchallenges for a quantitative sound field characterization. The high sound pressure levels in combination withthe higher harmonics generated by nonlinear effects require both, a wide dynamic range and a wide
bandwidth of the measuring chain. Furthermore, the small wavelengths and the focusing result in spatiallystrongly varying sound fields. In the present case, a 40 kHz carrier signal was focused on a focal point using
a transducer array with 256 elements.
Different microphone types and analysis methods were investigated with respect to their suitability inmeasuring high-power airborne ultrasonic fields. Using the portal scanner of the PTB, a spatial
characterization of the ultrasonic field was performed. The results for different microphones andmeasurement setups were compared and will be presented and discussed in this paper. Due to the nonlinear
propagation of the sound wave, peak sound pressure levels of more than 160 dB were measured in the focalregion. To evaluate the exposure of potential users the measured sound pressure levels in the focal region
were compared with currently existing limit values.
Keywords: Ultrasound, Metrology, Airborne

1. INTRODUCTION
Ultrasonic fields have been used to generate local audio sources for many years (1). The generalidea behind this so-called parametric acoustic array is based on nonlinear acoustic effects of soundwave interaction in air (2). An ultrasonic carrier signal is amplitude modulated by an audio signal.
When nonlinear interaction of sound takes place, the modulated audio signal is demodulated in the air.
In this way audio sources with very sharp directivity pattern can be realized. The generation ofnonlinear effects requires high sound pressure levels of the ultrasonic carrier signal.
Recently, a comparable technology has been developed to create haptic feedback during gestureinteraction (3,4). The basic idea behind this application is to generate a tactile feedback in free spaceby using an ultrasonic phased array to focus high-intensity sound waves on user’s hand. The
mechanoreceptors within the skin are sensitive to frequencies below 1 kHz. Therefore, a common
approach is to modulate the ultrasonic carrier frequency by a low frequency signal.
Both applications use the ability of ultrasound to concentrate acoustic energy for the creation of
localized low frequency sound sources. For this purpose ultrasonic fields with high sound pressurelevels are used. In order to assess the potential noise exposure of users of such systems, quantitative
sound field measurements must be performed. This paper describes a first approach and discusses
challenges of measuring high-intensity focused airborne ultrasonic fields.

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Marko.Liebler@de.bosch.com

2. EXPERIMENTAL SETUP
2.1 Ultrasonic Array
High-intensity focused ultrasonic fields were created using the commercially available
Ultrahaptics STRATOSTM explore development kit. It consists of a 16 x 16 transducer array and acontrol board. The piezoelectric transducers (muRata MA40S4S) have a resonance frequency of40 kHz. The provided software tools allow the generation of different pre-defined sound field
distributions. The sound field of all measurements presented here was focused stationary on a single
focal point at a distance of 20 cm centered above the array. With a normalized intensity parameter,
defined by the control software, the sound intensity could be adjusted between the values 0 (no sound)and 1 (maximum acoustic power).
2.2 Measurement Microphones
Measurements for sound field characterization were carried out using different microphones atdifferent laboratories in order to examine the influencing factors of microphone selection,measurement setup and the entire measuring chain on the results. The specifications of themicrophones used by Robert Bosch GmbH and by Physikalisch-Technische Bundesanstalt (PTB) are
given in Table 1.

Table 1 – Specifications of the microphones used
Microphone type

Microphone

Preamplifier

cartridge

Frequency

Dynamicrangerange

Sensitivity

Institution

4 mV/Pa

Robert(± 3 dB)
GRAS 46BE IEPE

1/4" 40BE

1/4" 26CB(prepolarized)

4 Hz –

35 dB(A)

100 kHz– 160 dB

6.5 Hz –52 dB(A)140 kHz– 175 dB

6.5 Hz –52 dB(A)140 kHz– 178 dB

Bosch

free-field microphone
GRAS 46DD IEPE

1/8" 40DD

1/4" 26CB(prepolarized)

0.62 mV/Pa

RobertBoschpressure microphone
GRAS(externally polarized)

1/8" 40DP

1/4" 26AS

1 mV/Pa

PTB

pressure microphone
1/4" and 1/8" measurement microphones were used for appropriate spatial resolution when
scanning the sound field and to keep the influence of the directivity of the microphone as low as
possible (5). The 1/4" free-field microphone set has a higher sensitivity and a lower intrinsic noise than
the 1/8" microphones. The free-field microphone is designed to determine the sound pressure as it wasin the absence of the microphone. In contrast, the 1/8" microphones as pressure devices measure theactual sound pressure on the surface of the microphone´s diaphragm, including sound pressure
disturbances induced by the microphone itself. Since in the present case measurements are made underfree-field conditions, the measured values determined with the 1/8" pressure microphones must
undergo a free-field correction. As advantages over the 1/4" microphone, the 1/8" microphones offer ahigher dynamic range, a wider frequency range and, due to their smaller diameter, a better spatial
resolution including a lower influence of the directivity. A disadvantage of the 1/8" microphones is thelower signal-to-noise ratio of the measurement.

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2.3 Measuring Quantities
As characteristic values for the evaluation of the measurements, the acoustic properties given in
Table 2 were used. These measuring quantities are generally used in literature for the evaluation ofairborne ultrasound exposure on users.
Table 2 – Measuring quantities and guiding limits (5,6,7,8)
SymbolTermGuiding limit‫ܮ‬Zeq,Terz

Z-weighted energy-equivalent continuous

110 dBsound pressure level in one-third octaves‫ܮ‬ZpeakZ-weighted peak sound pressure level

140 dBAs a measure in a limited frequency band around the 40 kHz center frequency of the carrier signal
the one-third octave level ‫ܮ‬Zeq,40kHz was used, whereas the peak sound pressure level ‫ܮ‬Zpeak wasapplied as a broadband measuring quantity covering the whole frequency range including also higher
harmonics.
Several standards and regulations issued by various institutions and valid in different countries
exist which recommend ultrasound exposure limits. The guiding limits given in Table 2 are considered
as a general consensus in literature (5,6,7,8). These values are derived for workday exposure
conditions and hold at workplaces. Some published standards have exposure time limits and others donot (6,7). Increased limit values for reduced time are based on the “equal energy hypothesis”, which isapplied in the audible frequency range. It is not proven, if this hypothesis is also valid for ultrasoundexposure. It is therefore recommended to adhere to the conservative standards that define guiding
values independent of exposure duration (6).

3. FOCAL POINT MEASUREMENT RESULTS
For the measurement results presented in this section one of the microphones was placed in thefocal point, respectively. The intensity parameter was varied between 0.1 and 1.0 and the focalpressure signal recorded. Due to the nonlinear propagation effects, higher harmonics occur in addition
to the carrier frequency. Preliminary investigations had shown that at least the first harmonic of
80 kHz should be sufficiently captured by the bandwidth of the measuring chain. Therefore, the dataacquisition was done at Robert Bosch GmbH using a sampling rate of 384 kHz (measurement module
GIM 282, ZODIAC DATA SYSTEMS GmbH) and at PTB with a sampling rate of 192 kHz (DataTranslation DT 9847).
In Figure 1 a comparison between the recorded pressure time signals at the geometrical focus forthe two microphones GRAS 1/4" 46BE and GRAS 1/8" 46DD is given. The measurements wereperformed with attached microphone protection grids. No free-field correction was made for theresults of the 1/8" microphone in that case.

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Figure 1 – Comparison of pressure-time signals for two different microphones (intensity setting = 0.8).
Measurements done with microphone protection grid.
A 200 Hz amplitude modulation of the ultrasonic carrier signal can be seen. There are clear
differences in the results of the two microphones both in terms of amplitude and signal shape. This iscaused by the overestimation of the high frequency components due to the pressure microphone aswell as by the influence of the microphone grid. Figure 2 shows for the same microphones thecomparison of the measured one-third octave sound pressure level ‫ܮ‬Zeq,40kHz with and without grid asa function of the intensity setting.

Figure 2 – Influence of the microphone protection grid on the measured one-third octave sound pressure level‫ܮ‬Zeq,40kHz as a function of the intensity setting. The grid leads to an overestimation of the measured values,
where the effect on the 1/4" microphone is greater than on the 1/8" microphone. In this case no free-fieldcorrection for the pressure microphone has yet been taken into account.
The microphone protection grid leads to an overestimation of the measured pressure (9),independent of intensity. The effect varies in its impact for both microphones. In addition to theinfluence on the frequency response, the protection grid also has an influence on the microphone
directivity at ultrasonic frequencies. For this reason, all further measurements were carried out without
the grid.
Figures 3 and 4 compare the measurement results of the three microphones for the two acousticparameters. The recorded raw signals were weighted with the respective transfer functions and themeasurement data of the pressure microphones obtained a free-field correction. Overall, a very good
agreement of the measurement results can be seen.

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Figure 3 – One-third octave sound pressure level ‫ܮ‬Zeq,40kHz measured at the focal point as a function of theintensity parameter setting. The results of the three different microphones show an excellent agreement witha maximum deviation of less than 0.6 dB. The measurements were carried out on different days and indifferent laboratories at Robert Bosch GmbH and PTB.

Figure 4 – Peak sound pressure level ‫ܮ‬Zpeak measured at the focal point as a function of the intensity
parameter setting. For intensity parameters up to 0.8, the results of the three microphones show very good
agreement with a maximum deviation of about 1 dB. For the highest intensity range, larger deviations result
because the dynamic limit of the 1/4" microphone was reached here. The measurements were carried out on
different days and in different laboratories at Robert Bosch GmbH and PTB.

4. SPATIAL SOUND FIELD DISTRIBUTION
Besides measurements at the focal point, the spatial sound field distribution in areas in the vicinityof the focal point was also determined. For this purpose, the sound field was scanned in high resolution
at different cutting planes. A computer-controlled 3D positioning system with three linear axes wasused for automated scanning. The zero point of the coordinate system used corresponds to the center ofthe array surface. The focal point is located 200 mm above the center of the array ((‫ݔ‬, ‫ݕ‬, ‫= )ݖ‬(0,0,200)) and thus, the array is located in the (x, y)-plane at z = 0.
Figure 5 depicts the sound field distribution measured in the (‫ݕ‬, ‫)ݖ‬-plane and in the focal plane((‫ݔ‬, ‫ݕ‬, 200)-plane) at maximum intensity setting. The measurements were done with the 1/8" 40DP
microphone using the PTB portal scanner. The spacing of the measuring points was 1 mm in a regionof ‫ݔ‬, ‫ = ݕ‬±20 mm or ‫ݕ‬, ‫ = ݖ‬±20 mm around the geometrical focus and 5 mm outside. In propagation
direction ((‫ݕ‬, ‫)ݖ‬-plane) the focal volume has a conical shape with long extent. A sharp focus in lateral

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direction can be seen from the results in the focal plane. Besides the strong main lobe, pronounced side

Figure 5 – Spatial distribution of one-third octave sound pressure level ‫ܮ‬௓௘௤,ସ଴௞ு௭ measured in (‫ݕ‬, ‫)ݖ‬-plane(left) and (‫ݔ‬, ‫)ݕ‬-plane at z = 200 mm (right) at maximum intensity setting. Measurements were done with
1/8" 40DP microphone using the portal scanner of the PTB.
lobes around the symmetry axes occur which are caused by the quadratic array arrangement. On thesymmetry axes and in the (‫ݕ‬, ‫)ݖ‬-plane the guideline limit for ‫ܮ‬୞ୣ୯,ସ଴୩ୌ୸ is clearly exceeded in a large
area. There are additional maxima with zones of high sound pressure levels clearly outside the arrayarea. These grating lobes appear because the spacing between the array elements is greater than awavelength. The diameter of the transducers is in the order of the wavelength, leading to a directive
radiation pattern with a considerable part also in horizontal direction. Therefore due to the directivity
pattern of the single elements and the array factor constructive interferences in undesired directions
occur generating grating lobes with noticeable energy. The described effect also becomes clear in theillustration of the axial distribution in Figure 6. In addition to the narrow main lobe with a diameter(full width at half maximum) of about 14 mm grating lobes occur at a distance of approximately
200 mm away from the array center.

Figure 6 – Axial distribution of one-third octave sound pressure level ‫ܮ‬Zeq,40kHz measured at z = 200 mm andmaximum intensity setting. Measurements were done with 1/8" 40DP microphone using the portal scanner of
the PTB.

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The basic physical approach to generate a localized low frequency sound source by modulating a
focused ultrasonic carrier signal becomes very clear from the results in Figure 7. Here the measured
pressure signals in the focal plane (‫ݔ‬, ‫ݕ‬, 200) at maximum intensity setting are evaluated at different
frequency bands. The measurements were performed at the Robert Bosch GmbH, using the 1/4" 46BE
microphone and a 3D positioning unit. The spacing of the measuring points was 0.5 mm in a region of‫ݔ‬, ‫ = ݕ‬±20 mm around the center and 1 mm outside. The frequency bands with the specified nominal
center frequencies represent the 200 Hz modulation frequency, the 40 kHz carrier frequency and theharmonic component at 80 kHz. The results demonstrate the ability to generate a focused sound fieldwith a low frequency component by modulating an ultrasonic carrier and use of nonlinear effects. The
existence of higher harmonics is also the result of nonlinear effects. In a purely linear system, neitherthe modulation frequency nor a harmonic component would occur in the frequency spectrum of amodulated carrier signal. Compared to the spatial distribution at 40 kHz center frequency themaximum of the harmonic field component is about 12 dB lower.
200 Hz40 kHz80 kHzFigure 7 – Spatial distribution of one-third octave sound pressure level ‫ܮ‬୞ୣ୯,୘ୣ୰୸ evaluated at the frequency
bands with the specified nominal center frequencies according to the 200 Hz modulation frequency (left), the40 kHz carrier frequency (middle) and the 80 kHz harmonics (right).

5. CONCLUSIONSThe strongly focused airborne ultrasound fields with very high sound pressures characterized inthis study require a careful selection and use of suitable measurement microphones. When measuring
without a protection grid and applying a free-field correction for pressure microphones, very good
agreement between the measured sound pressure levels can be achieved with both 1/4" and 1/8"
microphones. In terms of spatial resolution and when multiple harmonics need to be considered, 1/8"
microphones are preferred.
With regard to user exposure the guiding limits are clearly exceeded in the focal region as well as
far outside the geometrical focus due to the effect of grating lobes in the operation mode of a stationary
single focal point considered in this study. The intended use of the investigated ultrasonic array is tocreate haptic feedback by projecting one or multiple focal points on user’s hand. Further investigations
under real use conditions including shielding effects by user’s hand are necessary to evaluate thepotential exposure of users to hazardous high ultrasound pressure values.

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