PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Ultrasound noise policy and assessment: Canada Safety Code – 24,
a Canadian perspective
Lixue Wu1
1

National Research Council, Canada

ABSTRACTUltrasound is extensively used nowadays in work and home. Scientific evidence on the health effects of
exposure to ultrasound recommends that caution should be taken when using ultrasound. This leads to safety
regulations established and enforced by government agencies in some countries. In Canada, early regulations
respecting occupational safety and health made under Part IV of the Canada Labour Code, were substituted
by Canada Occupational Health and Safety Regulations (SOR/86-304), effective March 31, 1986. Where adevice that is capable of producing and emitting energy in the form of non-ionizing radiation is used in thework place, the employer shall implement the applicable document, in respect of ultrasound equipment,
Safety Code - 24, dated 1990. The National Research Council Canada (NRC) is one of the government
agencies who provided reviews and comments in the preparation of the Code. In this paper, the Canadianrecommended human exposure limits for airborne ultrasound and its measurement techniques are discussedwith the focus on challenges. To implement the safety code, a measurement system for ultrasound pressurelevel measurements was developed recently at the NRC. This paper presents the details of the system with thefocus on measurement uncertainties. An on-site noise assessment example is given using the system.
Keywords: Ultrasound, Noise, Safety

1. INTRODUCTION
Ultrasound is extensively used nowadays in work and home. Scientific evidence on the healtheffects of exposure to ultrasound recommends that caution should be taken when using ultrasound.
This leads to safety regulations established and enforced by government agencies in some countries.
However, these regulations are not well-known to the public. A recent article (1), for example, statesthat so far there are no real national or European laws or regulations, but only recommendations issued
by health organizations in several countries. This is certainly not true in Canada. In Canada, early
regulations respecting occupational safety and health made under Part IV of the Canada Labour Code,were substituted by Canada Occupational Health and Safety Regulations (SOR/86-304), effective
March 31, 1986 (2). Where a device that is capable of producing and emitting energy in the form of
non-ionizing radiation is used in the work place, the employer shall implement the applicable
document, in respect of ultrasound equipment, Safety Code – 24, dated 1990. The National ResearchCouncil Canada (NRC) is one of the government agencies who provided reviews and comments in the
preparation of the Code.
Ultrasound applications generate ultrasound waves. The human body may contact with these waves
through two means of transmission of these waves: direct contact and airborne ultrasound. Directcontact exposure to high-power ultrasound must be avoided at all times as stated in Safety Code 24.
For low-power ultrasound, such as that used in non-destructive testing, unnecessary direct contact
exposure should be avoided. Therefore, direct contact exposure to ultrasound is not the scope of thispaper.
In this paper, the Canadian human exposure limits for airborne ultrasound and its measurement
techniques are discussed with the focus on challenges. To implement the safety code, a measurement
system for ultrasound pressure level measurements was developed recently at the NRC. This paper
presents the details of the system with the focus on measurement uncertainties. An on-site noiseassessment example is given showing how the noise levels from the noise-generating equipment aremeasured and how the effectiveness of protective measures is compared using the system.
1

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lixue.wu@nrc-cnrc.gc.ca

2. SAFETY CODE - 24
2.1 Safe Use of Ultrasound
Safety Code – 24 (3), hereafter the Code, aims to assure the safe use of industrial and commercial
ultrasound. It provides information on the health effects of both contact and airborne ultrasoundexposure and recommends safety procedures and protective measures. Most important, the Codeincludes exposure limit criteria.
2.2 Human Exposure Limits
For contact exposure, there is no exposure limit given in the Code. Instead, the Code states that
direct contact exposure to high-power ultrasound must be avoided at all times and recommends
avoiding unnecessary direct contact exposure to low-power ultrasound. There is no numericaldefinition to the power level, high or low. But typical high-power applications are given in the Codewith frequency range and power range specified. For these reasons, direct contact exposure toultrasound is not the scope of this paper.
Exposure to airborne ultrasound, when sufficiently intense, appears to result in a syndrome
involving manifestations of nausea, headache, tinnitus, pain, dizziness, and fatigue. The type ofsymptom and the degree of severity appear to vary depending upon the actual spectrum of theultrasonic radiation and the individual susceptibility of the exposed persons, particularly their hearing
acuity at high frequencies (3).
Based on interpretations and analyses of the biological effects studies, The Canadian exposure
limits (3) are given in Table 1. These are given in 1/3-octave bands from 16 kHz to 50 kHz. The
exposure limits are independent of time as subjective effects can occur almost immediately.
Table 1 – Canadian exposure limits for airborne ultrasoundFrequency (kHz)SPL (dB)

16

75

20

75

25

110

31.5

110

40

110

50

110

2.3 Airborne Ultrasound Measurement
Measurements shall be made in the place where exposure of a person occurs. The angle of incidence
shall be estimated for a directional signal unless the ultrasonic field is determined to be better
approximated as a random incidence (diffuse) field. This can be done by placing the ultrasound sourcein an anechoic chamber if possible or by observing changes in the sound pressure levels as a functionof orientation of the microphone (3). In order to make a reasonable judgment of whether the limits
have been exceeded it is important to find the position for the microphone which gives the largest
sound pressure level. This requires determining whether the ultrasonic field is well approximated as afree field or random incidence field by means of approximate field mapping. That is, the knowledge ofthe angular and distance dependence of the sound pressure level. If the conditions were always freefield, then a 1/4-inch microphone designed for flat free field frequency response with protecting gridremoved and operating at normal incidence would be suitable up to the 50 kHz 1/3-octave band (3).
2.4 Challenges
There are several significant challenges when attempting to make reliable measurements ofairborne ultrasound in the frequency range up to 50 kHz. These include the lack of widespread
literature on the subject (4). The most significant challenges are the calibration of measuring
instruments and the estimation of ultrasound pressure level measurement uncertainty.
The measuring instrument is often calibrated at a single frequency, 1 kHz or below. The Coderecommends adding systematic corrections for ultrasound frequencies. The correction can be obtained

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from manufacturer’s specifications. This practice is technically reasonable in early 90s; however, itcannot produce a traceable sound pressure level measurement. Over the years, the research advances inmetrology have realized the acoustical standards for frequencies up to 100 kHz. Several primary andsecondary microphone calibration systems have been developed (5-8). Traceable ultrasound pressurelevel measurements became possible.
Sound pressure level (SPL) measurements are key points to determine whether there is a breach ofnoise limits. Any measurement made without the knowledge of its uncertainty is completely
meaningless. On the other hand, overestimation of uncertainty may result in unnecessary measures toreduce noise that can be very expensive. The Code states that it is essential to make reasonableestimates of the measurement uncertainty. This is rather informative and does not meet therequirements of ISO/IEC Guide (2008). With the recent research advances (9, 10), an accurate
estimation of measurement uncertainties can be done. With the measurement uncertainty, one candetermine whether an ultrasound noise source conforms the noise regulation, the Code, according to
the Joint Committee for Guides in Metrology document, JCGM 106:2012, Evaluation of measurement
data - The role of measurement uncertainty in conformity assessment.

3. IMPLEMENTATION
3.1 Ultrasound Pressure Level Measurement System
The ultrasound pressure level, hereafter the sound pressure level, measurement system developed isan open system that includes the exposure of the data at every stage. The uncertainty contribution ateach stage can then be assessed. The schematic diagram of the sound pressure level measurement
system is shown in Fig. 1.
ComputerFrequency
Weighting

Squaring

Digitizer
Time
Weighting

Summing
Preamplifier

Scaling

Microphone

(a)

(b)

Figure 1 – A measurement system for ultrasound pressure level measurements. (a) Signal waveformrecording unit with sound receiving device. (b) Post data processing unit.
The sound wave recording unit is a digital sound recording with its schematic diagram shown in Fig.
1a. The digital recording converts the analog sound signal picked up by a microphone and a
preamplifier to a digital form by the process of digitization in a digitizer. This lets the sound data be
stored and processed by a variety of algorithms. The microphone can be either a ¼-inch free-field orpressure-field microphone depending on the application of interest. The digitizer is a high-speed
analog-to-digital converter offering 4 channels of simultaneous sampling at 20 MSa/s or above with 14
or 16-bits of resolution (Keysight L4534A or GaGe CS8349).
Shown in Fig. 1b is the schematic diagram of the post data processing unit. The frequency
weighting and time weighting are optional depending on the application. The basic problem inଵ ்
implementation is to compute an approximate solution for the definite integral given by ଴ ଶ 
்where ଶ  is the squared instantaneous sound signal that may be frequency-weighted and/or
time-weighted and T is a stated time interval. The numerical integration is implemented through the7066squaring, summing and scaling as shown in Fig. 1b. Different methods for approximating the integral
to a desired precision are realized in the summing with different numbers of subintervals for a given
sampling rate (9).
3.2 Measurement Uncertainty
There are many factors to be considered when evaluating the measurement uncertainty associatedwith the measurement of sound pressure levels. The major contributing factors impacting uncertainty
significantly are microphone including amplifier, quantization, numerical integration, and numericalcalculation listed in line with the data flow shown in Fig. 1.
With the sound signal samples available from the sound wave recording unit, the task of the post
data processing unit is to calculate the sound pressure level according to its definition given in
ANSI/ASA S1.1-2013. That is, to calculate the definite integral. This is implemented by numericalintegration. The error bonds of this approximate solution have been studied for various oversampling
rates and approximation methods (9). It is found that the error bond can be less than 0.06 dB for a tentimes oversampling with Simpson’s approximation method.
Quantization errors are introduced by quantization in the analog-to-digital conversion (ADC) in themeasurement system. The error bounds of the quantization for different digitizers with different input
ranges have also been studied (9). Here a simple method is presented using uncertainty propagation.
First, the absolute standard uncertainty d of quantization is calculated for a digitizer with given
resolution b. That is, = 1 ௕ିଵ √12 . Then, the relative standard uncertainty d i of quantization for2
sample x i is obtained by ௜ = ⁄| ݅ | . Finally, using uncertainty propagation, the uncertaintyଶ
contribution q due to quantization is = ௡ ∑௡௜ୀଵ ௜ , that depends on sound wave samples. Table 2 lists
the standard uncertainties of quantization for different sound wave signals with different ADC
resolutions.
Table 2 – Standard uncertainties for different signals with different ADC resolutions
ADC resolution bStandard uncertainty (dB)(bits)

SquareTriangle

Sinusoidal

120.00240.01990.0127140.00060.00580.0037160.00020.00170.0011240.00000.00000.0000The sound pressure level is first calculated by summing squared samples ௜ଶ . Numericalcalculation errors, or rounding errors, may occur because of the computing device's finite word sizethat is incapable to deal with certain numbers. Such numbers need to be rounded off to their nearest
approximations that can be represented by the device's word. A partial sum technique was thenproposed to eliminate rounding errors (9). For ultrasound application, there is no need to use thistechnique. The maximum integrating (summing) time without introducing any rounding error for a
16-bit ADC with a sampling rate of 2.5 MS/s (50 times oversampling) is about 28 minutes for 64-bit
computing devices. This is way beyond the summing time (exposure time) required for ultrasound,normally in a few seconds, as specified in the Code for monitoring subjective effects that can occur
immediately. Therefore, numerical calculation errors will never occur for sound pressure levelmeasurements in ultrasound.
The most significant uncertainty contribution is from the acoustic front-end that consists of a
microphone, a microphone preamplifier, and a bandpass filter (analog or digital). The gain of thepreamplifier and the bandpass filter can be calibrated accurately with an uncertainty of less than 0.004
dB. The uncertainty of a primary 1/4-inch microphone calibration is less than 0.4 dB for airborne
ultrasound up to 63 kHz (8). If a measured sound pressure level is way below the exposure limit, one
can use the worst-case scenario, a concept in risk management wherein the planner considers the most
severe possible outcome. This method would use the largest measurement uncertainty of the measured
frequency response of a microphone, normally occurring at the low or high frequency end, as theuncertainty contribution of for all frequencies (11). This will lead to an overestimation of theuncertainty contribution if the acoustic signal occupies a mid-frequency band. If the measured sound7067

pressure level is near the exposure limit, then an accurate estimation method (10) should be used to
avoid unnecessary noise reduction measures that can be very expensive.

4. ON-SITE ASSESSMENT USING THE SYSTEM
4.1 Ultrasound Source
The major difficulty with measurements of airborne ultrasound arises when the ultrasonic field isbest described as a superposition of a random incidence and free field and it is not possible todetermine the magnitude of each component (3). For this reason, the sound field of an ultrasoundsource (an ultrasound cleaner with nominal power of 160 W) was studied in an anechoic chamber asshown in Fig. 2. The information obtained can be used to estimate the random incidence component of
the ultrasound source at a non-free field.

(a)

(b)

Figure 2 – Measurement setup in anechoic chamber. (a) Receiving microphones at a fixed distance of
2 m. (b) Receiving microphone at a variable distance from 0.5 to 2 m.
Radiation beam patterns of the ultrasound source (ultrasound cleaner) were measured and plotted inFig. 3 for two typical operator positions (sit and stand). The directional variation of the sound radiationis less than 1.5 dB for both positions. The inverse law was also verified. It has negligible effects onsound pressure level measurements.

(a)

(b)

Figure 3 – Beam patterns of an ultrasound source. (a) On a horizontal plane (base plane of cleaner). (b)On a 60º cone with the apex at the acoustic center of the source.

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4.2 On-site Measurements
The on-site setup of the ultrasound source in a laboratory room is shown in Fig. 4. The source was
first placed on a granite table (Fig. 4a). Sound pressure levels were measured at the operator ear’sheight for different distances away from the acoustic center. The measurement results are plotted inFig. 5a. It is noted that the sound field does not follow the inverse law. This is mainly due to the nearby
screen doors of another apparatus. The reflected sound waves were added to the direct wave
constructively. For this setup, anyone who stays in the room without additional ear protection will
exceed the exposure limit. The source was then placed in a fume hood (Fig. 4b). Sound pressure levels
were measured at the operator ear’s height for a distance of 35 cm away from the acoustic center.
Figure 5b plots the sound pressure levels measured for two cases, the fume hood sash window open
and closed. The attenuation for the sash closed is about 6 dB. Thus, addition protective measures areneeded for this setup.

(a)

(b)

Figure 4 – On-site setup of ultrasound source. (a) Source on a granite table. (b) Source in a fume hood.
130

130
129

sash open

30 cm
60 cm
120 cm128

sash closed

125

126

SPL (dB)

SPL (dB)

127

125
124

120

123
122
121
120

0

5

10

15

20
25
Time (sec)

(a)

30

35

40

45

115

0

5

10

15

20
25
Time (sec)

30

35

40

(b)

Figure 5 – Sound pressure level measurements for two setups. (a) Fig. 4a setup with microphone at the
distances of 30 cm, 60 cm and 120 cm. (b) Fig. 4b setup with the fume hood sash open and closed.
4.3 Protective Measures
Enclosures are simpler and more economical in the ultrasonic range since the higher ultrasonicfrequencies tend to be much more readily absorbed and reflected inward by enclosures. An enclosure,used and shown in Fig. 6, was effective in attenuating the sound pressure level from 46 to 74 dB in the
1/3-octave frequency band from 16 kHz to 50 kHz. The enclosure was made of 1/2" steel, wrapped7069

with two-inch Styrofoam TM and fitted with a hinged front panel with a stainless steel runners on theoutside of the panel for the door. The enclosure was perfectly sealed as it was a pressure chamber usedin the investigation of microphone pressure coefficients (12).

(a)

(b)

Figure 6 – An enclosure for hazard isolation. (a) Inside the enclosure. (b) Front door closed.
Figure 6 shows the photos of an enclosure for ultrasound hazard isolation. Instead of placing theenclosure in an anechoic chamber for measuring attenuation, a simple and more practical method wasused. The method is to detect any effect on ambient noise when the source is turned on. To test whether
the two cases, the source on and off, leads to the same results, measurements on the ambient noiselevels were performed through switching the source on and off. The measured sound pressure levels
were then tested for the null hypothesis that the true mean difference is zero. The t-test results arelisted in Table 3. It is concluded that there is no significant mean difference between the two samples.
That is, there is no significant difference between the results obtained from the two cases, the sourceon and off. In other words, the ultrasound waves were confined in the enclosure, or the leakage is notdetectable.
Table 3 – Results of t-test for SPLs measured with ultrasound cleaner on and off
MethodAssuming equalvariances
Assuming unequalvariancest-test results
degrees of freedom

t-statistic

t-critical

48

1.491

2.011

48

1.491

2.011

In some cases where engineering controls are not feasible, reduction of ultrasound at the operator
by locating the ultrasound source in an isolated area is simple and effective. The operator can operate
the equipment remotely or use a timer.

5. CONCLUSIONSIn Canada, early regulations respecting ultrasound safety was established in 1986. The Canadianhuman exposure limits for airborne ultrasound and its measurement techniques were discussed withthe focus on challenges. To implement the safety code, a measurement system for ultrasound pressurelevel measurements was developed at the NRC. The details of the system were presented. The
uncertainty contribution of the digital part of the system can be controlled to negligible small. The
uncertainty contribution of the analog part of the system can be estimated using worst-case scenario orexact modelling depending on how close to the exposure limit. An on-site assessment shows how
environment (free field vs. non-free field) affects the sound pressure level measurements and how
protective measure reduces the ultrasound noise level. A normal laboratory room should not be

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considered as a free field room at ultrasound frequency. A well-sealed (pressure sealed) steel enclosure
can confine ultrasound radiation well to an undetectable leakage level. The ultrasound leakage of theenclosure can be tested simply by comparing the measurements of ambient noise with or without
ultrasound.

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