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Appendix I:D. Ultrasonics

Applicability of OSHA's Noise Standard, 1910.95

OSHA's noise 1910.95 addresses airborne ultrasonic noise because of OSHA's criterion parameters.

At 20 kilo-Hertz (kHz), an A-weighted sound pressure level is 10 decibels (dB) below an unweighted sound
pressure level.

At 50 kHz, an A-weighted sound pressure level is 25 dB below the corresponding unweighted level.
 Example: If an A-weighted instrument is used to measure a 50 kHz, 110-dB tone, the instrument
will indicate 85 dBA. (Note that the American National Standards Institute's (ANSI) S1.4-1976 or
S1.4-1983 standard, which sets the performance and accuracy tolerances that all OSHA sound-level
meters must meet, requires a Type II microphone accuracy of only ±5 to 4 at frequencies above
10,000 Hz).

Health Effects and Threshold Limit Values (TLVs®)

Research indicates that ultrasonic noise has little effect on general health unless there is direct body
contact with a radiating ultrasonic source. Reported cases of headache and nausea associated with
airborne ultrasonic exposures appear to have been caused by high levels of audible noise from source
subharmonics.

The American Conference of Governmental Industrial Hygienists (ACGIH®) has established permissible
ultrasound exposure levels. These recommended limits (set at the middle frequencies of the one-third
octave bands from 10 kHz to 50 kHz) are designed to prevent possible hearing loss caused by the
subharmonics of the set frequencies, rather than the ultrasonic sound itself.
®
 These TLVs represent conditions under which it is believed that nearly all workers may be
repeatedly exposed without adverse effect on their ability to hear and understand normal speech.
Previous TLVs® for frequencies in the 10 kHz to 20 kHz range, set to prevent subjective effects, are
referenced in a cautionary note below (1). The 8-hour time-weighted average (TWA) values are an
extension of the TLVs® for noise, which is an 8-hour TWA of 85 dBA for sound below 10 kHz. The
ceiling values may be verified by using an integrating sound level meter with slow detection and 1/3
octave bands. All instrumentation should have adequate frequency response and should meet the
specifications of ANSI S1.4-1983 and International Electrotechnical Commission (IEC) 804.
®
 Measuring any source suspected of producing sound at levels exceeding the ACGIH recommended
limits requires the use of a precision sound level meter, equipped with a suitable microphone of
adequate frequency response, and a portable third-octave filter set. Consult with the Assistant
Regional Administrator for Technical Support for guidance.

TLVs® for Ultrasound

Measured in Air in dB Measured in Water in dB

Mid-Frequency of
Ceiling
Third-Octave Band 8-Hour TWA Ceiling Values
Values
(kHz)
10 105A 88A 167
12.5 105A 89A 167
16 A A 167
105 92
20 A A 167
105 94
 25 110B -- 172
31.5 115B -- 177
40 115B -- 177
50 115B -- 177
63 115B -- 177
80 115B -- 177
100 115B -- 177
A
Subjective annoyance and discomfort may occur in some individuals at levels between 75 and 105
dB for the frequencies from 10 kHz to 20 kHz especially if they are tonal in nature. Hearing
protection or engineering controls may be needed to prevent subjective effects. Tonal sounds in
frequencies below 10 kHz might also need to be reduced to 80 dB.

BThese values assume that human coupling with water or other substrate exists. These thresholds
may be raised by 30 dB when there is no possibility that the ultrasound can couple with the body
by touching water or some other medium. [When the ultrasound source directly contacts the body,
the values in the table do not apply. The vibration level at the mastoid bone must be used.]
Acceleration Values 15 dB above the reference of 1g rms should be avoided by reduction of
exposure or isolation of the body from the coupling source. (g = acceleration due to the force of
gravity, 9.80665 meters/second; rms = root-mean-square).

Source: ACGIH® Worldwide. 2003 TLVs® and BEIs®: Threshold Limit Values for Chemical
Substances and Physical Agents & Biological Exposure Indices, p.107.

Controls

High frequency noise is very directional and is relatively easily reflected or blocked by any type of barrier.
The wavelength of a 16 kHz tone, for example, is about 3/4 inch, so a barrier of one to two inches higher
than the source is generally sufficient to reflect noise of approximately the same frequency away from the
nearby worker. Such barriers are inherent in some of the machines and surfaces themselves.

High frequency audible noise is also easily absorbed by any of the so-called acoustical materials (for
example, glass fiber or foam).

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