Guidelines for the Safe Use of Ultrasound:
Part II - Industrial & Commercial Applications - Safety Code 24

3. Human Exposure to Airborne Ultrasound

There have been a number of reports and investigations into the exposure of workers to airborne ultrasound (Sk 65, Pa 66, Mi 74, Ac 67, Do 67, Gr 80, Sw 82, An 86). A wide variety of SPL values were obtained depending on the device, the location of measurement relative to the device, and whether the device is enclosed. Selected examples of SPLs, as measured at the operator's position, are given in Table 2.

Grzesik and Pluta (Gr 80, Gr 83) measured 1/3-octave band SPLs at the operator's position from ultrasonic cleaners at 25 and 28 kHz and at the subharmonic frequencies of 12.5 and 14 kHz. The SPLs at the fundamental frequencies were between 100 and 116 dB and the subharmonics had SPLs of 80 -102 dB. For ultrasonic welders, the average frequency was 21 kHz and the highest SPL was 106 dB in the 20 kHz 1/3-octave band. The results obtained over the past 25 years have for the vast majority of cases remained in agreement with Grigor'eva's statement that sound pressure levels from ultrasonic machinery in the ultrasonic frequency range rarely exceed 120 dB (Gr 66).

Since 1976, a number of measurements of airborne ultrasound SPLs from commercial devices have also been made by the Bureau of Radiation and Medical Devices (BRMD) in Canada and by the Center for Devices and Radiological Health (CDRH) of the Food and Drug Administration (FDA) in the U.S.A. Table 3 contains several examples of the largest SPLs found for typical exposure positions. Since these devices emit pure tones, the side band values are not given. Exposures from commercial devices are usually from single pure tones (although sometimes frequency sweeps are also used) and tend to be at lower SPLs than exposures from industrial devices.

Table 2. Selected SPLs in dB at the Operator's Position in 1/3-Octave Bands from Industrial Ultrasonic Sources

Reference	Ultrasonic Device	1/3 Octave Band Cente Frequency (kHz)
		10	12.5	16	20	25	31.5
(Sk 65)	Drill (no enclosure)	91	85	90	81	108	84
(Sk 65)	Welder	76	95	115	93	73	69
(Sk 65)	Cleaner	<61	<61	<61	83	67	<61
(Sk 65)	Cleaner	77	78	79	96	77	77
(Pa 66)	Jet engine (after-burner) power	114	113	11	108	106	103
(Cr 77)	Cleaner	86	77	86	105	97	80
(Ac 67)	Drill	75	67	75	97	75	78
(Ac 67)	Cleaner (small)	53	61	91	83	75	105
(Ac 67)	Cleaner (large)	83	86	91	96	91	102
(An 86)	Welder	N/A	N/A	N/A	127	N/A	N/A
(An 86)	Welder	N/A	N/A	N/A	106	N/A	N/A
(Sw 82)	Cleaner	78	66	76	97	74	70
(Sw 82)	Welder (no enclosure)	106	114	119	96	80	56

Table 3. Exposure SPLs in dB for Various Commercial Devices.

Reference	Ultrasonic Device	1/3-Octave Band Center Frequency (kHz)
		10	12.5	16	20	25	31.5	40
BRMD	Video display terminal (VDT)	-	-	66	-	-	-	-
CDRH(a)	VDT	-	-	61	-	-	-	68
BRMD	Ultrasonic person sensor	-	-	-	-	94	-	-
CDRH(b)	Ultrasonic intrusion alarm	-	-	-	93	-	-	-
CDRH(b)	Ultrasonic dog repeller	-	-	108	-	-	-	-

• (a) From (FDA 81(a)). Results for two separate VDTs at widely separated frequencies are shown here.
• (b) From (FDA 81(b)). The exposure site for the ultrasonic dog repeller is atypical, in this case, being five feet in front of the repeller. In most cases, the dog, rather than a human, would be at this position.

In summary, exposure to industrial ultrasonic devices rarely exceeds 120 dB. Exposure to commercial devices designed to emit ultrasound rarely exceeds 110 dB. Commercial devices which incidentally emit ultrasound, such as VDTs, yield SPLs at the operator's ear of l es s than 70 dB and only rarely exceed 65 dB.

Industrial ultrasonic devices also produce audible noise. These sonic exposures must not be ignored in considering the hazards of industrial ultrasonic equipment. There is ample evidence in the studies noted in Section 2 that the upper sonic and sonic emissions from ultrasonic devices are considerably more hazardous than ultrasonic emissions for equivalent SPLs.