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

1. Introduction

These guidelines, including exposure limit criteria, aim to assure the safe use of industrial and commercial ultrasound. They cover a large number of varied applications of ultrasound, which use quite different mechanisms to produce their desired effects. Table 1, from Shoh (Sh 75) and Michael (Mi 74), lists the major applications of high-power ultrasound together with the ultrasound frequency, power range and a brief description of the process involved in the application.

The physical and biophysical effects of high-power ultrasound generally depend on the following complex, vibration-induced phenomena in matter:

1. cavitation and microstreaming in liquids;
2. surface instability occurring at liquid-liquid and liquid-gas interfaces;
3. heating and induction of fatigue in solids;
4. heating in liquid and liquid-like media.

In high-power industrial ultrasound the objective is to expose the workpiece to vibratory energy of sufficient intensity to bring about a permanent physical change. The main hazard to the user is from accidental contact exposure to the ultrasonic wave. However, many industrial and commercial uses of ultrasound also incidentally generate and propagate high sound-pressure levels in the air in the sonic and ultrasonic range. When this happens, a hazard may also arise from the ear's reception of the airborne ultrasound.

Table 1. Industrial Applications of High Power Ultrasound [From Shoh (Sh 75) and Michael (Mi 74)].

Application	Description of Process	Frequency (kHz)	Intensity Range (W/cm2)
Cleaning and degreasing	Cavitated cleaning solution scrubs parts immersed in solution	20 - 50	Approximately 1 - 6 W/cm2 ofdriving area
Soldering and braising	Displacement of oxide film to accomplish bonding without flux	-	1 - 50 W/cm2
Plastic welding	Welding soft and rigidplastic	About 20	Approximately 100 W/cm2 at theweld (700 W)
Metal welding	Welding similar and dissimilar metals	10 - 60	Approximately 2000 W/cm2 at the welding tip
Machining	Rotary machining, impact grinding using abrasive slurry, vibration assisted drilling	Usually 20	-
Extraction	Extracting perfume,juices, chemicals from flowers, fruits, plants	About 20	About 500 W/cm2 (100 - 500 watts)
Atomization	Fuel atomization to improve combustion efficiency and reduce pollution and dispersion of molten metals	Between 20 and 300	-
Emulsification, dispersion, and homogenization	Mixing and homoge-nizing of liquids, slurries, creams	-	-
Defoaming and degassing	Separation of foam and gas from liquid, reducing gas and foam content	-	-
Foaming of beverages	Displacing air by foamin bottles or container prior to capping	-	-
Electroplating	Increases plating rates and produces denser, more uniform deposit	-	-
Erosion	Cavitation erosion testing, deburring, stripping	-	-
Drying	Drying heat sensitive powders, food stuff, pharmaceuticals	-	-

A number of direct commercial applications of airborne ultrasound, including burglar alarms, automatic door-openers, TV converters and automatic camera-focusing devices, can also, in some cases, yield relatively high sound-pressure levels at ultrasonic frequencies. Again, the hazard from the airborne ultrasound may arise from reception by the ear.

To make allowances for differences between these two types of hazards, separate guidelines and rationales are given for contact and for airborne ultr asou nd exposures.