Is ultrasound audible?
Sounds with a frequency of 20 kHz and higher are referred to as ultrasound (or ultrasonic sound). High frequency sound is sound of which the frequency lies between 8 and 20 kHz. High frequency sound with a frequency over 16 kHz can hardly be heard, but it is not completely inaudible. High frequency sound and even ultrasound in the lower frequency zone (up to 24 kHz) can be audible if the sound level is high enough. The sound threshold (the sound level where sound can be perceived) rises sharply once the frequency (and therefore, the tone) becomes higher. Younger persons hear high frequency sound better and their hearing range is greater toward the high frequencies.
Ultrasound (and sound of high frequencies) are widely applied in the industry and health care, but is also used more and more in the consumer products (“Mosquito”- products, products for repelling animals and birds, movement detectors, range finders and remote controls). Typical for these applications is that their sound levels are low and that the sound frequency usually lies below 100 kHz. Medical and industrial applications make use of a wider range of ultrasound frequencies (up to MHz) and usually use much higher sound levels.
More information concerning products which use ultrasound, and the possible nuisance thereof, you find in the section “Ultrasonic products”. In this section we are dealing with the everyday consumer products.
Health effects and their threshold values
It is assumed that high frequency sound and ultrasound with sufficient intensity can be traumatic for the hearing, and can cause other effects as well. High frequency sound causes two types of health effects: on the one hand objective health effects such as hearing loss (in case of protracted exposure) and on the other hand subjective effects which may already occur after a few minutes: headache, tinnitus, fatigue, dizziness and nausea. Depending on the personal sensitivity (this is also age-related) people may already experience these subjective effects at a sound level of 75 dB. Note that ultrasound (and sound of high frequencies) is expressed in dB contrary to normal sound which is measured in dB(A). Read more in “Explanation of technical terms concerning sound”.
With protracted exposure, inaudible ultrasound can also contribute to hearing loss. If the ultrasound is too strong, in the ear sub-harmonic vibrations are generated, which are audible and which can cause hearing damage.
For the frequency range from 16 to 20 kHz two agencies, the International Non-Ionizing Radiation Committee(INIRC/IRPA, 1984) and the Canadian government (1991), impose a limiting value of 75 dB for the sound pressure level with an occupational exposure. The INIRC/IRPA also provides a value for the exposure of the general public, namely 70 dB with 20 kHz. Respecting these limits would offer protection against subjective effects and possible hearing loss as a result of protracted exposure.
For ultrasound between 25 kHz and 100 kHz INIRC/IRPA advises a limiting value of 100 dB.
Standardization of high frequency sound and ultrasound
There is no specific regulation for high frequency sound and ultrasound. High frequency sound must comply with the same laws as all other sound. Ultrasound is not taken into account. Manufacturers which market ultrasonic products must consider available scientific recommendations.