Ultrasonic noise refers to sound waves with frequencies beyond the range of human hearing. These high-frequency sounds are generated by various sources in daily life and industrial settings. The safety of this noise for humans and animals is a common concern. Understanding ultrasonic noise and its potential effects is important for assessing its impact.
Understanding Ultrasonic Noise
Ultrasonic noise encompasses sound frequencies above 20 kilohertz (kHz). While its physical principles are similar to audible sound, these higher frequencies are not perceived by the human ear. This type of sound can originate from both intentional and unintentional sources.
Intentional sources include technological devices that produce ultrasound for specific processes. Examples are ultrasonic cleaning devices, often operating around 40 kHz, and industrial applications like welders, drills, and galvanizing pots, some generating noise levels up to 140 dB. Unintentional sources include high-speed machinery such as milling machines, grinders, circular saws, and pneumatic tools. Medical imaging, like diagnostic ultrasound, uses much higher frequencies, typically ranging from 2 MHz to 40 MHz.
Effects on Human Health
Exposure to ultrasonic noise can lead to various reported symptoms in humans, though these are often subjective. Individuals exposed to ultrasonic sound, particularly at higher intensities, have reported headaches, fatigue, nausea, dizziness, and tinnitus (ringing in the ears). These effects are often temporary, resolving once exposure stops.
High-intensity industrial exposure can have more pronounced effects than lower-level consumer device exposure. Workers using ultrasonic devices have experienced functional changes, including neurasthenia, cardiac neurosis, and central nervous system disturbances. Exposure to very high-frequency sound and ultrasound with sufficient intensity can be traumatic for hearing and cause other health effects. While severe, permanent harm from common sources is not widely documented, discomfort remains a possibility.
The concept of “silent sound” highlights that even sounds inaudible to humans can still affect them. Sensitive individuals might experience symptoms like ear fullness and headache, even from signals exceeding 17 kHz at levels above 70 dB. Some studies suggest that subjective symptoms may be more related to audible components of the noise spectrum accompanying ultrasound rather than the ultrasound itself.
Impact on Animals
Many animals possess a broader hearing range than humans, making them more sensitive to ultrasonic frequencies. For example, while humans typically hear up to 20 kHz, dogs can hear up to 60 kHz, and cats up to 64 kHz. Mice and rats have even higher ranges, up to 91 kHz and 79 kHz respectively.
Ultrasonic pest repellers, designed to deter rodents and insects, operate within frequency ranges audible to many animals, often between 25 kHz and 65 kHz. This overlaps with the hearing ranges of common pets like dogs and cats. While some manufacturers claim these devices are harmless, pet owners report signs of irritation, anxiety, or behavioral changes in their animals.
Pets might display behaviors such as panting, pacing, or attempting to hide, indicating discomfort or stress. Smaller pets like guinea pigs, hamsters, and rabbits are particularly sensitive to high-frequency sounds, and constant exposure may cause them greater stress. The intensity and variability of the ultrasound can influence how different animals react.
Safety Standards and Prevention
Regulations and guidelines exist to manage exposure to ultrasonic noise, particularly in occupational settings. The American Conference of Governmental Industrial Hygienists (ACGIH) has established permissible ultrasonic exposure levels, and the Occupational Safety and Health Administration (OSHA) acknowledges these guidelines. These limits are generally set to prevent potential hearing loss caused by subharmonics of the frequencies, rather than the ultrasonic sound itself. Most guidelines recommend an upper limit between 100 and 115 dB for frequencies above 20 kHz as an admissible sound pressure level that should not cause audiometric hearing loss.
For the general public, minimizing exposure to ultrasonic noise from consumer devices involves practical steps. Maintaining a reasonable distance from devices known to emit ultrasonic frequencies, such as certain pest deterrents or cleaning equipment, can help reduce exposure levels. Checking product specifications for information on emitted frequencies and sound pressure levels can also inform consumer choices. Awareness of one’s environment and any potential sources of high-frequency sound, even if inaudible, contributes to prevention.
For industrial applications, where exposure levels can be significantly higher, more stringent controls are applied. These include engineering controls to reduce noise at the source, administrative controls to limit exposure duration, and the use of personal protective equipment. While direct body contact with a radiating ultrasonic source should generally be avoided, airborne ultrasonic noise usually has little effect on general health unless there is such direct contact.