What harm does ultrasound have to the human body? Soun […]
What harm does ultrasound have to the human body?
Sound waves are not a source of radiation and are generally harmless to the human body. We know that when an object vibrates, it emits sound. The number of vibrations per second is called the frequency of the sound, and its unit is hertz. The frequency of sound waves that we can hear is 20 to 20,000 Hz. If the vibration frequency of the sound wave is greater than 20000 Hz or less than 20 Hz, we will not be able to hear it. Sound waves with a frequency higher than 20000 Hz are called "ultrasound". The frequency of ultrasound usually used for medical diagnosis is 1 to 5 MHz. Ultrasound has the characteristics of good directionality, strong penetrating ability, easy to obtain concentrated sound energy, and long distance in water. It can be used for distance measurement, speed measurement, cleaning, welding, stone crushing, etc. It has obvious effects in medicine, military, industry, and agriculture. High-power focused ultrasound can be used to treat tumors. It can rapidly increase the temperature of the target area tissue to above 70°C within 0.5 seconds, causing the intracellular protein to rapidly appear coagulation. Necrosis (the critical point of lethal temperature for tumor cells is 42.5~43.0°C, while that of normal cells is 45°C. For liver tissue, a local temperature exceeding 58°C can cause tissue coagulation necrosis). The local sound intensity is as high as more than 10,000 watts/cm2, of course, it may damage the local normal tissues, and precise positioning is required. In terms of ultrasound protection, it is necessary to pay attention to people suffering from hearing diseases and neurological disorders not to work around such equipment. High-intensity ultrasound equipment should be shielded to block ultrasound. At present, there is no exact data to prove that the ultrasonic welding machine is too harmful to the human body, but the only disadvantage is that you must wear earplugs when welding to prevent long-time work and affect your hearing!
For industrial ultrasonics, such as ultrasonic welding and ultrasonic cleaning, the ultrasonic frequency and sound intensity must also be considered. In many cases, the exposure time and the coupling of the medium must also be considered (for example, if there is water between the human and the sound source, the ultrasonic sound intensity will be There will be danger). In a word, low-frequency ultrasound (around 20kHz) needs earmuff protection. Although it is harmless in theory, it sounds disturbing. . .
There is no exact report that air-borne ultrasound can cause hearing damage, but there are some subjective discomforts such as headaches and ear discomfort. Even so, the sub-harmonics caused by ultrasound can still cause damage to hearing when the sound intensity is large enough.
Therefore, the industry now has clear standards to limit the intensity of ultrasound. Compared with high-frequency ultrasound, the intensity of low-frequency (about 20kHz) ultrasound is more stringent, because some people may still hear this frequency. The most loose standard is currently revised by the American Government Industrial Health Association in the early 21st century. For 20kHz ultrasound, the maximum allowable sound intensity is 135dB, and when exposed for 8 hours, only 94dB is allowed. For high-frequency ultrasound, this standard will be appropriately relaxed, and if there is a liquid coupling between the human and the sound source, it must be reduced by 30dB in this standard.