MadSci Network: Anatomy |
Ultrasound is above the 20 kHz limit of our hearing, so normally, humans do not hear them, but some animals (like dogs) can hear these high frequencies. For instance, dog whistles work because dogs can hear up to about 40 kHz. Less well known is infrasound, or sound at low frequencies, below about 20 Hz. Some animals hear these frequencies; one is the elephant, which can detect 15 Hz tones at sound levels inaudible to humans. Whales produce low-frequency sounds, which are thought to be a means of communication over distances of kiolometers. Low-frequency vibrations are also produced by the earth, and it is thought that some animals may sense an impending earthquake by hearing such a sound. Even though we usually cannot hear very low frequencies with our ears, they are present in our environment and can have unpleasant subconscious effects. Infrasound is produced by such devices as air conditioners, boilers, aircraft, and automobiles. Though intense infrasound from these machines does not cause hearing loss, it can produce dizziness, nausea, and headache. Many cars produce low-frequency sound when they're moving at highway speeds, making sensitive people carsick. At very high levels, low-frequency sound may also produce resonances in body cavities such as the chest and stomach, which can damage internal organs. You might want to think twice before standing in front of a large speaker at a concert! By the way, in case you are not familar with the term "hertz": sound produces variations in air pressure that are periodic. The frequency of the sound is the number of compressed patches of molecules that pass by our ears each second. One cycle of sound is the distance between successive compressed patches; the sound frequency, expressed in units called hertz (Hz), is the number of cycles per second. Our auditory system can respond to pressure waves over the remarkable range of 20 Hz -20,000 Hz (this can also be expressed as 20 kilohertz, abbreviated kHz). Thus, ultrasound is above 20kHz (20,000 Hz); infrasound, under 20 Hz. This information was taken from Neuroscience: Exploring the Brain, by Mark Bear, Barry Connors, and Michael Paradiso; William and Wilkins, Baltimore, MD; 1996.
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