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Hearing Sound
Sound is energy that travels as longitudinal waves containing regions of high and low pressure.
The audible region of the sound spectrum for humans extends from about 16 Hz - 20 kHz.
Test your hearing:
http://www.noiseaddicts.com/2009/03/can-you-hear-this-hearing-test/
In the 1930s recordings had a frequency range of only 3 kHz which was good enough to
understand speech, but music was muffled. Today, frequencies from about 20 Hz up to
25 kHz can be provided. There is no interest in developing this further since only young
children and dogs can hear up to 25 kHz.
Frequencies below 16 Hz are called infrasonic. The vibrations of a bee's wings, a pendulum, or
water waves before reaching shore are all infrasonic. Many birds and animals can detect
these frequencies. Rhinoceroses even communicate using infrasound. Because of the
behaviour of animals before an earthquake, scientists believe that earthquakes may produce
low-frequency ultrasound before the main shock waves begin.
Frequencies higher than 20 kHz are called ultrasonic. Ultrasound is used to clean jewellery,
detect internal flaws in castings, break up gallstones, and accelerate the healing of damaged
tissues such as muscles. Bats use ultrasound to locate objects, and dolphins and porpoises use
ultrasound for navigation as well as communication. Because of the short wavelength, the
waves do not diffract around small objects, and reflect strongly back to the source.
The amount of energy passing through a unit area per second is called the intensity. The
human ear is quite sensitive, so we use the decibel (dB) as a unit of measurement for
intensity. (The bel (B) is named after Alexander Graham Bell. 1 B = 10 pW/m2)
Sound intensity depends on the strength
of the source of the sound, how far away
the listener is from the source, and the
ability of the medium between the listener
and the source to transmit sound energy.
Source
Intensity
Loudness is a measure of the response of the ear to sound.
A person with acute hearing can detect a frequency of 1 kHz at a minimum intensity of 0 dB,
whereas the average person must receive 12 dB to be able to hear the same frequency.
Since humans cannot hear a frequency of 25 KHz, this sound has a loudness of zero, even
though its intensity might be 100 dB.
Structure and Function of the Human Ear
The eardrum vibrates in phase with
the incoming sound wave.
If the Eustachian tube is blocked, the
pressure inside the ear cannot adjust
quickly to changes in the atmospheric
pressure. This can lead to earaches
when suffering from allergies or a cold,
or during take-off in an airplane.
The middle ear transmits and amplifies the pressure variations received from sound waves.
Vibrations are passed to the fluid inside the cochlea, where hair-like cells respond to
different frequencies. Their vibrations stimulate nerve cells to emit electric impulses which
are carried along the auditory nerve to the brain.