The Science Behind Hearing

The ear is one of the most important sensory organs. It can withstand the pounding of sound waves strong enough to make our body shake. At the same time it can hear even a pin drop. In a gathering, the ear allows to listen to some one speak while suppressing the noise of he room. It is beyond ordinary comprehension, how the ear detects minute sound vibrations and translates them into blended perceptions of a symphony orchestra. The ear co-ordinates our auditory experiences and co-relates them as pitch, loudness and timbre.

We all know that the pitch of a sound is related to the frequency of the waves hitting our ear. The range of frequency that a human ear can detect is from 16 to 20,000 cycles per second. Animals like dogs can hear frequencies much higher. This is the reason why a dog responds to a whistle of a high frequency that a man cannot hear.

The range of pressures that can produce various degrees of loudness is enormous. Auditory intensity is measured in terms of dynes per square centimeter of pressure, expressed in terms of the decibel. This is a unit stating how many times a sound is more intense than another, as experienced by the listener. As a standard, the absolute intensity is measured in relation to the lower threshold of hearing at 1000cycles per second. This intensity is taken as 0 decibels.

A quite office has a sound intensity of around 40 decibels. The vacuum cleaner creates an intensity level of 50 decibels whereas a busy street is more intense in sound with a decibel range of about 60 decibels. The noise level from a bolt of lightning is some 120 decibels and a jet plane engine produces a sound level of 14 decibels which is near the threshold of pain for a human ear.

We recognize sounds. The characteristic of a tone by which we recognize sound is called its timbre. Thus we identify the sounds of violin, the piano or the guitar. The timbre of a tone is identified by its form or pattern. The lowest frequency is the basic which determines the predominating pitch of the tone. The partial vibrations come as the overtones, which enrich the auditory experience

When two tones of a very small frequency difference are sounded together, we get the beats. For example, when we sound a tone of 256 cycles together with that of 260 cycles, we hear a pulsation beating four times per second. These pulsations in loudness equal the difference between the frequencies of the tones. Beats are thus caused by the interaction of two sound waves as they alternately reinforce and interfere with each other. Two tones of greater frequency difference will give a pleasant third tone whose frequency will be equal to their difference.

Also, a louder noise drowns out a weaker one rendering you disabled to hear the weaker noise. For example, it is our experience that our conversation is masked while a jet plane flies overhead. This is because a masking tone drowns out higher frequencies more than tones lower to itself. It will be interesting to note that the closer the frequency of the masking tone to the tone being masked, the greater the effect of masking.

Similarly, low pitches and noises are comparatively easer to locate than high frequencies. It is found that most sound localization is difficult except when the source of the sound is to the left or right of the ear. The location of sounds which come from the front, back, top or below is usually confused by the listener. Also, sound localization using only one ear is more difficult and less accurate than the localization by two ears. Successful sound localization can be done by turning one's head and by a familiarity with one's surroundings. That is to say, if we know the location of the fire station, our ears will easily detect the direction of the wail of the siren.

As we grow older, most of us lose some auditory sensitivity. In such cases, tones of 500 cycles and above are affected. Because of individual differences, it will be difficult to predict one's hearing loss due to age. However, studies have shown that in general, women tend to show greater losses with age in the range of 500 to 2000 cycles where as men show losses of frequencies above 2000 cycles per second.

Can hearing be improved? Very rarely if the abnormality is due to nerve deafness. If the deafness is due to conduction, there is a chance to restore hearing by an operation or by using a hearing aid.