After reading this article you will learn about:- 1. Introduction to Auditory Sensation 2. Dimensions of Auditory Sensations 3. Sound Mixture or Tonal Mixture 4. Auditory Adaptation 5. The Auditory System 6. Theories of Hearing.
Introduction to Auditory Sensation:
The auditory sensation or the sensation of hearing is next in importance only to visual sensations. In a way, however, auditory sensations are even more important than visual sensations. They have the ability to respond to stimuli from a much longer distance than visual sensations.
From the point of view of evolution, auditory sensations are more primary than visual sensations. In fact, in some of the lower organisms where the visual sensation is not fully developed, auditory sensations are fairly well developed. The stimuli for auditory sensations are sound waves.
The human ear can respond to a wide range of sound waves ranging from sixteen decibels to nearly twenty-two thousand decibels. Sound waves below sixteen and above twenty two thousand are not generally heard.
Dimensions of Auditory Sensations:
The various sound sensations we receive, differ in a number of dimensions. The major dimensions are duration, pitch, loudness and timbre. Different sounds last for different durations. Some sounds are heard for a short time and some for a long time, some have a high pitch being very shrill like a scream while others have a low pitch and are not shrill.
Some sounds are loud and others not so loud. Some sounds are pure while other sounds are mixed, thus, showing different degrees of timbre or quality. We can, therefore, see that the different sounds we experience differ in many dimensions. These dimensions or characteristics of the sound sensations correspond to different characteristics of the sound stimuli which impinge on the human ears and the receptors therein.
The pitch of a sound is dependent on the frequency of the sound waves. Sounds are categorized as low pitched and high’ pitched. As the frequency increases the pitch gets higher and higher. The loudness of a sound depends on the amplitude of sound waves. The amplitude of soun4 waves determines the pressure on the human ears and this, in turn, determines the loudness of the sound.
The range of amplitudes or the amount of pressure that can operate on the human ear is very wide. This intensity is measured in terms of decibels. It should also be pointed out that to some extent the pressure or amplitude also influences the pitch. Thus, pitch as well as frequency is determined by amplitude.
The timbre of a sound is the typical, qualitative characteristic of a sound. Thus, the sound of a violin is different from the sound of a flute. Similarly, we talk of rough voice, smooth voice, harsh voice etc. differentiating between different kinds of voice qualities. The timbre of a tone or a sound is determined by its pattern.
Out of these patterns emerge what are known as overtones. These overtones determine the quality of a sound sensation. Sometimes we refer to a sound as noise. Noise differs from sound. What differentiates a noise from the sound is the high degree of irregularity characterizing the former. Noises have mixed frequencies and are highly heterogenous.
Sound Mixture or Tonal Mixture:
While talking about visual sensations, it was pointed out that different kinds of light rays can be mixed to give different colours. Similarly, different auditory experiences also combine and mix together.
Some of the experiences arising out of a combination or mixing of sound experiences are as follows:
When two sounds of different frequencies are combined in quick succession, we hear a beat. We are all familiar with the beating of a drum. The loudness of this sound is equal to the difference between the two sounds. This is called the difference tone. Similarly, it is also possible to produce a summation tone where the two tones add up to produce a combined effect. This type of combination is an important element in orchestral music.
We have all experienced that when two-tones or sounds occur at the same time; one drowns or silences the other. This phenomenon in which one tone or sound covers the other is called masking. Usually tones or sounds with lower frequency are likely to mask those with higher frequency.
Another phenomenon which one notices in relation to sound sensation is that of adaptation. If we continue to listen to a sound for some time, it appears to become less and less loud though the stimulus characteristic remains the same. This phenomenon of adaptation, it is a general characteristic of all sensory experiences, though there may be exceptions.
The Auditory System:
The auditory system or the system of hearing consists of the receptors in the human ear, the auditory nerve and the temporal lobe of the cerebral cortex. The functioning of the system in brief is as follows. The sound waves activise the eardrum in the human ear. These motions of the eardrum or tympanic membrane as it is called are further transmitted through a chain of three bones in the middle part of the ear.
These bones are known as ossicles. They are popularly known as the hammer, the anvil and the stirrup because of their shapes. Their technical names are the incus, malleus and stapes. The three bones conduct the disturbances caused by the stimulations to another membrane in the inner part of the ear.
This membrane is known as the basilar membrane. The actual receptors which are in the form of hair-like structures, also known as organs of corti varying in length are located along the basilar membrane. The basilar membrane actually forms the lining of a snail-like structure called the cochlea.
The basilar membrane is set in vibration by the sound impulses transmitted by the ossicles. These are, in turn, transmitted by the auditory nerve to the temporal lobe of the cerebral cortex. Sound sensations are experienced when the temporal lobe is activated.
It can be seen that the human ear consists of three broad divisions; the outer ear, the middle ear and the inner ear. The pinna or the outer ear serves to funnel the sound waves into the middle ear.
The middle ear consists of the auditory tube or canal ending in the eardrum or the tympanic membrane followed by the ossicles. The inner ear consists of the cochlea, the basilar membrane and the three semicircular canals. The actual receptors, are in the inner part of the ear.
Theories of Hearing:
Just as in the case of visual sensations, a number of theories have also been developed to explain auditory sensations. We may consider some of them briefly.
1. Helmholtz or Place Theory:
This theory, originally formulated by the great physiologist Helmholtz, has subsequently come to be known as the place theory. Evidence for the theory comes from studies on people with partial deafness and also experimentation on animals.
According to this theory, different regions or places in the basilar membrane are sensitive to and respond to sound waves of different frequencies. Thus, the receptors in the basal end and near the three bones vibrate more when sound waves of high frequency enter.
On the other hand, the receptors at the top end or the end farthest from the three bones respond more to sounds of low frequency. This means that receptors of sound waves of different frequencies are placed at different places of the basilar membrane.
The theory also states that the experience of loudness of a sound depends on the total amount of the basilar membrane stimulated. Thus an intense sound would stimulate the membrane to a greater extent than a less intense sound. The timbre of the sound heard is dependent on the overall patterns of vibrations.
2. Frequency Theory:
Another theory attempting to explain the different characteristics of sound sensations is the frequency theory. No sensation can reach the brain directly. This theory says, the hair cells of basilar membrane respond to the stimulus like the diaphragm of the telephone-vibrating to the frequency of the wave (i.e. pitch). According to this theory, the pitch of a sound sensation depends on the frequency of the impulses reaching the auditory area of the brain rather than anything that happens at the basilar membrane.
Loudness would depend on the number of the nerve fibres which participate in the process. Thus a weak sound would involve fewer fibres compared to a stronger sound. This theory, however, faces some difficulty in explaining the hearing of higher levels of pitch because of limitations in the number of impulses that can be transmitted by the nerves. However, it can be seen that this theory emphasises brain action rather than any activity at the level of the receptors.
3. Volley Theory:
A third theory which has been found to be a little more satisfactory is known as the volley theory originally suggested by wever. This theory holds that the nerve fibres of the auditory nerve are activated in volleys or squads.
Different nerve fibres participate in different volleys. The volley theory therefore combines, in a way, the place theory and the frequency theory. It, on the one hand, emphasises the fibres participating in a particular volley and on the other, the volley taking place at different levels and intensities.
It may therefore be seen that the volley theory explains more satisfactorily the different characteristics of auditory sensations by combining the advantages of the place theory as well as that of the frequency theory.