Hearing and Personal Response to Sound
Dr Bob Thorne
This Paper has two objectives. The first objective is to illustrate the processes involved in personal hearing and the importance that these processes have in the measurement, characterisation and assessment of sound. The second objective is to identify the essential processes needed to transform the 'sound' from air movement to personal information and reaction to bring forward the information needed to develop an assessment methodology for perceived sound and noise.
The auditory system takes the mixture of sound that it derives from our complex natural environment in a process termed auditory scene analysis[201]. Alain et al.[202] describe auditory scene analysis methodologies identifying the content ('what') and the location ('where') of sounds in the environment. An example of auditory scene analysis is the ability to hear, identify, locate and track different sounds in an environment at the same time and over time. This form of analysis is critical to our sense of hearing and informational responses.
Personal hearing response
The external ear consists of the pinna (the fleshy 'ear'), the concha (opening to the ear canal), the ear canal and the ear drum. The pinna, with the head and torso, collects and diffracts sound (or acoustic) waves into the ear canal. At this stage of 'hearing' sound is the variation in sound pressure at the ear and within the ear canal. Both the sound pressure levels and the phase of the sound waves change while being propagated within the ear canal to the eardrum. The changes vary with the frequency of the sound and for each direction of the sound waves. It is the external ear and middle ear together that allow the transmission of acoustic energy at each frequency from the free-field to the inner ear. The energy at the eardrum is now a modified image of the free-field sound level at the ear that is, itself, a modified image of the characteristics of the sound source. That is, the sound in the ear being transferred to the brain is not the same as at the sound source.
Once the acoustic stimulus reaches the eardrum, sound can be transmitted through the middle ear to the inner ear in three ways:
- By bone conduction, with the stimulus bypassing the middle ear and travelling by way of the bones in the skull to the inner ear
- By way of the air in the middle ear
- Across the middle ear to the inner ear by way of the ossicular chain. This is the most effective means of transmitting sound to the inner ear...
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