Explain briefly the physiological process of hearing in humans. Which factors affect our hearing in terms of perception of loudness and pitch, and what part does psychoacoustics play in our perception of sound? Then, give an overview of the main historical developments in recording sound. Which development do you think is the most significant and why?
Marking Tutor: Stuart Avery
Date of submission: 23/11/2011
Word count: 1629
It is possible to explain shortly the hearing process, fundamental for any human being, as it follows:
As a sound reaches the ear, this gets directed into the ear canal by the pinna, a funnel-like structure. The ear canal increases the sound pressure within the range of frequencies corresponding to the human voice, then the tympanic membrane, or eardrum, a boundary between outer and middle ear located at the end of the canal, reacts to the sound and starts to vibrate sympathetically.
The tympanic membrane's vibrations set in motion the middle ear's mechanism, which comprises three ossicles (stapes, incus and malleus) and two small muscles (tensor tympani and stapedius). Tensor tympani and stapedius normally allow for the ossicles' free motion , but they can tighten up and inhibit their action when the sound gets too loud, in order to prevent damage.(1) The role of the middle ear's mechanism consists in transmitting the tympanic membrane's vibrations to the fluid filling the inner ear. 10
As the fluid is being set in motion, a real time spectral decomposition of the acoustic signal processed by the ear is carried out by the cochlea, an 'hydromechanical frequency analyzer' (2), which further provides a spatial frequency map of the sound to the vestibular nerves linked to the brain. The cochlea has a coiled shape, and it is divided in three ducts, the vestibular canal, the tympanum canal and the cochlear duct. The floor of the cochlear duct is formed by a basilar membrane where lies the organ of Corti, consisting of two rows of rod cells, arranged on the membrane to form a minute arch to which four rows of hair cells are fixed finally, a minute fibre of the cochlear nerve is attached to each cell.(3) This is how the brain analyzes the sound: as the fluid is moving, the hair cells react to different frequencies, as each is “tuned” to a specific one.
Perception related to:
Loudness and pitch
The frequency response of the ear is not flat, and as we can notice by observing the curve of Fletcher & Munson, the loudness perception of the ear varies accordingly with the frequency of the sound perceived Referring to this graph, it is clear how
high and low frequency sounds have to be at a higher sound pressure level in order to be perceived with the same loudness as a medium frequency sound.
It is proved that the bandwidth of a sound affects its perceived loudness, so generally, if two sounds with the same sound pressure level are playing together, the one with the widest frequency range will sound louder than the other. This is because the ear has a set of filters called critical bandwidths which vary with frequency and whose width is 1/3 of octave intervals. Therefore, if two sounds with the same sound pressure level fit both within a critical bandwidth, they will sound one as loud as the other. On the contrary, if another sound with the same sound pressure level has a bandwidth that crosses the critical bandwidth in someway, it will sound louder than the other ones within it. The phenomenon of masking is strictly related to this, as it occurs whenever two sounds, which are close in frequency but have different sound pressure levels, play together. What happens is that the louder one will mask the quieter.(4)
Another factor to be taken to consideration is the duration of the sound perceived, as it affects its perceived...