Hearing and Audiology

Listening to music means that your ears act as a sensor for sound which is processed further by the brain and, as result is recognised as sound. The way in which this process takes place is the study of audiology. Audiology is concerned about the physics of the hearing organ (the peripheral hearing system), the processing of sounds by the brain (cognition) and auditory brainstem (the acoustical nerve) and all other aspects that deals with sound processing in the human hearing system. It deals with the physics behind these topics. It also includes the understanding of hearing impairment and the digital signal processing (DSP) in hearing aids to recover such impairment. This site will only deal with those aspects of audiology relevant to musical perception such as psychoacoustics and the basics of hearing.

Basics of Hearing

Sound perception starts at receiving sound energy at the pinna. The remarkable thing about the pinna is, that it works similar to a horn. Since the inverse square law also applies for sounds (sound is radiated energy) in a 3-dimensional space, the ear must work most efficiently to receive as much energy as possible. The auditory canal functions as a tube in which standing waves or resonances occurs. The acoustical characteristics of the pinna and the canal fulfills an important role in the discrimination of speech by specifically receiving those frequencies that are crucial for speech.earo.gif (18608 bytes)

From an evolutionary point of view this can be understood as mankind who evolved towards the understanding of complex sounds, called speech (in contrast to other species). Speech is the most important communication mechanism for mankind. As a result, hearing of the human being reaches its highest sensitivity for frequencies between 200Hz-8kHz. This bandwidth is typically the bandwidth in which normal phonetics takes place. The bandwidth of music, however, is much larger (20-20kHz) than the one for speech. From this we can understand that the ear is not "designed" for perception of music. Especially bass is badly covered by the ear. So, when we want to listen to good bass sounds we need to know how the sensitivity of the ear is for frequencies between 20-20kHz).

The audiological correlate for intensity (in dB-SPL) is the loudness. Loudness is measured in phons A frequency of 1kHz is used as reference in which the phon level equals the intensity level. If we look at the sensitivity curves of the human ear, we see large variation between high and low frequencies. The graph indicates different curves for equal loudness i.e for a specific loudness level, the curve indicates what sound intensity or SPL is required at the outer ear to perceive an equal loudness sensation  for all frequencies. The graphs indicate the average dynamic range of the human hearing. The level of 0 dB corresponds to the threshold of hearing while 120 dB corresponds to pain threshold of hearing.

eqlou.gif (24803 bytes)These graphs indicate some interesting audiological effects which are also important for music listening. For example, we see that much less intensity is required in the speech area. The resonance frequency of the auditory canal can also be observed as a peak at 3-4kHz). We also observe that for low loudness, frequencies below 100Hz needs much more power but that the curves becomes more flat for higher loudness. This effect is recognisable for music; at higher volumes the bass is more pronounced. Above 10 kHz loudness becomes irregular and decreases indicating that the ear is not dedicated to perceiving these high frequencies.

These iso-loudness curves are often referred to as the A-weighted curves and are generally used in audio engineering. It actually reflects the manner in which we perceive frequencies vs loudness. To get an impression of loudness levels:

10       dB hearing threshold
40-50  dB level of whispering
60-70 dB level of normal speech conversation
80 dB Crowded room
90-100 dB Busy road
110 dB Level of a disco
110-120 dB Level of a popconcert
>130 dB Pain threshold

Like loudness is the audiological correlate for the physical quantity intensity, we also use pitch for the frequency. In addition, timbre or quality are also used to quantify sounds in psychoacoustics.

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