Drawings: S. Blatrix


Sound is the audible band of the mechanical wave spectrum, similar to the band of visible light within the electro-magnetic wave spectrum.
Hearing is mainly concerned with two parameters of sound : its frequency (wave/sec = Hertz), which allows differentiation of pitch, and its pressure level (dB), which allows differentiation of intensity.

The period is the inverse of the frequency (T=1/F) :

short periods (high frequencies) are representative of high pitch tones, and long periods (low frequencies) are representative of low pitch tones.

You can activate the next 3 drawings and hear: a) a 3 kHz sound; b) a 300 Hz (low pitch) sound; c) the same low pitch (300 Hz) modulated in intensity.

a) High pitch:
this wave corresponds to a pure tone of 3,000 Hz (3 kHz);
i.e., a pitch corresponding to half an octave above the soprano top C ( 2,090 Hz)

b) Low pitch:
this wave corresponds to a pure tone of 300 Hz (pitch common to most voice registers).
The lowest pitch of a singing voice (bass) is 65 Hz.
c) Same low pitch at high (black) and low (blue) intensities

Pure tones, musical sounds and noise

Pure tones: regular wave of a single frequency. i = intensity,
p = period, t = time

Musical sound: the wave is made up of a fundamental frequency (pitch) and harmonic caracteristics of the timbre.
Upgrading a sound by one octave means increasing the fundamental frequency twofold.

 Noise: no characteristic frequency.

Human voice

Sonogram of the French word "cochlée" (= cochlea).


Spectrogram of the same word "cochlée" showing the frequency complexity of a voice sound: phonem having a fundamental and many harmonics.

Sounds at human level
The human ear recognizes frequencies between 20 to 20,000 Hz as sounds. All frequencies below 20 Hz we call 'infrasounds' , and all frequencies above 20 kHz 'ultrasounds'. Not all animal ears work within the same frequency range. The mole rat ear, for instance, perceives sounds 2 octaves below the lower limit of the human ear (below 10 Hz). On the other hand, a dog or a cat ear is able to perceive sounds 1 octave above the human frequency range (40 kHz), and a bat ear may react to sounds 3 octaves above that which we can hear (up to 160 Kz).

Audiometric curve for a normal hearing subject

The human auditory field (green) is limited by the threshold curve (bottom) and a curve giving the upper limit of sound perception (top). At each frequency, between 20 Hz and 20 kHz, the threshold of our sensitivity is different. The best threshold (at around 2 kHz) is close to 0 dB. It is also in this middle range of frequencies that the sensation dynamics is the best (130 dB). The conversation area (dark green) demonstrates the range of sounds most commonly used in human voice perception; when hearing loss affects this area, communication is altered.

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