Sound Production and Perception Flashcards
Sound Waves
A single sound wave is caused by an increase in pressure at a certain point in an elas&c medium which causes a “domino effect” outward. If the perturba&on is repeated periodically, then it generates a series of sound waves: The crests correspond to the high pressure points and the troughs correspond to the low pressure points.
Propaga&on and speed of Sound
In atmosphere: sound propagates from the source at equal speed in all three dimensions, therefore sound waves are spherical waves.
Speed of propagation depends on the type, temperature and pressure of the medium through which it propagates.
In dry air at 20。C the speed of sound is approximately 343m/s.
That’s approximately 1 meter every 2.9 milliseconds.
Waveforms
Sound waves can be represented as the temporal variation of sound pressure at a fixed point in space U for example the membrane of a microphone.
Most useful sounds are periodic
Most sounds are generated by oscillators (strings, vocal folds, resonators, etc…)
Therefore most natural sounds are are periodic.
The pressure varia&on of a periodic sound is an oscillation with a given period and a given amplitude
Period
The period of a sound wave is the dura&on of an oscillation cycle Can be measured as the &me between two peaks.
Frequency
The frequency of a sound is the number of air pressure oscilltion cycles per second. It is the multiplacative inverse (or reciprocal) of the period: F = 1/T
Amplitude, SPL and loudness
The amplitude is the magnitude of the change in sound pressure within the wave. It corresponds to the maximum amount of pressure at any point in the sound wave.
It is also called “Sound Pressure Level” and measured in decibels, a logarithmic (perceptual) scale.
“Loudness” is the perceptual correlate of amplitude … it is a subjective, non linear perceptual attribute of sound (varies with people, frequency, distance…)
“Simple” soundwaves: pure tones
Pure tones are single frequency tones with no harmonic content e.g. whistles
Complex sounds
Animals, humans and most musical instruments usually generate periodic sounds which have energy at more than one frequency.
These sounds are called “complex sounds”
Sound perception
We can hear sounds with frequencies ranging between 20 and 20 000Hz.
The mammalian ear is characterised by very a high sensitivity and a sharp frequency resolution (we can differentiate up to 1500 different pure tones).
Outer, mid and inner ear
Outer: Captures sound signals, Amplify mid-freqs, Vertical direction coding
Mid: Protection, Impedance match
Inner: Frequency analysis, transduction
Outer ear
The pinna affects the high frequency sounds by interference between the echoes reflected off its different structures. The meatus (ear canal) links the pinna to the eardrum.
Middle ear
Three ossicles: the malleus (hammer), the incus (anvil) and the stapes (stirrup).
Transmit vibrations from eardrum to smaller oval window.
Impedance matching: enables air vibration (sound) to be efficiently transformed into fluid vibration.
Inner ear (cochlea)
The function of the cochlea is to transform a mechanical signal into neural responses in the 8th cranial nerve (auditory vestibular nerve).
Organ of Corti
Seperates inner hair cells from outer hair cells.
