Hearing / Acoustics
Acoustics
- INTENSITY: decibels: dB=20*log(P/P0)
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log scale: loudest sound detected by human ear is approx. 10 million times louder than the quietest sound
- ratio: various scales use different P0 reference values (relevant for the interpretation of audiograms)
- dB SPL: relative to 2x10-5 N/m2 - same reference level for all frequencies - expresses intensity in absolute terms
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dB HL (or SL): relative to the lowest sound pressure detected by normal individuals - different reference level for each frequency - expresses intensity in relative terms
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- FREQUENCY
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complex sounds can be broken down into their component frequencies via a practice called spectral analysis - this type of analysis is the key to understanding cochlear frequency processing
- another fundamental principle is the concept of linearity (i.e., "what you put in is what you get out")
- in a linear system such as the middle ear, if you put in a signal that has two frequency components (e.g., 2 kHz and 5 kHz), what you get out is two frequency components (2 kHz and 5 kHz)
- in a nonlinear system such as the cochlea, if you put in two frequency components, what you get out is multiple frequency components (2, 5, 1, and 3 kHz)
- the added components are called DISTORTION PRODUCTS - the distortion products created by the cochlea can be recorded as otoacoustic emissions
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