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Week4 outline

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Paul Faure

This outline summarizes major points covered in lecture. It is not intended to replace your own lecture notes. Filters  Cut-off frequency (f ) is frequency where signal energy = ½ input signal energy (i.e. the -3 dB frequency) c  Low pass – allows frequencies below cutoff frequency to pass through unattenuated  High pass – allows frequencies above cutoff frequency to pass through unattenuated  Band pass – allows frequencies between lowpass cutoff and highpass cutoff to pass unattenuated  Band reject – attenuates frequencies between lowpass cutoff and highpass cutoffs  A linear filter does not affect frequency component of signal, only amplitude and/or phase  Band pass and band reject filters have both a low-pass and a high-pass cutoff frequency  Roll-off rate: amount of attenuation (in dB) per doubling of frequency (octave)  Roll-off rate is measured starting from the cut-off frequency (c )  Filter bandwidth: difference in Hz between high pass and low pas cut off frequencies (only applies for band pass and band reject filters)  Filter bandwidth determines the pass band range of frequencies (i.e. the frequencies that pass through the filter with little or no attenuation) Concept of Linearity  A device is linear when it fulfills the relationships of: 1. Superposition 2. Homogeneity  Superposition. A system (e.g. device) is linear when the output of a device in response to a number of independent inputs presented simultaneously is equal to the sum of the outputs that would have been obtained if each input had been presented alone  Homogeneity. If the input to a device is changed in magnitude by a factor of k, then the output should also change in magnitude by a factor of k, but otherwise be unaltered  The output of a linear device never contains components that were not present in the original signal How sound is heard: EAR  Mechanical energy reaches the eardrum, moves to the middle ear, and then to the inner ear  Energy changes from mechanical to electrochemical at nerve impulse stage in inner ear Outer and Middle ear  Transform energy from mechanical air pressure to motion of fluids in inner ear that leads to the generation of action potentials  Most important function: overcome impedance mismatch between air and bone and inner ear fluids  Sound travels through ear starting at pinnae  ear canal (external auditory canal)  ear drum (tympanic membrane) Outer ear  Air pressure collected by pinna (plural = pinnae)  Major outer ear function is sound amplification  The outer ear also has protective functions (e.g., wax captures invading foreign bodies)  The outer ear is composed of epidermal tissue  Going from external to ear to internal: at the centre of pinna is concha, then external auditory meatus, then ear canal (see diagrams)  External auditory canal ~2-3 cm long  Only mammals have pinnae, and only high frequency hearing mammals have mobile pinnae  Resonance of concha and external auditory canal produce 5-20 dB SPL gain from 500 Hz – 9 kHz. Sound alterations by body  Head, ear, body act as a filter that influences received signal amplitude and phase  Impinging sound is altered in amplitude and time by head and torso  Head Related Transfer Function (HRTF) describes how these effects vary with source location as a function of frequency (i.e. how amplitude and phase are modified by impinging on listener) Psych 3A03 28 September 2012 Week 4 Dr. Paul A. Faure Rayleigh head model  Rayleigh head model describes properties of HRTF  Transfer Function describes the relationship between the input and output of a system (can also think of it as the transfer characteristics of a system  Pinna and head have a large influence on received sound, depending on angle and frequency  Amplitude changes through pressure gain (dB) depends on angle of incidence and signal frequency  Measured SPL at ear usually increases in amplitude between 0 - 90 degrees Function of outer ear  Sound frequencies vary in received sound pressure and time of arrival depending on source location  Interaural Level Difference (ILD) – difference in signal amplitude between two ears  Interaural Time Difference (ITD) – difference in time of arrival between the two ears  ITD is mostly constant as a function of frequency (b/c speed of sound is also constant)  Most amplification occurs between 1-6 kHz, and this coincides with speech spectrum  HRTF is a nice summary of how ITD and ILD cues vary at listener’s ear with position of source Function of the Outer ear (cont’d)  Interaural time difference (ITD) & Interaural level difference ILD – main cues to sound localization  ILD (or IID) = interaural level difference (ILD) or interaural intensity difference (IID) — level/intensity is difference in
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