Frequency selectivity of the auditory system

1. Frequency selectivity of the auditory system

2. Frequency selectivity • Important for aspects of auditory perception such as, pitch, loudness, timbre, melody, harmony, consonance and dissonance. • frequency resolution of the auditory system refers to its ability to resolve the frequency components of a complex sound • Depends to some extent on the frequency analysis that takes place on the BM

3. BM • Each point on the BM responds maximally to a particular characteristic frequency (CF) • However each point also responds to frequencies close to the CF, but its response is less • Each point of the BM may be thought of as a filter with a different centre frequency and bandwidth.

4. The operation of the BM may be thought of in terms of a bank of bandpass filters with overlapping passbands – (auditory filters) • The bandwidth of each auditory filter is called the Critical Bandwidth (CB) • The frequency resolution of the BM is generally described in terms of the CB.

5. important – auditory filters are continuous not discrete – we have a CB for any frequency within the audible range (20 Hz - ~20 kHz*)

6. Some findings from studies on measuring the CB • CB varies as a function of frequency • The Equivalent Rectangular Bandwidth (ERB) is often used as a practical measure of the CB • Equation describing the ERB as a function of centre frequency: ERB = 24.7(4.37F+1); ERB is in Hz, F is in kHz • The ERB, when measured in Hz increases with increasing centre frequency.

8. Critical Bandwidth • The effects of the CB may be demonstrated through the perception of beats and roughness. • Asa demo (62)

9. Beats • Add together two pure tones of similar amplitudes, with slightly different frequencies (difference between them <~20 Hz*) • one may not hear two separate tones but rather a single tone periodically fluctuating in loudness with a pitch equal to the mean of the two frequencies. • The fluctuations in loudness are called beats.

10. Beats • These beats are perceived because the frequency selectivity of the auditory system is finite - the two frequencies fall into the same critical band and are not processed separately

11. Roughness • When the frequency difference between the two tones is increased above about 20 Hz* the sound may be perceived as rough. • Why? • At this and larger frequency differences the auditory system may be unable to follow the rapid amplitude fluctuations individually - the sensation of fluctuating loudness is replaced by a ‘rattle-like’ sensation called roughness.

12. Diagram: The perceived effect of two simultaneous pure tones when their frequency difference is increased from 0 to above 1 CB.

13. CB CB

14. 2 pure tones F1 and F2 sounded together • F1 = F2 – single tone is heard • small frequency difference between F1 and F2 – beats are heard • Frequency of the beats is equal to F1-F2 • Beats are usually heard when the frequency difference is less than about 12.5 Hz *

15. Perception of a rough and fused tone when the frequency difference is above 15 Hz* • Further increases in frequency difference – 2 separate tones – rough and separate – further increase – smooth and separate • *The precise range of frequency differences for the perception of beats and roughness varies among listeners

16. 2 separate tones – tones resolved by the auditory system • CB – the frequency difference when the percept changes from rough and separate to smooth and separate. • The effects of the CB is also demonstrated in our ability to hear out partials in complex tones

17. Hearing out partials in complex tones • Main findings of these studies: • Our ability to hear out partials of a complex sound depends on the CB (i.e. frequency resolution) as well as other factors • Other factors include musical training and the location of the partial within the complex. • A partial can be heard out from a complex sound when its separation exceeds the CB - when separation is at least 1.25 times the ERB

18. Binaural beat • Binaural beats may be heard when a pair of tones of slightly different frequencies are presented one to each ear via headphones. • frequency difference < 2 Hz the sound appears to move right and left across the head. • larger frequency difference the sound no longer appears to move but is rough with a fixed location. • Binaural beats differ from monaural beats in that the binaural beat depends upon the interaction in the nervous system of the neural output from each ear.

19. Binaural beat • provide evidence that phase information is preserved in the discharges of the neurons in the auditory nerve and that the auditory system can detect the phase difference at the two ears. • Binaural beats are not as distinct as monaural beats and are generally present at just low frequencies, being heard best for frequencies between 300 and 600 Hz(Moore, 2003)

20. Beats of mistuned consonances • Beats may also be heard when the frequencies of the two pure tones are slightly mistuned from a small integer ratio • Asa demo 63 • Strength of these beats compared to the monaural beats? • Indicates – sounds interact over distances much larger than the CB

21. Consonance and dissonance • The perceived consonance or dissonance of a pair of simultaneous pure tones may be explained in terms of the critical band. • Plomp and Levelt (1965) demonstrated that the CB might provide the psychoacoustic basis for the perceived consonance or dissonance of a sound. • listeners judged the perceived consonance and dissonance of two simultaneous pure tones

22. Their results: • the frequency difference required for maximum dissonance and maximum consonance was dependent on the frequencies of the pure tones • the consonance and dissonance of two simultaneous pure tones was best quantified in terms of the CB. • The intervals formed by pure tones were considered as most consonant when their frequency difference exceeded the CB.

23. The authors interpreted this as indicating that the perceived consonance of simultaneous pure tone intervals is related to the absence of beats and roughness. • The maximum perceived dissonance corresponded to a frequency difference of about one quarter of the CB.