Physics Acoustics for Musicians selected slides, March 5, 2002 Loudness at different frequencies Critical band Masking The ear Neurological response of the ear

Critical Bands of Sound Perception for pure tone of some frequency, nerve cells of the basilar membrane are excited over a small region. approx 30 regions each covering about 1/3 octave Adding more sound in a different band causes much more increase in subjective loudness than adding sound in the same band. Subjective loudness of complex tones is measured in SONES (see Fig 4, page 99 Backus to find SONES from Phones. Difficult and subjective - when are tones judged “twice as loud”? demo: f

critical band width as a function of frequency critical bandwidth (Hz) 1/4 octave band

Critical Band and Masking when will one tone mask another? (a) for small loudness (mf or below) another tone is masked only if it's frequency is within the same critical band. Calculations: if the softer tone adds less than 0.5 dB to the total intensity level it will not be heard. (b) for loud sound such as ff: tones of LOWER pitch are masked very little tones of HIGHER pitch are masked (can't be heard) importance for orchestration! see Fig. 5, p. 102

(may explain some aspects of masking) Human basilar membrane response for different frequencies

Neural Response of Ear: the Neural Transmitters sound is transmitted to the brain not as a wave shape but as individual pulses ("discharge" of nerve cell). Pulses have same height independent of stimulus, average pulse rate depends on loudness level. time distribution of pulses encodes wave shape

365 Hz 460 Hz 600 Hz 167 Hz Rose et al. J. Neurophysiology time interval between nerve impulses for different frequencies ms rate 120 spikes/s at 70 dB 217 Hz 192 Hz ms ms rate 30 spikes/s at 70 dB

ms 20dB 30dB 70dB 60dB