Physics 371 - 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.
demo:
f
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”?
critical bandwidth (Hz)
1/4 octave band
critical band width as a function of frequency
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
Human basilar membrane response for different frequencies
(may explain some aspects of masking)
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
Rose et al. J. Neurophysiology
time interval between nerve impulses for different frequencies.
rate 30 spikes/s at 70 dB
rate 120 spikes/s at 70 dB
217 Hz
600 Hz
192 Hz
460 Hz
167 Hz
365 Hz
0
0
0
5
10
15
ms 20
5
5
10
10
15
15
ms
ms
20
20
30dB
70dB
60dB
0
5
10
20dB
15
ms 20