Lecture program Psychoacoustics and instrumentation

1. PSYCHOACOUSTICS
OF DISCRIMINATING AND
IDENTIFYING SOUNDS
Perceptual attributes of
acoustic waves

2. Loudness
■ This attribute of sound waves related mainly to intensity.
■ In general, large intensity values result in 'loud' sounds while
low intensity values result in 'soft' sounds.
■ More specifically, loudness is a perceptual manifestation of how
the auditory system represents sound wave intensity and
makes intensity comparisons among signals across frequency
and time.
■ In terms of physiology, loudness may be a measure of the total
activity of the basilar membrane.

3. Sound Intensity Levels (SILs)
■ SIL’ s are measured on a logarithmic scale because
a) Perceived intensity is related logarithmically to the
physical magnitude of intensity and
b) Logarithmic scales can represent very large ranges of
values more efficiently.
■ For example, 10dB SIL increase represents 10 times increase in
intensity, 20dB SIL increase represents 100 times increase in
intensity, and so on.

4. Dynamic Range of the ear
Defined as:
■ Lower and upper intensity limits that the auditory system can
process effectively.
■ The dynamic range of (functional and safe) hearing extends
from 0dB (at 1000Hz) to 120dB (at all frequencies).
■ Sounds with intensity levels below 0dB are inaudible while
sounds with intensity levels above 120dB can be damaging to
the ear.

6. ■ The lowest audible intensity is different for different
frequencies
■ The upper limit of the ear's dynamic range may be defined in
several ways:
a) Intensity level at which we experience an unpleasant, even
painful sensation, and which can cause physiological damage
(~120dB)

7. b) Intensity level beyond which telling intensity level
differences apart becomes very difficult.
■ The dynamic range of the ear is therefore ~120dB in the mid-
frequency region (1000-6000Hz) and decreases at low and high
frequencies

8. Loudness and frequency
Effects of Frequency on
Loudness
■ The equal loudness contours illustrate that
loudness does not only depend on intensity;
it also depends on frequency and does so
differently at different intensities.
■ According to these figures, for example, a
pure tone with SIL = 60dB will sound
moderately loud at 1000Hz but will be
barely audible at 50 Hz.

9. ■ At high frequencies (approx. between 6000 and 15000Hz), the
drop in sensitivity is not as dramatic.
■ The fact that we are most sensitive to frequencies in the range
1000Hz - 6000Hz (approx.) may have some evolutionary
significance, since speech sounds have most of their energy
within this range

10. Loudness level unit & Loudness unit
■ The loudness level unit is called Phon. Loudness level in Phons
and SIL in dBs are equivalent only at 1000Hz
■ The loudness level of a given frequency in Phons is equal to the
SIL of an equally loud 1000Hz tone.

11. Loudness magnitude estimation:
■ Listeners are asked to compare the loudness of two tones and
determine how much louder/quieter the second tone is relative to the
first.
Loudness magnitude production:
■ Listeners are asked to adjust the SIL of a tone so that it becomes,
say, twice as loud as another tone with known and fixed SIL.

12. Loudness magnitude estimation & production experiments use as
standard reference a 1000Hz tone at 40dB SIL.
The sound loudness level (SLL) of any tone is then described in
relation to this reference standard, whose loudness is defined as:
1 Sone (Loudness unit): Loudness of a 1000Hz tone at SIL (or SPL)
of 40dB.
The advantage of the Sone scale is that it is based on loudness
units that are proportional to loudness and can be manipulated
arithmetically rather than logarithmically (e.g. two sones sound
twice as loud as one, three sones sound three times as loud, etc.).

14. Critical Bands
■ The critical band corresponds to a
pooling along the basilar membrane:
the width in terms of frequency
corresponds to an estimate of the
physical length, along the membrane,
over which auditory nerve signals are
pooled. For a center frequency of 1000
Hz the critical bandwidth is 150 Hz;
that corresonds to a 1.3 mm stretch
along the basilar membrane. Likewise
for a center frequency of 8000 Hz the
critcal bandwidth is 800 Hz, but this
frequency range also corresponds to
1.3 mm along the basilar membrane.

15. ■ Summary: a neural code for both pitch and loudness. Pitch
depends on both a place code and a temporal code. Loudness
depends on firing rates and the number of neurons firing.

16. Effects of Spectrum (bandwidth) on
Loudness
■ For complex tones, loudness also depends on spectrum (i.e. on
the way energy is distributed among the complex tone's
components).
■ Suppose that two complex tones have the same intensity level
(in dB) but they have different spectra. The complex tone with
more 'spread-out' spectrum (i.e. with components spreading
across many critical bands) will sound louder than the one with
a less 'spread-out' spectrum (i.e. with components spreading
across fewer critical bands).

17. ■ If all frequencies in a complex tone are within a single critical
band, the total loudness can be approximated by the sum of all
intensities.
■ If the frequency components are spread over multiple critical
bands, the total loudness is larger.

18. For moderate to high SILs, increasing the noise bandwidth beyond
that of a single critical band will result in an increase in loudness.
Explanation: Spreading a given noise intensity over, say, double
the bandwidth, will half the intensity exciting each relevant
portion on the basilar membrane (B.M.). At the same time, due to
the B.M.'s compressive response at moderate-to-high SILs, the
decrease in response will be less than the decrease in
stimulus. With total loudness being the sum of B.M. excitation
per critical band, the decrease in SIL per B/M portion will be
matched by the increase in the number of critical bands excited
and overcompensated by a smaller decrease in local (i.e. per
critical band) response than in stimulation. The final result will be
a net increase in loudness.

19. Phenomenon of simultaneous masking
■ The phenomenon of simultaneous masking (perceptual erasure of
one sound by a more intense/loud sound) is an additional example
of the dependence of loudness on spectral distribution.
■ The disturbance pattern on the basilar membrane is wider and the
response is faster for high intensity sounds than for low intensity
sounds, facilitating masking.
■ In addition, due to the general asymmetry in the disturbance
pattern of the basilar membrane, strong low-frequency sounds will
mask weak high-frequency sounds more efficiently than the other
way around.

20. The dependence of
loudness on frequency and spectrum and the phenomenon of
masking play important roles in
sound (data) compression algorithms.