SOUND PRESSURE,
POWER AND LOUDNESS
MUSICAL ACOUSTICS
Science of SoundChapter 6
FREE FIELDI = W/4πr2
at r = 1 m:
LI = 10 log I/10-12
= 10 log W/10-12 – 10 log 4
= LW - 11
HEMISPHERICALFIELD
I = W/2r2
at r = l m LI = LW - 8
Note that the intensity I α 1/r2 for both free and
hemispherical fields; therefore, LI decreases 6 dB for each doubling of distance
SOUND PRESSURE LEVEL
Our ears respond to extremely small pressure fluctuations p
Intensity of a sound wave is proportional to the sound Pressure squared: ρc ≈ 400 I = p2 /ρc ρ = density c = speed of sound
We define sound pressure level:
Lp = 20 log p/p0 p0 = 2 x 10-5 Pa (or N/m2)(or SPL)
TYPICAL SOUND LEVELS
MULTIPLE SOURCES
Example:Two uncorrelated sources of 80 dB each will produce a sound level of 83dB (Not 160 dB)
MULTIPLE SOURCES
What we really want to add are mean-squareaverage pressures (average values of p2)This is equivalent to adding intensities
Example: 3 sources of 50 dB each
Lp = 10 log [(P12+P2
2+P32)/P0
2] = 10 log (I1 + I2 + I3)/ I0)= 10 log I1/I0 + 10 log 3 = 50 + 4.8 = 54.8 dB
SOUND PRESSURE and INTENSITYSound pressure level is measured with a sound level meter (SLM)Sound intensity level is more difficult to measure, and it requiresmore than one microphoneIn a free field, however, LI ≈ LP
FOUR ATTRIBUTES USED TO DESCRIBE A SOUND:
•Loudness•Pitch•Timbre•Duration
EACH OF THESE DEPENDS ON ONE OR MORE PHYSICAL PARAMETERS THAT CAN BE MEASURED:
•Sound pressure•Frequency•Spectrum•Duration (measured) •Envelope
Relating the SUBJECTIVE QUALITIES to the PHYSICAL PARAMETERS that we can MEASURE OBJECTIVELYIs an important problem in PSYCHOACOUSTICS
DEPENDENCE OF SUBJECTIVE QUALITIES OF SOUND ON PHYSICAL PARAMETERS
LOUDNESS LEVEL
Contours of equal loudness are labeled phons At 1000 Hz, Loudness Level = Lp
PLOT YOUR OWN FREQUENCY RESPONSE
ASSIGNMENT: Plot your own frequency response curves by using
www.phys.unsw.edu.au/~jw/hearing.html
HOW DOES LOUDNESS DEPEND ON
FREQUENCY?
LOUDNESS SCALING
LOUDNESS RESPONSE OF THE EAR
LOUDNESS OF COMPLEX TONES
Loudness depends mainly on SOUND PRESSURE.
but it also depends on FREQUENCY, SPECTRUM and DURATION
DEPENDENCE OF LOUDNESS
ON BANDWIDTH
CRITICAL BANDS
LOUDNESS OF COMBINED SOUNDS
JUST NOTICEABLE LEVEL DIFFERENCE
LEVEL INCREMENT NEEDED TO DOUBLE LOUDNESS
RANGE OF FREQUENCY AND INTENSITY OF THE EAR
MUSICAL DYNAMICS AND LOUDNESS
HOW DOES LOUDNESS DEPEND ON
PARTIAL MASKING?
HOW DOES LOUDNESS DEPEND ON DURATION?
LOUDNESS RECRUITMENT
UNUSUALLY RAPID GROWTH OF LOUDNESS ABOVE A CERTAIN THRESHOLD
GENERALLY ASSOCIATED WITH HEARING LOSS,BUT NORMAL LISTENERS EXPERIENCE IT FOR
TONES OF VERY HIGH OR VERY LOW FREQUENCY
MONAURAL vs BINAURAL LOUDNESS
FOR SOFT SOUNDS (~20dB) BINAURAL LOUDNESS EXCEEDS MONAURAL LOUDNESS
BY A FACTOR OF 2
(CORRESPONDS TO ΔL = 8dB)
FOR LOUD SOUNDS (~80dB) BINAURALLOUDNESS EXCEEDS MONAURAL
LOUDNESS BY A FACTOR ~/.4
(CORRESPONDS TO ΔL = 6dB)
Zwicker & Fastl (1990)
INTENSITY DISCRIMINATION AND CODING
AT LOW LEVELS, INTENSITY CHANGES CAN BESIGNALLED BOTH BY CHANGES IN FIRING RATES OF
NEURONS AT THE CENTER OF THE EXCITATIONPATTERN AND BY THE SPREADING OF THE EXCITATION
PATTERN (TO INCLUDE MORE NEURONS)
AT HIGH LEVELS, MOST NEURONS AT THE CENTER OF THE EXCITATION PATTERN ARE SATURATED, BUT
INTENSITY CHANGES ARE SIGNALLED BY CHANGESIN FIRING RATES AT THE EDGES.
AN INCREASE IN LEVEL ALSO MAY BE SIGNALLED BY INCREASED PHASE LOCKING TO THE TONE WHICH
RESULTS IN TEMPORAL REGULARITY OF NEURAL FIRINGS
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