Acoustics/Psychoacoustics
Huber Ch. 2
Sound and Hearing
Sound
• Sound-pressure waves
• Compressions and rarefactions
• Wave propagation
Waveform Characteristics
• Amplitude
• Peak
• RMS (root mean squared): Peak * .707
• Frequency
• Velocity
• 1138 ft/sec; 344 m/s at 68° F (20° C)
• 1.1 ft/s increase for every ° F (2 ft/s ° C)
Waveform Characteristics (2)
• Wavelength
•  = V/f
• T = 1/f
• Phase
• Offset, or delay between two waveforms
• Degrees
• Shift/distortion/cancellation
• Harmonic/spectral content
• (Envelope)
Reflection/Diffraction
• Reflection
• Equal to, and opposite of, initial angle of
incidence (=)
• Diffraction
• Reconstruction around obstacle
• Through small opening, reconstruction but
new sound source
Frequency Response
• Frequency response of a system
measured as the output amplitude at
each frequency from 20 Hz to 20,000
Hz.
• Usually done in chart form.
Loudness Levels
• Decibel (dB): tenth of a Bell
• dB: Logarithmic value that expresses
differences in intensities between two
levels (SPL, IL, Power)
• Hearing range 1013:1
(note book typo)
• Logarithmic scale maps wide value
range to our perception
Logarithmic Basics
• Log of 2 is 0.3
• Log of any number that can be
expressed as an integral exponent of 10
is the exponent. (Count the zeros)
• Numbers greater than one have positive
logs; less than 1 will have negative logs.
Sound Pressure Level
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Reference SPL is threshold of hearing
SPL refers to pressure vibrations in air.
dBspl = 20 log SPL/SPLref
SPL levels change with square of the
distance (the “20” in the equation)
• Weighting: compensates for nonlinearity in hearing.
Voltage (Intensity)
• Refers to electrical charge
• dBv = 20 log V/Vref
Power
• Measures current, or flow of electricity
over time.
• dBm = 10 log P/Pref
• 3 dB change represents a doubling or
halving of power (the “10” in the
equation).
dB: Practical Considerations
• Power level is greatest concern during
recording engineering
• 1 dB change is barely noticeable
• +/- 3 dB change doubles or halves the
signal level (!= doubling loudness)
• A 10 dB increase is 10 x original signal
level; 20 dB is 100 x, etc.
• Developing a listener’s feel for dB.
Ear
• Threshold of Hearing is the reference
SPL. ( 0 dB)
• Threshold of Feeling is the SPL that
causes discomfort 50% in listener (118
dB SPL between 200 Hz and 10 kHz)
• Threshold of Pain, same 50%, but now
pain (140 dB SPL)
Auditory Perception
• Non-linear, and varies according to
frequency
• Fletcher-Munson equal loudness curves
(p. 59)
• Beats (slight freq differences)
• Combination tones
• Masking
Perception of Direction
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Interaural intensity differences (higher f)
Interaural time differences (low f)
Effect of pinnae (outer ear)
In mixing, panning refers to placement
of sounds between speakers.
Perception of Space
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Direct sound (no reflections)
Early sound/reflections (within 30 ms)
Reverberation (after 50 ms)
Doubling (actual and perception, 4 to 20
ms)
• Echo (discrete, limited delays after 35
ms)