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 :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 Reference SPL is threshold of hearing SPL refers to pressure vibrations in air. dB spl = 20 log SPL/SPL ref SPL levels change with square of the distance (the “20” in the equation) Weighting: compensates for non- linearity in hearing.

Voltage (Intensity) Refers to electrical charge dB v = 20 log V/V ref

Power Measures current, or flow of electricity over time. dB m = 10 log P/P ref 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 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 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)