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In this section, we consider a few psychoacoustic aspects of our hearing process. Most of these characteristics are subjective in nature and the reported capabilities generally represent averages determined from measurements of large populations. Psychoacoustics is an immense field of study which has gained much recognition recently in conjunction with sound data compression and 3-dimensional audio recreation systems. If you are intrigued by this research, consider taking Music 151: Psychophysics and Cognitive Psychology for Musicians.
Equal-loudness curves for pure tones (from Rossing, 1990).
- Sounds of equal SPL but differing frequencies do not necessarily sound equally loud.
- Our auditory system is particularly insensitive to low-frequency sounds of moderate to low intensity.
- The sensitivity maximum near 4000 Hz and the local maximum near 13 kHz are related to the first two resonances of the ear canal.
- The contours of equal loudness are labeled in units of phons, which are determined by the SPL in decibels at 1000 Hz.
- A reference loudness, called the sone, is defined as the loudness of a 1000 Hz tone at a sound level of 40 dB (or a loudness of 40 phons).
- In general, a loudness of four sones sounds twice as loud as a sound of two sones.
- The total loudness of two pure tones, each having the same SPL, will be judged equal for frequency separations within a critical bandwidth. Once the frequency separation exceeds the critical bandwidth, however, the total loudness begins to increase.
- Broadband sounds will generally sound louder than narrow band (less than a critical bandwidth) sounds.
- The determination of loudness for complex sounds can be reasonably approximated using a sound pressure meter (set for A-weighting) and the following conversion: 30 dBA = 1.5 sones and then double the number of sones for each 10 dBA increase.
- A louder sound may ofter render a softer sound inaudible, depending on the relative frequencies and loudnesses of the two sounds.
- Pure tones close together in frequency mask each other more than tones widely separated in frequency.
- A pure tone masks tones of higher frequency more effectively than tones of lower frequency.
- The greater the intensity of the masking tone, the broader the range of frequencies it can mask.
- Loudness grows with duration, up to about 0.2 seconds.
- Up to about 20 dB of protection is provided by the muscles attached to the eardrum and ossicles of the middle ear when exposed to sounds in excess of 85 dB or so. This reflex does not begin until about 30 to 40 ms after the sound overload and does not reach full protection for another 150 ms or so. Thus, loud impulsive sounds such as explosions or gunshots occur too quickly to prevent injury to the ear.
- Minimum perceptible difference between two stimuli is called just noticeable difference (jnd).
- The jnd for pitch depends on frequency, sound level, duration, and suddenness of the frequency change.
- Same mechanism appears responsible for critical bands and pitch discrimination.
- Approximately 30 jnd's within a critical band for all center frequencies.
- A sense of pitch can develop after as little as three or four cycles for low-frequency tone bursts (50-100 Hz), but this duration increases to about twelve cycles at 1000 Hz (and continues increasing with frequency).
- If the tone has a soft onset, tone recognition times as short as 3 ms are possible.
- A test tone will be perceived to increase in frequency in the presence of an interfering tone of lower frequency.
- A test tone will be perceived to decrease in frequency in the presence of an interfering tone of higher frequency.
- A test tone will be perceived to increase in frequency in the presence of interfering noise with lower frequency components. If the noise contains higher frequency components, a shift in either direction is possible.
- The perceived shift in pitch increases with the amount that the interfering sound amplitude exceeds that of the test tone.
- In a harmonic sound, the lowest common factor of the component frequencies will be identified as the pitch (even if it is very weak or missing altogether - the ``missing fundamental'').
- In a nonharmonic complex tone, the ear will pick out a series of nearly harmonic partials somewhere near the center of the audible range and determine the pitch as the largest near-common factor in the series.
- Tone quality or tone color.
- A subjective attribute of sound based, in large part, on frequency content.
- Various subjective rating scales attempted (Dull - Brilliant, Cold - Warm, Pure - Rich, Dull - Sharp, Compact - Scattered, Full - Empty, Colorful - Colorless).