Sounds affect many hair-cell receptors {hearing, physiology}. Hearing finds intensities at frequencies and frequency bands (sound spectrum).
properties: fundamental missing
If people hear harmonics without the fundamental frequency, they hear the fundamental frequency, probably by temporal coding. Amplifying a chord tone causes hearing both tone and its fundamental tone, though fundamental frequency has zero intensity.
properties: octave
Animals conditioned to respond to pitch respond almost equally to its above and below octaves.
properties: phase differences
People cannot hear phase differences, but hearing can use phase differences to locate sounds.
properties: rhythm
Hearing can recognize rhythms and rhythmic groups.
properties: timing
People perceive two sounds less than three milliseconds apart as the same sound.
processes: contrast
Hearing uses lateral inhibition to enhance contrast to distinguish sounds.
processes: damping
Later tones constrain basilar membrane. Lower-frequency later tones constrain basilar membrane more. If later tone is more than 1000 Hz lower than earlier tone, to hear first tone requires high loudness. If later tone is more than 300 Hz higher than earlier tone, to hear first tone requires moderate loudness.
processes: filtering
Hearing integrates over many neurons to filter frequencies to find their individual intensities. Hearing performs limited-resolution Fourier analysis on sound frequencies [Friedmann, 1979].
processes: important sounds
Important sounds use more neurons and synapses.
processes: memory
Previous sound experiences help distinguish current sound patterns.
brain
Because brain is viscous, sound cannot affect brain tissue.
For short sounds in noisy backgrounds, hearing can complete missing sounds or sharpen noisy sounds {continuity effect} {perceptual restoration effect}. Hearing does not fill in short silences with sounds, but sharpens temporal boundaries. Hearing does not know when it fills in.
Sound radiates in all directions from sources and reflects from various surfaces back to ears {echo perception}. Hearing can distinguish echoes from their source sounds. Hearing uses binaural signals to suppress echoes.
Body and head, including pinnae and ear canals, transmit and absorb different-frequency, different-elevation, and different-azimuth sounds differently {head-related transfer function}.
People can perceive sound frequency {pitch, sound}|.
frequency
People can hear ten frequency octaves, from 20 Hz to 20,000 Hz. Lowest frequencies, 20 Hz to 30 Hz, are also highest vibrations detectable by touch.
Shortest hair-cell hair lengths detect highest frequencies. High-frequency tones vibrate basilar-membrane stiff narrow end, far from oval window. Above 3000 Hz, higher hearing neurons respond to frequency, tone pattern, or intensity range.
Low-frequency tones activate all hair cells, with greater activity near oval window and its long-hair hair cells.
sensitivity
People are most sensitive at frequency 1800 Hz.
neuron firing
Maximum neuron firing rate is 800 Hz. After sound frequency and firing rate reach 800 Hz, firing rate drops abruptly, and more than one neuron carries sound-frequency information. After sound frequency and firing rate reach 1600 Hz, firing rate drops abruptly.
Auditory neurons have frequency {characteristic frequency} (CF) at which they are most sensitive. The characteristic frequency is at the maximum of the frequency-intensity spectrum (threshold tuning curve). For CF = 500 Hz at 0 dB, 1000 Hz is at 80 dB, and 200 Hz is at 50 dB. For CF = 1100 Hz at 5 dB, 1500 Hz is at 80 dB, and 500 Hz is at 50 dB. For CF = 2000 Hz at 5 dB, 3500 Hz is at 80 dB, and 500 Hz is at 80 dB. For CF = 3000 Hz at 5 dB, 3500 Hz is at 80 dB, 700 Hz to 2000 Hz is at 50 dB, and 500 Hz is at 80 dB. For CF = 8000 Hz at 5 dB, 9000 Hz is at 80 dB, 1000 Hz to 3000 Hz is at 60 dB, and 500 Hz is at 80 dB. For CF = 10000 Hz at 5 dB, 10500 Hz is at 80 dB, 5000 Hz is at 80 dB, 1000 Hz to 2000 Hz is at 60 dB, and 500 Hz is at 80 dB.
Auditory-nerve channels carry frequency-range {critical band} information.
For 100-Hz to 6000-Hz sound stimuli, basilar membrane has electric pulses, with same frequency and intensity, caused by potentials from all hair cells, that do not fatigue.
For 20-Hz to 900-Hz sound stimuli, auditory-neuron axons have electric pulses {microphonic electric pulse}, measured in cochlear nerve, with same frequency and intensity [Saul and Davis, 1932]. For 900-Hz to 1800-Hz sound stimuli, auditory-neuron axons have electric pulses with same frequency and one-half intensity. For 1800-Hz to 2700-Hz sound stimuli, auditory-neuron axons have electric pulses with same frequency and one-third intensity. For above-2700-Hz sound stimuli, auditory-neuron axons have electric pulses that do not correlate with frequency and intensity. Perhaps, auditory nerve uses summed potentials of microphonic-electric-pulse envelopes.
For below-500-Hz sound stimuli, auditory-neuron-axon signals have same frequency and phase {phase locking, hearing}.
Similar frequencies group together to make increasing loudness {recruitment, hearing}.
Tones that share one octave have perceivable sound features {tone chroma}.
Tone frequency determines low or high pitch {tone height}.
Noise or tones within two octaves of stimulus frequency can interfere with stimulus perception {critical band masking}. Pure tones mask high frequencies more than low frequencies, because higher frequencies activate smaller basilar-membrane regions. Complex tones mask low frequencies more than high frequencies, because lower frequencies have more energy than higher frequencies [Sobel and Tank, 1994].
Previous-tone {preceding tone} intensity-frequency spectrum affects neuron current-tone response.
Different later tone can decrease auditory-neuron firing rate {two-tone suppression}.
At each audible frequency, people have an intensity threshold {audibility curve}.
At each audible frequency, specific sound-pressure levels (SPL) cause people to hear equal loudness {equal loudness curve}.
At constant amplitude, auditory-neuron firing rate depends on frequency {isointensity curve}. For amplitude 20 dB at characteristic frequency, firing rate is 180 per second. For amplitude 20 dB at 500 Hz below or 500 Hz above characteristic frequency, firing rate is 50 per second. For amplitude 20 dB at 1300 Hz to 1400 Hz above characteristic frequency, auditory neurons have spontaneous firing rate.
At each frequency, people have a sound-intensity threshold {threshold tuning curve}.
Same-intensity-and-pitch sounds can have different harmonics {timbre, sound}|. Rapid timbre changes are difficult to perceive.
Clear tones {clarity, tone} have narrow frequency band. Unclear tones have wide frequency band.
Full tones {fullness, tone} have many frequency resonances. Shallow tones have few frequency resonances.
Shrill tones {shrillness} have higher frequencies. Dull tones have lower frequencies.
Sounds with many high-frequency components seem sharp or strident {stridency}. Tones with mostly low-frequency components seem dull or mellow {mellowness}.
People can hear sound energies as small as random air-molecule motions. {hearing, intensity} {sound intensity}. Because oval window is smaller than eardrum, sound pressure increases in middle ear. Middle-ear bones increase sound intensity by acting as levers that convert distance into force.
distortion
High sound intensities can strain materials past their elastic limit, so intensity and/or frequency change.
frequency
For same stimulus-input energy, low-frequency tones sound louder, and high-frequency tones sound quieter. Smaller hair-cell hairs have faster vibrations and smaller amplitudes.
maximum sound
Maximum sound is when physical ear structures have inelastic strain, which stretches surface tissues past point to which they can completely return.
pain
Maximum sound causes pain.
rate
For amplitude 40 dB to 80 dB at frequency between 2000 Hz below and 50 Hz above characteristic frequency, maximum firing rate is 280 per second {rate saturation, hearing}.
temporal integration
If sound has constant intensity for less than 100 ms, perceived loudness decreases {temporal integration, hearing}. If sound has constant intensity for 100 ms to 300 ms, perceived loudness increases. If sound has constant intensity for longer than 300 ms, perceived loudness is constant.
At loud-sound onset, stapedius and tensor tympani muscles contract {acoustic reflex}, to dampen stapes and eardrum vibration.
Tones can rise quickly or slowly from background noise level to maximum intensity {attack, hearing}| {onset, hearing}. Fast onset sounds aggressive. Slow onset sounds peaceful.
Tones can fall slowly or rapidly from maximum to background noise level {decay, hearing} {offset, hearing}.
Hearing perceives sound-source locations {source location} {sound location}, in space. Most space locations are silent. One space location can have several sound sources. Hearing determines sound location separately and independently of perceiving tones.
azimuth
Hearing can calculate angle to right or left, from straight-ahead to straight-behind, in horizontal plane.
elevation
Hearing can calculate height and angle above horizontal plane. People perceive lower frequencies as slightly lower than actual elevation. People perceive higher frequencies as slightly higher than actual elevation.
frequency and distance
Sound sources farther than 1000 meters have fewer high frequencies, because of air damping.
sound reflection and distance
Sound energy comes directly from sources and reflects from other surfaces. Close sounds have more direct energy than reflected energy. Far sounds have more reflected energy than direct energy. Reflected sounds have fewer high frequencies than direct sounds, because longer distances cause more air damping.
Hearing can separate complex sounds from one source into independent continuous sound streams {auditory stream segregation}.
Sound grouping has same Gestalt laws as visual grouping.
If one ear hears melody with large ascending and descending tone jumps, and other ear hears another melody with large ascending and descending tone jumps, people do not hear left-ear melody and right-ear melody but hear two melodies, different than either original melodies, that depend on alternating-tone proximities.
People separate sounds from multiple sources into independent continuous sound streams {auditory scene analysis} {source segregation}. Hearing separates sounds from different locations into independent continuous sound streams {spatial separation, hearing}.
Having two ears {binauralism} allows calculating time and amplitude differences between left-ear and right-ear sound streams from same space location.
Hearing can reject unwanted messages {focusing, hearing}, using binauralism to localize sounds.
The same sound reaches right and left ear at different intensity levels {interaural level difference} (ILD). Level difference can be as small as 1 dB. Intensity difference reflects stimulus distance, approaching or receding sounds, and body sound damping. Slight head movements are enough to eliminate direction ambiguity. Intensity differences due only to sound distance, or to approaching or receding sounds, are useful up to one or two meters. Beyond two meters, differences are too small to detect.
damping
Pinnae and head bones absorb sounds with frequencies higher than 1500 Hz, according to their frequency-related dampening function. Pinnae and head-bone damping differs on right and left, depending on source location, and hearing uses the intensity differences to determine space directions and distances beyond one or two meters.
brain
Lateral superior olive detects intensity-level differences between left-right ears and right-left ears, to make opponent systems. To find distance, two receptor outputs go to two different neurons, which both send to difference-finding neuron. Opposite-ear output goes to trapezoid-body medial nucleus, which lies beside pons lateral superior olive and inhibits same-ear lateral-superior-olive output. Interaural time difference and interaural level difference work together.
The same sound reaches right and left ear at different times {interaural time difference, hearing} (ITD), because distances from source location to ear differ, and ears have distance between them. Hearing can detect several microseconds of time difference. Slight head movements are enough to eliminate direction ambiguity. Interaural time difference uses frequencies lower than 1500 Hz, because they have no body damping.
Medial superior olive detects time differences between left-right ears and right-left ears, to make opponent systems. To find distances, two receptor outputs go to two different neurons, which both send to difference-finding neuron. Interaural time difference and interaural level difference work together.
In a cone {cone of confusion} {confusion cone} from head center into space, sounds have same intensity and timing, because ear timing differences (interaural time difference) and intensity differences (interaural level difference) are zero.
Electronic instruments {audiometer}| can test hearing.
Amplified auditory-nerve signals played through speakers sound same as stimulus sounds {microphone effect}.
People can study subjective sense qualities or psychological changes evoked by sound stimuli {psychoacoustics}.
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Date Modified: 2022.0225