Second half of final Flashcards
Physical qualities of sound waves:
Amplitude: The magnitude of displacement of a sound pressure wave.
Intensity: The amount of sound energy falling on a unit area.
Frequency: For sound, the number of times per second that a pattern of pressure repeats.
Units for measuring sound:
Hertz (Hz): A unit of measure for frequency. One Hz equals one cycle per second.
Decibel (dB): A unit of measure for the physical intensity of sound.
Psychological qualities of sound waves:
Loudness: The psychological aspect of sound related to perceived intensity or magnitude.
Pitch: The psychological aspect of sound related mainly to the fundamental frequency.
Timbre: Psychological sensation by which listener can judge that two sounds that have same loudness and pitch are dissimilar, determined by the harmonic structure of the sounds.
Sine wave, or pure tone:
One of simplest kinds of sounds
Sine wave: Waveform for which variation as a function of time is a sine function
Time taken for one complete cycle of sine wave: Period
There are 360 degrees of phase across one period
Decibel (dB)
A unit of measure for the physical intensity of sound:
Named after the inventor* of the telephone, Alexander Graham Bell.
Decibels define the difference between two sounds as the ratio between two sound pressures: dB = 20 log10 (p1/p0)
Each 10:1 sound pressure ratio equals 20 dB, and a 100:1 ratio equals 40 dB
Doubling in sound pressure corresponds to 6 dB
*Bell actually just stole the patent [citation needed]
Pure tone
A tone with a sinusoidal wave form
Complex sounds
The summation of pure tones
Most sounds in world
Complex sounds can be described by Fourier analysis
A mathematical theorem by which any sound can be divided into a set of sine waves.
Combining these sine waves will reproduce the original sound
Results can be summarized by a spectrum
Harmonic sounds with the same fundamental frequency can sound different because amplitudes of harmonics here three different instruments.
Harmonic spectra: Typically caused by simple vibrating source, (e.g., string of guitar, or reed of saxophone)
First harmonic: Fundamental frequencylowest frequency component of the sound
First harmonic: Fundamental frequency
lowest frequency component of the sound
Timbre
Psychological sensation by which listener can judge that two sounds that have same loudness and pitch are dissimilar – defined by the shape of the harmonic spectrum.
Auditory system is acutely sensitive to natural relationships between harmonics
Harmonic
Harmonic spectra: Typically caused by simple vibrating source, (e.g., string of guitar, or reed of saxophone)
First harmonic: Fundamental frequencylowest frequency component of the sound
Timbre: Psychological sensation by which listener can judge that two sounds that have same loudness and pitch are dissimilar – defined by the shape of the harmonic spectrum.
Auditory system is acutely sensitive to natural relationships between harmonics
VOICES are HARMONIC SOUNDS!
If the Fundamental is taken away from a sound, people will still HEAR IT.
Interaural time difference (ITD):
The difference in time between a sound arriving at one ear versus the other.
Medial superior olives (MSOs): First place where input converges from two ears.
ITD detectors form connections from inputs coming from two ears during first few months of life.
Azimuth
Used to describe locations on imaginary circle that extends around us, in a horizontal plane
Let’s analyze ITD:
Where would a sound source need to be located to produce maximum possible ITD?
What location would lead to minimum possible ITD?
What would happen at intermediate locations?
Medial superior olives (MSOs)
First place where input converges from two ears.
ITD detectors form connections from inputs coming from two ears during first few months of life.
Interaural level difference (ILD):
The difference in level (intensity) between a sound arriving at one ear versus the other.
Lateral superior olives (LSOs): Neurons that are sensitive to intensity differences between two ears
Excitatory connections to LSO come from ipsilateral (same side) ear
Inhibitory connections to LSO come from contralateral (opposite side) ear
ITD and ILD compared:
Low frequencies are diffracted by the head (like an ocean wave around a pylon), high frequencies are absorbed.
Low Frequencies / Timing Cues Dominate
High Frequencies / Intensity Cues Dominate
Stimuli on headphones, where ITDs pointing to the left are offset by ILDs pointing to the right, so the sound is perceived as coming from the midline.
Low Frequencies
Timing Cues Dominate
High Frequencies
Intensity Cues Dominate
Subwoofer placement is less important in a home theater setup due to our inability to accurately localize the low frequencies.
…
Cone of confusion
Regions of positions in space where all sounds produce the same time and level (intensity) differences (ITDs and ILDs)
Experiments by Wallach (1940) demonstrated this problem
THE MOST CONFUSION CONE:
ABOVE-infront-below-behind!!!
Directional transfer function:
Shape and form of pinnae helps determine localization of sound
Describes how pinnae, ear canal, head, and torso change intensity of sounds with different frequencies that arrive at each ear from different locations in space (azimuth and elevation)
Sometimes called Head-Related Transfer Function
BINAURAL RECORDINGS
Recording through microphones inside your head, near the ear drums
Direction transfer function preserved. Then you feel sound as coming from outside of your HEAD!!
Inverse-square law
Sound intensity decreases with 1/d2 with increasing distance d in 3D space.
A sound 1 meter away is 6dB louder than 2 m
A sound 39m away is only 1dB louder then 40m
Relative amounts of direct vs. reverberant energy also help evaluate distance.
Reverberations that occur in a room can severely distort localization cues.
One strategy that listeners unconsciously employ to cope with this is to make their localization judgments instantly based on the earliest arriving waves in the onset of a sound.
This strategy is known as the precedence effect, because the earliest arriving sound wave—the direct sound with accurate localization information—is given precedence over the subsequent reflections and reverberation that convey inaccurate information.
Asymmetrical ears for localization of elevation
For example, Barn owls’ asymmetry is such that the center of the left ear flap is slightly above a horizontal line passing through the eyes and directed downward, while the center of the right ear flap is slightly below the line and directed upward.
Sound originating from below the eye level to sound louder in the left ear, while sound originating from above the eye level to sound louder in the right ear.
Shepard Tone
“Sonic Barber’s Pole” illusion.
The tone sounds as if it is continually ascends (or descends)
Consists of a superposition of sine waves separated by octaves.
Batpod™ sound effect in The Dark Knight ®
Perceptual Segregation
In complex auditory environments, humans are able to focus their attention on one source while ignoring sounds on other sources.
Perceptual segregation is often based on physical properties of a sound, but it is also facilitated by tracking the meaningful aspects of speech sounds.
Auditory Grouping Principles
- Good continuation
- Similarity of pitch
- Temporal proximity
- Similarity of timbre
- Location
Outer ear:
Sounds are first collected from environment by the pinnae
Sound waves are funneled by the pinnae into ear canal
Length and shape of ear canal enhance sound frequencies
Main purpose of canal is to insulate structure at its end: Tympanic membrane
Tympanic membrane
Eardrum; a thin sheet of skin at end of outer ear canal; it vibrates in response to sound
Common myth: Puncturing your eardrum will leave you deaf
In most cases it will heal itself
It is possible to damage it beyond repair
Earwax
Known by the medical term cerumen
Secreted in the ear canal.
Assists in cleaning and lubrication.
Provides some protection from bacteria, fungi, insects and water.
Naturally removed by the “conveyor belt” like regenerative growth process at ear drum center. (and jaw movement)
Cause of 60-80% of hearing aid faults
Middle ear:
Tympanic membrane is border between outer ear (ear canal) and middle ear
Consists of 3 tiny bones:
Ossicles, that amplify sounds
In Latin: Hammer, Anvil & Stirrup
Ossicles
Malleus, incus, stapes
smallest bones in body
Amplification provided by ossicles is essential for ability to hear faint sounds.
Stapes transmits vibrations of sound waves to oval window, another membrane which represents border between middle ear and inner ear.
Middle ear: Two muscles:
tensor tympani and stapedius
Purpose: To tense when sounds are very loud, muffling pressure changes.
However, acoustic reflex follows onset of loud sounds by about one-fifth of a second, so cannot protect against abrupt sounds, (e.g., gun shot).
Inner ear:
Fine changes in sound pressure are translated into neural signals
Function is roughly analogous to that of retina
Cochlea
Spiral structure of the inner ear containing the Organ of Corti.
Cochlea is filled with watery fluids in 3 parallel canals.
Tectorial membrane
A gelatinous structure, attached on one end, that extends into the middle canal of the ear, floating above inner hair cells and touching outer hair cells.
Vibrations cause displacement of the tectorial membrane, which bends stereocilia attached to hair cells and causes the release of neurotransmitters.
