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Sound is really tiny fluctuations of air pressure– units of pressure: N/m2 or psi (lbs/square-inch) Carried through air at 343 m/s (770 m.p.h) ascompressions and rarefactions in air pressure
Wavelength () is measured from crest-to-crest– or trough-to-trough, or upswing to upswing, etc. For traveling waves (sound, light, water), there is a speed (c) Frequency (f) refers to how many cycles pass by per second– measured in Hertz, or Hz: cycles per second– associated with this is period: T = 1/f These three are closely related:f = c or Tpressure
We hear frequencies of sound ashaving different pitch. A low frequency sound has a lowpitch, like the rumble of a bigtruck. A high-frequency sound has ahigh pitch, like a whistle or siren. In speech, women have higherfundamental frequencies thanmen.
When we hear complex sounds, the nerves in the earrespond separately to each different frequency. The braininterprets the signals from the ear and creates a “sonicimage” from the frequencies. The meaning in differentsounds is derived from the patterns in how the differentfrequencies get louder and softer.
How we hear the loudnessof sound is affected by thefrequency of the sound aswell as by the amplitude. The human ear is mostsensitive to sounds between300 and 3,000 Hz. The ear is less sensitive tosounds outside this range. Most of the frequencies thatmake up speech are between300 and 3,000 Hz.
1. A common way to record sound starts with amicrophone. A microphone transforms a soundwave into an electrical signal with the samepattern of oscillation.
2. In modern digital recording, a sensitive circuitconverts analog sounds to digital values between 0and 65,536.
3. Numbers correspond to the amplitude of the signaland are recorded as data. One second of compact-disk-quality sound is a list of 44,100 numbers.
4. To play the sound back, the string of numbers isread by a laser and converted into electrical signalsagain by a second circuit which reverses theprocess of the previous circuit.
5. The electrical signal is amplified until it is powerfulenough to move the coil in a speaker and reproducethe sound.
A sound wave is a wave of alternating high-pressureand low-pressure regions of air. Sound is a longitudinal wave, meaning that the motionof particles is along the direction of propagation
Waves in air can’t really be transverse, because theatoms/molecules are not bound to each other– can’t pull a (momentarily) neighboring molecule sideways– only if a “rubber band” connected the molecules would this work– fancy way of saying this: gases can’t support shear loads Air molecules can really only bump into one another
The pitch of a sound depends on the frequency ofthe tone that the ear receives. High notes areproduced by an object that is vibrating a greaternumber of times per second than for a low note The intensity of a sound is the amount of energycrossing a unit area in unit time or the powerflowing through the unit area. The SI unit is wattsper square meter. The loudness of the sound depends upon thesubjective effect of intensity of sound waves on thehuman ear.
The shift in frequency caused by motion is called theDoppler effect. It occurs when a sound source is moving at speeds lessthan the speed of sound.
Any waveform can beanalyzed as the sumof a set of sine waves,each with a particularamplitude, frequency,and phase.
The speed of sound in air is 343 meters per second(660 miles per hour) at one atmosphere of pressureand room temperature (21°C). An object is subsonic when it is moving slower thansound.
We use the term supersonic to describe motion atspeeds faster than the speed of sound. A shock wave forms where the wave fronts pile up. The pressure change across the shock wave is whatcauses a very loud sound known as a sonic boom.
A complex wave is really a sum of component frequencies. A frequency spectrum is a graph that shows the amplitudeof each component frequency in a complex wave.
A single frequency by itself does not have much meaning. The meaning comes from patterns in many frequenciestogether. A sonogram is a specialkind of graph that showshow loud sound is atdifferent frequencies. Every person’s sonogramis different, even whensaying the same word.
The eardrum vibratesin response to soundwaves in the ear canal. The three delicatebones of the inner eartransmit the vibrationof the eardrum to theside of the cochlea. The fluid in the spiralof the cochlea vibratesand creates waves thattravel up the spiral.
The nerves near thebeginning see arelatively large channeland respond to longerwavelength, lowfrequency sound.The nerves at the smallend of the channelrespond to shorterwavelength, higher-frequency sound.
The pitch of a sound is how high or low we hear itsfrequency. Though pitch and frequency usually meanthe same thing, the way we hear a pitch can beaffected by the sounds we heard before and after. Rhythm is a regular time pattern in a sound. Music is a combination of sound and rhythm that wefind pleasant. Most of the music you listen to is created from apattern of frequencies called a musical scale.
Harmony is the study of how sounds work together tocreate effects desired by the composer. When we hear more than one frequency of sound and thecombination sounds good, we call it consonance. When the combination sounds bad or unsettling, we call itdissonance.
Consonance and dissonance are related to beats. When frequencies are far enough apart that thereare no beats, we get consonance. When frequencies are too close together, we hearbeats that are the cause of dissonance. Beats occur when two frequencies are close, butnot exactly the same.Beats are created by the interference of two waves with different frequencies.
A listener will hear the alternating loudness, knownas beats. The number of beats per second, called the beatfrequency, equals the difference between thefrequencies of the two individual waves. To tune an instrument accurately, a musicianlistens carefully and adjusts her instrument toeliminate beats between the instrument and a givenpitch.
Echolocation is the methodof detecting objects byemitting a sound, receivingthe echo and correctlyidentifying the location, sizeand structure of the object. These sound waves are veryhigh-pitched, and mosthumans are unable to hearthem.
The same note sounds different when played on differentinstruments because the sound from an instrument is nota single pure frequency. The variation comes from the harmonics, multiples ofthe fundamental note.
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