Sound

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Waves

WHAT IS SOUND?

• 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) as compressions and rarefactions in air pressure

PROPERTIES OF WAVES

• 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 T

pre

ssu

re

THE FREQUENCY OF SOUND

• We hear frequencies of sound as having different pitch.

• A low frequency sound has a low pitch, like the rumble of a big truck.

• A high-frequency sound has a high pitch, like a whistle or siren.

• In speech, women have higher fundamental frequencies than men.

COMPLEX SOUND

When we hear complex sounds, the nerves in the ear respond separately to each different frequency. The brain interprets the signals from the ear and creates a “sonic image” from the frequencies. The meaning in different sounds is derived from the patterns in how the different frequencies get louder and softer.

SENSITIVITY OF THE EAR

• How we hear the loudness of sound is affected by the frequency of the sound as well as by the amplitude.

• The human ear is most sensitive to sounds between 300 and 3,000 Hz.

• The ear is less sensitive to sounds outside this range.

• Most of the frequencies that make up speech are between 300 and 3,000 Hz.

RECORDING SOUND

1. A common way to record sound starts with a microphone. A microphone transforms a sound wave into an electrical signal with the same pattern of oscillation.

RECORDING SOUND

2. In modern digital recording, a sensitive circuit converts analog sounds to digital values between 0 and 65,536.

RECORDING SOUND

3. Numbers correspond to the amplitude of the signal and are recorded as data. One second of compact-disk-quality sound is a list of 44,100 numbers.

RECORDING SOUND

4. To play the sound back, the string of numbers is read by a laser and converted into electrical signals again by a second circuit which reverses the process of the previous circuit.

RECORDING SOUND

5. The electrical signal is amplified until it is powerful enough to move the coil in a speaker and reproduce the sound.

SOUND WAVES

A sound wave is a wave of alternating high-pressure and low-pressure regions of air.

Sound is a longitudinal wave, meaning that the motion of particles is along the direction of propagation

WHY IS SOUND LONGITUDINAL?

• Waves in air can’t really be transverse, because the atoms/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

INTENSITY AND PITCH The pitch of a sound depends on the

frequency of the tone that the ear receives. High notes are produced by an object that is vibrating a greater number of times per second than for a low note

The intensity of a sound is the amount of energy crossing a unit area in unit time or the power flowing through the unit area. The SI unit is watts per square meter. 

The loudness of the sound depends upon the subjective effect of intensity of sound waves on the human ear.

THE DOPPLER EFFECT

• The shift in frequency caused by motion is called the Doppler effect.

• It occurs when a sound source is moving at speeds less than the speed of sound.

FOURIER ANALYSIS

• Any waveform can be analyzed as the sum of a set of sine waves, each with a particular amplitude, frequency, and phase.

THE SPEED OF SOUND

• The speed of sound in air is 343 meters per second (660 miles per hour) at one atmosphere of pressure and room temperature (21°C).

• An object is subsonic when it is moving slower than sound.

THE SPEED OF SOUND

• We use the term supersonic to describe motion at speeds faster than the speed of sound.

• A shock wave forms where the wave fronts pile up.

• The pressure change across the shock wave is what causes a very loud sound known as a sonic boom.

SOUND SPECTRUM

• A complex wave is really a sum of component frequencies.• A frequency spectrum is a graph that shows the amplitude

of each component frequency in a complex wave.

SOUND, PERCEPTION, AND MUSIC

• A single frequency by itself does not have much meaning.

• The meaning comes from patterns in many frequencies together. A sonogram is a special

kind of graph that shows how loud sound is at different frequencies.

Every person’s sonogram is different, even when saying the same word.

HEARING SOUND• The eardrum vibrates

in response to sound waves in the ear canal.

• The three delicate bones of the inner ear transmit the vibration of the eardrum to the side of the cochlea.

• The fluid in the spiral of the cochlea vibrates and creates waves that travel up the spiral.

HEARING SOUND

• The nerves near the beginning see a relatively large channel and respond to longer wavelength, low frequency sound.

The nerves at the small end of the channel respond to shorter wavelength, higher-frequency sound.

MUSIC

• The pitch of a sound is how high or low we hear its frequency. Though pitch and frequency usually mean the same thing, the way we hear a pitch can be affected 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 we find pleasant.• Most of the music you listen to is created from

a pattern of frequencies called a musical scale.

CONSONANCE, DISSONANCE, AND BEATS

• Harmony is the study of how sounds work together to create effects desired by the composer.

• When we hear more than one frequency of sound and the combination sounds good, we call it consonance.

• When the combination sounds bad or unsettling, we call it dissonance.

CONSONANCE, DISSONANCE, AND BEATS Consonance and dissonance are related to

beats. When frequencies are far enough apart that

there are no beats, we get consonance. When frequencies are too close together, we

hear beats that are the cause of dissonance. Beats occur when two frequencies are

close, but not exactly the same.

Beats are created by the interference of two waves with different frequencies.

BEATS

A listener will hear the alternating loudness, known as beats.

The number of beats per second, called the beat frequency, equals the difference between the frequencies of the two individual waves.

To tune an instrument accurately, a musician listens carefully and adjusts her instrument to eliminate beats between the instrument and a given pitch. 

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ECHOLOCATION

Echolocation is the method of detecting objects by emitting a sound, receiving the echo and correctly identifying the location, size and structure of the object.

These sound waves are very high-pitched, and most humans are unable to hear them.

HARMONICS AND INSTRUMENTS

• The same note sounds different when played on different instruments because the sound from an instrument is not a single pure frequency.

• The variation comes from the harmonics, multiples of the fundamental note.

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