sound vibration and motion. sound waves sound is an important part of existence for many living...

Sound

Vibration and Motion

Sound Waves

Sound is an important part of existence for many living things

From your everyday experiences, you are already familiar with several of the characteristics of sound, including volume, tone, and pitch

You can use these, and other characteristics, to categorize many of the sounds that you hear

Properties and Detection of Sound

Sound Waves

Sound is a type of wave

As the bell shown in the figure moves back and forth, the edge of the bell strikes particles in the air


Sound Waves

When the edge moves forward, air particles are driven forward; that is the air particles bounce off the bell with a greater velocity


Sound Waves

When the edge moves backward, air particles bounce off the bell with a lower velocity

The result of these velocity changes is that the forward motion of the bell produces a region where the air pressure is slightly higher than average

The backward motion produces slightly below-average pressure


Sound Waves

Collisions among the air particles cause the pressure variations to move away from the bell in all directions

A pressure variation that is transmitted through matter is a sound wave

Sound waves move through air because a vibrating source produces regular variations, or oscillations, in air pressure.

The air particles collide, transmitting the pressure variations away from the source of the sound


Detection of Pressure Waves

The frequency of the wave is the number of oscillations in pressure each second

The wavelength is the distance between successive regions of high or low pressure

Because the motion of the air particles is parallel to the direction of motion of the wave, sound is a longitudinal wave



The speed of sound in air depends on the temperature, with the speed increasing by about 0.6 m/s for each 1°C increase in air temperature

At room temperature (20°C), sound moves through air at sea level at a speed of 343 m/s

The speed of sound is greater in solids and liquids than in gases

Sound cannot travel in a vacuum because there are no particles to collide



Sound waves share the general properties of other waves

Reflected sound waves are called echoes

The time required for an echo to return to the source of the sound can be used to find the distance between the source and the reflective object

Two sound waves can interfere, causing dead spots at nodes where little sound can be heard

The frequency and wavelength of a wave are related to the speed of the wave by the equation λ = v/f



Sound detectors convert sound energy—the kinetic energy of the vibrating air particles—into another form of energy

A common detector is a microphone, which converts sound waves into electrical energy

A microphone consists of a thin disk that vibrates in response to sound waves and produces an electrical signal


Perceiving Sound – Pitch

Marin Mersenne and Galileo first determined that the pitch we hear depends on the frequency of vibration

Pitch can be given a name on the musical scale. For instance, the middle C note has a frequency of 262 Hz

The ear is not equally sensitive to all frequencies


Perceiving Sound – Frequency

Most people cannot hear sounds with frequencies below 20 Hz or above 16,000 Hz

Older people are less sensitive to frequencies above 10,000 Hz than young people

By age 70, most people cannot hear sounds with frequencies above 8000 Hz

This loss affects the ability to understand speech


Perceiving Sound – Loudness

Frequency and wavelength are two physical characteristics of sound waves

Another physical characteristic of sound waves is amplitude

Amplitude is the measure of the variation in pressure along a wave



In humans, sound is detected by the ear and interpreted by the brain

The loudness of a sound, as perceived by our sense of hearing, depends primarily on the amplitude of the pressure wave

The human ear is extremely sensitive to pressure variations in sound waves, which is the amplitude of the wave



The ear can detect pressure-wave amplitudes of less than one-billionth of an atmosphere, or 2×10−5 Pa

At the other end of the audible range, pressure variations of approximately 20 Pa or greater cause pain

It is important to remember that the ear detects only pressure variations at certain frequencies

Driving over a mountain pass changes the pressure on your ears by thousands of pascals, but this change does not take place at audible frequencies



Because humans can detect a wide range in pressure variations, these amplitudes are measured on a logarithmic scale called the sound level

The unit of measurement for sound level is the decibel (dB)

The sound level depends on the ratio of the pressure variation of a given sound wave to the pressure variation in the most faintly heard sound, 2×10−5 Pa

Such an amplitude has a sound level of 0 dB



Most people perceive a 10-dB increase in sound level as about twice as loud as the original level

In addition to pressure variations, power and intensity of sound waves can be described by decibel scales



Exposure to loud sounds, in the form of noise or music, has been shown to cause the ear to lose its sensitivity, especially to high frequencies

The longer a person is exposed to loud sounds, the greater the effect

A person can recover from short-term exposure in a period of hours, but the effects of long-term exposure can last for days or weeks

Long exposure to 100-dB or greater sound levels can produce permanent damage



Hearing loss also can result from loud music being transmitted through stereo headphones from mp3 and CD players

Cotton earplugs reduce the sound level only by about 10 dB

Special ear inserts can provide a 25-dB reduction

Specifically designed earmuffs and inserts can reduce the sound level by up to 45 dB



Loudness, as perceived by the human ear, is not directly proportional to the pressure variations in a sound wave

The ear’s sensitivity depends on both pitch and amplitude

Also, perception of pure tones is different from perception of a mixture of tones


The Doppler Effect


The Doppler Effect

For both a moving source and a moving observer, the frequency that the observer hears can be calculated using the equation below

The frequency perceived by a detector is equal to the velocity of the detector relative to the velocity of the wave, divided by the velocity of the source relative to the velocity of the wave, multiplied by the wave’s frequency


The Doppler Effect

In the Doppler effect equation, v is the velocity of the sound wave, vd is the velocity of the detector, vs is the velocity of the sound’s source, fs is the frequency of the wave emitted by the source, and fd is the frequency received by the detector

This equation applies when the source is moving, when the observer is moving, and when both are moving

The sound waves will be approaching the detector from the source, so the velocity of sound is always positive


The Doppler Effect

For a source moving toward the detector (positive direction, which results in a smaller denominator compared to a stationary source) and for a detector moving toward the source (negative direction and increased numerator compared to a stationary detector), the detected frequency, fd, increases

Similarly, if the source moves away from the detector or if the detector moves away from the source, then fd decreases


Sound Waves

Assume that the speed of sound in air is 343 m/s. at 20°C. You are listening to an outdoor concert on a day when the temperature is 0°C. The sound of a wavelength of 0.490 m is emitted by a flute on a stage 125 m from where you are standing. What is the time elapsed before you hear the sound emitted from the stage? What is the frequency of the sound?


The Doppler Effect

Assume that the speed of sound in air is 343 m/s. at 20°C. A species of bat navigates by emitting short bursts of sound waves that have a frequency range that peaks at 58.0 kHz

If a bat is flying at 4.0 m/s towards a stationary object, what is the frequency of the sound waves reaching the object?

What is the frequency of the reflected sound waves detected by the bat?

What is the difference between the frequency of the sound waves emitted by the bat and the frequency of the sound waves detected by the bat if the bat is flying at 4.0 m/s and the object is a moth approaching at 1.0 m/s?


sound vibration and motion. sound waves sound is an important part of existence for many living...

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