Chapter 15

SoundSound

What is sound?

Sound is a compressional wave which travels through the air through a series of compressions and rarefactions.

Sound travels through different media.

We hear sound which usually travels through air. Sound travels through other media as well, such as water and various solids.

Sound travels different speeds in different media. Sound typically travels faster in a solid that a liquid and faster in a liquid than a gas.

The denser the medium, the faster sound will travel.

The higher the temperature, the faster the particles of the medium will move and the faster the particles will carry the sound.

The velocity of sound in air The velocity of sound in air dependsdepends

on the air temperature. The speed on the air temperature. The speed ofof

sound in dry air is sound in dry air is 331 m/s331 m/s at at 0 0 ººCC..This speedThis speedincreasesincreases

with with temperature: temperature: about about 0.6 m/s0.6 m/sfor every 1 for every 1 ººC C increase in increase in

temperature.temperature.

Determine the speed of sound, through air, when the temperature is 20°C.

V = 331 m/sec + (.6 m/sec x 20°C)

velocity at 0°C + velocity change per °C

V = 331 m/sec + 12 m/sec = 343 m/sec

What is sound intensity?

Sound intensity is the energy that the sound wave possesses. The greater the intensity of sound the farther the sound will travel and the louder the sound will appear.

Loudness is very closely related to intensity.

Loudness is the human perception of the sound intensity. The unit for loudness is decibels.

Loudness

• Loudness is determined by the amount of pressure produced by a wave measured in decibels

• An increase of 10 decibels equals twice the volume

– Threshold of hearing (0 dB) is the softest audible sound– Threshold of pain is 120 dB. – A normal conversation is 65 dB above the threshold of

hearing

Loudness in Decibels

The human ear relatesThe human ear relatesamplitudeamplitude to toloudnessloudness

andandfrequencyfrequency to to

pitchpitch..

Pitch

• A measure of how high or low a sound is• Pitch depends on the frequency of a sound wave• For example,

- Low pitchLow pitch

- Low frequencyLow frequency

- Longer Longer wavelengthwavelength

- High pitchHigh pitch

- High frequencyHigh frequency

- Shorter Shorter wavelengthwavelength

How is frequency related to pitch?

The pitch (highness or lowness of sound) of a sound wave is directly related to frequency. A high-pitched sound has a high frequency (a screaming girl). A low-pitched sound has a low frequency (a fog-horn).

A healthy human ear can hear frequencies in the range of 20 Hz to 20,000 Hz. Although most humans can not hear above 16,000 Hz.

We are most sensitive to the 1000-5000 Hz range.

Normally after age 70, we usually do not hear sounds above 8000 Hz.

The “mosquito” tone is approximately 16,000 Hz.

Humans cannot hear below 20 Hz. Sounds below this frequency are termed infrasonic.

Sounds above 20,000 Hz are termed ultrasonic. Some animals, such as dogs, can hear frequencies in this range in which humans cannot hear.

What is the Doppler Effect?

The Doppler Effect is the apparent change in frequency detected when the sound is moving relative to the hearer.

The Doppler effect-Change in frequency due to the source or receiver.The greater the speed of the source, the greater the Doppler

effect Stationary bug

Bug swimming

Blue Shift-increase in frequency Red Shift- decrease in frequency

Bow waves

V-shaped pattern made by overlapping crest

Shock Waves Produced by supersonic aircraft,

three-dimensional cone shaped Sonic boom – sharp crack heard

when conical shell of compressed air that sweeps behind a supersonic aircraft reaches listeners on the ground below.

Supersonic

- slower than the speed of sound• Subsonic

• Supersonic - faster than the speed of sound

Mach Number =speed of sound

speed of object

Mach 2 means twice the speed of soundMach 2 means twice the speed of soundMach 5 means five times the speed of soundMach 5 means five times the speed of sound

What is Resonance?

Many objects have a natural frequency – vibrates in a regular pattern.

Resonance occurs when whenever a sound wave has the same frequency as the natural frequency of an object. The sound will cause the object with the same natural frequency to vibrate.

What are echoes?

An echo is a sound wave that has been reflected.

All objects have a naturalAll objects have a natural

frequency of vibration.frequency of vibration.

ResonanceResonance - the inducing- the inducingof vibrations of a naturalof vibrations of a naturalrate by a vibrating sourcerate by a vibrating source

having the same frequencyhaving the same frequency

““sympathetic vibrations”sympathetic vibrations”

The The interferenceinterference of sound of soundwaves can cause “beats”waves can cause “beats”

Beats are produced when two Beats are produced when two slightly different frequencies are slightly different frequencies are

added and reach your ear added and reach your ear simultaneously.simultaneously.

UCSD: Physics 8; 2006UCSD: Physics 8; 2006

Sound Wave Interference and Beats• When two sound waves are present, the superposition

leads to interference– by this, we mean constructive and destructive addition

• Two similar frequencies produce beats– spend a little while in phase, and a little while out of phase– result is “beating” of sound amplitude

signal Asignal A

signal Bsignal B

A + B beatA + B beat(interference)(interference)

in phase: addin phase: add

out of phase: cancelout of phase: cancel

Using Sound

What is Acoustics?

Acoustics is the study of sound and ways to optimize the hearing of sound inside various structures.

What is echolocation?

What is sonar?

Sonar is a system that uses the reflection of underwater sound waves to detect objects. This has been used to find sunken ships and schools of fish.

Sonar

- An instrument that uses reflected sound waves to find underwater objects

- For example,

Animals use sonar or echo location to find their Animals use sonar or echo location to find their prey; these sounds have such a high pitch or prey; these sounds have such a high pitch or frequency that the human ear cannot hearfrequency that the human ear cannot hear

Humans use sonar Humans use sonar to locate or map to locate or map

objectsobjects