SOUNDS

is a wave that is created by vibrating objects and propagated through a medium from one location to another.

is a mechanical wave.

is a disturbance that is transported through a medium via the mechanism of particle-to- particle interaction.

The generation and propagation of a sound wave

The vibrations of the object set particles in the surrounding medium in vibrationmotion, thus transporting energy through the medium. For a sound wave traveling through air, the vibrations of the particles are best described as longitudinal.

Sound created from a vibrating string

Sound is a pressure wave.

Compressions are regions of high air pressure.

Rarefactions are regions of low air pressure.

The diagram below depicts a sound wave created by a tuning fork and propagated through the air in an open tube.

Nature of Sound

1.There is a medium that carries the disturbance from one location to another.

2. There is an original source of the wave, some vibrating object capable of disturbing the firstparticle of the medium.

3. The sound wave is transported from one location to another by means of particle-to-particle interaction.

Sound Properties and their Perception

FREQUENCY AND PITCH

refers to “how often” the air particles vibrate.

The sensation of a frequency is commonly referred to as the pitch of a sound.

A high pitch sound corresponds to a high frequency sound wave and a low pitch sound corresponds to a low frequency sound wave.

Any sound with a frequency below the audible range of hearing(i.e., less than 20 Hz) is known as an infrasound and any sound with a frequency above the audible range of hearing (i.e., more than 20 000 Hz) is known as an ultrasound.

Hearing Capacity

Dogs ( 50 Hz – 45 000 Hz )Cats ( 45 Hz – 85 000 Hz )Bats ( > 120 000 Hz )Dolphins ( > 200 000 Hz )Elephant ( 5 Hz – 10 000 Hz )Human ( 20 Hz – 20 000 Hz )

AMPLITUDE AND INTENSITY

• The amount of energy that is transported past a given area of the medium per unit of time is known as the intensity of the sound wave.

• The greater the amplitude of vibrations of the particles of the medium, the greater the rate at which energy is transported through it, and the more intense that the sound wave is.

• Intensity is the energy/time/area; and since the energy/time ratio is equivalent to the quantity power, intensity is simply the power/area.

• Typical units for expressing the intensity of a sound wave

are Watts/meter2.

• The scale for measuring intensity is the decibel scale.

• The faintest sound that a human ear can detect is known as the threshold of hearing. 

• While the intensity of a sound is a very objective quantity that can be measured with sensitive instrumentation, theloudness of a sound is more of a subjective response that will vary with a number of factors.

• Despite the distinction between intensity and loudness, it is safe to state that the more intense sounds will be perceived to be the loudest sounds.

Speed of Sound

v = 331 m/s + (0.6 m/s/C)•T

SPEED OF SOUND IN DIFFERENT MEDIA

Medium Speed of sound, (m/s)

Air 330

Seawater 1 540

Freshwater 1 410

Brass 3 650

Copper 4 000

Wood 4 650

Steel 4 950

Iron 5 000

Aluminum 5 140

Glass (soda) 5 150

(Vacuum) - 0 -

The Human Ear

Behavior of Sound

1. Reflection of Sound

Reflection of sound waves off of surfaces can lead to one of two phenomena - an echo or a reverberation.

A reverberation often occurs in a small room with height, width, and length dimensions of approximately 17 meters or less.

An echo is produced in an empty room.

2. Refraction of Sound

Sound waves are known to refract when traveling over water.

3. Diffraction of Sound Waves

Sound waves moving around corners or edges.

4. Interference of Sound Waves

5. Beat refers to the rate at which the volume is heard to be oscillating from high to low volume.

Doppler Effectcan be described as the effect produced by a

moving source of waves in which there is an apparent upward shift in frequency for the observer and the source are approaching and an apparent downward shift in frequency when the observer and the source is receding.

Sample Problem:

  On a hot summer day, a pesky little mosquito produced its warning sound near your ear. The sound is produced by the beating of its wings at a rate of about 600 wing beats per second.

a. What is the frequency in Hertz of the sound wave?

b. Assuming the sound wave moves with a velocity of 350 m/s, what is the wavelength of the wave?

Check Your Understanding

Identify the following statements whether TRUE or FALSE.

1. A sound wave is a mechanical wave.2. A sound wave is a means of

transporting energy without transporting matter.

3. Sound can travel through a vacuum.4. A sound wave is a transverse wave.5. To hear the sound of a tuning fork, the

tines of the fork must move air from the fork to one's ear.

6. Intense sounds are usually perceived as loud sounds. 7. The speed of a sound wave depends upon its frequency and its wavelength. 8. Sound waves would travel faster on a warm day than a cool day. 9. A high pitched sound has a low wavelength. 10. Ultrasound waves are those sound waves with frequencies less than 20 Hz.