Properties of Sound – Chapter 14

Sound Frequency

• Infrasonic Below 20 Hz• Audible Range 20 Hz to 20 000 Hz• Ultrasonic Above 20 kHz• Bats can detect frequencies as high as 120 000 Hz. • Dolphins can detect frequencies as high as

200000 Hz.• Elephants possesses the unusual ability to detect

infrasound, having an audible range from approximately 5 Hz to approximately 10 000 Hz.

Pitch and Frequency

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

• A high pitch sound corresponds to a high frequency sound wave

• A low pitch sound corresponds to a low frequency sound wave.

Example 1

Two notes which have a frequency ratio of 2:1 are said to be separated by an octave. A frequency which is separated by an octave from middle C (256 Hz) is

a. 128 Hzb. 254 Hzc. 258 Hzd. 345 Hze. none of these

Example 1

Two notes which have a frequency ratio of 2:1 are said to be separated by an octave. A frequency which is separated by an octave from middle C (256 Hz) is

a. 128 Hzb. 254 Hzc. 258 Hzd. 345 Hze. none of these

Two notes separated by an octave have a frequency ration of 2:1. If a note is one octave below 256 Hz, then it must have one-half the frequency.

Sound Intensity• The amount of energy which 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 of a point source is inversely proportional to the square of the distance from the source.

Intensity of Sound

Intensity of a point source is inversely proportional to the square of the distance from the source.

Threshold of Sound

• The faintest sound which the typical human ear can detect has an intensity of 1*10-12 W/m2.

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

• The most intense sound which the ear can safely detect without suffering any physical damage is more than one billion times more intense than the threshold of hearing. 1*104 W/m2 or 160 dB

Sound intensity

• The scale for measuring intensity is the decibel scale.

• The threshold of hearing is assigned a sound level of 0 decibels (abbreviated 0 dB); this sound corresponds to an intensity of 1*10-12 W/m2.

Source Intensity Intensity Level

# of Times Greater Than TOH

Threshold of Hearing (TOH) 1*10-12 W/m2 0 dB 100

Whisper 1*10-10 W/m2 20 dB 102

Normal Conversation 1*10-6 W/m2 60 dB 106

Busy Street Traffic 1*10-5 W/m2 70 dB 107

Vacuum Cleaner 1*10-4 W/m2 80 dB 108

Large Orchestra 6.3*10-3 W/m2 98 dB 109.8

Walkman at Maximum Level 1*10-2 W/m2 100 dB 1010

Front Rows of Rock Concert 1*10-1 W/m2 110 dB 1011

Threshold of Pain 1*101 W/m2 130 dB 1013

Military Jet Takeoff 1*102 W/m2 140 dB 1014

Instant Perforation of Eardrum 1*104 W/m2 160 dB 1016

Hearing Loss Is Permanent

Example 2

A mosquito's buzz is often rated with a decibel rating of 40 dB. Normal conversation is often rated at 60 dB. How many times more intense is normal conversation compared to a mosquito's buzz?

a. 2b. 20c. 100d. 200e. 400

Example 3

On a good night, the front row of the Twisted Sister concert would surely result in a 120 dB sound level. An IPod produces 100 dB. How many IPods would be needed to produce the same intensity as the front row of the Twisted Sister concert?

SPEED OF SOUND

• The speed of the sound wave depends of the properties of the medium it travels through– IMFs and elasticity (called elastic modulus)– Inertial properties and Density

• In general, solids have the strongest interactions between particles, followed by liquids and then gases

• A highly elastic material causes vibrational disturbances to propagate faster because the restoring forces are higher

• vsolids > vliquids > vgases

SPEED OF SOUND IN AIR

• The speed of a sound wave in air depends upon the properties of the air– temperature and pressure

• What is the speed of sound in air at room temperature (20 oC)?

USING THE SPEED OF SOUND TO MEASURE DISTANCES

• At normal atmospheric pressure and a temperature of 20 degrees Celsius, a sound wave will travel at approximately 343 m/s

• Light travels through air at a speed of approximately 300 000 000 m/s

• The arrival of the light wave from the location of the lightning strike occurs in so little time that it is essentially negligible. Yet the arrival of the sound wave from the location of the lightning strike occurs much later.

EXAMPLE FOUR: THE LIGHTENING STRIKE

• During a storm, the air temperature is 21 C. A lightening strike is observed and only 3 seconds later, a clap of thunder can be heard.

• How far away did the lightening strike occur (in meters and in miles)?

ECHOLOCATION

• Echo: perceived time delay between the production of a sound and it’s reflection from a boundary.

• EXAMPLE 5: If a person on one side of a canyon hollers and the echo is heard 1.40s later, how far away is the other canyon wall (at 21o C)?

v = fWAVE SPEED CAN BE CALCULATED BY

FREQUENCY AND WAVELENGTH• the wave speed is not dependent upon these

quantities. • An alteration in wavelength DOES NOT affect wave

speed. Rather, an alteration in wavelength affects the frequency in an inverse manner.

• The speed of a sound wave depends on the properties of the medium through which it moves and the only way to change the speed is to change the properties of the medium.

Example SixAn automatic focus camera is able to focus on objects by use of an ultrasonic sound wave. The camera sends out sound waves which reflect off distant objects and return to the camera. A sensor detects the time it takes for the waves to return and then determines the distance an object is from the camera. If a sound wave (speed = 340 m/s) returns to the camera 0.150 seconds after leaving the camera, how far away is the object?

Example Seven

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?

The Doppler Effect

• The Doppler effect is a phenomenon observed whenever the source of waves is moving with respect to an observer.

• There is an apparent upward shift in frequency for the observer when the source of sound is approaching and …

• an apparent downward shift in frequency when the observer and the source is receding

• As the car approached with its siren blasting, the pitch of the siren sound (a measure of the siren's frequency) was high; and then suddenly after the car passed by, the pitch of the siren sound was low

During target practice, a man shoots a 7 g bullet with a horizontal velocity of 150 m/s directly at a 3 kg pumpkin sitting on a post.– If the bullet embeds in the pumpkin, how fast will the pumpkin

be knocked off the post? – If the post is 1 meter tall, how much time will it take the

pumpkin to strike the ground?– How far from the base of the post will the pumpkin strike the

ground?– What will be the pumpkin's resultant impact velocity just as it

strikes the ground? – If the dirt where the pumpkin strikes the ground offers an

average resisting force of 750 N, how deep of a "dent" will the pumpkin make upon impact as it is being brought to a rest?

– The shooter heard the bullet strike the pumpkin 0.9s after releasing it. If the air temperature on that day was 78 oF, how far away was the pumpkin on the post?

Independent Practice

• P. 498-504; 1, 2, 3, 29, 30, 32, 34, 41, 45