Sound

Sound is a longitudinal wave that travels through a medium.

The speed of sound depends on the properties of the medium. It travels faster in a solid than in a liquid, and it travels faster in a liquid than a gas.

Air: 330 m/s Water: 1500 m/s

Aluminum: 6400 m/s Wood: 4000 m/s

Poll

You tap the edge of the pool. Friend A is under water 10 m from where you tapped. Friend B is out of the water 10 m from where you tapped. Who will hear the sound first?

1. Friend A

2. Friend B

3. Neither, because they will hear the sound at the same time.

Sinusoidal sound wave

A continuous, single-frequency tone is described by a sinusoidal wave. It can be produced by a speaker that oscillates in simple harmonic motion.

Superposition

Most periodic sound waves are a superposition of sine waves. Superposition means to add sine waves.

Examples: voice recognition, BOSE noise- canceling headphones

Beats

Beats occur when two sine waves of different frequencies are added together. The amplitude of the sum of the waves varies between zero and a maximum value with a frequency called the beat frequency.

t

y1

t

y2

1.0 s

1.0 s

Two subsonic (below frequencies audible to humans) signals are shown here over a one second time interval. A creature that can hear at these frequencies hears a beat frequency of _______ Hz.

A. 1 B. 2 C. 12 D. 24 E. 25

Poll

A person is tuning his guitar. The low E string is tuned to 156 Hz. When playing the same note on another string, he hears a beat frequency of 2 Hz. What is the frequency of the other string?

1. 158 Hz

2. 154 Hz

3. It could be either 154 Hz or 158 Hz.

4. None of the above.

Poll

Suppose that the guitarist in the previous question decides to increase the tension on the string he is tuning (in order to get it in tune). He hears the beat frequency increase as he increases the tension. What should he do?

1. Continue to increase the tension until the strings are in tune.

2. Turn the key the other way and instead decrease the tension until the strings are in tune.

3. Give up and hire someone else to tune the guitar.

Sound level (loudness)

Sound level is related to intensity. Intensity is proportional to amplitude squared. The units of sound level are decibels.

where

Example

To increase the sound level by 20 dB, by what factor must you increase the sound intensity?

1. 2

2. 20

3. 10

4. 100

5. 200

Typical sound levels

Weakest sound heard 0dBWhisper Quiet Library 30dBNormal conversation (3-5') 60-70dBCity Traffic (inside car) 85dBSubway train at 200' 95dBLevel at which sustained exposure may result in hearing loss 90 - 95dBPower mower at 3' 107dBMotorcycle 100dBPower saw at 3' 110dBRock Concert 115dBPain begins 125dBpermanent damage - Loudest recommended exposure WITH hearing protection 140dBJet engine at 100' 140dBDeath of hearing tissue 180dBLoudest sound possible 194dB

Example

What is the intensity of sound with a measured sound level of 100 dB?

Poll

The sound level of a motorcycle is about 100 dB. The sound level of normal conversation is about 70 dB. How much more intense is the sound from a motorcycle than the sound from a person’s voice during conversation?

1. 30 times more intense

2. 1.4 times more intense

3. 300 times more intense

4. 3 times more intense

5. 1000 times more intense

Doppler Effect

The frequency of sound heard by the detector is different than the frequency of sound emitted by the source, depending on the relative motion of the source and detector.

Doppler shift

The upper sign in each case corresponds to the detector and source moving away from each other.

The lower sign in each case corresponds to the detector and source moving toward each other.

The speaker below is mounted on wheels. When it is being pulled in the direction shown at constant speed, the frequency that the ear hears will be be ____________ when the speaker is stationary.

v

A. lower than B. the same as C. higher than

NYPD NYPD

An ambulance and a police rescue vehicle are traveling in the same direction toward the scene of an accident. The ambulance has its siren on.

When the ambulance is going faster than the rescue vehicle, the siren frequency heard by the police officers in the rescue unit will be _________________ when both vehicles are stationary

A. higher than B. the same as C. lower than

NYPD NYPD

An ambulance and a police rescue vehicle are traveling in the same direction toward the scene of an accident. The ambulance has its siren on.

When the rescue vehicle is gaining on the ambulance, the siren frequency heard by the police officers in the rescue unit will be _________________ when both vehicles are stationary

A. higher than B. the same as C. lower than

Example

Sound of frequency 1000 Hz is coming toward you with a velocity of 20 m/s. You are at rest. The speed of sound in air is about 330 m/s. What frequency will you hear?

Doppler Shift for Light

Light from a star moving away from us is shifted to lower frequency (longer wavelength) which is toward the red end of the visible spectrum -- red shifted.

Light from a star moving toward us is shifted to higher frequency (shorter wavelength) which is toward the blue end of the visible spectrum -- blue shifted.

We use Doppler shifts to measure (radial) speeds of stars.