Sound wavesPg. 45in NB

Objectives• Investigate and analyze

characteristics of sound waves: frequency, wavelength, and amplitude.

• Examine and describe sound wave propagation in a variety of media.

Physics terms• pitch

• speed of sound

• decibel (dB)

Equations

The speed of a sound wave equals the product of its wavelength and frequency.

𝑓 =𝑣𝜆 𝜆=

𝑣𝑓

Sound is a longitudinal wave, like this compression wave on a Slinky®.

What is sound?

The difference: it is AIR that is being compressed.

Sound is a tiny oscillation of air pressure.

Imagine this cymbal vibrating up and down when struck.

What is sound?

When the surface moves up, the air above it is slightly compressed (slightly higher pressure).

When it moves down, the air is drawn out (slightly lower pressure).

The result is an oscillation of air pressure – a sound wave!

What is sound?

• Sound waves are traveling oscillations of air pressure.

• Sound waves can interfere.

• Sound obeys a wavelength and frequency relationship like a wave.

Why is sound a wave?

List some properties of sound.

If two musical notes sounded different to you, what words could you use to describe that difference?

Describing sound

• Loudness

• Pitch – the perception of high or low

• Timbre –

Describing soundSome properties of sound:

the property that makes a piano note sound different from the same note produced by a guitar or a vocalist.

The loudness of a sound wave depends on its amplitude.

Louder sounds waves have larger amplitude pressure variations.

A stereo’s speakers move back and forth a greater distance when producing a loud sound than when producing a soft sound.

Loudness and amplitude

Humans can hear sound frequencies from about 20 Hz to 20,000 Hz.

Pitch and frequencyThe pitch of a sound depends on the frequency of the sound wave:

• Low-pitched sounds have lower

frequencies.• High-pitched sounds have higher

frequencies.

Most sounds contain multiple frequencies at the same time.

Musical instruments produce a fundamental frequency and many overtones (additional frequencies).

Overtones give the sound its timbre, its “piano-ness” or “guitar-ness”.

Timbre and overtones

In Investigation 16A you will experiment with the wave characteristics of sound.

Click to open the simulation on page 441.

Investigation

http://www.essential-physics.com/TX/sbookStudent ID login: 742  379  6973-Click on the “investigation” icon-Scroll down & click on Investigation 16A- Sound Waves-Select the investigation icon-Complete simulation & record all data & answer all questions on the provided Investigation sheet.

1. Choose a note and adjust the speakers so you can hear it.

2. Set time and amplitude values on the graph until you can see at least a few cycles of the wave.

3. With time plotted on the horizontal axis, try to make the black wave match the red sound wave by adjusting the frequency and amplitude.

Part 1: Matching the parameters of a sound wave

Investigation

4. Switch to distance for the horizontal axis. Match your black wave to the red sound wave by adjusting the wavelength and amplitude.

InvestigationPart 1: Matching the parameters of a sound wave

A good match has a score of greater than 95%. You must match both frequency and wavelength in order to score 100%.

InvestigationPart 2: Going further with octaves

1. Devise an experiment to determine what happens to the frequency and wavelength when you set the octave to different values.

Record your data and conclusions on your assignment sheet.

Sound waves are harder to visualize than waves in a string.

Low pressure

High pressure

High pressure

Visualizing sound waves

Here, a vibrating surface, such as a speaker, produces a pressure wave that travels to the right.

Amplitude on the graph below represents pressure, NOT distance!

Visualizing sound waves

It is the wave that travels, not the air molecules. The air moves in a tiny back-and-forth motion as the wave passes by.

Visualizing sound waves

There are three key characteristics of sound waves: • frequency

• speed

• amplitude

Characteristics of sound waves

Sound has a huge frequency range.• Humans can hear sounds in this

frequency range: 20 Hz < f < 20,000 Hz.

• By middle age, most people can only hear sounds less than about 12,000 Hz.

• Click on this Sound wave generator on page 440 to test your own hearing range.

Frequency

Some animals can hear higher and lower frequencies than humans:

Audible frequencies

Medical ultrasound technology uses very high frequency sound waves.

Differences in tissue density reflect ultrasound waves back to a detector and allow sophisticated imaging without harm to the patient.

Ultrasound technology

Sound waves are fast.

The speed of sound in air is 343 m/s (767 mph!)

Many military jets are capable of supersonic flights.

Speed

Sound travels even faster in water, or ice, or steel. The stiffer the medium, the faster the sound speed tends to be.

When sound passes from one medium to another . . .

• speed and wavelength change

• frequency stays the same

Speed in various materials

A 1000 Hz sound in …

AIR has a speed of 343 m/s and a wavelength of .34 m.

WATER has a speed of 1480 m/s and a wavelength of 1.5 m.

ICE has a speed of 3500 m/s and a wavelength of 3.5 m.

Speed in various materials

Sound can’t travel in a vacuum.

The loud explosions from space battles in science fiction movies are not realistic.

If you were actually watching a space battle from a distant space ship, you would hear total silence.

No sound in a vacuum

Sound waves have small amplitudes.Amplitude

BUT our ears are extremely sensitive and can easily detect these tiny pressure oscillations.

Typically the variation in pressure is about 0.0001 atmospheres, far below our ability to detect through our sense of touch.

The amplitude of a sound wave determines its loudness. Larger amplitude means louder sound.

BUT, to a human ear, frequency also matters.

A high amplitude sound at a frequency of 40,000 Hz is silent to a human ear but quite loud to a bat!

Amplitude and loudness

The Equal Loudness Curve shows how sounds of different frequencies compare in perceived loudness to an average human ear.

Examine the graph. Which frequencies do we hear the best?

Loudness and frequency

Our ears can detect an enormous range of pressures.

For this reason, the logarithmic decibel (dB) scale is used to measure loudness.

On the decibel scale, an increase of 20 dB means the wave has 10 times greater amplitude (and 100 times greater power).

The decibel scale

Assessment1. Based on this

graph:

a) What is the frequency of the sound wave?

b) Is this a transverse or longitudinal wave, and why?

c) What can you say about the loudness of this sound?

1. Based on this graph:

Assessment

a) What is the frequency of the sound wave? 400 Hz

b) Is this a transverse or longitudinal wave, and why? longitudinal (sound)

c) What can you say about the loudness of this sound? It is constant.

d) Is this a sound humans can hear? Why or why not?

Maybe. At 400 Hz, it is within our frequency range—but we don’t know if it is loud enough to hear. The pressure axis has no numbers or units.

2. Sound waves with a frequency of 172 Hz have a wavelength of 2.0 meters in air. When these waves enter water, their wavelengths change to 8.7 meters. What is the speed of sound in water?

A. 19.7 m/s

B. 40 m/s

C. 1500 m/s

D. 2990 m/s

Assessment

2. Sound waves with a frequency of 172 Hz have a wavelength of 2.0 meters in air. When these waves enter water, their wavelengths change to 8.7 meters. What is the speed of sound in water?

A. 19.7 m/s

B. 40 m/s

C. 1500 m/s

D. 2990 m/s

Assessment

The frequency stays the same.