Sound Waves

Physic 11 · Chapter 9

Joke:

Sound is made when something vibrates!

The vibration disturbs the air around the source.

This causes changes in air pressure.

These changes in air pressure move through the air as sound waves.

Sound is a longitudinal wave… That is carried through air as compressions and rarefactions in air pressure

wavelengthcompressed gas

rarefied gas

Properties of sound:

Loudness (volume)

Pitch Quality

Loudness:

Perceived intensity Loudness is related to

amplitude Large amplitude =

loud sound Small amplitude =

quiet sound

Human hearing:

Humans do not perceive sound intensity linearly.

For us to perceive a sound as twice as loud its intensity must be ten times greater.

The perceived intensity level of sound is measured in a logarithmic scale using a unit called the decibel (dB).

Human hearing (cont’d):

The scale begins (0 dB) on the softest sound that a person can hear. This is called the threshold of hearing.

The scale ends at the volume that causes pain (120 dB) and is therefore called the threshold of pain.

From the perspective of the logarithmic scale the threshold of hearing pain is

1,000,000,000,0001,000,000,000,000

times as great as the threshold of hearing.

Pitch:

Pitch is related to frequency High frequency = high pitch sound Low frequency = low pitch sound

Pitch:

We can hear frequencies between 20 hertz or cycles (vibrations) per second (low pitches) to 20,000 Hz (high pitches).

When the frequency of a sound doubles we say that the pitch goes up an octave.

We can hear a range of pitches of about ten octaves. Many animals can make sounds and hear frequencies that are beyond what we can hear!

Quality:

Quality is related to wave form Simple = pure (ex. flute) Complex = rich (ex. Organ)

Speed of Sound:

The speed of any wave is determined by the properties of the medium it is traveling through.

Sound waves travel through the air by moving molecules Molecular speeds in gas depend on

temperature. Speed of sound increases with temp increase.

Sound travels fastest in solids, then liquids and slowest in gas.

Examples:

Medium sound speed (m/s)

air (20C) 343

water 1497

gold 3240

brick 3650

wood 3800–4600

glass 5100

steel 5790

aluminum 6420

Echoes:

An echo is the sound you hear when you make a noise and the sound wave reflects off a distant object.

Remember: When the wave hits a different medium, some of it is reflected and some is transmitted through the material. In the case of a sound wave in air hitting a

solid object, most of the sound is reflected.

Echoes (cont’d):

Echoes can be used to easily estimate the distance of an object using time and the speed of sound

Ex: The temperature was 11° as a hiker passed through a canyon. She shouted at the canyon wall and 4.99 second later heard and an echo. How far away was the canyon wall?

v = 331 + 0.59Tc

= 331 + 0.59(11°) = 331 + 6.49 = 337.49 m/s

v = d t

337.49 = d (4.99)d = 1684 m

2 = 842 m away

Try it :

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