The Nature of Sound

Students will describe how sound is caused by vibrations, how it is transmitted through a

medium.

Students will also compare the speed of sound in different media, explore the doppler effect,

and explain how echoes are produced.

Students will discuss the range of hearing for humans, and the threshold of pain.

Sound is caused by vibrationsThe medium is vibrating and particles

are compressed and decompressed.

Another view of the particles vibrating back and forth

A cross section of a sound wave

Tuning Fork – notice how the sound spreads out in all directions. You can also see the reflection of some of the sound waves, as well.

Tuning Fork Sound WavesTime-lapse showing the compressions

moving through the medium

First second

Second2

Third second

Fourth second

Fifth second

Sixth second

Seventh second

Sound Spreads Out in All Directions

Can you name the compressions and the rarefactions?

Amplitude of Sounds

Both waves have the same frequency, but the top one has a much greater amplitude. This means that they have the same pitch (or “note” on a musical scale), but the top one is louder than the bottom one.

Sound vs LightLight travels at 3x108 m/s, whereas sound only travels at about 340 m/s

That is 880,000 times faster!!!!

So the music you hear out of your radio was transmitted through the air from radio towers as a form of light (radio waves) not sound waves

3x108 =

300000000

The speed of sound

Medium Speed of soundAir 343 m/s

Helium 1005 m/s

Water 1482 m/s

Sea water 1522 m/s

Wood (oak) 3850 m/s

Glass 4540 m/s

Steel 5200 m/s

Also…the speed of sound depends on temperature!

The cooler the media, the slower the speed of sound.

Remember, the particles are moving slower when it is cooler, so they also transfer sound energy at a slower rate!

How much can the teacher really hear in class???

The average human can detect frequencies of 20 Hz to 20,000 Hz. (Augustyn can detect 2 Hz to 200,000,000 Hz)

Sounds that are lower than 20 Hz are called infrasonic

Sounds that are higher than 20,000 Hz are called ultrasonic

Gum being chewed is about 5 Hz

Doppler Effect

The Doppler Effect (for sound waves) is the apparent change in pitch as a sound approaches and then passes by

Example: Train whistles at you while your standing by the tracks – it seems to be higher in pitch as it approaches, and lower after it passes by.

The frequency (pitch) does not really change, it only appears to change.

Loudness is related to AmplitudeThe higher the amplitude, the louder the soundLoudness of sound is measured in decibels (db)• Near total silence - 0 dB • A whisper - 15 dB • Normal conversation - 60 dB • A lawnmower – 80-90 dB • A car horn – 110-115 dB • A rock concert or a jet engine - 120 dB • Threshold of pain - 120 dB • A gunshot or firecracker - 140 dB • Jet engine 15 miles away – 140 dB• Instant perforation of the ear drum – 160 dB

EchoesEchoesEchoesEchoesEchoesEchoes

Sound waves bouncing off a surface.

Best when sound bounces off smooth surfaces.

Which provides a better echo? An empty house, or a house full of furniture and carpeting?

Students will describe how sound is caused by vibrations, how it is transmitted through a medium.

Students will also compare the speed of sound in different media, explore the

doppler effect, and explain how echoes are produced.

Students will discuss the range of hearing for humans, and the threshold of pain.