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School-college Physics Notes: SOUND 5. Echoes and soundproofing

SOUND  5. Echoes are reflections of sound waves bouncing off a hard surface - reflection versus sound absorption (soundproofing) with respect to different surfaces

Doc Brown's Physics exam study revision notes

5. Echoes are reflections of sound waves bouncing off a hard surface

Reflection from surfaces

Sound waves are reflected of hard flat surfaces eg walls, but tend to be absorbed by rough soft surfaces eg like foam -used in ear protectors.

Note the difference in echoes between an empty bare room in a house and when it is carpeted and filled with furniture and curtains etc.

The sound is dispersed by both reflection (bouncing off surfaces) and diffraction (bending round corners)

Echoes are heard when you shout towards a hard flat surface and you then hear the reflected sound waves impacting on your inner ear drum.

The further away a reflecting surface is, the longer the time interval between your shout and hearing the echo.

If the wall or side of a mountain or valley is 340 m away, it is 2 seconds before you hear the echo (speed of sound 340 m/s and the sound has travelled 680 m).

If the reflecting surface is a km (1000 m) away, its about 6 seconds before hear the echo.

speed = total distance / total time, time = distance/speed, time = 2000/340 = 5.9 s

You can hear sounds from some distance throughout a building or even a wide area outside because the sound waves are reflected and bounced around by all hard flat surfaces BUT sound waves are also diffracted and can therefore bend round corners into your ear!

The further away you are from the sound source, the fainter it will sound for two reasons:

(i) on every reflection some of the sound wave energy is absorbed

(ii) waves naturally spread out from a central source.

Echoes can be used to measure the speed of sound in air in part 6

and see also uses of ultrasound in section - medical scanning and echo-location

Soundproofing - where you want to minimise sound wave reflection from surfaces (methods of sound insulation)

The energy transferred by sound waves is more readily absorbed the softer and rougher the surface material impacted by sound waves.

This is process is used to minimise reflection. i.e. echoes, where sound proofing (sound insulation) is required.

The same principle is used in ear defenders to prevent ear damage from loud sounds.

Rooms whose floors are covered in soft layered carpets are much quieter than wooden or tiled floors which will tend to reflect the sound waves allowing them to 'echo' around the house.

Apart from the home, soundproofing is used in noisy nightclubs, recording studios and airports.

Soundproofing materials include polystyrene foam and acoustic mineral wool, corrugated cardboard (like egg box materials)

Keywords, phrases and learning objectives for sound waves

Know that echoes are reflections of sound waves bouncing off hard surface/

Be able to discuss reflection versus absorption depending on the nature of the surface the sound wave strikes.

Be able to describe and explain soundproofing (sound insulation) and examples of where it is used.

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