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
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
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 =
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
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
Soundproofing - where you want
to minimise sound wave reflection from surfaces (methods of sound
The energy transferred by sound waves is more
readily absorbed the softer and rougher the surface material impacted by
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
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)