Transcript Music_and_Sound_More_Waves

More Waves in Music and
Sound
Decibels, Interference
and Doppler Effect
Measuring Sound
Intensity:
Sound "intensity" is
measured in Watts,
watts/m2 or in Decibels
(dB).
P0 = 10-12 W/m2 and is the
threshold of human
hearing. (The lowest
amplitude humans can
hear)
Wave Addition:
The displacement (maximum
displacement is amplitude) of two
similar Waves will add for the same
location and time.
If two waves are "out of phase"
they will cancel each other out with
"destructive" interference.
If two waves are in phase they will
have constructive interference.
The total amplitude will be the sum
of the two waves' amplitude
Destructive Interference
Constructive Interference
If waves have a different wavelength (different frequency)
They will add together in the same way, except they will
not remain in or out of phase.
If two notes have similar, but not identical frequencies (or
wavelengths) the time in or out of phase will be audible as
the tone getting softer and louder (interfering destructivly
then constructively) this is called a "beat frequency". The
closer the tones are to being the same frequency (or
wavelength) the longer the period of the beat frequency,
where the beat frequency will be the difference between
the two frequencies:
A guitar's "A" playes a tone of 436 Hz, and a tuning fork is
played at 440 Hz. The "beat frequency" is 4 Hz.
Doppler Effect
As a sound source moves while producing sound waves, the location of
each wave peak is modified by the amount and direction that the source
is moving. This will increase the wavelength (decrease the frequency)
behind the object and decrease the wavelength (increase the frequency)
in front of the object.
λ = original wavelength
λ’ = wavelength in front of moving sound source
λ’ = λ(1-vsource/v) (in front)
λbehind = λ(1+vsource/v)
http://en.wikipedia.org/wiki/Doppler_effect