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# LPH 105 W15 12 intro

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## Richard Datwyler

on 28 June 2016

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#### Transcript of LPH 105 W15 12 intro

Characteristics of sound
Decibel
Sources of sound
Interference
Doppler
Characteristics of sound
Speed of sound
Loudness - intensity
Pitch - frequency
Depends on density (skip) and temperature (here)
v = 343 m/s (20 C)
Conversion - big numbers - logarithmic
Intensity
v = .2 miles/s = 1 mile / 5 seconds
We mentioned intensity of a wave in the
last chapter

We have a phenomenal range of intensities
that we can hear. From
12 orders of magnitude is
intense.

Thus we use a logarithmic scale to measure
sound levels.
Intensity
Sound level is calculated using a log scale.
But it needs a starting point.
Threshold of hearing.
Sound below this we can't hear.

It uses SI units of decibel dB
If I double the intensity
then the sound level (dB) doubles as well.

A. True
B. False

If one table saw produces 75 dB noise, what would be the sound level of three saws?
take my 75 dB saw, and do both
2 saws
and 4 saws.

8
16
....
Sound sources
So far we have just played the piano, guitar, violin.... stringed instruments.
Sound is heard by the moving of air molecules, eventually reaching our ears.
Strings were closed on both ends, having nodes at the end points.
A pipe allows the air molecules to move
the most at the ends (anti nodes)

but the pipe can be blocked off on one side
Demo
Demo
open open pipe
Open closed pipe
OVERTONES
HARMONICS
HARMONICS
Because of this we need two sets of equations for standing waves.

one for Open-Open pipes / closed-closed pipes/strings

one for Open-closed pipes
material moves
air moves
string moves
Air moves more, anti nodes,
Sound quality
If a flute plays and A and a clarinet plays an A they sound different, but the same...
Say a Flute plays an
A it would have the
Fundamental and
a few of these overtones
added in, and it looks like
this
A frequency analysis
looks like this:
It breaks the sound
down into its separate
frequencies and
plots their amplitudes
Superposition and interference.
you noticed that the waves 'added and subtracted' just right, such that it can cancel in some places, and add in others.
one source of sound might compress the air,
while another rarefacts it.

Together it cancels out
One dimension interference is nice
Two dimension a bit harder
if the difference between the "speakers" is 1 wave length....
is 1/2 a wave length....
if the difference between the
"path lengths to each speaker"
is 1 wave length...
is 1/2 a wave length...

Mathematically
Note these 'n' values are counting numbers, just like standing waves,
But they start at zero.
Simple demonstration of interference is Beat Frequency
If one frequency is a little different than another, an overall 'beat' frequency is seen or heard.

Demo
Math is simple

If one frequency is playing 334 Hz and the other is 332 Hz What is the beat frequency?
A. -2 Hz
B. -1 Hz
C. 0 Hz
D. 1 Hz
E. 2 Hz

If the sound emitted from two speaker both have a wavelength of .5 meters and the difference between the path lengths from the point in question to the two sources is 1.5 meter will there be:
A. Constructive interference
B. Deconstructive interference

Higher frequency in front, lower in the back
What if the observer moved either towards or away.

This is a Doppler shift as well.
"can you explain overtones question from the quiz? I also don't understand the doppler effect equations"
"When sound passes through different objects, like a solid to a liquid, what changes in it?"
"I don't really understand harmonics still."
"Can you describe the difference between an open - closed pipe and an open - open pipe?"
"can you explain the doppler effect?"
"Harmonics vs overtones?"
Full transcript