Twin speakers interference pattern.
Using two speakers, driven by a frequency generator, you can create
an audible interference pattern that students can walk through and
get a vivid experience of nodal and antinodal lines.
You need
Function Generator ( the orange Unilab one is excellent)
Two decent speakers (identical is best) ( 10 cm is fine)
Extension cable ( minimum 10 m )
An RCD ( for safety - you will be on a field)
Maybe an amplifier and a multiplug box.
Pair of stools ( to stand the speakers on)
The demonstration has to be outside, on a grassy field, at least 10 m
from buildings . We have found that reflections from hard court areas
and from walls mask the effect we are looking for.
Mains
RCD
10m
Function
Generator
INSIDE
OUTSIDE
on field
The speakers should be about 1 m apart
The function generator should have its frequency set for about
3.3 kHz, which gives a wavelength of about 0.1m ( and is quite
unpleasant and memorable)
The listeners should be about 10m in front of the speakers and moving
up (or down) the page.
The effect is quite spectacular (should that be auracular?) and well
worth the effort.
Consider "mapping" complete nodal lines with a line of students to
see if they can observe the hyperbolic curve of the lines.
Discuss and then demonstrate the effect of altering the separation of
the speakers . What is the effect of altering the frequency?
Run across the sound field to hear beat like effects.
You could stay out all day!
Finally, in the old Barber and Osborne "Advanced Senior Physics"
page 280, is a description of a twin speaker demo where one speaker
is placed behind the other and the distance set at half a wavelength.
This should result in quiet for ALL the listeners in front of the
speakers, as the two signals will be half a wavelength out of step.
We haven't tried this yet.
Student recording log sheet
Name.........................
Class..................
Date..................
Interference Pattern from twin speakers
Sketch the anti-nodal lines
d
L
listener
Speed of sound (c)
Distance from speakers (L)
Distance between speakers (d)
Distance from one node to the next (x)
Frequency (read from generator) (f)
=
=
=
=
=
(Watch your units)
The wavelength () of the sound can be calculated in two ways
Using the interference pattern,  = x d
L
Using wave equation
=c
f
=
=
Comment on these results ...................................................................
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The Laws of Thermodynamics - You cannot win. You cannot break even. You cannot get out of the game. (Anon)