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# Acoustic interferometer

##### Demonstration

waste pipe is used to illustrate interference of sound waves coming from a single source by two different paths. By identifying nodes and antinodes, you can find the speed of sound.

#### Apparatus and materials

1.25 inch waster pipe, 2m

1.5 inch waste pipe, 1m

90 degree bends, 2 of each size (2 x 1.25inch + 2 x 1.5 inch)

1.25inch T-pieces, 2

Transparent scale of paper scale

Signal generator and loudspeaker

Small microphone and oscilloscope )optional)

metre rule

#### Health & Safety and Technical notes

Use a frequency range of 1k-10kHz.

#### Procedure

a Connect up the pipes as shown in the photo. Fix the metre rule parallel to the sliding tube.

b Put a small speaker operating at about 2 kHz under the bottom T-piece, and a detector (your ear or a microphone and CRO) near the top T-piece.

c Starting with the slider fully closed, carefully move it out until you find the first position of minimum intensity (node). Record the position of the slider.

d Continue moving the slider out, until you reach the next nodal position. Again, record the position of the slider.

e Repeat step d, recording the position of the slider at each node you find.

f Plot a graph of slide position against node number. The gradient of this graph is the distance between nodes = 0.5 wavelength.

g To calculate the speed of sound, multiply wavelength by frequency.

#### Teaching notes

1 This is known as the Quincke's tube method for finding the speed of sound. Students are interested by the unusual use of waste pipes and the clear pattern of nodes and antinodes.

2 The sound from the speaker has two possible paths to the detector. One path (pipe A) has a fixed length and so at the detector the phase of the wave travelling through pipe A does not change.

The path along the side with the slider (pipe B) changes length as you move the slider, and so at the detector the phase of the wave travelling through pipe B does change. Nodal positions occur when waves reaching the detector by the two different pipes are in anti-phase (compression from one path meets rarefaction from the other).

As you extend pipe B, the distance between positions of that wave in the same phase is one wavelength. This is twice the distance the pipe is extended, because both sides of the pipe extend at the same time.

Note: there is not a standing wave pattern in the pipes.

3 You could repeat the whole procedure for other values of frequency.

This experiment was submitted by David Ferguson, the physics technician at Uppingham School.