Acoustic standing waves

Introduction

All waves can be reflected. This means that a wave that is reflected back onto itself will ‘interfere’ with itself. This interference will be both constructive & destructive, which will give rise to points of small or no activity (nodes) and points of large activity (antinodes).

This experiment will use the microphone on a DrDAQ to measure the intensity of sound produced by superposition of a transmitted sound wave and a reflected sound wave, giving a clear picture of the standing wave.

The experiment will also give a good insight into potential dividers.

Equipment required

• A PC with PicoLog data logging software installed
• A DrDAQ data logger
• A sound source with a frequency of approx 1 kHz
• 1m board, with thick (18 swg) unsheathed wire mounted along its length
• A supply of thin (24 swg) hook-up wire
• A large wooden board to reflect the sound waves off
• 2 x 1.5 volt cells and a switch

Experiment setup

Connect the 2 cells and switch to the opposite ends of the wire mounted on the board to create the circuit pictured.

Connect the Dr.DAQ to the PC and load up the PicoLog software. Mount a piece of the thin hook-up wire (unsheathed) to the bottom of the DrDAQ (as pictured) and attach the other end to the ‘ground’ of the unit

It is important that this runs from the parallel port to the opposite end, so that your hand does not get in the way of the microphone when you move the DrDAQ down the board. Attach a ‘flying’ lead of over 1m to the voltage input of the DrDAQ and the other end to the start of the thick wire on the board.

Place the DrDAQ on the board so that the wire on the board and the wire on the underside of the DrDAQ are perpendicular and touching. This creates a potential divider; the voltage being read by the DrDAQ will be proportional to the distance the unit is from the start. This gives a means to measure the distance of the microphone from the sound source. At this point you should place the DrDAQ at both ends and note the voltage that corresponds to 0 m and 1 m from the source. (NB this circuit will place a high current drain on the batteries due to the small resistance, so the switch should only be closed while voltages are being measured).

Place the sound source at the start of the track and put the reflector at the end of the track.

Set the PicoLog software to record both voltage and sound intensity and make sure that ‘readings per sample’ is set to single. Record 200 samples at 100 ms intervals, this will give you 20 seconds to traverse the full length of the track.

Carrying out the experiment

Place the DrDAQ at the start of the track and close the switch. Start the PicoLog logging and slowly and firmly move the DrDAQ along the track (use the parallel cable, so that your hand is out of the way of the microphone). You should aim to take 15–20 seconds to move the full metre. If you finish the track before the 20 seconds you should keep the Dr.DAQ at the end of the wire but still connected to the board wire until PicoLog has finished logging. It is unimportant if you vary the speed with which you move down the track, as all you are measuring is position, though a slow steady pace will produce clearer results.

Questions and discussion of results

1. How would the graph change if the frequency of the sound source was increased / decreased?
2. Can you determine the wavelength of the source?
3. Why does the intensity of the antinodes reduce?
4. The nodes that are closest to the reflector are more ‘quiet’ — why?
5. What would the graph look like if there were no reflector? Try it!