Swanee Whistle (slide whistle)

Harry the HATS modelling the swanee whistle device

The sound has to pass through the pipe to reach the ear, and so is altered by the resonances of the pipe. You can easily change the length of the pipe on this device, which clearly reveals the effect of the pipe resonances. The video below will give a sense of what it sounds like to use this in a noisy crowd.

You will need:

  • 1x cheap pair of hearing defenders (e.g. only £6)
  • 1x right angled joints (for 21.5mm overflow waste pipe)
  • 1x length of 21.5 mm overflow waste pipe (e.g. 44cm)
  • 1x length of 25 mm round plastic conduit (e.g. 42cm)
  • 1x large drill bit (26mm for the connectors we used)
  • Plastic glue (e.g. from hot glue gun)

How to make it:

Pull the foam insert out of the ear defender. Carefully drill a hole in each of the ear defender cups. Push one right-angled connector into each of the cups. Glue the right angled connector into place (we used a hot glue gun). You need to make sure there is a good seal around the tube so sound doesn’t leak in. If that isn’t achieved by the glue, Plasticine can be used to ensure a seal. Cut a hole in the middle of the foam insert (for the end of the plastic connector) and put back in the cup.

The two pieces of pipe are just the right size for one to fit inside the other. Put the overflow waste pipe into the end of the right-angled connector. Slip the electricity conduit over the top. Pull it up and down to change the length of the listening tube.

The science

Like a trombone, the pipe in this device has a series of resonances. The sound reaches the ear through the pipe, so you’re listening through the resonances. Any parts of the crowd hubbub that are close to one of the resonant frequencies gets amplified. By changing the length of the pipe, you change the amplified frequencies, and hence the whooshing sound shifts about. Below you can see what frequencies were present in the sound picked up by the microphones in the manikin’s ears. The yellow lines that form a ‘W’ are the resonant frequencies of the pipe. So the pipe was pulled up and down twice in the video, causing the resonances to drop in frequency twice.

Spectrogram showing how the sound changes as the pipe was moved up and down

2 Comments

Outreach event at school for the hearing impaired | Acoustical Society of America- Penn State Chapter · August 15, 2019 at 3:24 pm

[…] and cello while discussing string instrument design and mode shapes. They also got to try out confusaphones to show how much we rely on our hearing to localize our surroundings. The chapter had a great time […]

PSU ASA at Grange Fair 2019 – Ask a Scientist | Acoustical Society of America- Penn State Chapter · September 6, 2019 at 9:30 pm

[…] was excited to interface with a lot of kids & adults from the local area. We showed off the Confusaphones again, as well as demonstrated and explained the basic concepts behind Active Noise Cancellation […]

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