Funnelling sound

Funnel hearing device
Funnel hearing device

These boost your outer ears and amplify sound. If you’re using them to amplify someone talking, they’re most impressive if you wear them back-to-front (the other way round to the picture). If you use them facing forward, then the effect is less dramatic because you’ll also be lip reading.

How to make it

You need:

  • 1x cheap pair of hearing defenders (e.g. only £6)
  • 2x right angled joints (for 21.5mm waste pipe)
  • 2x funnels (e.g. 18mm diameter)
  • 1x large drill bit (26mm for the connectors we used)
  • Plastic glue (e.g. from hot glue gun)
Harry the HATS modelling the funneling devices

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. Place the end of the funnels into the right-angled connectors and glue in place. If necessary seal around with Plasticine.

The science

The external part of the ear (pinna) has two functions:

  • It amplifies the sound (along with the ear canal, this boosts the sound by about 10-15 decibels from 1,500, – 7,500 Hz)
  • It helps us detect where sound is coming from. The reason the external part of the ear (pinna) is a strange shape is to help us sort out whether sound is coming from the front or the back. If the pinna was a simple cone, then sound from the back and front would be the same. The asymmetric shape makes the sound from the front and back slightly different.

Adding funnels helps amplify the sound, it’s like having giant pinna. Someone once remarked that listening through these devices is like being in an aquarium because the sound is muffled. The reason for this is that the sound is coloured by resonance. The graph below shows what we measured on manikin’s mics. At the left side you’ll see a series of six peaks, these are the resonances of the funnel where the sound is being especially amplified. The ambient sound has to pass through the funnels to reach your ears. Parts of the sound that are close to the resonant frequencies are amplified, hence you get a bit of a bass boost with the funnels.

Spectrum for sound of a crowd with (red) and without (blue) drilled ear defenders


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