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Auditory Illusions & Binaural Recording

  • Introduction
  • Video & Results
  • Materials
  • Procedure
  • Preparation & Notes
  • Download Files

Introduction

In these activities, students will:

tubeface
  1. Learn about the how the brain can tell the direction from which a sound originates by tapping a tube in different that is held up to both ears.
  2. Listen to recordings that have made use of the human physical anatomy to create a sense of direction in the sounds.
  3. Listen to recordings that demonstrate the dominance of one ear over the other for hearing high pitched sounds.

Interpreting the Direction from which a Sound Originates

dummyhead

The brain is extremely adept at interpreting the direction from which a sound originates. Without our reallizing it, the brain detects that sound arrives at our ears at very slightly different times and then subconciously signals that it came from the left or the right. The difference in the time it takes for the sound to get to one ear first in this exercise is approximately 8 milliseconds (8 one millionths of a second!), yet we can tell that it's source is closer to the right or left ear! These spacial cues and other more complex ones are reconstructed in a process known as binaural recording.

Binaural Recording

Holophonic and binaural recordings are both made using a technique in which stereo microphones are placed inside of a model human head. The model contains features of the human head with microphones placed inside 18 cm apart and facing outwards, like human ears. Sounds that are then recorded have subtle features (or nuances) that mimic the human experience of hearing and add a three-dimensional aspect to the recording. Binaural recording techniques date from the late 19th century but were not used extensively because of the expense of the equipment. Modern technological advances have made the costs of the technique a minor consideration. As a result, binaural recording has gained in popularity with recordings in the 1970's by Lou Reed and a CD released by Pearl Jam, entitled "Binaural" in which the technique was used in several tracks.

The term holophonic was introduced by Hugo Zuccarelli in a 1988 paper of the Journal of the Acoustical Society of America. Zuccarelli later patented a device designed to encode spatial relationships in sound. Zuccarelli sought to recreate interference patterns and acoustic effects generated inside the head with his recording method. The technique in practice is very similar to binaural recording, and it appears that there is some controversy as to whether the two techniques differ.

 

 

Where did this sound come from?
Without looking most people can tell which side of the tubing has been tapped, even very close to the center line.

How it works:
The brain can identify the ear that detects the sound first. This feat is quite amazing! Our hose is approximate 1.22 m long. If the tap occurs 5 cm the left of the center, the sound travels 117 cm to the left ear and 127 cm to the right. We can calculate* that it will take 551 microseconds for the sound to reach the left ear and 559 microseconds for it to reach the right! This difference is only 8 microseconds (8 millionths of a second)!

* (speed of sound in plastic is ~2270 m/s)

Binaural Recordings

Both of the recordings illusions require the use of headphones. The recordings are so vivid that the observer often experiences physical sensations that accompany the actions on the recording. These feelings demonstrate the collaboration between the sense of hearing and touch. Enjoy.

Cambiata Illusion cambiata

The Cambiata Illusion is created by playing alternating high and low notes simoultaneously in left and right speakers (top right). When the Cambiata Illusion is played most listeners will hear the higher tones on in one ear despite the fact that they are actually being played alternating in left and right ears, as illustrated in the bottom two lines. When the headphones are swapped, the listener will hear the high pitch tone in the same ear. Perception is influenced by how the brain organizes the information, which is often different in right-handed and left-handed observers. Right-handed listeners commonly perceive the displayed pattern; whereas there is more variability among left-handed observers. For many left-handed observes, the higher pitch tones are perceived in the left ear. More reading at Diana Deutch's website.

 

 

 

Materials

Where did the sound come from?

tubeface
  • 8 ft of 1" plastic tubing
  • 2 funnels to fit in the ends of the tube
  • pencils for tapping
  • duct tape

Audio illusion listening

  • stereo headphones
  • a computer to plug the headphones into
  • a headphones base so multiple students can listen at the same time (optional)
  • Audacity software for listening to .ogg audio files (Download)

 

Where did the sound come from?tubeface

  1. Hold the funnels up to your ears with the tubing hanging behind you.
  2. Have a partner lightly tap the tubing with a pencil. Don't look while they tap.
  3. Tell your partner which side of the tube was tapped.

How close to the center line can you get right?

 

Binaural Recordings

Listen to these recordings twice. First time listen to them without headphones or using only one ear piece. Then listen to the recording a second time using the headphones. Point your fingers in the direction of the sound.

  1. Shaking Matchbox
  1. Haircut

  2. Crumpling paper
    Additional binaural recordings can be found on the web. The wikipedia article has a good one with crumpling paper. You may need to download plugins to be able to listen to this recording.

Cambiata Illusion

The Cambiata Illusion described by Diana Deutsch illustrates how the brain can interpret patterns of sound differently. The detection of high and low pitch sounds can be influenced by handedness.

  1. Listen to the recording with headphones for 30 seconds and describe a pattern of high and low pitches. Which ear hears the higher pitch sound?
  2. Switch the headphones and listen again. Did the ear that heard the higher pitch change? Are you left-handed or right-handed?

  3. Listen to the recording with only one ear piece. Does the recording sound different? Describe how what you hear may be different from what you expected.

 

 

 

References

Shaking Matchbox Recording. This recording was made using the holophonic technique and was downloaded from the web but was credited to a Swiss Website (http://www.holophonic.ch/); however, the recording could not be found on their website.

Diana Deutsch's Website describes several auditory illusions.

Deutsch, D. Phantom Words, and Other Curiosities, 2003, La Jolla: Philomel Records.

Binaural Recording wikipedia article

 

Download files & Links

  1. Shaking Matchbox recording (.mov)
  2. Recording of crumpling paper, downloaded from wikipedia. (.oss file) Audacity software can open this file and can be downloaded for free.