1. The Spatial perception of sound<br />Introduction<br />Many people thought two ears were like having two kidneys or lungs. If something went wrong with one the other could still function. In 1907 a physicist, Lord Rayleigh, conducted an experiment to prove his theory that two ears work together in binaural localisation. People stood round him and tapped tuning forks; Lord Rayleigh stood in the middle with his eyes close and could locate the sound sourced source, proving his theory.<br />Two important features in sound localisation are interaural time difference (ITD) and interaural intensity difference (IID). ITD is produced as it takes longer for the signal to reach the more distant ear and therefore is delayed. IID is produced because our head blocks some of the energy that would have reached the far ear and therefore it is less intense in this ear.<br />Another important feature for locating sound is the pinna. This is the visible part of our ear that resides outside the head to act like a funnel to collect sound.<br />This experiment will hopefully prove Lord Rayleigh’s theory of sound localisation and allow us to learn how our ears locate sounds.<br />Practical<br />Before starting the practical we made sure all levels were at zero before plugging equipment in. Once we did this we also set a sensible listening volume so we cause no damage to our ears.<br />To perform the experiment we used a dummy head with an omni-directional microphone in each ear. The signal received by the microphone was sent to an amplifier and into headphones.<br />The diagram follow shows the setup we used.<br />HEADPHONESAMPLIFIERAMPLIFIERMICROPHONE IN EACH EAR <br />DUMMY-HEAD<br />For the experiment we will need three people. One person will click their fingers around the dummy head so the microphone will pick up the signal. Another person will plot the location of the sound and the final person will stand in another room with headphones on and plot where they believe the sound is coming from.<br />On the diagram I plotted my results, the actual results and a friend’s result to compare them.<br />Results in red were the sixth points I plotted, my friends results were in blue and the actual results were the points the sound source was actually from.<br />From the results locating the sound left and right were very easy. This is because of interaural intensity difference. For example if a sound if coming from the right, the sound will be less intense in the left ear. Therefore we know the sound is coming from the right and our brain can recognises this and we can locate the sound.<br />MORE INTENSE IN CLOSER EARLESS INTENSE IN FAR EAR<br />Another factor of Intensity differences is the degree to which the head casts a sound shadow. The greater the sound shadow cast by the head, the greater the level difference between the ears. The extent of the sound shadow cast by the head depends on the frequency of the sound.<br />Low-frequency sounds can bend around the head as they have a wavelength that is long compared to the size of the head, therefore there is little sound shadow cast. High-frequency sounds have a short wavelength which means the head casts a significant sound shadow.<br />This is shown in the diagram below.<br />http://openlearn.open.ac.uk/course/view.php?id=3373&topic=all<br />We can also easily locate sound left and right due to interaural time difference. If a sudden noise comes at you from the right, it will reach your right ear before it reaches your left ear. It is this delay that enables us to localise the source of a sound from left to right.<br />SOUND DELAYED IN THIS EARREACHES THIS EAR FIRST<br />Although easy to locate sound in a horizontal plane it was not the case locating sounds from the front and rear forwards and backwards. The results were very inaccurate when plotting where we heard the sound from. This is because the interaural time difference and interaural intensity difference do not help us locate sound forwards and backwards.<br />Interaural time difference does not apply because a sound coming from directly in front there will be no delay in each ear.<br />The interaural intensity difference will also not apply as a sound source from directly in front will have the same intensity in each ear.<br />BOTH EARS ARE RECIEVING THE SAME INTENSITY AND AT THE SAME TIME<br />The results also showed how difficult it is to locate sound source on a vertical plane. Most of mine and my friend’s results were incorrect.<br />IID and ITD are also not in use with the height of a sound source which explains why height was difficult for us to locate.<br />The major factor for sound localization from height and forward and backwards is head related transfer function. This is the reason we can locate sound in three directions. The brain, inner ear and pinna work together to derive information. This allows the direction of sounds to be perceived.<br />This explains why it was difficult to perceive sound during our experiment. The dummy-head has a pinna which is different mine. This means our own head related transfer function can not apply as it is something we develop our self.<br />Here we can see the different from the dummy head’s pinna to my own pinna.<br />Conclusion<br />This experiment has shown me that stereo gives a much better perception of the sound than mono.<br />It has taught me about binaural recording and how I would go about doing a binaural recording myself. This effect can be seen in popular viral video “Virtual barber shop” which has received over 6 million hits on popular video sharing website YouTube. The audio is binaural so when listening on headphone it sounds like the barber is speaking and cutting around your head.<br />Rrrasrd<br />This could be recreated by using the set up we used and recording us moving around the dummies head.<br />ReferencesCMU: Binaural Sound Localization [Online]Available at: http://www.cs.cmu.edu/afs/cs/user/robust/www/Papers/SternWangBrownChapter.pdf<br />[Accessed 1st December 2009]<br />Rossing, Moore & Wheeler - The Science of sound third edition, p. 585[Accessed 1st December 2009]<br />NC State: Sound Localization [Online]<br />Available at: http://www.ise.ncsu.edu/kay/msf/sound.htm<br />[Accessed 1st December 2009]<br />Wikipedia: Pinna [Online]Available at: en.wikipedia.org/wiki/Pinna_(anatomy)<br />[Accessed 2nd December 2009]<br />F. Alton Everest & Ken C. Pohlmann - Master Handbook of Acoustics, p.59[Accessed 3rd December 2009]Jens Blauert - Spatial hearing: the psychophysics of human sound localization p.50<br />[Accessed 3rd December 2009]<br />