Usually when we listen to sounds they appear to come from a particular location in space. However, when we listen to sounds over headphones the source appears as if it were inside our head. We are looking at what is different in these two listening situations that results in our different perception of the sound location. The most basic piece of information that we need is how the outer ear changes the sound waves under both free field conditions and when listening to sounds over headphones. To do this we place a small probe coupled to a miniature microphone in the ear canal and record a test sound that is presented for a large number of locations about the subjects head or over headphones
We make an impression of the outer part of the ear using a moulding silicone and use this to construct a small insert that fits into the outer part of the ear canal much like an 'in ear' hearing aid. This is used to guide the miniature microphone probe into the ear canal to a position about 6mm from the eardrum where we can make the acoustic measurements. By using an acoustic techniques we are able to determine the exact location of the microphone probe within the ear canal at all times. After the probes have been placed in both ears, the subject sits in the middle of an anechoic chamber while the test speaker is moved around in space about a metre form his/her head and the test measurements are made.
The recordings obtained using this method are referred to as the head related transfer function (HRTF). A considerable amount of morphometric data about the outer ear, their position on the head and the size of the head are also collected. Some of the objectives of this work are
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