HRTF Measurements at IAC. 8/11/2002

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From the beginnings of our project we have made numerous HRTF functions measurements of more than 30 volunteers. In September of 1998 we began to do it with a semiautomatic system which we still used, awaiting having a totally automated system in which we will use a robot at the ITC.

During the morning of 8th of November 2002 we made, in our experimentation room at the IAC, a series of HRTF funcionts measuremens in which 3 volunteers participated. This session is described below, to illustrate how we measure these so important HRTF functions for our research.

Before all the team and volunteers arrived at the room, Mónica Sanchez and Miguel Crespillo was in charge to mark in the ground the positions in which would be to go placed the loudspeakers during the experiment, as well as of regulating the chari in height as much as the loudspeakers and fixing the direction of these.

Once all the elements were in its positions and reunited the rest the team (Antonio González, Enrique Burunat, Miguel Charcos and the volunteers Libertad M.L., Miriam R.A. and Alicia P.S.) the last adjustments in height of the chair (due to the different spread from each volunteer) and in direction of the loudspeakers became.

Next it was come to place the necktie microphones (Senheiser MKE 2) to the first volunteer (Libertad M.L.). Occluding the auditory channel with auditory protectors (Derby), the microphone was stucked with tape to the face and it was placed supported in the tragus, just at the entrance of the auditory channel. This one is the more critical stage of the test, because on the correct positioning of the microphones it depends the reproducebility and the success of the measures.

When the necktie microphones were already correctly located, it was come to place the circumaurals earpieces Senheiser, being had much taken care of of not modifying the position of the microphones.

From the control room sounds were sent to the volunteer, who could listen to them through the earpieces while she always watched to the front. A sound is sent to left channel of the earpieces and another one to the right channel. First this operation was made with the loudspeakers columns at distance of 1 meter and changing the direction of the chair in 5 occasions. Later it was made the same but locating the central column of loudspeakers to distances of 2 meters and soon to 4 meters.

Our team, from the control room, was verifying that the measures were correct. In case that it were not, it would be repeated.

The measures made with Libertad M.L. were good since the first attempt. It did not happen thus in the case of Alicia P.S. In the case of Alicia we had problems when placing the microphones, because there was one microphote that was moving , which caused that the measures were not good. After three attempts we obtained a moderately good measurement with which we followed the experiment.

All the procedure previously described was repeated with the volunteers Alicia P.S. and Miriam R.A.

The results of these HRTF funcitions measurements kept for their later use in a RMF experiment. In this experiment we wil reproduce the processed sounds and they will be sent through earpieces to the same volunteers. We will try to visualize the zones of the brain that are stimulated with these sounds.

We want to thank to Libertad, Alicia and Miriam for the aid in this test.