LA SOLIDARIDAD EL VALOR FUNDANTE DE NUEVOS DERECHOS

The current listening tests have been carried out in a small anechoic chamber located in Rayleigh building at the University of Southampton, which approximately measures

seat is positioned, all the surfaces of the room are treated with absorptive foam wedges to prevent sound reflections. Although the result of any detailed acoustic survey is not available, it is supposed that the test room can be regarded as being reasonably anechoic

down to 200∼300 Hz. This chamber is annexed by a control room where most of the

equipment was placed, and the experimenter had a CCTV facility to monitor the subject inside the room.

An array of 19 loudspeakers has been located in the test room at every 10◦ _{from 0}◦ _to

180◦ _{with respect to the room coordinate system shown in Fig. 6.2(a). (The manufac-}

turer’s data sheet claims that the frequency response of the transducer unit is reliable

from 100 Hz to 20 kHz within ±2 dB.) The height of the array has been approximately

adjusted to the average height of the subjects, where the radius from the loudspeaker to the array centre was measured to be 1.5 m. Since the visual cues given by the loud- speakers can bias the subjective judgments of acoustic image locations, the array has been covered by thin black curtains with rigid metal wires placed on top of each loud- speaker unit (see Fig. 6.3), which was extended beyond the loudspeakers at both ends

approximately by∼20cm. In this way, empty space between loudspeakers can be fully

disguised, and therefore, the locations of loudspeakers may not be recognised by the subjects.

A switch box has been custom-made for the current listening test, which is equipped with a micro-processor to separately route two-channel output from the PC soundcard to a selected pair of loudspeakers. While the micro-processor also receives a signal from a push-button that enables the subject to notify that he/she made a judgement, the switch box contains a built-in amplifier to provide sufficient power to the loudspeakers. A serial port has been used for the communication between the switch box and the PC with Matlab 7.0.

The source signal presented to the listeners is identical to what has been employed in sections 5.3.2 and 5.3.3 for the model simulation, where 6-ms rise-fall ramps have been applied to a 100-ms white Gaussian noise in a total of 150-ms sequence [see Fig. 5.11(b)].

The amplitude gains, g₁ and g₂, which were randomly given by the test design, have

been then applied to this source signal, and the 2-channel output signals have been finally generated by a soundcard with D/A converter (RME ADI-2). When a single loudspeaker is used with a unit gain, the sound pressure level has been calibrated at the centre of the array to be 70 dBA.

FASTRAK, has been employed in the current listening tests, which consists of a trans- mitter and a receiver, both connected to a control box. The transmitter can be consid- ered as the origin of the coordinate system which, in this test, has been placed at the cen- tre of the loudspeaker array below the subject’s seat [see Fig. 6.3(a)], and the control box

obtained translational and angular positions of the receiver, (x, y, z,azimuth,elevation,roll)

relative to the transmitter. Since the current listening test is aimed to investigate the

subjective perception of image locations on the horizontal plane, onlyx,y and azimuth

information have been used.

Instead of wearing the receiver on the head, subjects held a wooden wand shown in Fig. 6.4(a) where the receiver and a laser pointer were attached to each end. In order to report the image location, the subject directed this device to where he/she perceived the acoustic image, and switched on the laser pointer to make a visible mark on the black curtain. Then, the subject pressed the push button [see Fig 6.4(b)], which triggered the control box of the head-tracker to send a single reading to the PC. Since the azimuth

angle, θ0

p given by the head-tracker is not identical to the perceived image position, θp

as illustrated in Fig. 6.4(c), a vector sum has been computed from the position vector

(x, y) and the radius of the loudspeaker array.

A total of 10 university personnel (7 male and 3 female) have been paid for their partic- ipation in the current listening test, who are identified in the following sections as SA, SC, SD, SE, SF, SG, SH, SI, SJ and SK. The first five subjects also participated in the HRTF measurements (chapter 3) and the listening test for the lateralisation of dichotic pure tones (chapter 4), while the distal-region HRTF has been additionally measured for the subject SG (not presented in chapter 3). In pure tone audiometry which has been carried out recently or during the course of the current test, all participants have shown acceptable hearing ability across the audible frequency (less than 20dB hearing level).

This experimental study has been approved by the Safety and Ethics Committee of the Institute of Sound and Vibration Research (ISVR), University of Southampton (Ap- proval number: 774).