Generation of Descriptive Terms

A number of reproduction systems were utilised from mono to a 3D surround system in order to elicit a full range of descriptors from the listening panel. The 3D system that was chosen is based on the previous experiments, since it has shown the most consistent rating from listeners.

8.3.1 Reproduction Systems

The systems used were:

 Mono: speaker at 0°

 Stereo: two speakers at ±30°

 5.0: two speakers at ±30°, one at 0° and two rear channels at ±110°

 Octagon: eight speakers equally dispersed at 45° intervals starting at 0°

 Oct_Cube: eight speakers equally dispersed on the horizontal at 45° intervals starting

at 0° with four channels above and below starting at 45° and dispersed at 90° intervals and an elevation of ±35°

8.3.2 Test Environment

The test was conducted in a semi anechoic chamber which conforms to the ISO 3744, ISO 3745, and BS 4196 standards.

8.3.3 Array Dimensions

The radius of the horizontal arrays was 1.35m, however the height stands for the elevated speakers cannot occupy the same position so the height speakers were at a slightly larger radius 1.7m. Therefore a delay of 1.2ms was inserted into the horizontal channels in order to insure that the sounds arrive at the listening position at the same time.

8.3.4 Calibration of individual Speakers

Each of the individual speakers was calibrated to a level of 71.8dB(A) by using a Bruel and Kjaer 2238 sound level meter, similar to the previous envelopment test by driving each speaker with pink noise of -20dB and measuring the resultant output at the listening position. 71.8dB(A) was chosen as a comfortable listening level, since it was found by (Airo et al., 1996) that most comfortable listening levels ranged between 52 and 88dB(A) and that 69dB(A) was the average most comfortable listening level for music.

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8.3.5 Stimuli

The stimuli below were all recorded in live spaces, the jazz ensemble were recorded in a small reverberant church using a soundfield microphone with the players assembled around the microphone in a semi-circular arc. The remaining two other recordings were recorded using an Eigemike, which has been described previously in Chapter 3, Section 3.5.4 the applause was recorded during the Proms festival in the Royal Albert hall as part of the FASCINATE project (Thomas et al., 2011), whilst the choir were recorded in the orchestra hall at the University of Detmold2. See table 8.1. As previously mentioned the availability of 3D surround scenes limited the test material, therefore this is not an exhaustive number of stimuli, but it is hoped by reproducing these over various systems will elicit a full range of descriptors from the listening panel.

Sound

Scene Description

Jazz Ensemble

Sound sources around the frontal arc, room ambience from rear and height

channels

Applause

Sound of applause from all around with ambience and reflections from hall in height and rear channels (microphone placed among the many standing spectators)

Choir

Choir distributed across the frontal sound stage with ambience from hall in height and rear channels

Table 8.1 Sound sources used in elicitation

8.3.6 Ambisonic Decoding

To decode each of the sound scenes to the relevant systems the optimised decoding scheme described in Chapter 3, Section 3.5.14 was used. The jazz band sample was recorded with a soundfield microphone therefore could only be reproduced in 1st order. However the applause excerpt and the choir excerpt was recorded using the Eigenmike therefore could be reproduced in 1st and 3rd order where applicable. See Table 8.2.

2

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System Ambisonic Order Stimulus 1 Stimulus 2 Stimulus 3

Mono W Component Jazz Band Applause Choir

Stereo Blumlein Pair Jazz Band Applause Choir

5.1 1st Order Jazz Band Applause Choir

Octagon 1st Order Jazz Band Applause Choir

Octagon 3rd Order Applause Choir

Octagon_Cube 1st Order Jazz Band Applause Choir

Octagon_Cube

3rd Horizontal/ 2nd

Vertical Applause Choir

Table 8.2 Systems and Decoding Order Used

8.3.7 Level Alignment between Systems

In order to minimise level differences between systems a subjective level alignment was carried out, the same method described in Chapter 7, Section 7.9. This was carried out by utilising a pre-test where two listeners were asked to compare the level of the mono centre channel with a signal input at 0dB to each of the multichannel systems in turn and adjusting the multichannel system to be of the same relative level to that of the mono centre channel.

See Table 8.3. Stereo -2.8dB 5.0 -7.1dB Oct_1 -6.9dB Oct_3 -3dB Oct_Cube_1st Order -6.9dB Oct_Cube_3rd order -3.8dB

Table 8.3 Gains of respective systems relative to mono centre channel

As before all speakers were screened from view so listeners had no visual cues as to the loudspeaker configurations, using an acoustically transparent but visually opaque curtain.

8.3.8 Collection of Individual terms

In order to collect each of the listeners descriptive terms an interface created in MAX/MSP was used. This was given to the listeners on an iPad which was connected to the playback computer using a wireless network. This allowed listeners to audition and enter descriptive

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words for all of the samples. Further, this would mean that there were no acoustic obstructions between the listener and the speakers.

8.3.9 Participants

Ten listeners who were either acoustic researchers or lecturers in the acoustic department took part in the elicitation. These are listeners who have taken part in listening tests before so can be classed as selected assesors (Bech and Zacharov, 2006). In the selection of listeners it is desireable to have listeners who are experienced and can describe what they hear, since naïve listeners may not be able to communicate adequately what they hear. Only some of the listening panel have previously worked in spatial audio or would be expected to be knowledgeable in the spatial audio literature.

8.3.10 Instructions to Participants

Each listener was given a printed instruction sheet describing the nature of the experiment and their task See Appendix 7. In this it was described that they should firstly focus on describing the spatial aspects of each sound scene, but also if necessary describe any timbral aspects also. Participants could also ask questions about anything they did not understand.