Situación de acción: apropiación de las tierras comunales 97

Stage 1 - Generation and Sounding of the MLS Signal

A 16383-point maximum length sequence (MLS) signal was generated at a sample rate of 44.1 kHz with a 16-bit resolution. On sounding the signal, eight periods of the MLS signal were generated to increase the signal to noise ratio. The signal was sounded using a PC with a multichannel soundcard connected to a digital to analogue converter. The analogue output signal was inputted to a power amplifier that was

connected to a dodecahedral omnidirectional loudspeaker placed at the centre of the stage at a height of 1.6m.

The measurements and recordings were taken in Peel Hall, a medium sized (384 seats) concert hall situated in the campus of the University of Salford. The hall is rectangular in shape with a semi-circular rear wall. The dimensions of the concert hall are approximately 20m wide, 35m long and 20m high with the seats arranged in 16 rows of 24 seats. The seats are set on a sloping surface with the front seats being at stage level and the rear seats being approximately 10m above the stage. A photograph of the hall, looking from the rear of the hall to the stage can be seen in Figure 4.9.

Stages 2A and 2B - Recording of the MLS Signal

The MLS signal was sounded twice to allow for two recordings of the signal. A Soundfield ST250 microphone was used for one of the recordings, allowing for B-format recording of the signal. A Bruel and Kjaer head and torso simulator (HATS) was used to record the binaural signals, allowing for the extraction of impulse responses from the MLS signals and ultimately for IACC measurements.

Using a multichannel soundcard, software and PC, simultaneous soundings and recordings of the MLS signal were taken at 24, evenly spaced seat positions and saved to the hard drive of the computer as 16- bit, 44.1 kHz PCM files. As the concert hall was symmetrical around the mid-line, measurements were only taken on the right-hand side (looking towards the stage) of the hall. Measurements were taken every fourth row (starting with the row nearest the stage and finishing on the last row) and every fifth seat (starting from the extreme right of the hall and finishing in the centre). This resulted in four measurements for each of the six rows.

Stage 3 – Sounding of the Decoded MLS Signal in the Reproduced

Concert Hall

The reproduced measurements took place in the semi-anechoic chamber of the School of Acoustics and Electronic Engineering, University of Salford. The decoded signals for each reproduction system and for each seat position were sounded. Whilst general ambisonic decoding details

can be seen in Section 2.3.8.2, details of the decodings of the B format signals for each system are shown in Appendix D.

The twelve loudspeakers required for auditioning the five non-periphonic systems were positioned upon stands, at head height and at distance of 1.15m from the listening position. The eight loudspeakers required for periphonic presentations were attached to a framework that surrounded the listening position. As the framework and the non-periphonic systems could not occupy the same space, the framework that supported the periphonic system arrangement did not form a perfect cube. In addition, the loudspeakers were placed a little further from the listening position (1.25m). To minimise reflections, the solid floor of the semi-anechoic room was covered with acoustically absorbent foam. All twenty of the loudspeakers used in the tests were Genelec 1029As that were level aligned using pink noise and a sound level meter. A photograph of the set up can be seen in Figure 4.10.

The decoded B format samples were replayed using a multichannel audio software package installed on a computer equipped with a multichannel soundcard that was connected to digital to analogue converters that, in turn, were connected to the loudspeakers. The computer and digital to analogue converters were located outside of the semi-anechoic chamber.

Stage 4 – Recording of Reproduced MLS Signal

The reproduced MLS signals were binaurally recorded in the same manner as described in Stage 2B.

Stage 5 - Extraction of the IACC Measures

Impulse responses were extracted from the binaural MLS recordings and then by utilising Equation 2.8 and octave band filtering, IACC and a number of variants of IACC were calculated for both the original and reproduced environments. These variations were included as certain

Figure 4.10 Photograph of the loudspeaker array used for replaying the MLS signals recorded in the concert hall. Also present is the dummy head used to binaurally

IACC measurement variations may be better suited to reproduced sound than others. The calculations were made using each IACC variant for the 24 seat positions in both the original and reproduced concert halls. The IACC variations (all of which have been previously used in concert hall acoustics) used in the test are listed below:

IACCFB - full bandwidth IACC with no time window

IACCE - full bandwidth IACC using a 0 to 80 ms time window

IACC3 - average of the 0.5k, 1k and 2k Hz octave bands with no

time window

IACCE3 - average of the 0.5k, 1k and 2k Hz octave bands using a 0

to 80 ms time window

IACCL3 - average of the 0.5k, 1k and 2k Hz octave bands using a

80 to 750 ms time window

Stage 6 – Comparison of Concert Hall to Reproduced Sound IACC

Measurements.

In order to evaluate the retention of spatial impression in each reproduction system, the original and reproduced IACC measurements were compared. A number of methods of comparison were implemented including correlation and SR. These are further discussed in the next sections.