A 3/4 surround sound setup was aligned according to the ITU-R BS.775-3 recommendations for a seven-channel arrangement. All loudspeakers are at least one meter far from any adjacent room boundary to avoid loudspeaker-room interactions since the loudspeakers used in this example The front soundstage comprises a center channel and two loudspeakers aligned at ± 30° to the 0° center.
The base width between the front left/right speakers is 2 m. Four surround loudspeakers are chosen to ensure envelopment to a broader listening area whereas two surround channels are located at ± 60° and an additional pair of rear speakers are arranged at ± 135°.
The rear side channels are delayed to provide additional decorrelation among the surround channels. The height of the front soundstage is 1.2 m. The surround loudspeakers are tilted towards the listeners and placed at 1.4 m above floor level. The higher placed surround speakers ensure better-unobstructed sound radiation thus leading to a better envelopment for the listeners
Subwoofer Crossover Frequency
The subwoofer crossover frequency was set to 125 Hz because of adjustment possibilities limited to octave bands in Catt-acoustic. Usually the crossover frequency should be set to 80 Hz due to localization effects above 80 Hz and yet allows the subwoofer to be placed independently from the other speakers at any location where a smooth low frequency performance can be ensured. In case of program material including a low frequency effect channel (LFE) one might consider to set the gain for the LFE-channel 10 dB higher compared to the other channels to obtain more power in the low end which can be favourable due to the special low frequency sound effects that are reproduced through this channel.
Although the measured output level will possibly be only around 4 dB more due to the narrower bandwidth of this channel.
Sound Pressure Level
The operational sound pressure level of each of the 3/4 surround sound channels at the distance of 1 m was set to 80 dB. Considering the combined levels of seven incoherent sound sources will therefore result in approximately 88.5 dB.
However, due to propagation loss over distance, the sound declines at 6 dB per double distance for a point source. Therefore, the level at the listening area (2 m far from the source) will be lower than 88.5 dB but may decline at a lower rate in a room due to wall reflections.
The level alignment of each of the channels should be performed by using band-limited (200 Hz – 20 kHz; 20 – 120 Hz for the subwoofer) pink noise as a test signal.
The test signal has to be set to -18 dBFs and fed to each channel of the reproduction system. Thus, the gain of each channel has to be adjusted so that each channel causes the following sound pressure level Lref at the listening location:
where n is the number of channels. This results in a sound pressure level of 76.5 dB (1 m) for each channel in a 3/4 surround sound set up thus leading to a combined level of 85 dB (1 m) in the case of seven channels performing simultaneously.
Furthermore, reasonable listening levels in home theaters are given by Beusch. This recommendation can be seen as a guiding principle dependent on the screen size and the listening distance.
In this post, we went to the Loudspeaker Arrangement and examined its considerations. Items needed to mention audio improvements and other parameters will be published in future posts. These items will pave the way for creating a home theater with the highest quality.
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