Tomlinson Holman, in his book entitled “5.1 Surround Sound Up and Running”, discusses the room acoustic requirements for monitoring a 5.1 mix. He states,
” Multichannel affects the desired room acoustics of control rooms only in some areas. In particular, control over first reflections from each of the channels means that half-live, half-dead room acoustics are not useful. Acoustical designers concentrate on balancing diffusion and absorption in the various planes to produce a good result.” If live end/dead end is not useful as it is in two channel monitoring, what are our options with multiple channel monitoring in terms of acoustical treatment?
It appears that primary reflections off of our multichannel monitors are as much of a concern as they are in two channel. Primary reflections distort the more pure (without the room), direct sound from the monitors. The engineer wants to hear all the vocals and instruments from the speaker without the room reflections. Room reflections add the sound of the room into the mix and that is not acceptable. Near field monitoring is a process to minimize room sound by sitting closer to the speakers to hear the wanted direct sound. It is important in two channel audio and it is even more important with multiple channels that are all producing energy at the same time. No need to add room sound to 2 channels, let alone 5.
He states that acoustical designers, “concentrate on balancing diffusion and absorption in the various planes to produce a good result”. If we look at each plane or room boundary surface for two channel sound, we can get an idea on how to balance absorption and diffusion in a multiple channel presentation. The ceiling must be a balance of diffusion and absorption in two channel sound as well as multiple channel monitoring and playback. We definitely do not want ceiling reflections in our mix, just as we don’t want them in our two channel presentations.
The same will apply to all the room surfaces. A balance of diffusion and absorption will work for all the room boundary surfaces. Each surface should have the same amount and type of diffusion and absorption to provide the engineer with a uniform and consistent room sound. In two channel sound, we tend to absorb the primary reflections and use diffusion in other areas. With multiple channel monitoring consisting of multiple, mono sources, we must have a balanced acoustical field for all channels.
Low frequency issues must also be addressed, so that the low frequency effects channel can be monitored correctly and produces the necessary energy level to fill a room with explosions and other special effects. Muddy and bloated bass energy will only confuse the listener and smother the middle and high frequencies. Vocals will be lost in a low frequency muck that may prevent localization of the vocals with the image on the screen. Video movements on the screen must have a corresponding audio movement from channel to channel. Low frequency absorbers whether freestanding or built in are a must.