Control rooms

Just as the studio where the voices or instruments will be recorded must comply with adequate acoustic characteristics, the Control Room, place of evaluation of the material to be recorded, mixed or broadcast, must also adhere to very precise technical requirements.

The Control Room should be seen as a system composed of the sound radiator elements (monitoring speakers) and the acoustically conditioned cabin.

A pair of speakers does not sound the same in different acoustic conditions such as outdoors (open field), in a reverberated room or in an aneoic chamber (minimal reverberation). The monitoring conditions then depend on the quality of the speakers, their location in the control room, the ideal place or mixing point and obviously, the acoustic parameters of the room. With the advent of stereophony, control rooms had to demand even more in terms of the types of speakers and the acoustics and shape of the control room.

Going back a little to what was seen in the article STEREOPHONY (sept./oct. 99), let's think about the location of the monitoring system depending on the mixing point or ideal hearing. The "law of the equilateral triangle" demands an equal distance between the two speakers and the operator-auditor. The stereophonic image reproduced by the speakers can suffer "deformations" due to phase problems between the speakers that make up the speakers (crossover or crossover frequencies), for example in the case of three-way monitors (woofer, mid-range, tweeter) that handle the low, medium and high frequencies separately.

The correct phase alignment of the tracks and monitors with each other does not ensure a stable stereophonic image if we have problems with absorption or reflection of certain frequencies from the monitors. Initially it is important to have a monitoring chain with flat frequency responses (amplifiers, monitors and even room). The best monitoring condition is presented in "free field", that is, when the auditor receives mainly the direct sound from the monitors with a minimal influence on the response of these during the journey of the sound to the ears.

A good symmetry in the acoustic response on both sides of the room, fairly uniform reverberation time over a wide range of frequencies and a very low noise level (NC 15) are requirements recommended by the EBU- European Broadcasting Union, as well as room volumes of the order of 80 cubic meters.

In any case, the size of the cabin will depend on the type of work to be done, the amount of equipment and people present during the productions. Small cabins present acoustic problems due to the presence of standing waves of great magnitude, created by reflections in short trajectories.

The acoustic response of the control room must meet the following requirements:

1. Avoid standing waves by selecting a volume of not less than 80 cubic meters, which also achieves a uniform distribution of the resonance frequencies of the enclosure, which cause the reinforcement or attenuation of certain frequencies at different points in the room. Avoiding parallel walls also helps.
2. The reverberation time (RT 60) should be short, between 0.3 and 1 second, depending on whether it is monitoring voices or music. In general, music requires slightly longer times than solo voices. The RT 60 is defined as the time it takes for a short-lived sound to reduce its intensity by 60 decibels (dB). The RT 60 is directly proportional to the volume of the enclosure, inversely proportional to the surface area and inversely proportional to the amount of absorbent material.
3. The noise level inside the room should not exceed NC Noise Criterion 15 (approximately 15 to 20 dB A). The noise level is formed by the sum of the transmission noises (walls), structural noises of the construction, as well as the noises of the ventilation system and those that come from adjacent spaces and that arrive through the ducts. It is recommended that the studio and cabin be isolated from the structure of the building as if it were a drawer inside another drawer with floor, walls and ceilings "floating" on neoprene blocks, for example, and massive materials such as plaster sheets.

Returning to the monitoring system it is good to take into account the directional character of the medium-high and high frequencies, which come out of the speakers. That's why the radiation axes must be directed head-on to the operator to better take advantage of radiation efficiency over the entire frequency range.

The way of anchoring the monitors to the walls should avoid possible resonances by acoustic coupling. If the monitors are recessed in the front wall, it is recommended that they are surrounded by a high absorption zone or if they are suspended, more than one meter away from the nearest wall and also surrounded by absorbent walls to avoid low frequency reflections that can alter the stereophonic image.

In 1978, Don Davis created the LEDE (Live End-Dead End) concept that consists of conditioning the front of the control room with absorbent material and the back reflective or with some degree of diffusion. This area behind the operator would be responsible for producing the first reflections that naturally reinforce the sound (sounds that arrive between 10 and 30 milliseconds after the direct wave. To achieve this, the distance between the operator and the back wall must be at least 3 meters (6 meters of round trip of the reflected wave).

Finally, the quality of the monitoring and especially the comfort of hearing during working hours will be linked to the level of sound intensity that comes out of the monitors. Each operator defines his level according to experience and can change by several decibels with respect to another operator. The idea is to have a comfortable monitoring system, thus avoiding hearing fatigue and loss of hearing ability, and therefore, judging the sound material.