Small multichannel control rooms for broadcast
DONATO MASCI, in collaboration with Genelec’s AKI MÄKIVIRTA, illustrates some of the solutions he has designed and the results obtained from small multichannel control rooms.
In the world of broadcast, postproduction and audio for video is always chasing new technologies yet when I visit these types of facility I come across innovative technological solutions built in to acoustic environments that have a lower quality than the monitoring standards I am used to in music studios. The worst problems stem from the fact that, except for large mix theatres, audio for video is increasingly produced in very small rooms — often with inappropriate dimensional proportions, such as square rooms or in OB vans. These rooms often do not use the same monitor type for all channels — at least for 5.1. In general, it is much more difficult to design a small room than a larger room with traditional dimensions built using a classic design (NonEnvironment, LEDE, RFZ, etc. that imply certain minimum dimensions). This is because room simulation methods are not typically used when designing control rooms having standard designs. The traditional acoustic modelling software based on ray-tracing methods does not properly work at low frequencies where the fundamental problems in small spaces obviously occur.
Yet the market demands that facilities adapt to smaller, usually very urban, working environments that have to look good enough to host client quality checks and sound good enough to pass client’s stricter QC departments. In this article I will illustrate some of the solutions I have designed and the results obtained from small multichannel control rooms, explain the project’s ideas, the design steps and the problems that I have encountered.
The problems of designing small rooms are the same as with larger rooms, but need more attention because the problems can be larger due to the small room size. To understand how a control room works you observe what happens in the acoustic eld generated by the monitors. The good features of rooms do not only depend on their passive acoustic properties, like absorption and diffusion, which determine the reverberant eld. Acoustic parameters (such as reverberation time etc.) can easily be optimised by choosing materials with proper absorption or diffusion characteristics. You can do this with the help of the Sabine or, even better, the Eyring law. It is challenging to understand some devices’ absorption properties, such as those for a bass-trap. The neutrality of the sound at the listening position depends largely on early re ections and the interaction between the sound source and the room. A studio monitor is usually considered neutral when the frequency response deviation is at to about ±2dB measured in an anechoic room.
An optimal reverberation time is not enough for a control room to be considered ‘professional’. The real target of creating the necessary neutrality for monitoring is obtained by having a at frequency response and the optimal values for the room’s acoustic parameters (reverberation times, early re ections, etc.). That’s why, in order to understand the quality of a control room, all paths that sound takes when leaving the monitor, through re ections in the room, to the listening area must be carefully analysed.
One of the key points in small room design is the study of low frequency reproduction and therefore the modal resonance analysis. This is a complex and specialised subject. Books can teach you to calculate the mode distribution showing locations of SPL maxima and minima for a rectangular room and there has been much discussion on how to choose optimal room dimensions to distribute the mode resonances evenly in frequency. Unfortunately in most cases the rooms are neither rectangular nor can their dimensions be easily chosen with an optimal dimensional ratio. This is not necessarily a big problem because the amount of sound absorption needed in these environments is considerable to achieve acoustic performance in accordance with AES, EBU, and ITU guidelines. This makes the choice of the optimal room dimensions less of a problem. It is also very dif cult to simulate standing wave distributions in a room because of varying geometry and the real-life acoustic impedance of the wall. Even with strong sound-proo ng, the walls can never be assumed rigid, especially when using a drywall construction.
The most practical way to address these problems is through the use of a FEM ( nite element method) simulator such as Comsol Multiphysics, which can also perform optimisations. This can provide important information about isolating layers, positioning of the sound-absorbing materials, and the optimal listening position.
Another key aspect of the room design is isolation between adjacent rooms. Usually rooms are placed close together to optimise space but have to operate independently and without sound leakage. Control rooms often share a vocal booth that is generally small with a high level of isolation. Isolation is the simple part of studio acoustics but determines the quality evaluation of the rooms. Choosing the right partition strategy is not enough, you have to pay attention to construction details, such as ceiling suspension with vibration damping hangers and the oating oor dampers, the HVAC system and audio cabling. All of them can transport noise between rooms.
The vertical partitions and ceilings are built with drywalls made with plasterboard, gypsum board, with different types of rubber and absorbent materials. Regarding the oating oors, the target is to build an ef cient ‘mass-spring’ system that has a resonance frequency at least half of the lowest vibration frequency you want to damp. To lower this frequency you add mass and this is the reason why instead of multilayered drywall ooring with MDF top and cork, rubber and mineral wool suspension, a concrete slab placed on isopads is preferred.
An important aspect of the acoustics is the interaction with the re ective surfaces that are always present in a control room. When the listening position is very close to the monitors it may seem simple to obtain a at frequency response but nearby hard surfaces can generate dips and comb lter effects. Aside from optimised acoustics, the room also needs good ergonomics for typically two people. We prefer to use projection on an acoustically transparent screen rather than a at TV screen, which is a re ective surface. In my opinion, the traditional quadratic residue diffusers (QRD or Skyline) are not suitable for small rooms because the distance between the listening position and the diffuser is not large enough to feel the diffusion effect. In small rooms my choice is usually to use a binary amplitude diffuser (E.C. Payne-Johnson, G.A. Gehring, and J. Angus — Improvements to Binary Amplitude Diffusers, Audio Engineering Society Convention Paper 7143). This is an absorber-resonator device with a panel featuring pseudorandom perforations, placed in front of a thick layer of sound absorbing material. This device provides good sound diffusion even at short distances and the effect extends over a much larger range of frequencies.
The last key aspect is choosing and placing the studio monitors. As discussed in my article Monitors in a room (Resolution V13.3), ush mounting can be really useful, even for small rooms as it eliminates many diffraction effects and improves the monitors’ low frequency output. With ush mounting it is also easier to insert a projection screen. However, you have to pay attention to the design of the baf e wall because the front wall is not the only location for sound sources as often the surround monitor interaction with the baffle wall in the front is problematic. In these cases it is preferable to finish the baffle wall with absorbent material. For stereo control rooms I usually prefer reflective finishes.
The studio monitor choice is challenging. In some applications 3-way monitors are needed, for example in postproduction for control of the midrange frequencies in voices. Then you have to pay attention to the distance from the listening position. Let me illustrate some of the factors affecting listening at close range and to help you understand if the setup is OK and why.
The Figures show how the relative listening direction becomes increasingly different for a listener when he approaches the monitor. This will increase problems with atness when the listener moves off-axis to some drivers (Figure 1); timing error between drivers particularly at the crossover frequencies; and the de nition of being on-axis becomes narrower when you approach the monitor (Figure 2). Driver directivity changes the frequency response in off-axis listening (Figure 3); the actual layout of the drivers on the front baf e can affect being more off-axis with certain drivers (Figure 4); when moving closer the direct sound to reverberation ratio changes so the in uence of reverberation goes down; and if there are diffraction effects in the monitor design, moving closer changes these in frequency, and their relative level compared to direct sound can change. Figure 5 shows the bene t of using a Directivity Control Waveguide. The DCW can maintain at frequency response off-axis and at close range can produce a more stable sound.
It is impossible to determine the minimum listening distance of a monitor by just looking at it as acoustical data and measurements are needed. Genelec 1238CFs were chosen for Fox International Channel UK Studios and these work at a range of 1.5m because the manufacturer has taken care of controlled directivity and minimised cabinet and other diffractions.
Case Study: beep! Studios, Rome – Gorilla room
This was the acoustic design of a 5.1 TV mixing room with a square area that was barely 9 sqm with the contractor Proaudio Consulting. The brief was challenging: they wanted top audio quality standards and unique aesthetics to meet the room’s name – it was used to host an ingest system called Gorilla and had to look like a jungle. The fundamental problem was to contain the annoying resonances of a square room. In a square room acoustic modes overlap creating very peaky maxima and minima and to minimise the effects an ‘old trick’ was used. We turn the room by 45° and the real improvement in this design is related to the front side of the monitoring system. Every corner in the room has its acoustic treatment with sound absorbing materials to give signi cant absorption at low frequencies. The front corner of the room has been closed with a 45° heavy multilayer structure with differing acoustic impedances. This is not possible with masonry, and MDF, plasterboard, and rubber were used.
This wall structure, as well as dampening typical resonances of the square room, also serves to minimise the effects of phase cancellation of the LCR speaker system so typical with a conventional room corner. I mentioned this effect in last year’s article Monitors in a room (Resolution V13.3). Applying the ITU circle with surround monitor angles of 120°, all monitors are now close to walls and the monitor arrangement is perfectly integrated with the ro25o0m structure.
The small console table is designed to minimise re ection with a circular armrest. Another possible improvement would be to make the table top slightly tilted, to reflect sound away from the listening point, but in this case it would not be ergonomic because the table is also used for notes and scripts. Auto-calibration can help by compensating the remaining interactions that otherwise can not be removed.
Proaudio’s project manager wanted to deliver a unique-looking room and this led me in collaboration with Valentina Cardinali to design palm shaped acoustic diffusers and to use a bamboo cane nish for a panel at the back of the ceiling.
Case study: FOX International Channels UK, London
This project was for the isolation and acoustic design of two 5.1 TV mixing rooms, sharing an isobooth and a video postproduction room, all to be hosted in a barely 42sqm area. The contractor was again Proaudio Consulting. The brief was close to impossible -– top audio quality standards and branded aesthetics and we were provided with guidelines to follow that were many inches thick. At a rst glance this was an almost hopeless assignment, especially considering that the volume needed for soundproo ng partitions had to be included in this space. I worked with Alessandro Travaglini, Fox’s resident sound supervisor and engineer in Rome from the very beginning, who was our client’s appointed technical interface.
We created a binocular-shaped shell to host twin 5.1 control rooms sharing an isobooth between (the postproduction room was easier to locate), optimising the use of the available space to the centimetre and considering all the technical requirements (D-Command, projector and projection screen, cabinets, extended table-console, 5.1, 2.0 and soundbar audio systems) as well as the ergonomics.
We chose a Genelec 5.1 audio system of three 1238CFs for LCR and two 8250As at the rear plus a 7270A subwoofer. This design was a great challenge and needed a careful study to understand if it was possible to insert 3-way monitors ush-mounted in such a small space. After a series of trial designs we noticed that the listening position was located about 1.5m from the monitors as the manufacturer’s minimum listening distance requirement and was enough to get a good listening experience. We rotated the DCWs because otherwise the tweeter would have been too high and would have been a problem for the projection screen placed in front of the monitors.
The front baffle wall is not only a technical element, but also a technical area for the cabling of the room. The baffle wall is partially covered and lled by damping material to avoid acoustic reflections of the surround speakers. At the back of the room there are the binary amplitude diffusers and absorbers I mentioned earlier, playing probably the most important role for creating the room acoustics.
The geometry of the side walls has been designed to not directly reflect audio waves to the listening position while creating a good view of the isobooth window. Another specular reflective surface was placed on the other side of the room to create symmetry with the window. This was used for the National Geographic rectangular logo and a Fox poster.
The ceiling has a thick absorbent layer and has reflective surfaces guiding waves coming from the front out towards the diffusers at the back, acting as a sort of geometrical ‘amplifier’ of the diffuser. The console was custom-designed to Travaglini’s concept and contained all the racks and workspace. For this reason the console has a very large surface and instead of tilting it I chose to cover specific areas with absorbing material to reduce first reflections from the L and R monitors. The reflections coming from the centre monitor are partially screened by the displays. This resulted in an extremely useful design. This project has been one of the most complicated I have ever dealt with but I considered it a challenge to overcome.
In conclusion, I’ve shown examples of how it is possible to build small control rooms for professional quality monitoring. Extreme care is needed at low frequencies and with acoustic reflections; a monitor auto-calibration system can reduce the inevitable remaining acoustic problems caused by reflections especially reflections from the console surface. However, I don’t want to be taken for an acoustician who prefers small rooms! Although these rooms sound very good and we are extremely proud of them, it is clear that working with traditionally sized rooms is much easier, the acoustic solutions are more natural and the performance results are higher. If you are not just looking at the acoustic parameters, this difference will be appreciated at the end of the work.
The next stage in the design of small monitoring rooms will surely be the inclusion of 3D immersive audio. Today there is no format that can be used in such small rooms as to-date only large mix theatres have these systems. But with the introduction of 3D audio in broadcast, for TV drama productions and sports events, the ITU recommendation for multichannel monitoring systems must be updated (or will need hybridisation) for systems such as Dolby Atmos and Auro-3D.
Donato would like to thank his business partner Francesca Bianco, Proaudio Consulting’s project manager; Christophe Anet and Aki Mäkivirta (Genelec); Valentina Cardinali and Roberto Magalotti (B&C Speakers); Matt Ward (Spark + Rumble Ltd) and his friend Christopher Martinuzzi for reviewing the text.