Acoustics For Small Studios

Traditionally, film and television post production has been carried out in large purpose-built studios, which makes sense from an acoustic point of view; large rooms generally sound better than small rooms, explains Chris Walls from Level Acoustic Design.

Dolby has set minimum dimensional requirements for film post production studios to help ensure an accurate monitoring response and achieve better translation from studios to cinemas. For example, a Dolby certified studio mixing feature films must have a minimum finished floor area of 45m2; once acoustic treatment, partitioning and access are taken into account, the total space required can be more than 80m2 (860sq ft). And that is without a projection room, machine room, breakout space, reception...

High property and operating costs combined with the capital cost of building and equipping studios mean building large studios is not always financially viable. It is therefore not surprising that there is an increasing trend towards building smaller studios in which the majority of audio post production can be completed before moving to a larger facility for final tweaks and print masters.

There are however a few inherent acoustic limitations in small rooms which require particular attention during design if accurate monitoring and reasonable translation are to be achieved:

• Low frequency anomalies caused by room modes
• Colouration due to reflections from room boundaries and equipment
• Achieving a balanced energy response within the room

Room modes
The low frequency monitoring accuracy in a studio is largely dictated by the modal behaviour of the room. Room modes are frequencies whose wavelengths have a simple correlation with the room geometry and at these frequencies the amplitude response is significantly affected. Modes are generally quite spread out at low frequencies which gives rise to an uneven amplitude response, with some frequencies being supported by modes and others not. The number and density of modes increases with increasing frequency to a point where there are multiple modes at each frequency and the sound field is much more even. The frequency at which this happens is directly related to the room size and is lower for a larger room, meaning larger rooms naturally have a more even low frequency response.

It is quite common for very small studios to have pronounced modal behaviour into the low – mid frequency region. When designing small studios, it is therefore particularly important to deal with the prominent modal frequencies to ensure they do not adversely affect the monitoring response. Where space is limited, this can only practically be achieved using resonant absorbers such as membrane, Helmholtz or panel absorbers; foam and rockwool will do very little to help at these frequencies. It is possible to mitigate the worst of the room mode effects with careful design and this is key to achieving good translation between small studios, large studios and, in the case of film mixing, cinemas.

Reflections
Reflections arriving at the mix position will arrive sooner and be stronger in a small room than a large room due to the closer proximity of the partitions. Strong, early reflections will combine with the sound arriving directly from the loudspeakers and cause errors in the frequency response and stereo imaging. Acoustic treatment in small rooms must therefore be designed to attenuate early reflections such that they do not adversely affect the direct sound, which in practice means attenuating them by at least10dB relative to the direct sound. This requires careful consideration of the loudspeaker, mix position and acoustic treatment positioning.

Another reflection-based problem is the boundary interference effect whereby sound emanating from a loudspeaker is reflected by nearby surfaces (typically the wall behind) and arrives back at the loudspeaker with a 180° phase shift causing cancellation and a resulting notch in the frequency response. The notch frequency is dependent on the distance between the loudspeaker and the offending boundary. In a large room it is generally possible to position the loudspeakers far enough from any room boundary that the notch is below the speaker's cut-off frequency, however in small rooms the notch frequency will generally be in the 80Hz – 150Hz region and often there will be several notch frequencies relating to several nearby boundaries. Dealing with boundary interference effects in small rooms can be tricky but considered positioning of loudspeakers can generally give a satisfactory low frequency response. The best solution to the problem is to flush-mount the loudspeakers in a solid wall, thereby eliminating the problematic rearward radiation. However the loudspeakers typically specified for small studios are not normally suitable for flush mounting.

Achieving balanced energy
The solutions to small room problems generally involve using acoustic treatment to absorb sound in the room. It is important when designing the absorption in any room that a suitable balance of low, mid and high frequency energy is maintained. If a room, particularly a small room, is treated solely with foam or mineral wool panels (which absorb primarily high frequencies) it will tend to have a lot of low frequency energy and very little upper mid and high frequency energy. This is generally what has happened when people complain of rooms sounding "boomy".

Acoustic treatment in small studios should comprise low frequency absorption, generally in the form of resonant absorbers, and judiciously placed foam or mineral wool absorbers. The former will deal with prominent room modes and the latter with problematic reflections. Together they give an even balance of low and high frequency energy. This, combined with careful positioning of the loudspeakers and mix position, will result in a great sounding studio.

When it comes to room acoustics for audio studios, bigger is generally better. Designing small studios largely involves mitigating potential problems thrown up by the reduced size, however with careful design it is possible to achieve a small studio which enables accurate monitoring and translates well to larger studios and the final delivery format.

Level Acoustic Design provides high end acoustic design for the music, post & broadcast industries. Clients include: NBC Universal, YouTube, ITV, Goldcrest, Halo, Molinare, Deluxe Technicolor & Abbey Road Studios.

www.levelacousticdesign.com

This article is also available to read in the October edition of Broadcast Film & Video here, page 35.

(JP)