Surround Sound at Strongroom - White Paper

STRONGROOM STUDIO TWO: SURROUND SOUND DESIGN CONSIDERATIONS

Spring 1998 

In May 1997, Strongroom studios approached White Mark Limited to comment on the acoustic performance of Studio Two at their Curtain Road Studios in London. The studio had invested in a 5.1 surround monitor system and Euphonix console in order to offer leading edge mixing capability in both the established stereo and emerging surround fields. It transpired that the room had difficulties born both of poor bass definition from the monitor system and difficulties with the console. Whilst significant upgrading was undertaken with the desk, the acoustic issues needed addressing and the feedback from considerable and informed client comment required careful integration. Strongroom’s MD Rob Buckler spent time carefully briefing the design team and the work was undertaken early in the summer. 

General Considerations

The design of control rooms for use in surround sound mixing raises a number of complexities over and above those normally encountered in acoustic design for stereo monitoring. Not only do these additional factors need careful consideration when designing for Dolby Stereo rooms with their L, C, R and single, band limited, surround channel, but more complex again are the more rigorous 6 or 8 channel systems such as those needed for the various standards relating to 5.1 or 7.1 be they administered by Dolby, Sony, DTS or THX. 

In the Dolby Stereo set up, the majority of image related signal is presented from the front with the single rear channel being reserved for atmospheric sounds such as rain, explosion reverberation and panning tails of moving images. Room design for these applications does not involve a significant departure from the basic tenants of good stereo monitoring environment design. These are held to be the minimisation of early reflections to produce a clean initial sound from the speakers followed by a clear reflection free period and as diffuse and even a reverberation tail as possible thereafter. The reverberant field decay should be even with frequency, to avoid unpleasant colouration of the environment. 

A practical approach

In practical design, the use of diffusion elements in the rear wall (coupled with a wide band absorptive front area) is often employed to satisfy these requirements. Additionally, everything possible should be done to maximise bilateral symmetry within the room. There is much interesting investigative work to be done in the Dolby Stereo monitoring field into the introduction of a rear facing rear monitor array for the surround channel and the use of wide band diffusion techniques to expand the virtual size of the monitoring environment. This has proved a more than acceptable technique thus far, although care must be taken that the frequency response of the combination of the rear speaker units and rear wall are is (the combination is singular) satisfactory. A more conventional approach is to use several small direct radiating units to produce the required diffuse response. 

It is, however, in the design of six channel monitoring environments that the requirements are fundamentally changed. What is required is true two-dimensional monitoring with full specification rear channel loudspeakers matched to those used both for front stereo monitoring and also to that used for the centre channel. A high specification sub bass unit makes up the sixth channel (or the "0.1" of the "5.1" generic name given to this set up). This has the practical effect of requiring that the performance of the front wall in both reflection suppression (and therefore in wide band absorption characteristics) and rigid monitor wall functions must be duplicated to the rear. Thus, the aim is that the image and monitor capability should be as good when using the rear speakers as it is when using the front. It can be seen that it becomes more difficult to produce the wide band diffusion surface usually employed to help with the creation of the even and uncoloured reverberation tail required. The risk is that the room becomes too dead in feel and moves toward the anechoic chambers of the past that were once perceived to be good monitoring environments by all, except those unfortunate enough to work in them. 

A further complication is the requirement for as consistent as possible a distance from the listening position to all of the individual loudspeakers. Thus, the ratio of the reverberant sound field to that of the direct sound field is maintained constant for all monitors. This is important in order to prevent any differential colouration effect of the room on the various units and consequent adverse effects on the accuracy of balance decisions. This, sadly, renders more difficult the creation of a large area of accurate monitoring within the room, especially if the reverberation characteristics are uneven or coloured or if the room is small. This should also be a factor when considering extending the room dimensions virtually by rear facing surround monitors. In the case of Dolby Stereo, the band-limited nature of the rear channels eases these difficulties. 

Results at the Strongroom

The recent application of these principals to the creation of Studio 2 at the Strongroom has produced a room that works very well as a stereo monitoring environment with excellent characteristics as a 5.1 monitoring facility. Although a fundamental aim of the redevelopment was to improve the stereo working environment, the opportunity was taken to include improvements to support the surround usage as well. The space is roughly square giving an even distance topology between the various monitors and the listening position and is fitted with a true matched array of 5.1 monitors. The set up of these monitors and their basic characteristics are very even indeed with the only difference being that the two main Left and Right units at the front are capable of delivering slightly higher power levels. This in keeping with the stereo monitoring requirements of the room when in its conventional music mixing mode and allows the high monitoring levels needed to be achieved. 

The nature of the front and rear walls of this room is basically similar, with their make up being based on a strong frame to rigidly support the monitors. This is principally to mount the loudspeakers to the room structure free of in band resonance that will selectively absorb energy otherwise output into the room. This usually manifests itself as a loss of bass energy coupled with a general unevenness of the low frequency response. An alternative is to suspend the units with an isolation system deigned to have a resonance well below the audio band but this also depends on a rigid front wall structure against which to work. It is also difficult to design a mount resonance that is sufficiently low. 

Monitor considerations

The monitors are fixed within heavily damped and tightly fitting loudspeaker enclosures closely set into a dead wall giving excellent wide band absorption and rigid, resonance free performance as a monitor boundary surface. Hard reflective areas are restricted to the side walls in the rear two thirds of the room and the necessary equipment racks, console and floor surfaces. The net result of this work has been a slightly dry monitor environment that is both colouration free and supportive of excellent surround mixing characteristics. 

The nature of the completed space places stringent demands on both the monitor system installed in the room and on its set up. As both ends of the room are configured as broadband absorbers, the reverberant field is considerably reduced and the monitor system has to produce generally higher output powers to give the same perceived sound levels. Further, as the principal difference in the characteristics from those of a conventional stereo room are in the lower end of the spectrum, this requires that the set up be done in situ and be permitted by the equipment’s adjustments and headroom to optimise for these changes. Thus, great care has to be taken in monitoring system selection and particular effort must be taken to line it up accurately once the room is completed. 

Conclusions arising from the project

The question has been raised whether there is a need to design using different and distinct criteria for rooms used for stereo music mixing and those used for the production of surround sound projects. It would appear that the design requirements are sufficiently different for this to be considered but that the excellent performance of Strongroom Studio 2 would tend to show that this is not the case. The advantages of being able to transfer projects from the music mixing stage into the post productions stage or, indeed, mix music directly into a surround format are many. The real world problems of justifying the expenditure on surround facilities appear to be addressed by the excellent stereo performance achieved coupled with the opening of new marketing possibilities for the studio into broader client bases. 

In this particular case the achieved characteristics have been very much to the taste of the Strongroom and their clients. There is further design interest in the possible installation of diffusion elements in the central portion of the rear wall. This would have effect on the spread of the stereo image when in use for straight stereo mixing and increase the perceived depth of the room in both modes of operation. In line with Strongroom’s policy of working with both it’s clients and consultants to allow development of facilities, the design of the room structure incorporated fixings for diffusion elements in the wall surfaces. In the near future, a period has been set aside for the installation of Diffractal units supplied for the purpose, and for a programme of measurement and subjective tests to research their effect. Separate analysis will be undertaken into changes in the two main operating modes of the room and on the set up of the monitors. Further exploration will therefore be possible of the optimisation of the function of the space and the production of quantified measurements will assist in the overall understanding of this interesting design issue.

For further information see:

• White Mark Clients: The Strongroom
• White Mark News: Strength to Strength