Reverberant spaces: Bouncing off the walls

Delivering intelligible audio in tricky spaces requires clever analysis and should make use of a range of tools at a system designer’s disposal says Paul Mac.

When it comes to bad audio spaces, blame is more often than not laid at the feet of reverberation. To tackle the problem of reverberation, you need to first break it down into a range of contributing factors.

A starting point is to consider whether reverberation is truly undesirable for the specific space in question. In fact, a long reverb time might be desirable if you’re looking to preserve the feel of a hall, or make sure a church continues to feel ethereal. On the other hand, a conference room with even a smallish reverb time might cause issues for clients at the other end of the line.

Alistair Meachin, one of the consultants at acoustics and AV specialists Harmonia Consulting, and an AV consultant for Media Powerhouse, says: “In many commercial spaces, the intelligibility targets are defined by the requirements for voice alarm systems. That is normally limited by two things – the background noise, and unwanted reverberation.”

Intelligibility can be quantified and measured. STI (Speech Transmission Index) and STIPA (Speech Transmission Index for Public Address Systems) are the main barometers for our purposes and they use modulated frequencies spread across a wide bandwidth to represent voice. STI ratings range from 0 (‘No idea what you just said’), to 1 (‘loud and perfectly clear’).

Requirements in different spaces around the world vary, but 0.5 (‘Fair’) is normally a minimum, with above 0.6 (‘Good’) being a requirement in many instances.

Different architectural and use case scenarios might specify sound levels, reverb times, ambient noise levels, but in the end, these ‘feed’ the STI.

The two primary strategies for dealing with poor intelligibility in reverberant spaces are acoustic treatment, and a high-quality sound reinforcement system with directional loudspeakers.

The blunt way to consider acoustic treatment for a reverberant space is to simplify it to an overall absorption coefficient, but no space is ever straightforward enough for that to be a wise approach.

Meachin says: “Absorbers work best where direct sound hits the absorber. They need to ‘catch’ the sound from the loudspeakers quickly, so they are more effective if they are placed where sound from your directional speakers is going to hit a surface, rather than up on the ceiling where they’ll catch reverberation but won’t stop early reflections.”

Of course, often you’re limited by the specific project. Plastering the wall of a church in acoustic tiles might not be acceptable, so ceiling treatment can be unavoidable.

Speaker in the house

Another factor to consider is directionality of loudspeakers. Line arrays, beam steering, Wave Field Synthesis hybrids, along with the ingenious mechanical and complex DSP solutions that enable them are becoming increasingly popular.

Put simply, a directional loudspeaker is a massive boon in reverberant spaces for a number of reasons. First, you can point it where you want the sound to go. So, with a bit of planning, you can direct sound straight to absorptive surfaces and non- parallel surfaces where early reflections are subdued or redirected to where they do less harm, thus lowering the energy in the reverberant field and shortening the reverb time.

Second, the ‘focused’ wavefronts of directional arrays mean that less power is lost to an expanding omnidirectional wavefront, thus extending the ‘throw’ of the array. In turn, that means you need fewer speakers to cover a given distance, which means a more coherent direct sound.

The directional speaker market is currently alive with the sound departments hunting for a piece of magic that will allow you to put any sound anywhere you want.

Holoplot is a German company that has made waves recently with its own unique approach to directional speakers which according to Roman Sick, the Holoplot CEO, is a new approach to Wavefield Synthesis in a single plane. The system is built from one or more of its Io modules and Sick explains: “In one type of use - for immersive sound - we create a wave front and purposefully create reflections to enhance the spatial effect. For speech intelligibility we can use the wavefield synthesis algorithm but make it very directional - effectively doing beam steering in the horizontal and vertical planes independently.”

The system can create up to 16 directional beams, and they don’t have to be the same either, so you can effectively map the beams to the space in front of the array. Active Audio’s core technology is a hybrid approach called DGRC (Digital and Geometric Radiation Control). This tech combines the advantages of geometrical arrays and electronic arrays, and uses a patented arrangement of speakers, arranged in groups, that reduces the number of amplifiers necessary to drive the loudspeaker system while still ensuring excellent directivity.

Eric Grandmougin, marketing director at Active Audio, says: “This also allows us to put the column loudspeakers in difficult areas, and even to have the electronics for the system in a separate technical room. Our Ray-On series is a passive column loudspeaker, while our StepArray series is digitally steerable column loudspeakers that incorporate DSP.”

Asher Dowson, house of worship segment manager for d&b audiotechnik, says that the company’s two main offerings in that market - the C16 and C24 column loudspeakers - typify the d&b approach: “It’s the story of directivity. It’s getting control, getting that energy to be intelligible, and getting it to where it needs to be.”

Meyer Sound has a wide range of product types from point source, to line array, to column products, including the new Ultra-X40 point source loudspeaker that comes in two different coverage pattern models – both thanks to the rotatable Constant-Q horn so you can choose the plane you need most or least directivity on. The company’s CAL column array loudspeakers are impressive too – boasting a vertical beam of programmable width which is then digitally steerable up to 30 degrees up or down for best intelligibility. It even has AVB connections so can be part of your digitally steerable network.

Another useful tool is room analysis. There are a number of options available and EASE by AFMG Technologies seems to be the most widely supported, though there are plenty more, like Odeon, and CATT-Acoustic; and of course there is acoustic modelling included in a number of the BIM (Building Information Modelling) applications such as Revit by Autodesk.

Many manufacturers’ products have associated files that plug those products into the modelling applications, such as Ease GLLL, BIM Revit, CLF, and AutoCAD files. In Meachin’s view, one of the most powerful tools in the room modelling kit, especially when it comes to convincing architects and customers that you’re right and they do have to spend more money, is auralisation. That is, the realisation of an acoustic with a generated sound file.

He says: “You can go straight to the customer and say ‘this is what it sounds like now, this is what it will sound like if you spend USD 3,775 on it, and this is what it will sound like if you spend USD 12,600 on it’.”

Whatever the challenge, it’s comforting to know that there is a spectacular range of options out there for battling problematic room acoustics, and particularly the ever-present issue of intelligibility. It’s also important to remember though that not only is this about correction, it can also be about creation.

Meachin concludes: “Often people aren’t very ambitious with the acoustic ambience that they’re going for. They think if it just passes the standards then it’s good enough. Actually, the acoustic can be part of the creative and architectural design in any space.”

Article Categories