21.05.19

Audio design tools and products to tackle unwanted reverberation

Sheikh Zayed Grand Mosque

Delivering intelligent 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.

The problem with considering audio in reverberant spaces is that the blame for a bad experience is always laid on the reverberation. But while a high-level reverberant field might be the dominant result of undesirable acoustic aspects of a room, to start solving the problem, you must break it down into a range of contributing factors.

 In fact, a long reverb time might be a desirable thing 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.

So when reverberation is the problem rather than part of the creative objective, intelligibility should be the ambition.

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

The good news is that intelligibility can be quantified and measured. STI (Speech Transmission Index) and STIPA (Speech Transmission Index for Public Address Systems) are the main ones 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 something above 0.6 (‘Good’) being a requirement in many instances.

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

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The right direction  

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.

“Absorbers work best where direct sound hits the absorber,” says Meachin. “They need to ‘catch’ the sound from the loudspeakers quickly, so they are more effective if they are placed where sound from your directional loudspeakers 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 case. Plastering the wall of a church in acoustic tiles might not be acceptable, so ceiling treatment can be unavoidable. There is a bewildering range of acoustic treatments available, from bespoke fabric-wrapped rockwool wall coverings to off-the-shelf tiles, and on to more specialist products like acoustic plaster, transparent absorbers, and so on. Absorbers can become part of the architecture, and they can be part of the décor.

Of course, humans are fantastic absorbers too, and should be considered. Remember, for low frequencies in large spaces, room modes become irrelevant, and absorptive panels on ‘normal’ thickness become invisible. “LF absorption becomes dominated by other things,” says Meachin.

“Those can include plasterboard walls, windows, and more – anything that makes a ‘lossy spring’ in the presence of low frequency sound waves.”

There is hope in the form of advances in uses of resonators and diaphragmatic devices, though.

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Speaker in the house  

Directionality in loudspeakers has become a very big subject. Line arrays, beam steering, Wave Field Synthesis hybrids, along with the ingenious mechanical and complex DSP solutions that enable them are 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 shorten 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 loudspeakers to cover a given distance, which means a more coherent direct sound.

Factory settings

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

You can’t really talk about directional loudspeaker arrays without mentioning L-Acoustics.

Dr Christian Heil pioneered modern line array technology, being the first past the post with WST (Wavefront Sculpture Technology) which was then applied to its original V-DOSC arrays.

The company has hewn plenty more milestones since then, with technologies such as the Constant Curvature Array and the latest K-series with the K transducer element - a leader in large-scale directional loudspeaker systems.

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.

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The system is built from one or more of its Io modules and powered by the Eta core. “In one type of use - for immersive sound - we create a wave front and purposefully create reflections to enhance the spatial effect,” he explains.

“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 are a passive column loudspeaker, while our StepArray Series are 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.”

Part of that approach is an emphasis on reducing energy escaping from the rear of the columns: “It's designed to reduce the amount of that energy that's exciting the surfaces directly behind it by 18dB, so it gives you a huge headroom to play with in front, in the congregational field. Our solution doesn't use any DSP to do that - it's mechanical. What comes out the back is almost exactly out of phase with what's coming out of the front.”

APG's primary business is in line arrays - the Uniline and Uniline Compact systems - and the company is careful to state it is as concerned about the horizontal plane as it is the vertical, as well as making sure the directionality of the system has stable relationship with frequency.

“The whole front faces of our line array elements are in fact horns,” says Grégory Dapsanse, marketing and business development director for APG. “That controls the directivity very precisely from the low-mid through the mid, and up to the high.

We think it’s important to keep the directivity constant. From the low mids up to the highs we have Laser-like thrown down the directivity corridor.

When you put this line array close to or far from the wall you get exactly the same contribution or energy from your reflections.”

 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.

Server side

One of the most useful tools for working with any space that you’re about to attack with loudspeakers and acoustic treatment is a room analysis.

There are a number of them about – L-Acoustics even has its own Soundvision software.

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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. “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 £3,000 (approximately €3,500) on it, and this is what it will sound like if you spend £10,000 on it’.”

Client side

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: “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.