Good vibrations

It used to be so simple, if you were building a concert hall you would tune the acoustics for live sound, whereas if it were a lecture theatre it would be optimised for intelligibility of the spoken word. But increasingly, new venues are being built to be multi-purpose and existing ones are broadening their scope to host a wider variety of events. This can create a challenge for sound engineers in achieving optimum sound quality for all types of performance.

The usual solution is to go for a ‘happy medium’ at the design stage, meaning the space will provide acceptable acoustics for most performances and optimum for a few. While this can be seen as a necessary compromise, a growing number of venues are choosing not to take this path. The traditional alternative was a mechanical solution, such as orchestra shells, retractable draperies or secondary chambers, but these can be costly and provide mixed results. An alternative to this is an electroacoustic architecture, where strategically placed microphones and speakers are used to alter the acoustic characteristics of the room.

One proponent of this technique is Californian loudspeaker manufacturer, Meyer Sound. It has developed Constellation electroacoustic architecture, and, since 2006, this system has been used in 25 installations around the world for a variety of room types where the aim was to increase the reverberation time of the building. ‘Constellation runs on two principals,’ explains John Pellowe, consultant engineer at Meyer Sound. ‘First, using a regenerative function, it can reduce the absorbance of the walls and make them more reflective.’

To achieve this, approximately 80 microphones are distributed around the building and over the stage to capture the natural acoustic. Then, using its digital signal processing algorithm, VRAS (variable room acoustic system), the sound is re-applied to up to 250 loudspeakers that are spread around the room. The basic design principal is that mics and speakers are evenly distributed over a set distance, one reverberation radius – the point in any building where direct sound and reverberant sound have equal value. Furthers Mr Pellowe: ‘As the gain of the system is increased, sounds around the room are captured and returned to the room but with natural reverberation.’ Essentially, this part of the system captures the natural reverberant energy of the hall and uses the multichannel gain of the mics and speakers to extend the reverberation time.

The second part of Constellation is a unitary reverberator. This takes the signals from the mics within a VRAS zone (which has a maximum of 16 mics) and sends them into a VRAS processor. The processor can be used to either create reverberation or generate early reflections. ‘Out in the auditorium we would use the VRAS processor as a reverberator. What we can do with that reverberator is build an electronic secondary chamber around the room, so we can actually make the room feel bigger than it really is,’ says Mr Pellowe. Essentially, by increasing the reverberation in a room the system makes it behave as a larger space would.

‘One of the failings of electroacoustic systems in the past was that they were always orientated around the stage,’ Mr Pellowe continues. ‘They would put a few microphones above the stage and distribute loudspeakers around the building. This is all fine but the audience don’t feel they are in the same acoustic space. Also, nothing patterned within the auditorium is time aligned correctly with the stage.’

To get around this, Constellation breaks the hall down into a number of VRAS zones (sized at 27m) with one VRAS processor per zone. ‘If a hall is broken down into four zones, we’ve actually got four coupled Constellation systems running. Each Constellation system will have up to 16 microphones and, if it’s an under-balcony system, it might even have 70 to 80 loudspeakers. The reality is that one zone will just cover an area where the microphones and loudspeakers are separated by no further than 27m.’ The idea is that by keeping the zones small, the time arrivals within each zone are manageable. ‘If sound from a Constellation system was arriving at the listener’s ear before the acoustic sound arriving from a performer, where ever they might be in the building, it would immediately localise the sound, you’d immediately know there was a speaker up there. In Constellation we ensure that there are no precedence issues by managing zone size.’

‘The auditorium has a combination of regenerative reverberation, where we are reducing the absorption of the walls, and in-line reverberation where we use our unitary reverberators to increase the volume of the room.’ The combination of these two factors allows Meyer Sound to create a number of preset reverberation times that would suit different types of performance.

While the number of presets available is unlimited, the company will usually try to persuade its customer to only choose a small number. ‘Typically we will give four basic reverberation times: very short, short, medium and long. For each of those reverberation times we’ll have an occupied and unoccupied setting.’ The reason behind this is that having people in a room increases the absorption. To counter this, Meyer Sound increases the gain on the system which effectively reduces the absorption, ‘that’s one thing that people with physically varying acoustics have difficulty doing.’

Changing between presets is done with the press of a button. When this is done the gain of the system and the reverberation time in the unitary reverberators are both changed. ‘When we go to a longer reverberation time, we increase the reverberation time in our unitary reverberators and we increase the gain to reduce the absorption of the walls.’ This process takes around 0.25s.

One of the critical factors within a Constellation system is that adjacent loudspeakers are de-correlated from one another. ‘The VRAS reverberators generate different reverberant outputs that enable us to run each loudspeaker separately and with a de-correlated signal.’ Each reverberator will produce 16 completely different reverberant outputs which can then be matrixed to any number of loudspeakers making sure that the same output is not sent to adjacent speakers.

The installation

When starting a new project, the first things required by the installation team are the volume of the hall and the unassisted reverberation time. Usually most multi-purpose venues will have between 1s and 1.2s as a reverberation time and will want to go to 2.3s to 2.4s for symphonic classical music. The amount of reverberation that needs to be added to the room decides the density of the system. ‘If they want to go from 1.2 to 1.6s we can do that with a comparatively low-density system. If they want to go from 1 to 2.5s then we’ve got a high-density system because the amount of gain increase that we can put into the system and the amount of reverberation that we can add are dictated by the system’s density,’ explains Mr Pellowe. The system is then designed from this information.

For the installation itself, omnidirectional microphones that will pick up sound from anywhere are put in the auditorium outside of the reverberation radius. Over the stage, cardioid microphones are hung within the reverberation radius, typically at around 5m or 6m. Signals from these cardioids mics are sent to a VRAS processor that does reverberation for the stage area, and a second that handles early reflections.

Early reflections are principally about clarity and immediacy. Constellation uses this to make the sound clearer deeper into the hall than you would be able to achieve with the natural acoustic. ‘What we do with early reflections is we capture early energy over the stage from the performers, and then by successively delaying those early reflections, we matrix them out into the rest of the system. As we go further out into the hall, we attenuate the early reflections and we successively delay them more and more so that they are always in the correct time domain to the stage.’ This is of particular use in fan-shaped halls, which are notoriously lacking in early reflections for the audience, the Constellation system is able to put these early reflections into areas where they formerly did not reach, such as the centre of the hall. ‘Constellation allows us to balance the amount of reverberation and the density of early reflections throughout the building in a way that as you do step away from the stage we can bring early reflections into the room that actually make the sound clear but at the same time reverberant. Through this, the listener is able to experience much more clarity than they would in any normal concert hall,’ explains Mr Pellowe.

 Hardware

After the design is accepted, the project is passed over to an installation contractor for the hardware and cabling to go in. The hardware behind the system is based around a number of digital processors that employ Meyer Sound’s VRAS algorithm. The Constellation processor contains the communications hardware required in the system. It receives the user’s preset selections and issues the commands to run them on the VRAS processors. The VRAS processor is the technical core of the system, providing the digital signal processing for the algorithm. The VRAS technology, originally developed by Dr Mark Poletti of Industrial Research Limited, employs a DSP engine capable of generating multichannel reverberation and early reflections, as well as mixing, processing, and routing them. MS-CONST-EXP expansion processors provide additional inputs and outputs for the VRAS processors.

For mics, the system uses precision-calibrated omnidirectional and cardioid Constellation condenser microphones. These are placed over the stage and spaced throughout a room in order to pick up both direct and reverberant sound.

To complete the package, a number of compact loudspeakers are required. The self-powered Stella-4 Installation Loudspeaker uses a four-inch cone transducer to deliver a maximum peak SPL of 108dB and a frequency range of 100Hz to 20kHz. It receives balanced audio and DC power through a single five-pin Phoenix connector. The UPM-1P ultra-compact wide coverage loudspeaker is a self-powered, bi-amplified, three-way system capable of high sound pressure levels with low distortion and uniform directional control. High-frequency reproduction is provided by a one-inch metal dome driver, while low-mid reproduction is handled by two 5-inch cone transducers. Both low-mid drivers work in parallel for low frequency power, with one driver rolling off above 320Hz to maintain a uniform directional pattern through the crossover region. In addition to a two-channel power amplifier (350W total), the internal electronics module also includes frequency- and phase-correction circuits, driver protection and a laser-trimmed, differential input stage for common-mode rejection.

The UPJ-1P compact VariO speaker is capable of 128dB SPL of peak output (at 1m) using a 10-inch neodymium magnet low/mid driver and a three-inch diaphragm high-frequency compression driver. An internal, two-channel class-AB power amplifier with complementary MOSFET output stages provides 300W of total output. The UMS-1P ultra-compact subwoofer completes the set. Housing dual 10-inch drivers in a bass reflex cabinet, it produces a peak SPL of 127dB (at 1m) over a range of 25Hz to 160Hz. An internal two-channel power amplifier provides 450W of total burst power.

The cabling that connect the speakers carries both the audio signal and the DC, making it comparatively easy to install and removing the need for the cables to go in trunking. ‘Because it’s low voltage, the safety people don’t mind you dropping them down between walls,’ explains Mr Pellowe. The system is controlled via a touchscreen controller or via a webpage should there be any problem with the controller.

Constellation is based on the same platform as Meyer Sound’s show control technology, Matrix 3. This means the technology can be used to create special effects. ‘As well as what we are doing, which is very pure acoustic work, we can leave them inputs to the system available to use for special effects. Using our Spacemaps technology, they can pan things around the room in multi-directions to create special effects.’

Tuning

When the equipment is in, the tuning of the room is divided into three sections. Tech support will first qualify the system and ensure the installer has fitted it correctly and make sure everything is working correctly. The next stage sees room calibration. This aligns the system to be unitary.

The final stage involves subjective testing, which is often done by the Grammy award winning Mr Pellowe himself. This is done with the artists that will be using the hall to make sure that they have an environment that they feel comfortable with: ‘My first point of focus is always to the stage and the orchestra pit to make sure that the musicians themselves are hearing well.’ After this, the sound in the hall is tested to ensure they are getting the correct balance of early reflections and reverberation. The final adjustments to the presets will also be made at this point. ‘I have to take an educated guess as to how the hall is going to behave with an audience, so I attend the first performances and sit with my computer at the back of the hall to make a couple of final tweaks while the audience is actually there. With that we’ll nail the presets for the hall when it’s occupied.’

Mr Pellowe can see a bright future for this area of acoustic technology. ‘Over the next 15 to 20 years, we are going to gradually get acceptance of electroacoustics as being a more sophisticated way of dealing with acoustic spaces than the old physical methods, and sound will become one of the important elements in the development of new technologies where we can go into virtual worlds.’

www.meyersound.com

Published in PAA January-February 2010