Active acoustic technology provides more flexible rooms.
By Steve Ellison & Pierre Germain
The geology professor shows the surface of Mars on the video displays behind her. Turning toward the screen, she describes the different layers in Gale Crater and launches into new theories about the forces that shaped the Martian surface. Turning back toward the class, she describes how our theories have been shaped by new data received by recent missions, such as the Mars Curiosity Rover.
The students in the back, 50 feet away, somehow feel much closer than they are, and are captivated by every word. After the presentation is complete, the class is instructed to pull out their laptops, check out an interactive lesson plan that has been prepared and work in small groups. At the press of a button, the room becomes quiet and acoustically tight. Each group works excitedly without disturbing each other.
A group in the back is ready to share their work with the class. Another button is pressed, and this time, acoustic energy from the back is transmitted to the front. Another button click, and lively discussion is supported as other students critique the first group’s work.
Or consider a Fortune 500 company product launch in a 250-seat presentation theater. Board members, shareholders and fans watch a new ad promoting the product, and are drawn in by the animation in the high-definition video and accompanying immersive soundtrack that surrounds and engulfs the audience. The CEO makes a presentation in the NC-20 room and the audience feels like they’re sitting in their living room. With a button press, a recording artist sings a couple of numbers accompanied only by his piano, and the audience now feels like they’re at a concert venue downtown.
In both cases, the participants may not be aware that acoustic changes are even taking place, but they are, naturalizing the room’s acoustics to the function and participants, rather than requiring the function and participants to adapt to the acoustics.
These scenarios are made possible by active acoustic technology that is seamlessly transforming lecture halls into concert halls, cinemas into classrooms, and back again. These systems provide programmable acoustics that are quickly changed and support ever more flexible rooms for educational and meeting environments. Let’s explore the concepts behind active acoustics, and begin by reconsidering reverberation.
When a sound is created in a room, it arrives at your ears using the shortest path first, known as the direct sound. Next to arrive are sounds that reflect off surfaces in the room, such as the ceiling, floor and walls. Because they’ve taken a longer path than the direct sound, these reflections will arrive slightly later than the direct sound. Once these early reflections have traveled past your ears, they continue to propagate throughout the room, reflecting again and again. The cumulative effect of these late-arriving reflections is what constitutes reverberation.
As the travel paths grow longer and longer, the reflections will lose energy to the point where they are no longer audible. Therefore, all sounds in a room follow the same structure at the point of reception: the direct sound, early reflections and reverberation. Reverberation time (RT) is the amount of time it takes for the sound to decay to the point of inaudibility.
Reverberation time is proportional to the size of the room, and inversely proportional to the amount of absorption. It’s no wonder that gothic cathedrals in Europe are among the most reverberant spaces. These large, awe-inspiring rooms are clad in stone and glass, with minimal absorption. Reverberation times in excess of six seconds surely inspired the liturgy and chants of the religious services designed for these spaces.
On the opposite end of the spectrum, cinemas are among the least reverberant spaces. To transport the audience into the story, the cinema must allow the multi-channel sound system to reproduce the filmmakers’ sound tracks without coloration by reflective architectural surfaces and with minimal background noise. In between the cathedral and the movie theater is a wide range of rooms supporting group activities in education, business, entertainment and worship, which typically have an acoustic signature optimized for their singular purpose.
Room reverberation helps audiences feel immersed in a musical performance and helps ensembles maintain their musical connection to each other when performing. However, in spoken word contexts such as classrooms, excessive reverberation can cause one word to blur into the next and thereby degrade intelligibility. Optimal reverberation times vary greatly for a wide range of uses. Even within the scope of spoken word applications, the range suggested varies by a factor of at least three, from the ideal near-anechoic condition of a teleconferencing room to the tight but moderately lively drama theater.
Teleconferencing systems must receive and transmit the signal faithfully between multiple people within a room and/or between different rooms. Microphones and loudspeaker systems might be distributed locally or linked over long distances, which may substantially degrade the remote talker’s voice transmission. Like the cinema, teleconferencing messaging is heard most clearly when the room leaves the signal alone. Such conversations are explicitly two-way communication, and therefore distributed microphones must maintain high direct-to-reverberant levels to allow participants in the room to freely change roles between talker and listener.
Early Reflection Benefit
Therefore, classrooms and auditoriums should be designed to be as acoustically dead as possible, correct? Well, not exactly. Early reflections hold the key. The first arrivals after the direct sound can be classified in complexity or “order,” with first order reflections being from a single surface, and second order created by two surfaces, such as a ceiling and a wall. The room’s geometry, as well as the acoustic properties of the surfaces, will dictate the strength, direction and arrival times of these reflections.
When a teacher is facing a whiteboard in a classroom while speaking to the class unreinforced, the class relies entirely on the early reflections created by the whiteboard surface, ceiling, floor and walls to understand what is being said. As it turns out, these early reflections can help make the spoken word easier to understand.
John Bradley of Canada’s National Research Council developed “Early Reflection Benefit” to quantify these useful early reflections as the ratio of early-to-direct energy. He carried out experiments in which word lists were played back by a loudspeaker in a non-reverberant environment, and found that introducing synthesized early reflections surrounding the listener could improve the same level of accuracy as increasing the direct signal.
He concluded that the speech energy arriving within the first 50ms after the direct sound is useful for intelligibility. This was corroborated in large part by recent work by a group led by Anna Warzybok at the Institute of Physics in Oldenburg, Germany, though their findings showed that the greatest benefit if the first reflection is received within 25ms of the direct sound, and from the same direction. Benefit is still achieved for slightly longer delays, but to a somewhat lesser degree.
However, direct and early energy may not be enough in a noisy environment. A concept known as the “Useful to Detrimental” ratio has been used to gauge the quality of an acoustic to support speech. Useful signals include the direct sound and early reflections, and detrimental signals include later reverberation and background noise. Clearly, a quiet environment is recommended in all cases.
Controlling Reverb, Early Reflections
Because reverberation time is inversely proportional to the amount of absorption, an absorption reduction is a reverberation increase. So, we’re left in a bit of a quandary. How do we keep reverberation time to a minimum and maximize the useful early reflections? Those same surfaces creating the useful first and second order reflections will continue their sonic paths and create an increasingly diffuse field, thereby transforming into intelligibility’s sworn enemy: reverberation.
Architectural solutions to maximize the projection of first order reflections to provide support for an acoustic source (our presenter) may include reflective walls and panels around the front of the room. Conversely, architectural solutions to minimize reverberation may include carpeting, absorptive walls and ceilings. The latter approach is the standard practice for cinemas, where loudspeakers have no need for reflection support to achieve sufficient levels.
So, herein lies the quandary. What if we want to turn this cinema into a meeting space? The utter lack of reflections will make the room stifling, unengaging and uninviting. One option would be to design the walls for meeting space acoustics and add removable heavy curtains and drapes to satisfy the cinema needs. Another is to attempt a compromise ceiling panel scheme that provides a balance of reflections and absorption.
Modern auditorium designs often include elements of both types of rooms: presentation spaces dressed up as high-definition theaters, and classrooms enriched with distance learning and telecommunication. Designers of a 400-seat cinema might strive for an RT of just 0.4 seconds. But the same size room for theater may approach 1 second. A great deal of variable acoustics would have to be employed to create this acoustic range.
Passive & Active Solutions
There is an alternative. Passive acoustic systems, such as movable curtains, drapes and panels are employed to take away energy from a room. Active acoustic systems use microphones, special-purpose signal processing and loudspeakers to add energy to a room. Thus, a room with a low baseline reverberation time can use such a system to add early reflections to the room, and provide configurations that change the direction, density and intensity of the early reflection patterns.
The patterns can be configured to send energy from the front of the room to the rest of the room for a presentation mode, and alternately configured to pick up the energy in the audience area, for a discussion mode. And it can be turned off during an AV presentation or when a remote talker is addressing the room in a teleconferencing setting. This means that a room with such a system can function as a cinema, presentation space and classroom.
How is an active acoustic system different from a typical sound reinforcement or public address (PA) system? In a PA system, the microphone needs to be relatively close to the talker so that it can deliver an amplified direct signal. This is achieved typically by a lavalier, handheld, or lectern microphone. The signal is amplified and sent to the audience via, typically, a relatively small set of loudspeakers, often replacing the acoustic direct signal.
The speaker system concentrates the sound into a minimum of source locations for maximum acoustic gain and uniform direct sound level distribution. Active acoustic systems, on the other hand, support the direct sound by picking up the sound from an area of the room with a group of microphones, and creating early reflections that are transmitted using a larger number of distributed loudspeakers at a lower volume. These early reflections are carefully adjusted to support the direct sound, and can create specific patterns of acoustic energy that sound very natural and help support both the talker and the listeners. Can traditional sound reinforcement and active acoustics co-exist? Absolutely. Active acoustic systems can be designed to deliver reinforcement as well as program audio. This is only possible due to advances in digital signal processing and linear systems that preserve the quality of the acoustic signals in the presence of other audio signals.
For a successful deployment of an active acoustic system for speech benefit, it is imperative to start with an acoustically well-behaved room. The noise floor should be no higher than NC-25 to NC-30, as it would be for a typical classroom. The nominal reverberation time should be less than a typical classroom because the system will add reverberant energy to the room. Access to the wall and ceiling spaces is important for installation and maintenance of the transducers. The sidewall speakers should be positioned low enough so as to have lateral energy arriving to as many seats as possible.
Microphones have to be close enough to participants so everyone is covered, and the direct to reverberant ratio is as high as possible. If there are conflicts with microphone positions and cinema screens, cable reelers can be used to deploy microphones when needed, and then retract them when not needed. Integration of active acoustics settings with other room functionality, such as lighting and video displays, can help ensure seamless transitions from one room mode to another.
Multipurpose Gathering Space
Not long ago, scientist, musician and author Jaron Lanier joined Stephen David Beck, Director of the School of Music at Louisiana State University (LSU), in an intimate Q&A session in the school’s new 200-seat Digital Media Center (DMC) theater. With the help of the room’s active acoustic system, the two carried on their conversation, with questions interjected by the audience, unencumbered by handheld or lavalier microphones. Audience members not only heard the presenters on stage clearly, but also each other’s questions. This helped encourage an organic flow of conversation, from the stage to the audience and back again.
“This auditorium gives us a strong sense of community, where the whole center can gather, have conversations, share presentations and enjoy the benefits of each other’s research,” said Beck. “We don’t have to worry about microphones. Just turn it on and it all works.”
The same room at LSU hosted a class recently on advanced multi-channel audio automation that also benefited from this. Students learned about manipulating audio in the very environment in which they were learning. The DMC theatre also hosts monthly screenings sponsored by the Digital Media Arts and Engineering center, with audio supported by an integrated SMPTE-compliant cinema audio system. Students at LSU now have the opportunity to not only manipulate sound in an immersive multi-channel audio environment, but also in a room with programmable active acoustics, as well. Here, the future and the present coexist as they demonstrate how a single room can support a wide range of collaborative activities more successfully.
Steve Ellison is Director of Applications and Pierre Germain is Senior Acoustic Engineer at Meyer Sound (www.meyer sound.com) in Berkeley CA.