Here is an overview of architectural acoustics. Hopefully, you can find your project here and fit it into the context of other spaces where listening is important.

What creates acoustics?

Few people have any conception at all of what factors affect the acoustics of a space, or what to take into account when contemplating the design or renovation of a space where people listen. In order of importance, here are the factors that determine the acoustics of a space:

1. Size,
2. Background noise,
3. Geometry,
4. Room surfaces.

Types of spaces

I list below some the types of projects on which I work, with some explanation of their acoustics. If you have contacted me, your project probably falls into one of these broad categories.

• Noisy spaces, such as cafeterias, work spaces, gymnasiums, pools;
• Meeting and video-teleconferencing rooms: rooms for discussion and dialogue;
• Rehearsal rooms: rooms for practicing music or theatre;
• Single purpose performances spaces: lecture halls, drama theatres, synagogues, recital halls, and concert halls;
• Multi-purpose performance spaces: multipurpose auditoriums and churches.

There are two basic tasks in these spaces:

1. Isolate the space from noise sources;
2. Reduce reverberation.

In this type of space, changing room surfaces is often the only solution. The basic problem is to determine how to get in as much sound absorbing material as possible at a reasonable cost. When designing such a space, size-height in particular-is also an important factor.

Meeting and video-teleconferencing rooms---rooms for discussion and dialogue

Size and background noise should be the first priority for meeting and video-teleconferencing rooms. The number of people may necessitate a certain size, but minimizing the size will always be acoustically beneficial. How much acoustical difficulty is encountered in an intimate living room? To optimize conversation among a large group of people, it is necessary to completely exclude background noise see Excellent Acoustics, below.

These days, acoustical design for meeting and video-teleconferencing rooms rarely goes beyond the use of sound absorption. This complete reversal of priorities is the reason for the poor acoustical quality of the vast majority of meeting and video-teleconferencing rooms, and the awkward over---reliance on sound systems for even modest---sized meeting and video-teleconferencing rooms.

Rehearsal rooms

Unlike most performance spaces, rehearsal rooms are usually too small. For these rooms, a combination of sound absorption and geometry that makes the room act like a larger room is the key to acoustical design. Sufficient height is critical. Sometimes, a music group that is too large or too loud will simply overwhelm the space.

Other important acoustical issues in rehearsal room design include:

• Sound isolation from other spaces;
• Mechanical (HVAC) noise control.
• Adjustable acoustics for different types of ensembles.

Single purpose performances spaces

The single purpose performance space is a delight---a real opportunity for a client with a limited budget to achieve acoustical excellence. Limit the size; exclude background noise; focus the design around a single type of performance, and excellence acoustics comes well within reach.

In real life, the true single-purpose performance space is rare. Nearly all performance spaces are multi-use in practice, if not by design.

Synagogues

The synagogue is category of single purpose performance space dear to my heart. Size, the elimination of background noise, seating configuration, attention to geometry, and reverberation are the keys---in that order.

Multi-purpose performance spaces

Multi-purpose performance spaces are often built to save the money it would entail to build several dedicated spaces. A typical high school, for instance, would probably be better served by a music recital hall, and a speech theatre rather than a large multi-purpose auditorium. This is very rarely done.

Because they are built for reasons of economy, these spaces are usually too large, with budgets too small to do what is necessary to make them work. The large multipurpose performance space is the most difficult acoustical challenge, even under the best of circumstances.

The keys to making a multi-purpose space work are limiting size, complete elimination of background noise, appropriate and adjustable geometry, and adjustable reverberation. These are all big challenges that grow as the space gets bigger.

Churches

Churches are an important category of multi-purpose space. Churches present additional acoustical challenges:

• The difference between the acoustical requirements for organ and choir on one hand, and speech intelligibility on the other is extreme, yet adjustable acoustics is almost never an option.
• There are severe aesthetic constraints on church design. For example, flying panels are an excellent tool for reconciling reverberance and clarity, yet church committees are reluctant to consider such visible measures. (There are ways to get around this.)
• Churches often have very constrained budgets.

Your Project

Your project is, of course, unique. I hope that this tour through the world of architectural acoustics will help you put your project in context, and help you decide the best approach to its acoustics.

What level of acoustics is appropriate for your project?

Contracting the services of an acoustical consultant---even a superbly talented, genius acoustician---is no guarantee of acoustical excellence, or even decent acoustics in your particular project. We are not magicians; we merely offer advice. Ultimately, acoustical quality is determined by the will of the client. I am so confident of this that that in certain projects, I offer a money-back guarantee on my services.

Before you hire me, or any other acoustician, I would like to give you some sense of what is involved in the acoustics of your particular project. My fee is just one of the costs involved. Acoustics will entail construction and design costs, and will profoundly affect other aspects of design.

In the third section in this report Acoustical Excellence, I discuss what it takes to achieve excellent acoustics for critical listening spaces. (Please forgive the awkward term, but there is no good word for the wide variety of space where acoustics is important).

For many projects, excellent acoustics may not be appropriate. Acoustics is, after all, only one item, and must be balanced with other aspects of the program. If it is important, you should realize just how many things affect acoustical quality-and how these in turn affect other aspects of the program-so that you can make an informed choice.

My fondest hope is that I can entice a few of you---some of who may not have even considered the possibility---to take the plunge and go for true acoustical excellence.

Excellent acoustics---what does it take?

This section applies mainly to critical listening spaces such as meeting and video-teleconferencing rooms, dedicated performance spaces, multi-purpose performance spaces and rehearsal rooms.

If your project is reasonable in size (under 1,500 people), and is intended to endure, I heartily encourage you to go for acoustical excellence.

Exclusion of all background noise

You may notice that I use this phrase rather often. It is fundamental to acoustical excellence.

Benefits of excluding background noise

For the long, poetic version, please see Silence is Golden Excluding background noise allows the following:

• crystal clarity of sound,
• maximum dynamic range,
• enhancement of the performer's freedom of expression,
• intimacy between the performer and audience,
• the ability to hear other more subtle acoustical virtues.

Exclusion of all background noise is the first and last precondition for excellent acoustics. A space with audible background noise is just not acoustically excellent. I feel so strongly about this that even in very tight budget projects, I encourage the client to spend most of the acoustics budget on noise control.

The positive side is that in a modest sized space, eliminating background noise brings you a long way towards the goal of acoustical excellence.

What does this entail?

Excluding background noise requires the following:

• Location of mechanical systems on a separate structure.
• Complete structural separation of the "noisy" structure, where all noise sources are located, and the "quiet" structure, where the critical listening space is located.
• Complete vibration isolation of all the machines in the building.
• A duct system designed to avoid ducting noise into the space, and to avoid creating noise. This requires oversized, lined ducts, additional sound attenuating devices, and sufficient duct distribution to avoid diffusers.
• Buffer spaces, such as corridors, protecting the critical listening space from the outside world.
• Sufficiently massive wall and ceiling constructions to protect the space from the outside world. To achieve these goals:
• The layout of the building must be taken into account from the very beginning;
• The HVAC budget must be much larger than for a typical building;
• Wall and roof constructions must be considerably heftier than for a typical building.

Size and the triangle

The preceding paragraph makes it clear that acoustical excellence strongly affects cost. Architects often use a visual device to illustrate the choices one must make in planning any building project.

Geometry and aesthetics

Sound waves are large. Thus, geometrical forms designed to affect sound must be large and visible. Large visible sound reflecting surfaces are imperative in larger spaces.

Clouds

Flying sound reflecting "clouds" are a tried and true method for dealing with the acoustics of larger rooms. Flying clouds are the key to the classic problem of clarity within a reverberant space. However, these clouds are visible. To be effective, they must be placed in highly locations, and be sufficiently large to function properly.

Audience configuration

Balconies are the best way to use geometry to improve acoustics, especially side balconies. Nearly every acoustically excellent concert hall, opera house, and drama theatre (above a certain size) ever built has side balconies. The most successful church designs also have side balconies. These balconies serve important functions:

• They make the space narrower;
• They serve as sound reflectors;
• They provide the best seats.

Room Surfaces and reverberation

Different performance types require different amounts of reverberation, sometimes radically different amounts. Broadly speaking, speech requires less reverberation; music requires more, with organ music requiring the most. This is less of a problem with smaller halls, but becomes critical with larger spaces.

Curtains

The best solution is adjustable reverberation: curtains. However, curtains are bulky, expensive, and awkward. Furthermore, in order to be effective, tremendous curtain surface area is necessary---more than most people realize. I know of many rooms where the amount of curtain is not sufficient to make much of an audible difference.

Sound absorbing Panels

Sound-absorbing panels are a very useful product, but like all "acoustical" items, they have to be used appropriately. In multi-purpose performance spaces, it is always better to change the reverberance of the room with adjustable curtains, rather than using static sound absorbing panels.

Conclusion

Only you can decide just how acoustics fits into the big picture of your project. After reading this you may decide that excellent acoustics is not worth the sacrifice. I am very happy to help clients make the best of difficult choices. My purpose in writing these three articles is to help you make the choice of the appropriate level of acoustics for your project, based on your priorities and resources.

I hope that just a few of you, realizing the costs and sacrifices, will be inspired to reach for the stars.