An auditorium is a “place for hearing.” It’s very definition suggests that any aspect of the space that has to do with communication should be given high priority.
Prior to the invention of the sound reinforcement system, an auditorium was a space designed to allow a large number of people to be addressed by an unreinforced talker. The room geometry and acoustical treatments in an auditorium provided natural reinforcement for the acoustic source.
The ancient Greeks even built megaphones into the masks worn by actors, allowing even larger audiences to be entertained. But even with these techniques, there was really no effective way to address a very large group of people-until the invention of the sound reinforcement system.
A sound reinforcement system becomes necessary for at least four reasons:
1. Room Size-The room is too large for the natural sound from a source to be heard at distant listener seats.
2. Room Geometry-Modern auditoriums are not geometrically compatible with the human talkers. The popular 180-degree fan-shaped room is at least 60 degrees wider than what the unaided human voice can cover. As soon as a talker turns their head, some of the audience will be missed.
3. Environmental Noise-The background noise produced by occupants, traffic, and heating and air conditioning systems masks the sound source for some or all of the listeners.
4. Acoustical Treatment-The type of acoustical treatment necessary to provide natural acoustical amplification is potentially expensive and not universally suitable for all musical styles.
When a society insists on packing large numbers of people into large spaces, sound reinforcement is necessary.
An Analogy
When illuminating a room with light, appropriate fixtures are used to beam light to where it is needed. Everyone understands this. The sound system designer must do the same thing with the selection and placement of loudspeakers.
Assuming reasonably good room acoustics, some major sound problems, such as poor gain-before-feedback and poor speech intelligibility can be directly attributed to inappropriate loudspeaker selection and placement. The most common reason for this is that the sound designer was forced to abandon their first choices regarding the “what” and “where” of the loudspeakers due to aesthetic concerns.
Architecture & Sound Reinforcement
Architects often view loudspeakers as something that is foreign to the space, an “add-on” that really doesn’t belong. Architecture schools do not teach sound system design, nor should they. They emphasize the visual aspects of the venue, with great attention given to the geometry and visual flow of the space.
This is their mindset, and it is what they were hired to do. Large, prominent loudspeakers interrupt their concept of the space.
As the room is designed, the architect doesn’t know what loudspeakers will be needed or how many, making it impossible for them to integrate them into the design. And even if they could, as we will soon see that their efforts to camouflage the loudspeakers will likely impair their performance.
For the loudspeaker to do it’s job, it must occupy some of the most visually important real estate of the room. It’s no wonder that architects and sound system designers are often in conflict.
Tools Of The Trade
The sound system designer’s job is to make sure that everyone can hear what is being reinforced. They have three major variables to work with in achieving even coverage of a seating area.
These include:
-Loudspeaker type
-Loudspeaker placement
-The number of loudspeakers
Designers do their best work when they’re given control all three. If the client or architect fixes one or more of these variables, then the system designer has less to work with and sound reproduction will suffer.
Loudspeaker Location
The ideal place for a loudspeaker isn’t available. It’s the location where the sound source being reinforced is positioned.
Imagine a small theatrical group or classical music ensemble performing in a small space. No sound reinforcement is required, and the sound from each source is heard in the proper spatial relationship to the other sources.
As the room size is increased, the instruments may require amplification. Musical ensembles have used localized amplification for many years, and “personal” sound systems are still marketed for small venues today - the current incarnation of the “Wall of Sound” used by the Grateful Dead several decades ago.
The location of the acoustical source is the natural place for the sound to come from. This approach can be impractical for many applications, and it is not very versatile, so a compromise is called for. The first choice is to place the loudspeaker directly above the source being reinforced, taking advantage of the human listeners poor ability to localize sound in the vertical plane.
If a loudspeaker is placed to the left or right of a talker, it will be obvious to the observer that the sound is not coming from the talker. If it is placed above the talker, the human hearing system is deceived into thinking that it is coming from the talker. The eyes effectively pull the sonic image down to correlate with the vision. Overhead placement also allows sound to be projected to the back of the room without overpowering the front rows of the audience.
With these considerations, it makes perfect sense to place loudspeakers directly above the stage.
Size vs Efficiency
Loudspeakers must be large to have control over where the sound goes-period. A tiny loudspeaker radiates sound in all directions.
As its size is increased the loudspeaker becomes more directional. Directivity control is mandatory in all audio systems, and the more control that is needed, the larger the loudspeaker must be. That’s just how it works.
Another reason for large loudspeakers is low frequency efficiency. Systems in many houses of worship are expected to produce very high sound pressure levels at very low frequencies.
This means that lots of air must move, which in turn requires a lot of piston surface area. There is simply no way to get high sound pressure levels in large rooms at low frequencies without a large loudspeakers-often used in multiples.
Each successively lower octave requires more radiating surface area. If you really do expect to reproduce thunder claps during the Easter musical, or the lowest pedals on an electronic organ, you had better plan on a very beefy low frequency loudspeaker system. It has to be big, and it has to be in the room.
Sound Problems
Many of the typical sound system ailments - lack of definition, muddiness, dullness and poor localization can be caused by reflected sound. Many of these detrimental reflections result from efforts made to disguise the loudspeaker.
Reflected sound is a part of life. We expect it, we use it, we need it. But not all reflections are good. The two types of reflections that sound designers try to avoid are those that come from objects very near the loudspeaker, and those that come from objects very distant from it.
The former produce colorations of the sound, the latter produce echoes that can garble the clarity of music and speech. Any attempt to disguise a loudspeaker invariably produces early reflections that change the loudspeaker’s response. This includes grills, fabric coverings, and cavities that were carefully designed to be aesthetically pleasing. In short, what makes a loudspeaker look better potentially makes it sound worse.
A careful and thoughtful design process is required to allow loudspeakers to be covered yet still perform acceptably.Unlike light waves, sound waves can cancel each other if the timing is appropriate. This is not always a bad thing.
Sound system designers use constructive and destructive interference to achieve the desired radiation pattern from a loudspeaker array. Good sound designers understand interference and how to use it to enhance the performance of a system.
Let’s look at some methods used to reduce the visual impact of loudspeakers.
Coverings-Grills always obstruct sound. The only questions are “How much?” and “Will it be audible?”
I have seen system designers go to great expense to extend the frequency response of the sound system to beyond 20 kHz, only to hide the whole thing behind a grill that is acoustically opaque to this part of the spectrum.
But even without the grill, there is little chance of sound energy above 16 kHz making it to anyone in the audience due to air absorption alone, so we can live with some fabric in front of the loudspeaker.
The fabric, of course, will require a frame. The frame is potentially more obstructive than the fabric that it supports. Frame members should be made as small as possible, and ideally not placed in the main path of the system’s high frequency drivers.
Cavity Placement-The worst place to put a loudspeaker is in a cavity. The loudspeaker itself is a carefully tuned resonant system. It’s size, volume, and openings are carefully selected to produce a desired response.
A cavity is essentially a “box around a box” and it will produce gross modifications of the loudspeaker’s response. Given the care and engineering that likely went into the design of the loudspeaker, placement in a randomly-sized cavity will usually destroy the response intended by the designer.
Grill Cloth-Never paint grill cloth, not even a light coat. Paint blocks the pores that the sound wave must pass through. If the grill is the wrong color, replace it.
Before you run out to your favorite fabric store to acquire a fashionable grill cloth, keep two things in mind. The grill cloth must be acoustically transparent, and it must meet fire safety codes. Unfortunately these criteria will rule out many of your first choices. It’s quite easy to test the audible effects of grill cloth.
Simply listen to some music and speech through a decent quality loudspeaker, and then drape it with a sample of the fabric. What you hear is what you get. If the cloth changes the sound, don’t use it. You will find that fabrics that are easy to blow air through produce little attenuation of sound and become good candidates for hiding loudspeakers.
Serviceabilty-Sooner or later it will be necessary to repair the loudspeaker. Keep this in mind when constructing the grill. It should be removable as a unit or in modular panels.
And Back to Location
These acoustical pitfalls can be avoided altogether by placing the loudspeakers in free space with no objects near them. A notable exception to this is the subwoofer, whose long acoustic wavelengths can be naturally amplified by placement near a room boundary.
But the very practice that can enhance the sound of a sub can wreak havoc on a mid/high frequency loudspeaker, whose shorter wavelengths do not couple well with walls, ceilings and other nearby surfaces. A good compromise is to simply have the contractor paint the loudspeakers and mounting hardware, including a light coat on the metal grill.
Most loudspeaker manufacturers can provide bare versions of their products. Veneers like Formica can be used on the cabinet surfaces, but remember that adhesives can dry out over time and a slab of veneer falling from the ceiling can be dangerous.
Loudspeakers aren’t the only add-ons required to make the room functional. Others include ceiling fans, HVAC vents, lighting fixtures, structural beams and support columns. No one questions that these must be present, and no one really minds that they are visible.
We would not ask the architect to completely hide the ceiling fans, or worse yet, reduce their size to some arbitrarily small value. The same is true for loudspeakers.
A Pattern Emerges
Having been in the sound business for quite some time now, I’ve seen a pattern develop in the implementation of sound reinforcement into public spaces.
The first design attempt is usually visually acceptable but sonically inadequate. The system undergoes upgrades and renovations as time progresses, and the final system sounds great but with visually prominent loudspeakers.
In an auditorium, the sound considerations must eventually rise above the aesthetic concerns. Just as head-worn microphones have become accepted due to their sonic benefits, large, visible loudspeakers are doing the same.
The main component of the sound reinforcement system is the loudspeaker. Just as a tour guide wishing to communicate with a group of people will stand in plain site, the loudspeakers in a sound reinforcement system work best if you can see them.
This creates aesthetic concerns from architects and end users alike. “Form follows function” applies here. The “what” and “where” of the audio aspects of a venue should be heavily influenced by the desired quality of sound reproduction.
There is nothing more natural than seeing where the sound that we are hearing is coming from. In day-to-day life we expect it. The reason that we have two ears is to be able to locate a sound source in space.
If the selection and placement of loudspeakers is driven by aesthetic concerns alone, then it is highly unlikely that the best possible sound quality will be realized. If fact, the room may prove useless for its primary function as “a place for hearing.”