Introduction Sound system design, and more specifically, loudspeaker systems design, is an interesting mixture of art and science. Some system characteristics follow predictable mathematical relationships, indicating an analytical approach by the designer. Others are subjective, and are more a matter of opinion than a matter of scientific fact. The would-be system designer must differentiate between the two, and arrive at a compromise that maintains the most important attributes of both. Here are some things to think about when selecting a sound reinforcement system for your auditorium.
This Ain’t Your Living Room
We all want our systems to sound good. Most people have a high-quality sound
system in their home in the form of a hi-fi system or even a home theater system.
When we think about achieving good sound in an auditorium, we naturally think
of the characteristics of our home systems.
Even though an auditorium system might have the same objective, these systems must work on a completely different scale than the home system. Auditoriums are much more voluminous, and the areas that need sound coverage are much larger. The room’s surfaces are farther apart, meaning that reflected sound will be more spread out in time. This makes it much more difficult to deliver even a single channel of quality sound to an audience, let alone the multiple channels (stereo and surround sound) that we listen to at home. When it comes to auditorium sound reinforcement, forget about your living room. There are few similarities between these types of environments and the systems used to serve them.
The 80/20 Rule
It is never possible to provide every listener in an auditorium with optimal
sound reproduction. System designers usually follow the 80/20 rule - if a system
characteristic can be delivered to 80% of the audience, then it is worth pursuing.
If not, then we should consider adopting different performance criteria for
the system. Any sound system can sound good at one seat. Successful auditorium
designs deliver adequate reproduction to the majority of the listeners, not
just those in the “sweet spots.” This can even mean compromising
the sound reproduction at some ideal listener locations in order to make it
better for the audience as a whole. In an auditorium, the sound must be acceptable
regardless of where the listener is seated.
Speech vs. Music
The system designer must consider the inherent differences between music and
speech when choosing the type of loudspeaker system. Comparing the criteria
for speech vs. music is much like comparing the criteria for a good traffic
sign to that for a good painting. Traffic signs convey information such as
Stop, Yield, Road Closed. The letters are easy to read, the color contrast
is high, the messages are simple and to the point, and all of the information
is coming from one location (you can only read one sign at a time). In fact,
when it comes to the design of a good road sign there is evidence to suggest
that what is good for one person is good for everyone.
Such is not true for a painting. Art interpretation is much more subjective. Beauty is in the eye of the beholder. One person may feel that the sound system sounds fine, yet another may think that it lacks punch, warmth, intimacy or whatever. It boils down to taste. Loudspeakers for auditoriums must be selected using a “road sign” mentality - it is more important to fulfill the criteria that are common denominators among listeners than to attempt to please the whims of the artists, which are usually a minority when considering the congregation as a whole. This is why we normally design an auditorium sound system with speech reproduction in mind, knowing that speech reproduction is usually much more critical to the listeners as a whole than the system’s artistic qualities. Don’t get me wrong, I’ m not saying that art (or music) isn’t important. It’s just that it is difficult to reach a consensus on what is “correct,” much like selecting the carpet color for the sanctuary or the fabric for the seat cushions. I would not abandon the artistic criteria completely. I just wouldn’t compromise the speech integrity in a house-of-worship system design to accomplish an artistic goal, which is exactly what can happen when music reproduction drives the design.
Speech also lends itself to scientific evaluation, meaning than there can be concrete design goals up front that can be measured after-the-fact to assure that they have been fulfilled. With music it usually boils down to one opinion vs. another.
Can sound systems produce good speech reproduction and also sound good for music? Read on.
Loudspeaker Characteristics that Matter
Upon examining the criteria for high-quality speech and music reproduction,
we will find that the possibilities for loudspeaker placements and types are
somewhat selfdefining. In other words, the desired end result determines the
means that can be used to accomplish it. This is exactly why the loudspeakers
from most manufacturers have strikingly similar physical characteristics -
they must if they are to accomplish the same end result. A related, but less
recognized fact is that the same is true for loudspeaker placement in an auditorium.
What are some desirable attributes for a good auditorium loudspeaker?
- Adequate sensitivity vs. power handling – The loudspeaker needs to be able to delivered the desired sound level to the farthest listener on a continuous basis without being destroyed by the applied power.
- Sufficient bandwidth – The spectral range of the program material must be reproduced by the system. More on this later.
- Adequate directivity – Directivity (sound radiation control) must be used to focus sound where it is needed and block it from where it isn’t. Small rooms don’t need loudspeakers with high directivity control, but large rooms do.
Directivity
The use of directional loudspeakers allows sound to be projected to greater
distances than is otherwise possible. Again, it’s the same principle
used with stage lighting - the further the fixture is from the stage the more
directional it needs to be - all the way to the follow spot back in the balcony!
One term that can be used to describe the directivity of a loudspeaker is its
“Q”. Low Q means low directivity, and high Q means high directivity.
Since auditorium loudspeakers are almost always placed at significant distances
from the listener, significant directivity (Q) is required. Loudspeakers for
auditorium use must be auditioned in auditoriums, since the acoustic environment
dramatically affects the sound energy ratios that will develop at a listener
seat.
Bandwidth
No, bandwidth isn’t how far the musicians are spread out on the stage.
The bandwidth of a system describes the range of frequencies that the system
can reproduce at about the same level. A speech system must have a bandwidth
of about one decade (a 10-to-1 frequency ratio). The telephone is a good example.
A full-range music system must cover most of 3 decades (about 10 octaves).
A good home theater system is an example of a full-range system. Even the best
auditorium sound system will not have the bandwidth of a good home listening
system due to air absorption, listener distances and other factors. Such systems
are usually “band-limited” to exclude the lowest and highest octaves
of the audible spectrum. We don’t want our sound systems to sound like
telephones, so even speech systems are designed to cover about two decades
of the audible spectrum (100 Hz to 10 kHz) in “acoustically friendly”
environments. Further bandwidth limiting may be required for hostile environments
such as factories and gymnasiums.
Intelligibility
An intelligible system is one that reproduces speech so that the listeners
can understand the words. This characteristic is obviously high on the list
for a house-of-worship system. It sounds simple, but this can be one of the
hardest attributes to satisfy for the audience as a whole. The intelligibility
criteria define the required directivity and minimum bandwidth of the system.
And many things that we can do to make music sound better, such as artistic
equalization, can reduce the intelligibility of the system. Ironically, speech
intelligibility and good sound quality are two completely different things.
What is good for one may not be for the other.
Clarity
Clarity is similar to intelligibility, except for music. The reinforced sounds
from musical instruments must be well-defined so that the listener can distinguish
between the various instruments. Unfortunately, clarity is as much a function
of the room’s acoustics as it is the sound system. System designers must
consider the reflections produced by the sound system at each listener location
to ensure that the system has a high degree of clarity. This often requires
suitable acoustic treatment of remote surfaces, such as back walls and balcony
faces. Don’t pull it from the budget - room treatment is every bit as
important as the sound system itself.
Imaging
Imaging is a term used to describe where the listener perceives the sound is
coming from. Sit in an auditorium and close your eyes while someone speaks
into the lectern microphone. Now point to where the speech appears to be coming
from. If it is in the direction of the live presenter, then the system has
natural imaging. System designers try to achieve good imaging for as much of
the congregation as possible - the 80/20 rule applies. Speech systems should
image (ideally) from the talker, or at least from in front of the listener.
Multi-channel music systems must have good imaging for each of the playback
channels to each seating position, something difficult to accomplish without
violating the 80/20 rule.
Gain-Before-Feedback
Why I am talking about acoustic feedback in an article on loudspeaker system
selection? Because loudspeaker selection and placement are the lifeblood of
system stability. Simply stated, the loudspeaker selection and placement determine
how loud you can operate the system. Period. No amount of electronic signal
processing can “correct” a loudspeaker system that is spilling
too much energy onto the stage. Using the stage lighting analogy, if the stage
lights are shining on the audience and producing an annoyance, then something
must be done to produce a shadow. This may be accomplished through increased
directivity of the lights or by constructing a barrier (which basically does
the same thing).
So Where Do I Place the Loudspeaker?
Let’s start with speech. Of the infinite number of possibilities, where
do the loudspeaker(s) need to be placed for the natural transmission of speech
information to an audience? The answer can be found by considering how humans
are designed to receive information: from a single source. We have two ears
(receivers), but only one mouth (source). And the reason for two ears is not
stereo listening (a relatively recent invention) but sound localization - determining
where the sound is coming from. Think face-to-face communication and you have
the required attributes of a good speech system. Humans are not designed to
deal with the same information coming from more than one location (which wasn’t
even possible before the invention of the public address system). Multiple
sources confuse the auditory system and produce problems with intelligibility
and sound localization. We are designed to get information from one place (I
am sure that there is a sermon in that statement!). In a Sunday school classroom,
this is accomplished with the natural voice of the teacher - no sound system
needed. Now let’s try to emulate this means of delivery on a larger scale.
In a small house-of-worship, this can often be accomplished with a single loudspeaker
placed above or behind the presenter. An array of loudspeakers may be required
if the coverage angles are too large for a single loudspeaker (shame on the
architect). If the differential in listener distances to the array are too
large, then a distributed system may be required - basically, additional single-source
systems spread through the auditorium. The goal of each of these approaches
is to allow each listener to receive speech information from one location,
emulating the highly successful and time-proven characteristics of face-to-face
communication.
Jesus understood this quite well, as he placed the masses on the hillside and spoke to them from a boat using the reflection from the water’s surface for amplification. And ironically, this is also exactly the goal of a good video projection system - to make sure that everyone can see the same image. And since the required characteristics of speech are wellunderstood, we can use scientific methods to assure that they are met.
In Part Two, which will appear in the May/ June issue of Church Production Magazine (CPM), we will explore the use of single- and multi-channel sound systems for churches, and more on loudspeaker placement.
Pat Brown teaches the Syn-Aud-Con seminars and workshops.
Synergetic Audio Concepts (Syn-Aud-Con) has been a leader in audio education
since 1973. With nearly 15,000 “graduates” worldwide, Syn-Aud-
Can is dedicated to teaching the basics of audio and acoustics. For more information,
visit their website at www.synaudcon.com or call 1-800-796-2831.
