Double Distance Rule Gets You.

You might face a situation where the people in the front row are being blasted right out of their seats while the people in the rear are hardly able to hear. This is because sound heard directly decreases as you move away from a sound source (your speaker system). In a non-reverberant (non-reflecting) environment, such as outdoors, sound pressure level from a simple source will be cut in half (drop 6 dB) every time the distance from the speaker is doubled. This is called the "inverse square law." Figure 5 shows the dB losses to be expected as distance from the speaker is decreased from the four feet used in E-V SPL specifications. To deal with this "law of nature" you need specialized speakers to project sound to the back of the audience while not hitting the people up front with the extra-high SPL levels this requires at the source.

Room Reverberation Swamps Your Voice.

Now you say, "Great, I have a good microphone, mixer, power amp, and an efficient speaker with reasonably flat frequency response and uniform dispersion." But when you use this system, people in the back of the room still can't understand the vocals or really hear the high frequencies. This involves not only the speaker but also the room in which it is operating.

Rooms have a phenomenon called "reverberation." Reverberation is the tendency for sound to continue within a room after the original sound has ceased. Outdoors, in an open field, is considered to be a "non-reverberant" environment, so this continuation does not occur. But, as you are supplying sound to a room, reverberation is occurring. The farther a listener is from the speaker, the better the chance he is in the "reverberant field" and the worse the chance he can understand what is being put out by the speaker itself.

If the listener is close to the speaker, he is said to be in the "direct field" of the speaker. This is where the sound coming directly from the speaker is much higher than the reverberant sound. But, as you move away from the speaker, the sound reflected from the floor, ceiling, and walls gets increasingly louder relative to the sound coming directly from the speaker. This is where trouble begins. In a reverberant environment, there is a point away from the speaker beyond which the "reverberant field" dominates the sound heard. It is interesting to know that the SPL tends to remain constant in the reverberant field, no matter where you're standing in it. Constant SPL throughout the room is, of course, a good thing but when the reverberant field is what's doing it you can get into trouble, as we will soon see.

The distance where the reverberant field begins to dominate is typically 10 to 20 feet from the speaker, and is longest for the least reverberant rooms and the most directional speakers. The distance from the loudspeaker where the direct sound and the reverberant sound are the same level is called the "critical distance." In Figure 6 you can see the two equal sound pressure levels add and become 3 dB higher at the critical distance.

When you are in the reverberant field part of the room, most of the sound you hear is reflected from the walls, floor, ceiling, etc., and only a small amount comes directly from the speaker. All these reflections cause the sound to reach your ears at slightly varying time intervals, and at a higher level than the direct sound. The result is that listeners in the reverberant part of the room find it very hard to understand what is being sung, or to hear clearly the various instruments being played. The music tends to become a confused jumble of sounds. The concept of the reverberant field is one of the most important concepts to understand in this whole guide. If we had a way to sound a siren to direct your attention to a particularly important problem which affects P.A. '5 in all rooms, it would go off now! When listeners are in highly reverberant rooms or reverberant parts of a room, you're going to need to do something about it. The larger the room is, the worse the problem is. Don't let this scare you - that's why we're making this guide so the problem can be recognized and conquered! A few methods of dealing with the reverberant field problem would be:

1. Make the room "anechoic," meaning "no echoes." This is probably not feasible and would result in a highly modified and bizarre appearing room.

2. Move outdoors to a big field. Remember outdoors is non-reverberant, with no walls or other surfaces to reflect sound. This is obviously an impractical solution - especially when it rains.

3. Select loudspeaker components which have appropriate directional characteristics for the room. These characteristics are inherent in the dispersion or coverage angle of the components. Ultimately, you would want sound to go only into the area where listeners are, so none is sent to bounce off walls, etc. (Listeners are excellent sound absorbers.)This can only be approached, in actual practice, but the results of paying attention to this factor in system design can be astounding! If you are putting together a system for portable use, by all means try to figure out the system that will conquer the reverberant field problem for most of the rooms in which you typically play. Your audience will love you even more for it.

Let's examine the solution outlined in Item 3, above, in some detail. The reverberant field problem is the basic reason why a single speaker system cannot be the answer to all sound problems, even if it has flat frequency response, high efficiency, uniform dispersion, and big amp driving it. And it's the reason why you need to augment the single speaker with the building-block components that E-V has to offer. This is the solution to one of the problems outlined at the beginning of this guide. If you think stacking up several of the kinds of systems you might use in a small room will work well in a large room, you are certain to be surprised and disappointed. This will increase the total sound pressure level, but it will probably be unintelligible in most of the room.

Large rooms require both narrower dispersion and higher efficiency than the best single speaker system can offer. For instance, when the listeners in the back part of the room cannot understand the sound because of reflections and reverberation, the solution is to have a narrow coverage system that will aim more direct sound at the back of the room. (Note: this also addresses the problem of reduction in SPL with distance from the speaker.) It should be noted that wide or narrow dispersion does not denote a good or bad loudspeaker, providing the system is designed to provide proper coverage in the room or hall. There are applications where one or the other is needed to best solve a specific sound problem.

Narrow dispersion devices are sometimes referred to as "long throw." The term "throw" is loosely used to describe how far sound will be clearly projected by the loudspeaker. This is directly related to dispersion. To describe this principle, think of a garden hose with a variable sprayer on the end. The water in the hose is being delivered to the sprayer with a constant pressure (the speaker or driver). The sprayer determines where the water will go (the horn). If you spray a wide pattern, it won't spray very far; but if you clamp down on the sprayer, it will spray a narrow stream and it will spray (throw, project, etc.) a heck of a lot farther. This is exactly what takes place in sound. Most direct-radiating speaker systems are classed as medium-to-wide coverage because they have coverage zones of approximately 90° or wider. However, special devices are needed to generate high SPL, and uniform, narrow coverage angles. These devices are usually horns. It is possible to have horn woofers, horn midranges or horn tweeters. For example, a midrange driver (such as the E-V DH1Ol2) can be coupled to a wide-angle horn (such as the E-V HR90 with 90° side-to-side coverage) for short4o-medium throw, or it could be coupled to a narrow-angle horn (such as the E-V HR40 with 40° side-to-side coverage) for long throw. By their principle of operation, long-throw devices take care of the reverberation problems of medium4o-large size rooms. By raising the sound pressure level of the direct sound at the rear of the room, program material will become more intelligible. The long4hrow device is not only used to create higher sound pressure levels away from the stage but also to aim or concentrate the sound on the listeners at a distance away. The direct sound will be kept high relative to the reverberant sound, and - WOW! - you'll be saying, "Can you believe we actually understand the words way back here?"

Now we know how to get good, clear sound to all listeners on paper; but how in the world can you apply this to your specific system and problems? Good question! Obviously, this material cannot give all the answers, but with a clearer understanding of the problem, the E-V engineering data sheets, some research, and a lot of common sense you can usually come up with a system that will get the job done effectively.

The next area is where we will give you some recommendations on total component system design and application.

Basic Approach to System Design

In the discussion that follows, some specific rooms will be selected and systems that could be used in them described. As room size grows, the problems involved in providing adequate sound pressure level plus clear and intelligible sound to the audience increase. This is because larger rooms usually require more acoustical output from the loudspeakers than smaller ones. (Crudely speaking, a doubling of the volume of a room of given absorption characteristics means about twice as much acoustic output would be needed to maintain a given sound pressure level in it.) It is also because more of the volume of a large room will be in the reverberant sound field of a given loudspeaker, making the generation of clear sound for far away listeners more difficult.

The range of room sizes that will be discussed extends from about 10,000 cubic feet (about three times the volume of a typical home living room) to 30,000 cubic feet, to 90,000 cubic feet. These room-size increases make a very big difference in how much acoustic power must be injected into the room by the speakers to get a given SPL in the reverberant field of the room. Let's look at an example where we will assume, for simplicity, that the same speaker system would work in both the small 10,000-cubic-feet room and the large 90,000-cubic-feet room. If a 100-watt amp could get 100-dB average level in the small room, the big room would take 10 times the power - or 1000 watts - to get the same 100 dB. What you would really get, of course, is a puff of smoke and a depleted bank account.

How much acoustic muscle you need is also heavily dependent on how loud you want to play your type of music. The specific room and system examples tell you the maximum loudness you can expect but you may not need that much. Figure 7 shows the long-term average sound pressure levels typical of various musical (and a few non-musical) situations. Peaks of a few milliseconds' duration will typically be about 10 dB above the average levels shown.

Normal talking at one foot is about 70 dB. A level of 120 dB is painful to most human ears. A room level of 90dB would usually be judged to be pretty loud except in the case of a full-tilt, high energy rock band where sound pressure levels are likely to fall into the 105-115 dB range. (The SPL's we're talking about are "A-weighted," where the bass below about 500Hz is rolled off. This makes the measurements correlate more closely to the loudness our ears perceive, since they are much more sensitive to midrange frequencies than to the bass.)

In most P.A. work a usable low-frequency capability of 50 to 75 Hz is quite satisfactory. All of the systems described in the following section fall well within this range. For extended low-frequency capability, on stage, a bass guitar speaker system can be employed (such as the E-V Bi 15-M or B215-M) or for synthesizer reproduction down to 40 Hz, a wide-range system (such as the S18-3) can be employed. Examining the specification sheet for each E-V speaker system or component will give you some insight to the sound levels and frequency range it is capable of producing in the application you are. interested in.