Collateral Coverage, Friend or Foe?

In Uncategorizedby tfwm

Ok, so you have the latest greatest loudspeakers available to mankind with all the processing and power that any road warrior would be envious of. You’ve professionally rigged your speaker system in an approved and secure manner, you’ve used the best money that wire can buy (the ones with the arrows in right directions), you insert your favorite CD (the one with the proper GREEN felt marker applied to the edge) in your high end CD player and spin it up, you dial the console up with great anticipation and… WHAT ON EARTH IS THAT SOUND?!! Kind of like that bunny on TV, it keeps going and going and going, and you are left wondering if you have the proper CD in the player or what; ya, blame it on the CD, that’s it. Maybe the effects were dialed up; nope, not today.

Disturbingly, this is not that far from the reality that so many churches experience, to one degree or another, upon the first use of their new facility and or sound system. This article will focus on the other part of the loudspeaker performance equation; the room and the speaker systems interaction with it. That’s right: the overall performance in a space is an amalgamation of the speaker system and the environment in which it is in; the room. Not considering the two as a whole is kind of like riding your bicycle without the seat; the possibility of things going terribly wrong is very high.

For purpose of this discussion, let’s get a couple of terms very basically defined before we proceed.

Direct Energy: The sound from the loudspeaker, or sound source, directly arriving or impacting on the listeners or architectural surfaces.

Reflected Energy: The sound that has impacted a surface, or multiples of surfaces, and is redirected elsewhere.

Inverse Square Law: The direct sound level will change by plus or minus 6dB for every doubling of the distance to or from an omni directional sound source in a free field; +20log D1\D2. (Ease up all you line array folks, the basic concept is the same for line arrays with different numbers (3dB); that is IF, in fact, you actually have a true line source.)

Ok, enough of the definitions for now.

One of the sages of the business and a mentor to me in my early development was Jim Carey (not to be confused with the movie star), who had a great knack for boiling the complex down into concepts that people can wrap their minds around. Two of the most important, in respect to speaker application, which have stuck with me are: “Rule#1: Put sound where people are.” and “Rule#2: Don’t put sound where people aren’t.” As time has passed, the third rule becomes apparent and so, Rule#3 is: Sometimes you have to bend Rule#2 to get Rule#1 to work out properly. Alas, the world is full of un-absolutes. The basic point of Rule#3 is that in order to make proper coverage of the places where people will be, it is often necessary to cover or “light up” places, with loudspeaker sound energy “coverage”, where people are not sitting. Management of this “collateral coverage” is necessary to ensure that the surfaces that are being lit up by the loudspeaker system do not redirect sound energy into places where that sound renders a negative impact on the use of the system. We’ll delve into both sides of Rule#1 and Rule#2 to see what the implication of Rule#3 actually is.

Let’s briefly plumb the surface of what, at first impression, it takes to satisfy Rule#1, shall we? This will not be an all encompassing dissertation of loudspeaker deployment, but rather a brief exercise in some of the major factors in using loudspeakers to provide uniform sound levels to a seating area. In order to consider an area to be adequately covered with reinforced sound the coverage should not vary in level more than 6dB that is 3dB; preferably I like to see coverage to be in the 4 to 5dB envelope. It should have that uniformity coverage over a wide frequency range as well. Sounds simple, but in application is harder than many people may think.

Let’s take a simple room and put some speakers in it and see what’s up. Our room (Figure-1) is roughly 110 feet wide, 75 feet deep and 28 feet tall inside with a platform on the long wall. You may recognize these dimensions; they are typical of so many Sanctanasium, Gymnatorium, or other intermediate step multipurpose rooms that are cropping up all over the place. One common problem with these kinds of spaces is that they are often considered as “temporary” and thus the funds are not made available to adequately equip or acoustically treat them.

We have placed a few speakers in it, but let’s just consider the center speaker for the moment. Every loudspeaker has a balloon of sound energy that radiates from it. This is roughly analogous to the pattern or puddle of light that emanates from a flashlight. The pattern form a loudspeaker is typically specified in horizontal and vertical angles, in degrees, where the balloon of energy has diminished by 6dB from the level of the on axis aspect of the source. The manufacturers generally specify a “NOMINAL” coverage in angle of degrees. This coverage balloon shape and size will vary at different frequencies and as the simple nominal coverage angle suggests is everything but rectangular. Figure-2 shows a fairly well behaved balloon at the 1/3 octave band of 2500Hz. Figure-3 shows a Cartesian view of the coverage, from the perspective of looking out from inside of the speaker, and has crosshairs that indicate the relative angle. There are three pattern lines indicating the coverage, relative to the on axis level, with the inner (red) representing a 3dB loss of level, the second (yellow) showing a 6dB drop and finally the third (green) indicating the point where the energy level has dropped by 9dB. This speaker is advertised as a 60/40 degree Nominal pattern and is fairly close to that at this frequency band.

Unfortunately it isn’t that simple, Figure-4 shows a typical two way loudspeaker at four different frequencies; what would you call the Nominal coverage angle of this speaker? The manufacturer specifies it as a Nominal 90/60 pattern; and this isn’t even the poorest example out there. There is a pattern of energy that emanates from a loudspeaker that almost universally varies with frequency and is not, necessarily, what the manufacturer published “Nominal” coverage angle specification says they are; so buyer beware.

Let’s step back to the room for a moment and determine what the room’s basic Inverse Square or distance vs. sound level losses or gains will be. If the sound source was omni directional, spherical radiator, placed at our speaker location the direct sound levels at our listening plane will follow the inverse square law relative to the distances from the sound source. Figure-5 shows the Inverse Square behavior of the room. There is roughly 10dB difference from the center line front seating to the center rear seating; if calculated using Inverse Square Law it calculates out at 9.65dB. If you use the level at the rear of the seating as your reference (D1) and move forward to the point where you are half as close to the sound source (D2) the direct sound level will raise by 6dB as shown in Figure-6; that’s the law and a physical certainty. The level increases purely as a function of the Inverse Square Law (+20log D1\D2) and is often referred to as inverse square gain or loss in respect to loudspeakers. Amongst some of my friends, we sometimes call this the “room” gain or loss, as it is related to the room rather than the loudspeaker. This is not to be confused with the term used in traditional acoustics to define the apparent rise in sound level by a listener at a location due to early reflections arriving within the inclusion time.

Fortunately our loudspeaker has some directivity, and that works in our favor to level out the inverse square gain as we move forward in the room. Ok, so we have taken our basic 60/40 degree speaker and aimed it into our room so that we have as reasonable coverage as possible at the rear of the room; which, by the way, is at the farthest part of the coverage. There are a number of reasons to ensure that there are good direct sound levels at the rear of the room. As a basic rule of thumb; in order to have the best level at the farthest seat, it is necessary to aim the speaker directly at that farthest listening point. This is to minimize the inverse square loss of the direct sound level at that location, thus delivering the maximum sound level, deliverable, from that speaker/location to the rear seats. The resulting vertical down angle of the speaker is -20 degrees. If a purely Inverse Square behavior were present, we would be -6dB at the position of D2. But look at Figure-7 and you will see that we are not up by 6dB, we are actually up by around 3dB. This is due to the inverse square gain and the compensating loss of the loudspeakers polar pattern. The yellow line is the 3dB down on the polar and the red line is the 6dB down polar line. So while the room level is going up, the loudspeakers level is going down and it compensates some. Actually, if you look at the numbers the loudest point on the seating is at 103.8dB, the rear of the seating is at 100.7 (-3.1dB) and the front row is at 97.5dB (-6.4dB); pretty close to covering the space, not withstanding the horizontal issues. We could use some moderate front fill or down fill speaker and nicely bring that level up by 3dB or so and have pretty nice coverage; Rule#1 satisfied, right?

Now, I recognize we have not considered the horizontal coverage issues and that has been intentional in an effort to keep the interactions as straight forward as possible. By adding the side speakers in order to make the horizontal coverage, the interactions only become more complicated and create more interactions with the acoustic space. Simply put, it only gets worse. As mentioned at the onset, this is not a comprehensive article on loudspeaker aiming and deployment; we’ll save that for another time.

What about Rule#2? Well, let’s see just what that speaker is covering in the room. Figure-8 is what you would see if you were looking out of the loudspeaker into the room with the polar isobars indicated for -3dB,-6dB and -9dB. You’ll note that most of the rear wall is within the -6dB isobar and better than half is within the -3dB isobar; I’d call that lit up, wouldn’t you? Is that a problem or not? The answer: it depends on what that wall is made of. Unfortunately, all too often that wall, in rooms like this, are made of concrete block, tilt-up concrete or at least gypsum board sheet rock and are very reflective. These kinds of surfaces are very efficient reflectors of sound energy and will dutifully redirect the sound that impinges upon them, without prejudice or compassion, to wherever the laws of physics dictate they should. You’re starting to see why I refer to it as collateral coverage; if not yet, you will.

In installment 2 of this article we will examine just what the implications of this collateral coverage is, and discuss possibilities for mitigating the impact of collateral coverage or possibly how to avoid it. Collateral Coverage, friend or foe? Are we doomed? Tune in next time to see if our caped crusader triumphs or gets mashed into unintelligible acoustic goo.