This month’s article will be on speaker placement, as in what to do and what not to do. Recently, we have been working with several groups, who I have noticed, don’t set their sound system speakers in a venue for the best optimum results. Now, back in the day when most groups packed around Altec A-7 “Voice of the Theatre” speaker systems, there was not much controversy over where to set these speaker cabinets. The consensus back then was the cabinets were big, heavy and bulky, and you set them in the easiest place possible. Well times have changed, and now as a rule, most groups use smaller main cabinets on tripods, backed up with subs located on the floor. Nothing is inherently wrong with this concept at all, if a few rules of physics are followed. However, there are a few hard and fast rules of placement that I see broken time and time again. Whether these gaffes are from lack of knowledge or simple apathy, I have no way of knowing.
Many of the venues of today are the wide, wedge shaped buildings. This architecture is very common in both churches and performance halls. The concept being that even the back rows are relatively close to the stage, though some seats are far to the right and left. We were recently on a program at just such a venue, where one of the groups on this particular evening, set up two main cabinets on either side of the stage for a total of 4 main speaker cabinets for the house. On each side of the stage they placed two tripod stands, each with it’s one speaker, setting somewhat side by side, aimed in a splayed out manner, in an attempt to evenly cover the audience in the performance hall. The one major flaw in this thought process however, is that with this speaker placement, there is only one extremely narrow set of seats in the building that will receive a decent mix. That seating, so aligned, square in the acoustic center, or middle between these two speaker cabinets will hear decent quality. Every other seat in the venue will hear two very phased, time offset signal sources. The final effect being a pinched, out of phase, harsh quality to the sound, resulting from the time smear caused by the two cabinets setting side by side. Those persons unfortunate to be setting in the middle of the performance hall receive a double dose of this less than stellar sound quality, as they are hearing sound coming from both sides of the stage, adding even further degradation to the sound quality and the enjoyment of the concert. Now, the idea to use 4 main cabinets to cover a very wide performance hall was in of itself, the right approach. Taking into account that most main speakers such as those used in this situation, have a horizontal coverage of about 90 degrees, and only two such cabinets, right and left, would never cover the seating area of such a wide venue, the group had little choice but to utilize four speaker cabinets. The idea in theory was correct, their execution of the idea was however, incorrect.
The better method for placement of the 4 speakers would have been first off, to use only one tripod on either side of the stage. Next, the depth of the cabinets in inches from the face of the cabinet to the mouth of the driver mounted on the rear of the horn must be known. This distance from the face of the cabinet to the mouth of the high frequency driver is crucial and must be very accurate, as this distance will become the acoustic pivot point. Secondly, after installing the first speaker cabinet on top of a tripod, the second speaker would then be turned upside down, so that the horn is on the bottom of the cabinet, and the woofer is at the top. This second cabinet is now set on top of the first cabinet. The top speaker cabinet and the lower speaker cabinet can now be rotated at the pre-determined pivot point until both cabinets are aimed in a way that ensures consistent coverage in the wide venue. In effect you have made a single, time aligned speaker cabinet, which has one woofer on top, two horns directly together, and another woofer on the bottom. Time smear is now virtually eliminated, as is the phase shift, and the resultant harsh pinched sound. Let me qualify this a bit. The acoustic image now created in the venue will be as good as the speaker system itself can generate, in combination with the acoustics of the building. We can never be any better than our source in the environment, but we certainly should not make the conditions worse by misguided speaker placement.
The next “misguided” speaker placement I see on a regular basis is how speakers systems are aimed at the audience. Most groups, including most professional groups, don’t have a clue how this should be done. The usual rule of thumb most use, is set one cabinet (or more) on either side of the stage pointed directly out. The acoustic image emitting directly from the center of each cabinet is essentially parallel to the other cabinet on the other side of the room, and if in a square venue, also parallel to the side walls of the church or venue, and then, many times striking the rear wall at a square on 90 degree angle. This is so wrong on so many levels I fret over to where to begin and what to expand upon.
First, you should always turn your main cabinets so that the center of the horns cross each other anywhere from 2/3 thirds toward the rear of the building, to as far away as the last row of pews. The amount of cross aiming one uses, depends upon the depth of the building, the horizontal coverage of your speaker cabinets, and the seating area to be covered. One very good rule of thumb is to restrict the amount of direct radiated energy hitting the side and rear walls of the venue. In this manner you eliminate much of the unwanted room modes where acoustic cancellations occur, and you also limit the “bounce-back” of reflected energy coming off any parallel surfaces. Think of your speaker system as holding down two points of an triangle. Where the far point of this triangle is located depends on the room you are in, and whether it is square, or fan shaped as many of the newer larger churches are these days. I recommend drawing a centerline across the top of each speaker cabinet, and then a second and third line depicting the angle of horizontal coverage of your particular speaker cabinet. If your speaker cabinet has coverage of 90 degrees, your left and right lines, either side of the center line, would reflect 45 degrees from center. You would be amazed how quickly these three lines can help you see problem areas in a room you are about to perform in.
And now, the one question I am asked probably more frequently than any other question, “Should I use pink noise to set my system?” The answer is never yes or no, instead it is more like “Do you know how to use pink noise properly?” The ONLY time pink noise will give you an accurate account of the frequency response of your speaker system is outside, or in an anechoic chamber. Knowing that few of us have access to a good chamber, outside becomes the only possible choice for doing extremely accurate pink noise sweeps. When I build a new system, or re-calibrate an older one, I take the speaker cabinets and test instrumentation outside so that I can get a very accurate account of what the entire system is doing. This includes some pink noise testing, but also includes time domain matching of crossovers to drivers, and tests that determine group delay before and after processing. I also use swept bandpass noise testing, which is far more accurate than pink noise alone. So, should you still use pink noise inside a venue to set your sound system? The answer is maybe. The problems with pink noise in a room environment are as complex as the acoustic domain within the room itself. If you use a high quality calibrated microphone, adjusting a pink noise source via a 1/3 octave equalizer to “tune the room”, you will succeed in ‘bending’ the frequency response of your system so that the “measured” response at the microphone is essentially flat, or to a curve of your liking at that single position in the room only. If you move the microphone an inch to the left or right, the response will change dramatically. Acoustics are complex, especially on the high end as wavelengths are so short that moving even a small amount can drastically change the measured response. If however, you use pink noise to determine the holes and the peaks of the acoustics of the venue, then some knowledge of what you need to do maybe obtained. Finding the problem areas in a venue can be done with limited success using pink noise, but bear in mind, pink noise does not take into account things like reflected versus direct signal, early reflections (structure born) or decay time (reverb time). All of these factors come into play when doing sound in a venue, and weigh heavily on the final quality of the performance within that venue. There are test methods to determine such artifacts in room acoustics, but they are expensive and require a great deal of training to learn the techniques in using these tools. New tools like Audio Precision’s new chirp technology and older methods such as TDS (time delay spectrometry) are two such high end tools that artists or even sound companies rarely, if ever, use.
Now, just a word about “room tuning”. Please keep in mind that any band on any equalizer causes phase shift to accomplish the eq it is directed to do. When you use an equalizer in a boost mode to fill in a “hole” that is seen by your RTA (real time analyzer) you are introducing phase shift within the system, and since you are in a boost mode, rather than a cut mode, the phase shift caused by the eq is quite easy to hear, since it is now increased in level. This increased phase shift is caused by the slight delay of the eq filters at the fundamental frequency at which it is set. The greatest phase shift will be at the center frequency, but phase shift will occur either side of the center frequency equal to the width of the band at which it works. In the cause of a 1/3rd octave eq, the phase shift caused by the frequency dependent filter will be 1/6th octave either side of the fundamental frequency, and will certainly cause a degree of harsh tonality to the overall sound. The filled acoustic hole versus the inherent phase shift maybe a great trade-off to make, but make no mistake, it is a trade off. The very term “Tuning a room” is of itself totally impossible with electronics. When you equalize a sound system for a certain desired response within a given room, you are not “tuning the room” but instead are severely bending the frequency and phase response of your speaker system in an attempt to offset anomalies within the acoustics of the room. The ONLY way to tune a room is to change the physical character of the room itself with acoustic absorbers, or acoustic deflection. Limiting the reinforced sound to the desired seating area only can be helpful, but it is not tuning the room. Ah, which brings us back to correct speaker placement.
Aiming your speaker cabinets in such a way as to minimize the contributing factors of the room itself will go a long way in accomplishing what many people attempt to do with this so-called “room tuning”. The concept is to cover the audience with sound while at the same time limiting the effects of reflected paths, parallel walls, floors and ceilings.
Speaker placement is an art that few really understand. Many Southern Gospel artists have become backyard sound companies not because they know what they are doing, but sadly, because they had little choice in the situation. I see many part time groups emulating the mistakes of their Southern Gospel heroes because they think their heroes know what they are doing. Thus, bad practice is learned and handed down time after time after time.
It reminds me of a recording session I did several years ago in Nashville. The drummer on the session that day was the famed Larrie London, and in my view, he was the best that has ever been. Another engineer who was to do a session in that same studio the next morning, came in during our session and went crazy for the drum sound we were getting. He wanted to know exactly how that drum sound was obtained. Later in the day, Larrie had to tear down his drums for a session at another studio that evening. So, I pushed the microphones out of his way so that the cartage company could remove Larrie’s drums without damaging some very expensive studio microphones. The next morning Larrie was booked back at this same studio, so his drums were back in place the following morning, albeit with the microphones still pushed to the side. The engineer came in and just assumed the drums had been there all night, and that the microphones were still placed just as they had been the day before. It was a terrible assumption, for he proceeded to use the mics just as they had been left, way out of position for Larrie London’s drums. The result was a poor drum sound and a wounded reputation for the engineer.
Many artists today are very much like that engineer. We tend to do things because we saw someone we admire and trust, using the same technique. Most professional artists, when it comes to acoustics and sound systems, are just like many of you, whistling in the dark. Sure, with success comes better equipment that is more forgiving, but that does not mean that proper knowledge of the art is very well known, if known at all. If you are one of those, simply mimicking procedures you have seen done, then you really need to become far more accomplished at the task. There are many great books on the subject, and an online search can provide you with tons of information on a very complex subject.
Ben Harris
http://www.southernsoundquartet.com
Author: Ben Harris
| Author's Website: www.southernsoundquartet.com
Written: 11/10/2009
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Category: Sound Advice
Comments: 3
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Good article. People used to wonder why I took so much time placing/aiming speakers. Is it not true though that inverting a cabinet on top of another cabinet narrows the coverage pattern of the HF horn? I was taught this technique as a way to get more throw out of a small system outdoors. And it worked.
No, it does not narrow the width of the horn at all. It simply doubles the acoustic energy available.....if setting exactly as the one below it. So in effect, you get more throw, but because of the increase in amplitude not the narrowing of the coverage angle. However, that is not the intent in this case. I would adjust the bottom of the two cabinets to cover the extreme left and right of the building, being careful to keep as much acoustic energy off the side walss as possible, and then rotate the top cabinets to cover the remaining. I want my speaker system to cross each other at least 2/3 of the way toward the rear of the building. Now as to throw, we use 120 degree horns on our system and getting to the rear of any building has never been a concern. I like the wide coverage because I can use just 2 cabinets and cover virtually any wide building and still fill the middle.
Invest in a pair of WorxAudio x2 Line Array Portable speaker cabinets and the coverage issues are over.Horizontal coverage is a true 160 degrees.A superb sounding cabinet.
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