Power Shading and More for Loudspeaker Systems
By Marty McCann
Power shading is something that many of us have been doing for years. It involves reducing the sensitivity controls of some of the power amplifiers driving the loudspeaker components, and is usually done top to bottom and/or outside to inside.
Power Shading Outside to Inside
For many years I did the sound at the International Steel Guitar Convention, a music event that is held in a nice size hotel ballroom in St. Louis, Missouri. The stage was set up on a long wall, and the distance to the back wall was about 1/3 shorter than the needed left to right coverage. Before I began to shade the levels to the power amps, I would receive complaints that the lead instruments could not be heard nearly as well in the seats that were off-axis to the stage (and of course we had speaker enclosures pointed at those seats).
The problem dealt with the FOH mix position. The mixer was located in the center of the room, and the sound coming down what some call "power alley" included the on-axis contribution of the lead instrument amplifiers on the stage. The mix was well balanced in the center of the room, but this included the contribution of the power alley sound. As you moved farther to the sides, and thus lost the contribution coming from power alley, the mix was less than ideally balanced.
The solution: power amp shading the system outside to inside. This is generally only done to the mid/high components, but the system that I used had three mid/high components, left to right. The outside enclosure was set at full sensitivity, the middle enclosure had the power amplifier sensitivity reduced -4 dB, and the inside enclosures had their sensitivity reduced an additional -4 dB (or -8 dB of power amp input sensitivity). Now when the mix was balanced, while listening and mixing from the FOH position, it was also well balanced as you moved away from the center.
If mixing in a room that is long and narrow, you may only want to reduce power amp sensitivities to the mid/high enclosures that are filling the front center of the stage by about -4 to -6 dB.
Power Shading Top to Bottom
To understand power shading top to bottom, you must first have a general understanding of near field/far field and the -6 dB Inverse Square Law. Very briefly, when you are close to the sound system, you are in the near direct field, which is actually frequency dependent. You are in the near direct field when you are closer than two wavelengths from the sound source, and you are in the far direct field when you are more than two wavelengths away.
To find the wavelength of any frequency, divide the speed of sound (1130 ft. per second) by the frequency of interest. For example, the wavelength of 1000 Hz is 1.13 ft. (1130/1000 = 1.13). The wavelength for 125 Hz is 9 ft. So, if you are 18 ft. away from the sound system, you are in the far direct field for all frequencies above 125 Hz, but you are still in the near direct field for all those frequencies below 125 Hz.
Also needing to be understood, the Inverse Square Law says that sound drops proportionally with the inverse of the square of the distance away from the sound source. (I know that sentence is as clear as a cup of Cuban coffee to some of us.) On the decibel scale, the Inverse Square Law means that the sound pressure level (SPL) will drop 6 dB every time the distance away from the source is doubled. Hopefully, near/far direct field is understood as well as the fact that sound drops in levels of 6 dB every time you double the distance away from a sound source.
Now that you have a basic understanding of near field/far field and the Inverse Square Law, lets move on to power shading top to bottom. There has been a lot of talk recently about Line Array technology. If you observe most city-fest type events done by local and even some regional sound companies, they will place their loudspeaker on the wings of the stage in a manner where they are four times wider than they are tall. People who even have trapezoidal enclosures amuse me, and when they choose to spread the speakers out horizontally as a "flat wall," instead of going vertical with the loudspeaker placement. (This is still being done by some of the finest minds of the mid-seventies.)
If you want to project sound a greater distance, the sound system should be at least three to four times taller than it is wide. The reason most people take the path of least resistance is that it is simpler and faster to spread out in the horizontal plane, but this is not "good science" acoustically. It does take more time and labor to go vertical and it also requires the use of scaffolding to secure the vertical array.
However, let's assume that you have arrayed your enclosures to where the vertical dimension is much greater than the horizontal, and you have a top, middle and bottom row of mid/high boxes. Many people would just operate the entire system at the same settings of the power amp sensitivities. There will be some additional SPL in the three rows of mid/high enclosures, but there will likewise be some comb filtering losses or cancellation due to the fact that the sources are not coincident, or originating from the same point source in time and space. So yes, with all mid/high enclosures operating at the same level, it will be louder at a distance but it won't necessarily be of the highest possible quality.
We should now examine conditions from the standpoint of those in the audience that are very close to the system (within 10 meters). These near field listeners are only benefiting from the output of the mid/high enclosure that is on the bottom row. If the system is NOT shaded in level, then they are forced to listen to an uncomfortably high SPL. If however, the systems bottom row of mid/high enclosures is reduced in level by -6 to -10 dB, then these near field audience members experience a more pleasurable SPL with less maiming and sterilization. The real advantage of this technique is that this bottom row is less loud, thus contributing to less interaction or much less comb filtering for the middle and top row of mid/high boxes.
Another thing that is often done today when employing modern digital signal processing (DSP) based crossovers such as the Peavey CEX-5, is actually delaying of the entire sound system to match the arrival of the signals that are originating at the backline of the musical instrument amplifiers on stage. For instance, if the backline of musical instrument amplifiers is physically 14 ft. behind the loudspeaker system, you can dial in 14 ft. of pre-delay in the input section of the CEX-5. This allows the arrival time of the sound from the line of stage amplifiers and the speaker system itself to be in sync. It can be precisely aligned by use of the delay locator in a measurement system such as SIA Live Smaart, or some other Impulse measurement system. (For more information on Impulse measurement and dual FFT, check out http://www.siasoft.com/. If you are interested in more information about the CEX-5 Digital Signal Processor check out http://aa.peavey.com/cex_5.html. You can also download free software for the CEX-5 by clicking on the software bar of this website page.)
Now, since this much time has been spent pointing out the basis of "power shading", I may as well add a couple of more thoughts. Those of us that do outdoor shows where the temperature is very hot and the lay of the land very flat, should actually configure the system vertically as described above. We should also have the ability to angle the mid/high enclosures downward to actually increase the projection or throw of the system. When you have a large crowd of people on a hot, flat surface, the heat will rise up from the mid/high boxes covering the far field are angled down considerably, then the otherwise lost information is still defracted, but it will defract in the direction of the far field listeners, and not as much to the birdies. However, to accomplish this form of downward mid/high enclosure placement, requires a lot of secured scaffolding, as well as cross or walk boards, to create a solid foundation. Dont forget to tie things off, especially if there might be any kind of chance that some crowd diver may scamper up the scaffolding.
A lot of this has been done for years by the most knowledgeable individual sound system designers. The future of sound reinforcement in the DSP world will include a DSP in the front of (or maybe built into) each power amplifier that drives the individual components of the system. Using dual channel FFT measurements, the system will be calibrated in its entirety to a precision that was never possible before DSP. Does All-Pass Filter mean anything to you? It should. The future is here!
Of course there are those out there that will think that I am as full of it as a Christmas turkey. They will continue to use obsolete analog 3rd order Butterworth filter crossovers on their not-very-tall, horizontally spread-out sound system, following in the footsteps of those finest minds of the 70s.
I hope this article is helpful to your ever-increasing understanding of the intricacies of electroacoustics and sound systems. The beauty of educating others (if they are ready for the information) is you can teach someone in fifteen minutes, something that it took you more than fifteen years to learn.