The Birth of the Array: Part 5
“How to get more coverage out of less boxes and still get paid”
The big conventional array is not dead. As much as we have seen their weaknesses, they still have virtues, one of which is flexibility; they will go more places a line array will.
This particular system is the Nexo Alpha. It is made up of building blocks of mid-high and bass boxes with the sub-bass on the floor. It’s likened to a ‘Lego’ PA that can be any size from a small club to a stadium. This type of PA, at a very big gig, would most likely get the distance covered with the addition of delay towers
Enter the Delayed System
We have seen what the effect of placing two speaker boxes next to each other does to the polar plot of an individual speaker box. Two speaker boxes reproducing the same frequency band will generate individual sound waves that will interact with each other and cause both partial cancellation and coupling effects, depending on frequency and distance. There are some high-profile speaker box manufacturers that will claim this phenomenon has no effect on the performance of their products, just as there are a smaller but growing number of manufacturers that “hang their hats” on the fact that they have solved this vexing technical hindrance to uniform coverage over a large area.
The fact is that the point source cluster will be around for a while yet, for the same collection of reasons as why most of us are still driving gasoline-powered cars. So until a more enlightened audio world is upon us, let’s look at how a technically savvy engineer/soundtech/roadie/PA putter-upper, can get paid for delivering quality audio with a big pile of speaker boxes. There is more to the bigger is better wall of boxes making a big noise; there’s a smaller pile, and a delay line. This is the delayed system. This combination when done correctly equals uniform coverage, and mitigates a few other multi box array problems as well.
What is a Delayed System?
Let’s look back at the inverse law chart we started with and a sound wave SPL of 120 dB.
Assuming we don’t mind some interference and incoherency from using multiple sound sources, we can get “loud enough” at the back of the listening area. What about the front? If our 50 m room had a flat floor, these days for OH&S reasons, we need to put up a fence in front of the loudspeaker at a distance of 5m to 6m in order to keep people from getting so close to the speakers that they would feel pain from the pressure in their ears. At 7 meters the peaks would be about 117 dB – just under the threshold of pain. The next pain is the litigation industry arriving at the promoter’s door. Even though the Nexo Alpha box is capable of 145dB peaks, at 40-50 meters we will have a big drop in level.
The answer is to add more boxes just before the physical point where the volume drops to too low a level. This is the delayed system.
Pictured is a front of house supported by a delayed system behind the mixing position. You don’t need to be doing arena shows to put up delays. Anywhere that an even coverage at a moderate level is required is a candidate for a delay.
It doesn’t need to be a really big event to need a delay
A big Hokker tent at the Port Fairy folk festival is a very good example where a delayed system is necessary even though the PA system is loud enough to throw to the back of the tent.
At the Port Fairy Folk Festival there is a volume problem of sound spilling between the tents, so the SPL per tent needs to be low enough for the inverse square law to attenuate the sound outside the tent and minimise spill. However it must be still loud and accurate enough to satisfy the crowd within.
Notice (in the picture below) the compact powered boxes hanging from the roof trusses in two concentric delay rings. Sound coverage will be even and feedback risk from the FOH system to the stage will be less, as the FOH will not be at a high level.
A front of house supported by two delay lines fixes the coverage and SPL problem. The bottom near front box is just in the picture
Acoustic festival gigs can be a real booby trap for the inexperienced, as there are often many open microphones on the stage set to high gain levels to hear a broad assortment of unusual (and quiet) acoustic instruments.
New Age Sensitive Cow Persons. There politically correct performers are easy to reproduce through most PA systems. Some folk and jazz acts are much more difficult
How is it done without the whole area being drowned in echoes and reflections?
We are back to the speed of sound (1150 feet per second), the inverse square law and Gaspare P Schotto’s 1657 book on sound propagation and reflections. Imagine a speaker system either side of the stage and some smaller arrayed speakers set back in the room, at the point where the sound level is 6dB above the minimum acceptable level.
The purpose of the delayed speakers is to ‘mesh’ in the sound from the satellite speakers with the front of house sound. The acoustic image must still appear to be coming from the stage. If your satellite speakers are more than about 10 meters in front of the main cluster from the listeners’ position, a distracting time shift will occur that will greatly degrade the sonic image from the FOH.
If you are to use small satellite arrays, you must delay the sound coming from the satellite speakers, or their output will reach the ears of the people in the back rows ahead of the sound from the front speakers causing all kinds of problems. The principles applied here can also be used to improve bottom end performance and ‘steer’ the sound around the room if necessary. At this point of time we are not talking about comb filters and phase shift, the main priority is to get even coverage with the acoustic image where it should be.
Loudspeaker Synchronization with a Digital Delay
As we know, sound travels at about 1,130 feet per second in air, or about 1 millisecond per foot. On the other hand, electronic signals travel almost one million times faster through your sound system to the loudspeakers, effectively instantaneous. The main task of digital delays is to synchronize multiple loudspeakers so the sound traveling different distances through air arrives at the listener’s ears at about the same time.
Let’s deviate from the multi box delay system for a short moment. The delayed system has other applications other than the concentric ring of small boxes to cover a venue area evenly.
A delay can also improve intelligibility by minimising comb filters.
Synchronizing the loudspeakers also reduces reverberation and echoes for improved intelligibility. System controllers that are mandatory for most modern concert speaker systems often do this job. Comb filtering is not just ½ wavelength problems between speakers in an array, it can also occur at the crossover points in a multi element speaker box. In a vertical array, the controller will not help if your array is incorrectly installed. And this is also where a delay can help.
How to unsynchronise your signals
(In other words, why your array boxes must be physically aligned)
Here’s a test you can do yourself to demonstrate phase shift and comb filtering due to acoustic centres out of alignment: Stack two identical full-range loudspeakers. Carefully align the HF horns and wire the speakers in mono. Stand in front while listening to pink noise. Get an assistant to move the top speaker slowly away from the target area. Comb filters will cause the degradation in sound quality you hear.
Using a digital delay or programmable speaker controller to fix array problems
There are times when it is not practical or possible to set up an array in the technically correct fashion due to venue unsuitability, sight lines, set and lighting design.
A common example is out-fill boxes way behind the main stack that will spill into the FOH coverage area. The same goes for a split FOH stack that may be partially flown to hit upper balconies leaving an area covered by both systems.
Now back to the multi box coverage problem.
A delay will effectively time shift the most forward speaker element into the same time domain as the primary sound source.
You will notice in the above diagram, the delay speakers are pointed in the same direction as the FOH speaker. It is important not to have delayed speakers pointed across the wave front of the preceding sound source. If you think you need to angle a box to get coverage, put up more boxes; don’t set your satellites at an angle across the main wave.
Setting the delay times, the Precedence Effect: Aligning the Acoustic Image
In the early days, a sound guy would be running around the building with a tape measure but now there is an easier way thanks to a guy called Haas. Helmut Haas (yet another German name shaping the audio world), published a study in 1951 describing a series of experiments that demonstrated how people perceive delayed signals and echoes. In his experiments, a listener was positioned between two speakers placed 3 meters away; one was placed 45 degrees to the right and the other was placed 45 degrees to the left.
When the same program was played through both speakers simultaneously, the listener perceived the acoustic image (the direction from which the sound seemed to be coming) centered between the speakers - in other words, a mono playback through two spaced speakers.
When Haas delayed the signal going to one of the speakers by somewhere between 5 to 35 milliseconds, the listener perceived a shift in the acoustic image to the speaker heard first. While the delayed speaker did not contribute to the apparent direction of the sound, it did make the program seem louder and “fuller.” (Get ready for the invention of stereo processors x years later).
Haas showed that you must increase the loudness of the delayed signal by about 8 to 10 dB (twice the perceived loudness) in order for the acoustic image to move back to the original center position. Increasing the loudness more than this, or increasing the delay somewhat more than 35 milliseconds, makes the delayed signal sound like an echo and ruins the effect.
The lesson here is that the human ear does not notice echoes under 35ms as an actual echo. Time shifts can be used as an aural effect to space a mix or synthesize image direction. Do we need to write this out 100 times? The phenomenon describing how the acoustic image follows the signal we hear first is called the Precedence Effect.
The phenomenon that makes two distinct sounds heard less than 35 ms apart seem like only one sound is call the Haas Effect.
Back at the gig
Above shows a typical situation where the performer is amplified by a center cluster hanging above the stage. Almost everybody in the audience will enjoy good sound, except those seated in the shadow of the balcony. So we add an under-balcony speaker to fill in the shadow.
Now we have sufficient volume under the balcony, but the sound from the two speakers arrives at the listener’s ears some 76 to 84 milliseconds apart. The two signals, along with their echoes, result in an unintelligible mush. We must delay the sound from the under-balcony speaker to synchronize the signals. Do we set the digital delay to 76 or 84 milliseconds? Obviously, the geometry will not allow us to exactly synchronize every location under the balcony; we have to compromise.
First, you must consider the program type. For spoken word programs, you will produce the best intelligibility if the signals from the under-balcony speakers arrive within 10 ms. of the signals from the center cluster. Therefore we should set the delay to 84-86 ms. you can allow a little more delay for programs that are mostly music.
In the final analysis, every setting is a compromise, and your ear has to be the final judge. Check the sound in several different locations throughout the auditorium and correct the most severe irregularities. The same principles for setting and testing delay times apply to outdoor shows.
Even with a big conventional array like this, there will be infill, some out-fill, stage-fill and delay towers. The same rules apply as if it was a tent at a folk festival. If arrayed and set correctly, there’s no reason it shouldn’t sound good everywhere. This assumes the band’s sound guy can mix and the band can play?
Synchronizing the output of a far-throw and short-throw loudspeaker.
In order to reach the proper coverage in larger venues, we often stack two arrays of full range speakers; a short throw center cluster or under-hang for the audience below and a far-throw speaker for the back of the auditorium. It is almost impossible to perfectly align the stacked speakers mechanically, so comb filter distortion becomes a problem in the area where the levels from both speakers are equal. The same thing happens with extra speakers mounted on the right and left sides as ‘infill’. We know it is impossible to remove comb filters with equalization, but a digital delay minimizes them without affecting the spectral balance for the rest of the audience.
Back with “The Bare-Naked Ladies”, FOH, out-fill and stage front-fill at the sides of the stage
Technique to time out-fill, or under-hang components of a primary array
Find the axis where the levels from the two speakers are equal. This is where the comb filters are most severe. Carefully adjust the digital delay so that the signal from both speakers arrives at precisely the same time. A quickie to do this is to run pink noise and walk across the coverage area. When the delay is correct, the pink noise will have minimal variation. Use the same procedure to align speakers within a cluster when necessary.
Remember we can also lower the comb filtering effect by turning down the ‘fill’ speakers 4-6 dB.
Remember, calculating delay time in terms of distance is a common and accepted method. For a good start, estimate the delay at 1 millisecond per foot between speakers and the audience.
In the situation where front fill is required, the front fill is closer than the flown cluster and must be delayed. The sound operator will estimate the distance to the flown boxes and add 15-20 ms. the image will then appear to come from the flown cluster and stage.
The key points for setting delays
1. Visualise how the eave is going to propagate from the primary system and locate the delay boxes so they will be pointed in the same direction as the radiating wave. A delay box will become an annoying echo if it radiated across the primary wave front and shift the acoustic image from the stage area.
2. Work out the distance and add 10-20 ms and stop when the image shifts from the delay box to the FOH. This is done by ear just as well as with a tape measure.
3. Delay boxes don’t have to be loud and in most cases can be high passed at 100 Hz for better intelligibility. It is the high frequencies that have trouble making the distance, generally not the bass.
The trick is to mesh the delay boxes into the advancing wavefront seamlessly while retaining the acoustic image at the stage. The only problem with perfectly timed and positioned delay boxes is the constant flow of ‘helpers’ that will be telling you, “Hey mate, them boxes up the back are off!” Treat it as a complement. Try it yourself. Set up some boxes as described and listen to a delay line in and out of sync.
These days’ digital processors are not so expensive. Especially if your system amplifiers are all together somewhere accessible or you have a distribution network running powered boxes. All you need is time. The effects will range from subtle to considerable improvement.
So what does it all mean?
So here we are with a smaller array at the front of the stage that can be focused to where it’s needed and with less interference effect (with luck) due to its smaller number of elements. The distributed single source boxes spread around make up for the falling SPL and are easier to EQ as they are only single boxes. None of them will be pushed too hard and so long as you have time to test the system sufficiently, a seamless field of accurate coverage should be your reward.
Next month we will look at the anatomy of the “dud gig” as far as reverberant spaces are concerned.
Recognise this? Remind you of any venues you have battled with lately? Some performance space designers seem to aspire to find the most reflective surfaces possible and marvel at the three seconds of reverb they have created. Enter the unfortunate sound guy who will have to put up a sound system one day. It will all end in a crescendo of loud, unintelligible abuse.
The Birth of the Array: Part One
The Birth of the Array: Part Two
The Birth of the Array: Part Three
The Birth of the Array: Part Four
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