To design a large front of house system, you need to start with the size of the audience and then work backwards. For the purposes of this article, we will assume that you understand the basics of sound propogation and have some basic math skills. We heartily implore you to ask the advice of a professional for your specific situation, since making mistakes can be very costly. This article is designed to give you an overview of the thought process needed to begin the design of a large system.
So, let's start with the task of designing an FOH system for 2000 people, for a typical rock band, and at an outside venue. So, each person takes up about a very comfortable 4'x4', which is 16ft square. Usually, the audience area is in a variation of a triangle or parallelogram, but it's way faster to solve for the sides of a square and then draw a line from two opposite corners giving you a triangle. Think of a square, multiply by two, the find the square root and that should be your maximum throw. Here is the example for this situation:
Total area needed = 16ft x 2000 = 32,000 sq feet
32,000 x 2 = 64,000
square root of 64,000= 252
Therefore, the longest throw should be about 250 feet from 45 degrees off of each side of the stage. This should be an adequate approximation for the purposes of this design, and typically you would have more specific site information when choosing a venue.
Next, is to determine the Sound Pressure Level, or SPL that you would like to achieve. If you wish the back of the house to experience a pretty full sound at 95db, then you use the db math of 6db of inverse square law and work backwards to 1 meter in front of the stage. This will result in the following graph:
125 ft = 101 db
62.5 ft = 107 db
31.25 ft = 113 db
15.625 ft = 119 db
7.8125 ft = 125 db
3.906 ft = 131 db
Therefore, we should be designing a system that can deliver about 130 to 135 db of sound at 1 meter, and this is the distance that most speaker cabinets have specifications for.
Now, let's pick a professional grade speaker (the manufacturer will rename nameless). This particular box has two 15in subwoofer drivers and a high frequency driver with a beam of 70x50 degrees. The specs also rate it at 99db 1m/1W, a peak of 136db, and a power handling rated as 1200W/2400W/3600W. This gives us an approximate value of 123 db at the RMS rating of 2400W. When you couple a sound source, that gives you a 3db boost, therefore you would need about 4 cabinets to give you 12 additional db's, which brings the SPL level back to 135 db, the SPL level we were designing to.
The next issue to tackle is whether or not we have sufficient dispersion. If the beam spread is 70 wide by 50 high, and the crowd radiates out around 45 degrees, and you have a stage that is somewhat around 50 feet wide, then you will need more speakers. High frequencies are very directional, and you will need to focus the high frequency drivers to specific places in the audience. If you have a Left Right stereo image, then you have to recalculate the distance from each side to the center line, and also along the outside edge, and the additional length of one side of the stage to the far end of the other. Since most stages would have this left right configuration, by simply grouping 2 cabs per side, you would lose a fair amount of high frequency power along the outside lines of the audience. Putting 4 speakers per side would grow the system beyond the SPL requirements along the center audience line. Therefore, here is where we can compromise and determine that need 3 cabinets per side for a total of 6 for the entire system. Also remember, that you will also have to include the very front rows in the beam pattern of these high frequency drivers, or the people who paid the most for the concert will not understand a word of the lead singer!
Are we done yet? Not yet. Most likely, you will also be adding subwoofers to increase the bass response of the system, since the double fifteen cabs will be driving mostly mid and high frequency information. Kick drum, and bass, will need the low frequencies that only subs can deliver. So, we will choose a double 18in cabinet that is rated 1600w/3200w/6400w and an SPL rating of 98db to 136db. Therefore, you can estimate that a single unit will produce 117db at average program level, and coupling will suggest that you need 6 units to get back up to 135db! However, we are going to separate the system into 3 bass cabinets per side since bass frequencies are very omnidirectional, and not need additional cabinets for dispersion.
Finally, we are near the end of our basic system, which will consist of a total of 6 tops and 6 bottoms, or 3 and 3 per side.
There are many, many other factors that will influence your system design, but this should be a useful tool for you in watching the thought process that goes into designing a system.
Please drop a note if you have comments or questions, I would love to hear them!
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This is very clear and simple.
ReplyDeleteThanks for this blog wich is very helpfull.
I have now two questions :
- How about corporate event as conferences :
example : 400 persons seated in 120x70 room
- I am looking for some noise signal to put in my FOH SPK to determine their response. Where can I find such things ?
Thanks,
info.produkson@gmail.com
A few points...
ReplyDeleteYou seem to have neglected headroom, in determining the sound levels required you would typically account for some desired headroom, maybe 10-20dB.
Why not do the distance right, as in 20log(d/dref). For speakers the 'dref' is typically 1 meter or 3.281 feet. Thus the loss for 252' (actually closer to 253') is 20log(252'/3.281') = 20log(76.8) = 37.7dB or approximately 38dB loss due to distance.
You seem to somewhat confuse coverage and 'coupling'. You may use multiple boxes to cover an audience area wider or deeper than a single box allows, but you typically do not necessarily use multiple boxes to get more output except under certain conditions.
When you utilize mutliple speakers for coverage the goal is that each speaker covers a different listener area and they do not overlap. Thus there is no 'gain' from the multiple speakers, only the improvements due to better coverage.
True coupling of multiple sources covering the same area, which is a 6dB gain per doubling of sources, only occurs when the individual sources are within 1/4 wavelength of one another over the operating range of the devices. This often works for subs and low frequencies where the wavelengths are long but at higher frequencies where the wavelengths are shorter this is difficult to do.
The result is that for much of your hypothetical speaker's range using mutliple speakers covering the same area would not result in coupling, but in overlap and combfiltering. You may get an overall 3dB increase in level but at any listening position various frequencies would be boosted and others reduced or even cancelled. Most system designers try to avoid this. So in your example you don't need three speakers per side, you need a different speaker.
A similar issue exists for the subs, the three subs on each side may couple but the two sets would be too far apart, instead they would interact and while you may get some overall average increase in level from that, you can also get the infamous 'power alley' where they soruces sum down the center of the audience, creating ver high levels on the centerline, but then have 'fingers' or lobes of varying levels off axis, including some areas of very low level. In fact, four subs tightly grouped together in the center would likely give about the same output as the suggested three subs on each side arrangment with two less boxes and no 'power alley'.
You say "The specs also rate it at 99db 1m/1W, a peak of 136db, and a power handling rated as 1200W/2400W/3600W. This gives us an approximate value of 123 db at the RMS rating of 2400W." That seems incorrect in more than one aspect. First, the 2400W rating would be the Program power rating, not an RMS rating. There really is no such thing as an RMS power rating, the ratings are Continuous/Program/Peak with the last two being calculated based on the measured Continuous rating.
Beyond that, s speaker rated 1200W/2400W/3600W and 99dB/1W/1m would have an output of 130dB, 133dB and 136dB, respectively. I'm not sure where the 123dB comes from, maybe this is wehere you were accounting for some headroom that you did not include in determining the desired SPL goal.
You note that "When you couple a sound source, that gives you a 3db boost, therefore you would need about 4 cabinets to give you 12 additional db's, which brings the SPL level back to 135 db, the SPL level we were designing to." and "Therefore, you can estimate that a single unit will produce 117db at average program level, and coupling will suggest that you need 6 units to get back up to 135db!" I think the math is off here. You do not get 3dB for each added source, coherent sources sum as 20log(number of sources) while random sources give 3dB per doubling of sources 10log(number of sources). So four sources gives 12db if they are truly coupled but 6dB otherwise, the latter is more likely with full range boxes. Six sources would be 16dB or 8dB, not 18dB as noted even for tightly grouped subs.
Not trying to pick this apart, but just hate to see misconceptions promulgated.
Dear Othmane,
ReplyDeleteIf you are having a typical conference which is mostly speech and some music beds, the system demands are much smaller. You could probably get by with a small system such as a couple of MI grade double 15s with horns per side, splayed for coverage. Don't forget to include monitors so the speakers can hear themselves clearly.
As to the second question, you might also want to include in your system a dbx Driverack PA, which includes a pink noise generator and you can get the matching RTA mic so the system can level itself off.
Thanks for writing!
Dear BPW,
ReplyDeleteWow! This is fantastic material, and I thank you for taking the time to write. I would like to rewrite the article, but I want to give you credit after the rewrite. Can you return email me directly with your information? Proaudionet@gmail.com.
I sincerely hope to hear from you.
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ReplyDelete