calculating total power in PA systems

[Disney_SK]

Hi all,

Does anybody know how they calculate an overall PA system's power ratings on big events? I mean, when I see for exapmple that there is an 20kW soundsystem at the event, how did they get this number?
I though it is about summing RMS power ratings of all the amplifiers used, but someone told me it is not true... So could anybody explain this?

[bear]

Depends on who "they" are.

The sum of all the amplification seems about right.

It is clearly not "acoustic watts" - which is a different animal.

Knowing the total amplification power, or even the power per frequency band tells you nothing really. Even assuming you are talking only the "main" array. (there are some other details, but they're not important right now)

One needs to also know the sensitivity and dispersion characteristics to know how much SPL is out in the audience and where.

If you really care or want to know about the SPL, bring an SPL meter with you to the concert. The SPL will fall off with the square of the distance, iirc... so it's way louder closer in.

I wear "musicians earplugs" to all amplified events. You should too.

_-_-bear

[pinkmouse]

The general rule of thumb for spec-cing a PA system using modern efficient systems is between 2-5W per person, depending on the music and venue. So indoor orchestral for instance, would be around the 2W, outdoor rock or rap, around 5W.

Usually, the 20K system spec is defined by the total power of the amps driving the FOH rig, monitoring doesn't count.

[Disney_SK]

Yes I know something about SPL, so I know that, for example 20kW amplified system can sound very different at different events, with different speakers in different enviroments. I think it is mainly because speakers may have very different sensitivity, and 3dB raise in sensitivity is equal to doubling the power of an amplifier.
I just wanted to know if there is any given formula to calculate an overall power...
What is the relationship between "accoustic watts" and amplifier wattage, or loudspeaker efficiency?

[bear]

Originally posted by Disney_SK
Yes I know something about SPL, so I know that, for example 20kW amplified system can sound very different at different events, with different speakers in different enviroments. I think it is mainly because speakers may have very different sensitivity, and 3dB raise in sensitivity is equal to doubling the power of an amplifier.
I just wanted to know if there is any given formula to calculate an overall power...
What is the relationship between "accoustic watts" and amplifier wattage, or loudspeaker efficiency?

Not much... there isn't a direct relationship unless one is created via the appropriate data being available to plug into multiple equations which will then arrive at the application's specific result(s).

You can get the general equations for converting to acoustic watts from any decent college level acoustics book. Berenek comes to mind, although I am sure there are many others, ummm... Olsen will have it too.

The polar response counts big time as far as what is going to be present at any given spot in the audience - and this varies all over the map.

The present "vogue" is these "line arrays" with nominally wide & controlled horizontal dispersion characteristics. You can read up on these, as there are a number of "competing" techniques being employed.

But the bottom line is that today's PA/SR stuff gets hugely LOUD from surprisingly small boxes as compared to even 15 years ago. Fairly amazing what can come out of a relative handful of cabinets these days.

Are you trying to figure out what you need in a PA for yourself or someone you know or are you just curious about PAs??

_-_-bear

[Disney_SK]

Are you trying to figure out what you need in a PA for yourself or someone you know or are you just curious about PAs??

Both I mean... Our team is doing mainly dance event sound reinforcing services. Not big, about 100 to 200 people are present at the events, but in a few days a bigger event is coming - we espect at least 600 people. The event is indoors and we have about 3kW bass and 2kW full-band + sat configuration. So I though it would be most accurate to tell it is 5kW total. But now someone told me it is not... So this is why I want to know more about this, I am very interested in this.
Pinkmouse saved me with his 5W per-person calculation, so now I am quite sure that 5kW for 600 people would me enough... I hope :-)

[bear]

Without limiters on at least ur mains out you don't have enough power or speakers in general. Hope you have limiters, soft knee.

Much depends on your speakers efficiency & dispersion and the room.

Me, I'd put the drums on a sub mix and put a limiter on that output too... but you can have at it as you wish...

Try to not hit the limiters too hard all the time, other wise the thing will sound kinda squashed.

I'd tell the non techies that your power is the sum of all amps...

_-_-bear :Pawprint

[still4given]

Out-of-doors? Indoors? Good acoustics? Poor acoustics? Subwoofers?

There are many things that come into play. Our church seats 2000 people. Our system uses 5 Yamaha PC2002 power amps, (200WPC), driving 2 each 2X18 subs, 4 each 2X15+horn and 6 each 115+horn all Bi-amped. It is more than adequate. This system would be considered 2kW. Unless you are driving a lot of subs, or low very low efficient speakers, I think you have plenty. YMMV

Blessings, Terry

[Disney_SK]

Well I think it would be good to write detailed configuration in here.
It is indoors, pretty good accoustics in there. Kind of wide, short room. We got one 2x1500W amp, we are going to hook two 1000W bandpass-type subwoofers with 107dB sens. onto one channel, with f<120Hz and three 800W bassreflex-type subwoobers with 100dB sens. onto the other channel, crossed probably at 500 or 1000Hz.
Then, we have four 400/500W 3-bands that we are going to power with two 400W/ch amps and they will be crossed at 120Hz.
All subs will lay on the floor, just in front of the stage and all four 3-bands will be hooked at the ramp in the front of the stage, about 8m high. The room is about 40m long and 80m wide.

Any comments on this planned configuration are welcome

[still4given]

Originally posted by Disney_SK
Well I think it would be good to write detailed configuration in here.
It is indoors, pretty good accoustics in there. Kind of wide, short room. We got one 2x1500W amp, we are going to hook two 1000W bandpass-type subwoofers with 107dB sens. onto one channel, with f<120Hz and three 800W bassreflex-type subwoobers with 100dB sens. onto the other channel, crossed probably at 500 or 1000Hz.
Then, we have four 400/500W 3-bands that we are going to power with two 400W/ch amps and they will be crossed at 120Hz.
All subs will lay on the floor, just in front of the stage and all four 3-bands will be hooked at the ramp in the front of the stage, about 8m high. The room is about 40m long and 80m wide.

Any comments on this planned configuration are welcome

I can't imagine that not being enough if the sens. specs for your speakers are accurate.

I have a couple of questions. Is the Sub amp 2x1500 @ 8ohms? If so, when you hook up two 8ohm cabinets to one channel you will be driving 4 ohms. Most SS amps but out almost double into 4 ohms if they can handle the load. On the other channel you are planning to hook up three cabinets. If they are 8 ohm you will get around a 2 1/2 ohm load. If they are 4 ohm cabinets you might be in trouble. You should check to make sure your amp can handle the lower ohm loads.


As far a loudness goes, I suppose it depends on what style of music. You probably won't make folks ears ring for hours afterward like mine have after some concerts, but I'd bet it will be adequate unless the guitar players are using stacks of Marshalls.

Blessings, Terry