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Author Topic: How do you calculate PA size?  (Read 13812 times)

Robert Piascik

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How do you calculate PA size?
« on: August 07, 2014, 11:30:11 PM »

I am mostly a LabLounger but recently am getting some calls to provide for larger events and I want to supply (or rent) the correct thing. How do you (or can you) calculate what a PA set up is capable of delivering (SPL) at a particular distance?

I have a rider calling for 118dB at the mix position 80-100 ft. from stage. I realize that is vague, but if I have top speakers that can deliver 130 dB peak and 124 dB continuous, how do I know if that is enough or not enough? What if I had two per side? How about four per side?
If the subs can deliver 140 dB peak and 134 dB continuous, how is that going to factor into the equation? How would it be different if speakers were a line array? How would you know how many to hang? Does the speaker sensitivity factor into the calculations as well?

Thanks for any advice.

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Cailen Waddell

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Re: How do you calculate PA size?
« Reply #1 on: August 07, 2014, 11:41:37 PM »

Outside, sound decreases at about 6db for every doubling of distance.  So if your speakers output is 124 DB at 1m, then at 2 it is 118, at 4 112, at 8, 94, and at 16 meters, 88.

Now that's assuming the numbers you are using and have provided by the manufacturers are accurate and helpful.  If your subs peak output is 140 dB but It can only do that at 500 hz - how is that number helpful. 

Most of the big boys use modeling software to predict peak output.  For line arrays, and trap boxes.  That software will help you know how many boxes you need

There are some far better educated people than me here who can get more specific.

Doing the reverse math 118 at mix is 124 at 50', 130 at 25, 136 at 12.5, and 142 at 6.25, and 148 at 3.125', or about 1 meter.  That's 18db greater than the peak output of your boxes, IF the number from the manufacturer is accurate.  That's a big difference.  When you start to get into dealing with riser acceptable gear, and meetig riders, I find it best to forward the rider to the company you are renting from and ask what they would recommend. 
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Tim McCulloch

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Re: How do you calculate PA size?
« Reply #2 on: August 08, 2014, 12:09:25 AM »

I am mostly a LabLounger but recently am getting some calls to provide for larger events and I want to supply (or rent) the correct thing. How do you (or can you) calculate what a PA set up is capable of delivering (SPL) at a particular distance?

I have a rider calling for 118dB at the mix position 80-100 ft. from stage. I realize that is vague, but if I have top speakers that can deliver 130 dB peak and 124 dB continuous, how do I know if that is enough or not enough? What if I had two per side? How about four per side?
If the subs can deliver 140 dB peak and 134 dB continuous, how is that going to factor into the equation? How would it be different if speakers were a line array? How would you know how many to hang? Does the speaker sensitivity factor into the calculations as well?

Thanks for any advice.

Was the required 118dB A weighted, C weighted or flat?  Slow or fast response?  Realize that this "spec" is also probably copy and paste bullshit from some other bands rider from 20 years ago...

That said, 102dBA, slow, is loud enough that you'll need to holler to be heard by someone 4 feet away, and 110dBA will require you to shout at a person 1' foot from you.  Set up your rig and play the band's recording, turn it up until you run out of PA, begin to worry about the rig's safety or the police show up (or something breaks).  Use your trusty Radio Shack SPL meter (not a phone, seriously) to measure.

PM me with the name of the act and I'll tell you what I know about them, if anything.
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Mark Cadwallader

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Re: How do you calculate PA size?
« Reply #3 on: August 08, 2014, 12:30:02 AM »

A not-too-serious way to meet an unreasonable spec:  set up a remote speaker (suitably delayed, of course) 2 meters in front of the mix position. If somebody insists on a stupid requirement, they should expect a stupid solution to meet the requirement.

Please note that I don't really suggest that as a solution -- it is just what I wish I could do under those circumstances. Mark C.
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Tim McCulloch

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Re: How do you calculate PA size?
« Reply #4 on: August 08, 2014, 01:18:30 AM »

A not-too-serious way to meet an unreasonable spec:  set up a remote speaker (suitably delayed, of course) 2 meters in front of the mix position. If somebody insists on a stupid requirement, they should expect a stupid solution to meet the requirement.

Please note that I don't really suggest that as a solution -- it is just what I wish I could do under those circumstances. Mark C.

The other helpful possibility is moving the mix position closer to the stage, say 70' instead of 100' (you don't want to schlep a console up into the grandstand anyway).  While not 6dB worth of movement, if you're on the edge every little bit helps.
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Scott Holtzman

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Re: How do you calculate PA size?
« Reply #5 on: August 08, 2014, 01:22:15 AM »

A not-too-serious way to meet an unreasonable spec:  set up a remote speaker (suitably delayed, of course) 2 meters in front of the mix position. If somebody insists on a stupid requirement, they should expect a stupid solution to meet the requirement.

Please note that I don't really suggest that as a solution -- it is just what I wish I could do under those circumstances. Mark C.

True story I am at dinner with a prospect.  He has some of his team with him.  I was in the SE role, our regional sales director and some engineers and other folks from the vendor.

One of the engineers from the vendor was enjoying his wine at a good clip.  He never had much patience with stupid questions.  The client asks this guy how big a router do you think we need.  This guy swills another drink, looks at the client and asks him "how big a router can you fit up your ass?" the sales guys look like stone and the engineer follows up with, it's as good a criteria as any when you don't give me any f***ing data.

Classic

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Brian Jojade

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Re: How do you calculate PA size?
« Reply #6 on: August 08, 2014, 11:40:06 AM »

I have a rider calling for 118dB at the mix position 80-100 ft. from stage. I realize that is vague, but if I have top speakers that can deliver 130 dB peak and 124 dB continuous

First off, 118dB at the mix position 100 ft out is friggin loud.  I feel for anyone that is positioned between the stage and FOH.  Provide plenty of cotton balls to soak up the blood from their ears. Just stating that they want 118dB tells me that the rider is just as good as toilet paper, but not as soft.  It probably says they want a frequency response of 20Hz-20kHz too, right?


Secondly, using the listed spec of 130dB peak and 124dB continuous is just reading the marketing bullshit that you are seeing on the speaker spec sheet.  Good luck getting those results real world.  Adding MORE speakers per side will just garble your sound.  If the speaker isn't up to the task, more of them won't help.

For sizing the gear appropriately, you need to provide WAY more information than you have.  What's the venue in question?  If you're in a narrow bar with low ceilings and FOH is just at the back of the bar, that's very different than if you're in a football stadium that holds 20,000 people.

At the very minimum, when discussing a sound system, you will need to know the following.

1. Size of the venue being covered. The rough room dimensions will work.
2. Type of room - i.e., a theater, metal shed, tent, outdoors, etc. Each will have their own requirements to make sound work correctly.
3. How much of the venue you want filled with sound.  Depending on the event, they may not want even coverage for the whole room.
4. What type of music/speech are you playing?  An acoustic set will need far less PA than a grunge metal system.

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Brian Jojade

Roland Clarke

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Re: How do you calculate PA size?
« Reply #7 on: August 08, 2014, 06:24:18 PM »

Firstly a ready reckoner is that sound drops off at around 6db a doubling of distance as a previous poster mentioned,  This changes to about 3db a doubling of distance if you are using a line array. Stuff like Ease is great for prediction, we had a very prominent noise consulting firm working on the particular project I did recently and they did some very detailed modelling, including buildings and other structures that came into the equation and my ready reckoner that I did in my head was within 1db of their model.  The most interesting thing was that on the event day the actual readings were within 1db of the modeled version.  Manufacturers are notorious for providing less than candid performance figures for their gear, however, most quality modern boxes are capable of making a lot of noise.  Knowing the rigs you use, and what they can do in real terms is a must in this game.  Many tours go out with a lot more boxes than they need, regional suppliers make their money by cutting it much closer and by consequence providing a more cost effective solution.  The good small suppliers do this successfully, the rubbish ones don't.
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Ivan Beaver

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Re: How do you calculate PA size?
« Reply #8 on: August 08, 2014, 07:46:08 PM »

Firstly a ready reckoner is that sound drops off at around 6db a doubling of distance as a previous poster mentioned,  This changes to about 3db a doubling of distance if you are using a line array.
Not in the real world. 

Maybe at some freq and the right position-but certainly not at a wide range of freq and at different points in the array.

That is just another "myth" that the marketing departments like.
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Ivan Beaver
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Ivan Beaver

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Re: How do you calculate PA size?
« Reply #9 on: August 08, 2014, 07:58:12 PM »

Just to add to the other comments, when looking at models or trying to use specs to determine the loudness you have to consider the following.

To get the peak SPL-you have to have an amp that is rated at the peak wattage of the speaker.  Most people do not use amps that are rated that high-so the peak SPL is not realizable.

DO NOT assume that the amp will automatically produce additional level above it rated wattage.

Years ago amps were rated different.  They were rated at sine wave output and could produce peaks that were A LITTLE BIT (NOT 6dB) higher than the rated wattage-for a very short period of time.

Today amps are rated WITH THOSE PEAKS-so no additional power can be assumed.

Also you have to consider the dynamic range of music.  Most music is between 10 and 20dB.

So if the peak output is 130dB peak and the music has a 20dB dynamic range, then the "average" or what people would typically read on an SPL meter is 110dB.  NOT 130dB.

Sub levels are typically MUCH hotter than the mains.  This can range from as little as 10dB to as much as 30dB hotter.  I would say 15-20dB is "fairly normal".

So if the tops are capable of 130dB, then the subs should be capable of 150dB.

Now this can vary depending on the situation.  Inside you can get some benefits of room gain-so less "calculated" sub output can be used.

But outside there is no room gain-so you need to typically plan on at least twice as much sub-if not 4 times as what you would use indoors.

If you simply double the full range cabinets-you will get "some" increase in level.  At the lower freq this could be as much as 6dB.  But at other freq where there is cancellation due to different arrival times at a particular listening position the actual output will be LOWER.  But there will be freq higher and lower that will actually be a little bit louder.

So the "overall" level will appear to be louder-but the sonic quality will be lower-WITH ANY LOUDSPEAKER BY ANY MANUFACTURER.

How much will it be affected?  It depends on a lot of factors of the loudspeaker design-how it is deployed and so forth.

As as usual-there is no "simple" answer.  It depends on what sort of answer you are looking for.
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Ivan Beaver
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ProSoundWeb Community

Re: How do you calculate PA size?
« Reply #9 on: August 08, 2014, 07:58:12 PM »


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