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Author Topic: Calculate needs given SPL  (Read 6130 times)

Clark Johnson

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Calculate needs given SPL
« on: November 30, 2011, 11:18:24 am »

I was contacted about putting in a sound system in a couple of gyms and a lobby at an university rec center.  I know that I would like to achieve a certain SPL, around 95dB.  I know the size of the space and I can look at the specs on speakers to determine sensitivity.  How do I go about finding how many speakers/wattage I will need for this project?  I can handle the math if someone can point me to a formula.

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Charlie Zureki

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Re: Calculate needs given SPL
« Reply #1 on: November 30, 2011, 11:28:58 am »

I was contacted about putting in a sound system in a couple of gyms and a lobby at an university rec center.  I know that I would like to achieve a certain SPL, around 95dB.  I know the size of the space and I can look at the specs on speakers to determine sensitivity.  How do I go about finding how many speakers/wattage I will need for this project?  I can handle the math if someone can point me to a formula.

  Clark,

   Have you ever Designed and Installed a Sound System before? To NEC code?
   Have you taken any data/ Measurements at this facility, and in all spaces that are to be covered?
   What is this (these) Sound System (s) to be used for?...It's primary purpose?

   If the answer to these questions is either.. No... or, I don't know....then, you should kindly refer the University to a qualified System Contractor.  See the post below regarding..."Hiring a Contractor to consult"....


  Hammer
 
   
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John Roberts {JR}

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Re: Calculate needs given SPL
« Reply #2 on: November 30, 2011, 12:04:10 pm »

In a highly reverberant space like a gymnasium, you can get the same SPL from a few big speakers bouncing sound all around, or a lot of smaller speakers pointed to hit the meat in the seats and not excite the room reverberation.

Same SPL, much different intelligibility and cost to implement. Are the systems just to make background music, where intelligibility is not much of a concern, or to make announcements also that the audience should understand?

 I am encouraged that you are thinking about SPL, the net result but even this needs more qualification and inspection to keep the customer happy.

JR
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Ivan Beaver

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Re: Calculate needs given SPL
« Reply #3 on: November 30, 2011, 12:55:41 pm »

I was contacted about putting in a sound system in a couple of gyms and a lobby at an university rec center.  I know that I would like to achieve a certain SPL, around 95dB.  I know the size of the space and I can look at the specs on speakers to determine sensitivity.  How do I go about finding how many speakers/wattage I will need for this project?  I can handle the math if someone can point me to a formula.
It is not a simple math formula.

You have to look at coverage patterns, location they are mounted in, "throw", is these full range systems, voice only systems, what kind of directivity will be needed (especially important in reverberant spaces), is localization important?  Is zoning important? and so on and so forth.

Each case is different and could/may require a different design approach.

If all you want to do is to make some noise at 95dB (whatever scale A or C fast or slow you mean the 95dB to be at-it can vary quite a bit-say 20dB?) then that is easy.  But if you want it to be a usable system, being able to understand it and so forth, then that gets harder.

Sorry, but there is no simple answer.
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Ivan Beaver
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Clark Johnson

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Re: Calculate needs given SPL
« Reply #4 on: November 30, 2011, 07:15:33 pm »

Clark,

   Have you ever Designed and Installed a Sound System before? To NEC code?
   Have you taken any data/ Measurements at this facility, and in all spaces that are to be covered?
   What is this (these) Sound System (s) to be used for?...It's primary purpose?

   If the answer to these questions is either.. No... or, I don't know....then, you should kindly refer the University to a qualified System Contractor.  See the post below regarding..."Hiring a Contractor to consult"....


  Hammer
 
 
Here is the background of the project.  I work for the universities audio visual department and do a lot of learning as I go.  We occasionally take some boxes on a stick (Mackie SRM350) or bigger (Mackie SA1532z, durable and cheap) for student groups to have dances or events like that.  Also, there is some announcements.  The director of the space wants to have some of her own system.  In the best case, it would be a permanent solution attached to the structural supports so that all that has to be plugged in is a mixer and some sources.

@Charlie - Yes, I've designed some sound systems before but nothing very complex.  I designed a 4 zone clubhouse, a temporary setup for a performing arts venue (a much need patch before a 27 million dollar renovation), a 36 seat cinema.  Mostly it was available equipment and or budgets that were the strongest consideration.  I've heard a lot about codes, but finding them can be difficult.  Where is a good place to locate the relevant codes? 

In terms of measurements, I'm relatively inexperienced.  The only pieces of equipment I have is a dbx reference mic and an old hardware RTA.  I don't have any software or interfaces.  I don't know the reverb time of the space.
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Clark Johnson

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Re: Calculate needs given SPL
« Reply #5 on: November 30, 2011, 07:21:30 pm »

In a highly reverberant space like a gymnasium, you can get the same SPL from a few big speakers bouncing sound all around, or a lot of smaller speakers pointed to hit the meat in the seats and not excite the room reverberation.

Same SPL, much different intelligibility and cost to implement. Are the systems just to make background music, where intelligibility is not much of a concern, or to make announcements also that the audience should understand?

 I am encouraged that you are thinking about SPL, the net result but even this needs more qualification and inspection to keep the customer happy.

JR

The space does everything, from dances to athletic camps to talent shows.  I would like to avoid carting over equipment for every event.  So the applications I want to consider are dances and athletic camps.  I can bring over more equipment as needed for talent shows.  What kind of questions do I need to ask the person with the money?  What kind of inspections do I need to make?  I'm only 1 person, so I can't do everything.
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Clark Johnson

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Re: Calculate needs given SPL
« Reply #6 on: November 30, 2011, 07:34:49 pm »

It is not a simple math formula.

You have to look at coverage patterns, location they are mounted in, "throw", is these full range systems, voice only systems, what kind of directivity will be needed (especially important in reverberant spaces), is localization important?  Is zoning important? and so on and so forth.

Each case is different and could/may require a different design approach.

If all you want to do is to make some noise at 95dB (whatever scale A or C fast or slow you mean the 95dB to be at-it can vary quite a bit-say 20dB?) then that is easy.  But if you want it to be a usable system, being able to understand it and so forth, then that gets harder.

Sorry, but there is no simple answer.

Ivan,
Danley sounds definitely has done it's share of installs.  That is quite a portfolio.  So I understand that you know what you are talking about where you say there is no easy answer.  The question I guess was mired in the details of the project.  I realize that the inverse square law can not be overcome.  The root question is, how can I go from "Let's throw up some speakers and see if it works" to "here is a system that has these specs" without having to use a computer to model a room?  How did sound system installers answer these type of questions before there were computers?
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Tom Young

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Re: Calculate needs given SPL
« Reply #7 on: November 30, 2011, 08:46:28 pm »

The root question is, how can I go from "Let's throw up some speakers and see if it works" to "here is a system that has these specs" without having to use a computer to model a room?  How did sound system installers answer these type of questions before there were computers?

On paper and/or with slide rules. The same way that any science was "computed" before computers came along.

Having computers and software does not free any of us from knowing the physics that are behind electroacoustics (which is what we do).

You, for example, cannot go out and buy modeling or measurement software and do anything with it unless, or until, you know what you are (and it is) doing.

Back to your dilemna: you have been tasked with providing a working live sound system. Fine. Except for the fact that you cannot possibly do this. No amount of brotherly advice from us is going to even come close to providing you with an appropriate design for a loudspeaker system for the space you are dealing with.

Then there is the rest of the sound system including the wiring infrastructure and AC power, plus rigging.

And then there are code, safety and other legal and ethical issues.

I am writing all this partly to try to enlighten you and partly because I realise you have to go back to the task-master and either bluff your way through this or convince him/her that the only way to accomplish the goal is to pay an experienced designer (consultant or contractor) to do this.

No easy task.

But easier than providing a mediocre sound system that no one can use effectively.
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John Roberts {JR}

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Re: Calculate needs given SPL
« Reply #8 on: November 30, 2011, 08:49:12 pm »

The space does everything, from dances to athletic camps to talent shows.  I would like to avoid carting over equipment for every event.  So the applications I want to consider are dances and athletic camps.  I can bring over more equipment as needed for talent shows.  What kind of questions do I need to ask the person with the money?  What kind of inspections do I need to make?  I'm only 1 person, so I can't do everything.

How much are they willing to spend ?

Is there an end with a stage? 

Does the audience sit everywhere?

Simple calculus is that you can fill a reverberant space with music more easily with a few large speakers, that make effecting speech reinforcement, since the reverb hurts intelligibility. If the audience area is modest you can point the speakers at it. The closer the speakers to the listener the more direct to reverberant sound ratio and better intelligibility

What works in there now?

JR

PS: The answer for how to do it before computers, is the same as after there were computers, "experience"... and remember that free advice on the Internet is worth exactly what you paid for it, even advice from me...  8)
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Brad Weber

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Re: Calculate needs given SPL
« Reply #9 on: December 01, 2011, 12:27:36 am »

... and remember that free advice on the Internet is worth exactly what you paid for it..
If you are lucky, it is also possible for it to end up being very costly.

As already stated, there is much more to it than may be readily apparent.  For example, you mentioned they want 95dB.  Is that a 95dB(SPL), dBA or dBC overall level or is it the level at any frequency over some specific frequency range?  Is it the average level, absolute maximum peak level, level exceeded some percentage of the time or what?  Is 95dB the minimum level anywhere in the listener area and what variance throughout the listener area is acceptable?  Do they care about anything other than obtaining 95dB, such as the frequency response of the system or being able to understand what is said?

And that's just defining what it is you are trying to do, then you also get into assessing practical limitations such as where you can physically mount speakers, interference from lights and backboards, where equipment can be located and so on.  You may also need to consider factors such as whether you need to be able to turn on and off or independently adjust the level of the speakers covering certain areas of the room in order to support some types of events, for example turning off speakers over the stage for talent shows.

Even then, you aren't close to predicting the system performance, more getting to a point where you can start to define something that stands a chance of actually working.
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Re: Calculate needs given SPL
« Reply #9 on: December 01, 2011, 12:27:36 am »


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