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Author Topic: Figuring my sub wattage based on FOH levels  (Read 2814 times)

Gordon Brinton

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Figuring my sub wattage based on FOH levels
« on: January 08, 2024, 09:37:55 AM »

I am trying to reverse engineer my apparent wattage feeding my subs if I measure 100dB at FOH position which is, let's say, 40 feet (12.192m) away from the stage. This is without being able to simply measure voltage during a show. I normally don't run that hot, but just creating a "what if" scenario.

My subs are JTR Captivator 212 Pro.
Specs (per box) are 800W RMS (56V), 3200W Peak (113V)
I believe cabinet sensitivity is 97dB @ 40 Hz, 102dB @ 100Hz. (I’ll assume 98dB as an average across the band for this experiment.)

Assuming it takes 1 watt to bring my subs to roughly 98dB at 1 meter away, using an Inverse Square Law calculator, I see that this will result in roughly 76dB at 40ft out. (I believe the ISL calculator is assuming outdoors with no boundaries.)

I entered a few different starting values into the calculator until I got as close as possible to 100dB at 40ft out (12.192m). I ended up at 122dB at 1 meter.

Using a speaker sensitivity of 98dB, I figure it must take roughly 256 Watts to equal 122dB at 1 meter, (adding 3dB for each doubling of watts).

Have I forgotten anything? Is 100dB at FOH an unreasonable level? How loud do you guys run your subs for bands in small/med venues? I usually shoot for 95dB, but it depends on the band, the venue, and energy level of the audience, and I do try to balance them with the level of my top boxes.
« Last Edit: January 08, 2024, 10:01:26 AM by Gordon Brinton »
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Art Welter

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Re: Figuring my sub wattage based on FOH levels
« Reply #1 on: January 08, 2024, 07:39:57 PM »


Using a speaker sensitivity of 98dB, I figure it must take roughly 256 Watts to equal 122dB at 1 meter, (adding 3dB for each doubling of watts).

Have I forgotten anything? Is 100dB at FOH an unreasonable level? How loud do you guys run your subs for bands in small/med venues? I usually shoot for 95dB, but it depends on the band, the venue, and energy level of the audience, and I do try to balance them with the level of my top boxes.
Gordon,

Your inverse distance calculations appear correct for outdoor use.
Indoors, room modes can make for drastic differences in level depending on sub and listener position.

You mention "subs", doubling cone area and cabinets (assuming mutual coupling) increases sensitivity by 3dB, so half the power is required for a pair of cabinets compared to using only one for the same level.

As far as level, 100dB SPL at 40Hz sounds about "equal loudness"  as 60dB SPL at 1000Hz.

95dBA (Slow) is the OSHA "permissible noise exposure" for a 4 hour daily duration.
"A" weighting progressively reduces the low frequency meter sensitivity below 1000Hz.

NIOSH levels are considerably lower.

I have to admit running well over 100dB at low frequencies at FOH, but consider above 95dBA "slow" dangerous, and have the noise induced hearing loss to prove it.

Art



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Gordon Brinton

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Re: Figuring my sub wattage based on FOH levels
« Reply #2 on: January 08, 2024, 08:37:37 PM »

You are correct. My calculations are for a single box, but I do have two and always cluster them together either center or to one side. I will add 3dB for that. My measurement at foh is for the entire rig. I am just trying to mathematically isolate the sub power. Maybe my efforts are all wrong due to the munson curve. I hadn't thought of that.

At any rate, I set my sub limiter to 800 Watts which is rms for the speaks, but I could raise it to 1200W if it is recommended to get more umph. Since I am getting older and don't run my rig hard anyway, I doubt that I am getting close to that. I was bored and curious. Thanks for the insight.
« Last Edit: January 08, 2024, 08:41:51 PM by Gordon Brinton »
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Ivan Beaver

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Re: Figuring my sub wattage based on FOH levels
« Reply #3 on: January 09, 2024, 08:13:38 AM »

Just to complicate things a bit.

You didn't mention the weighting or speed of your measurement.

There can easily be a 30dB (or more) difference between A slow and Z peak on the same program material at the same time.  So that can make a lot of difference in the calculations.

Normally subs have a crest factor of around 12 dB, so that is higher than the continuous/steady sensitivity readings/ratings.

Also the ratio of subs to tops can change things quite a bit.  Some styles like a big LF haystack, others like a more even response.

It just gets a bit more complicated the deeper you dig to get to reality.
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Gordon Brinton

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Re: Figuring my sub wattage based on FOH levels
« Reply #4 on: January 09, 2024, 10:05:20 AM »

Just to complicate things a bit.

You didn't mention the weighting or speed of your measurement.

There can easily be a 30dB (or more) difference between A slow and Z peak on the same program material at the same time.  So that can make a lot of difference in the calculations.

Normally subs have a crest factor of around 12 dB, so that is higher than the continuous/steady sensitivity readings/ratings.

Also the ratio of subs to tops can change things quite a bit.  Some styles like a big LF haystack, others like a more even response.

It just gets a bit more complicated the deeper you dig to get to reality.

My SPL meter is set to C weighted, Fast response.

I didn't mean for this to go down a rabbit hole. What I am really working toward is discovering where to set my limiter for the subs or if I even need limiting. In my mind, the only way to do that is to know how hard my subs are working during a typical show in a typical venue. In other words, If I rarely ever get close to driving my subs to 800 Watts, (I am usually in small venues,) then I will set it to 800 slow and forget it. If, on the other hand, I find that I am occasionally demanding more power, I may raise the limiter to peak at, say, 2000 with a faster attack. Jeff from JTR gives details on both RMS and Peak limiter settings, but he doesn't say which one to use.

The subs are passive and 800W rms, 4 Ohms, (each box).
The amp, QSC GXD 8 is advertised at 1200W Continuous and 2250W Dynamic at 4 Ohms per channel.

The limiter in the amplifier DSP can set a threshold by simply entering max watts along with Mild, Medium, or Aggressive modes.
The compressor/limiter in my Ashly Protea (speaker management) can be set using dBu, along with att/rel and detector type settings.

Thus, my quest is, (and what I should have asked to begin with,) which limiter to use and and where to set it? How do I discover how hard I am already pushing my subs?

EDIT: BTW, my top boxes are RCF NX-45a, powered with limiting built in.
« Last Edit: January 09, 2024, 10:14:03 AM by Gordon Brinton »
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Brian Jojade

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Re: Figuring my sub wattage based on FOH levels
« Reply #5 on: January 09, 2024, 02:00:11 PM »

I wouldn't want a limiter to be the deciding factor of how loud my show is. A limiter to protect against major oopses is fine, but if you're hitting the limiter AT ALL during normal operation, that's a problem.

I prefer to have an SPL meter at FOH and use that fancy MAIN slider to adjust downward if it's too loud.
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John Bosco

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Re: Figuring my sub wattage based on FOH levels
« Reply #6 on: January 09, 2024, 02:43:42 PM »

I prefer to have an SPL meter at FOH and use that fancy MAIN slider to adjust downward if it's too loud.

Hold on, wait, game changer here, those things go down too, I have to totally rethink the way I've been doing it.
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Jonathan Hole

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Re: Figuring my sub wattage based on FOH levels
« Reply #7 on: January 09, 2024, 04:26:29 PM »

My SPL meter is set to C weighted, Fast response.

I didn't mean for this to go down a rabbit hole. What I am really working toward is discovering where to set my limiter for the subs or if I even need limiting. In my mind, the only way to do that is to know how hard my subs are working during a typical show in a typical venue. In other words, If I rarely ever get close to driving my subs to 800 Watts, (I am usually in small venues,) then I will set it to 800 slow and forget it. If, on the other hand, I find that I am occasionally demanding more power, I may raise the limiter to peak at, say, 2000 with a faster attack. Jeff from JTR gives details on both RMS and Peak limiter settings, but he doesn't say which one to use.

The subs are passive and 800W rms, 4 Ohms, (each box).
The amp, QSC GXD 8 is advertised at 1200W Continuous and 2250W Dynamic at 4 Ohms per channel.

The limiter in the amplifier DSP can set a threshold by simply entering max watts along with Mild, Medium, or Aggressive modes.
The compressor/limiter in my Ashly Protea (speaker management) can be set using dBu, along with att/rel and detector type settings.

Thus, my quest is, (and what I should have asked to begin with,) which limiter to use and and where to set it? How do I discover how hard I am already pushing my subs?

EDIT: BTW, my top boxes are RCF NX-45a, powered with limiting built in.

I think the real question is what is the optimum amp for the 212Pro regardless of venue and desired levels - you already have the subs so whether it will run out of gas in your use case is a different question and might require changing subs entirely.  Jeff can get you the recommended DSP settings and/or you already know the max V and can enter those values - I don't know that amp but if the max settings are beyond it's capability then leave clip limiter on and go from there.  The 212Pro can benefit a good bit from low pass filtering and the NX45 allows you a lot of room there. 

FWIW:  For our "small system" I deploy 1 RCF NX945 over two powered 212Pro per side and for our large system I have Danley tops over JTR 218Pro subs unpowered, with K3 and K10 amps. 
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Scott Holtzman

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Re: Figuring my sub wattage based on FOH levels
« Reply #8 on: January 09, 2024, 08:10:49 PM »

Rabbit holes are our specialty.  Sorry to speak in the collective._


Sent from my iPhone using Tapatalk
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Steve-White

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Re: Figuring my sub wattage based on FOH levels
« Reply #9 on: January 09, 2024, 08:38:39 PM »

Rabbit holes are our specialty.  Sorry to speak in the collective._

Sent from my iPhone using Tapatalk

Yep.

To gain any kind of meaningful data, assuming it could be possible, would require the room to be acoustically mapped which renders this adventure below zero sum.
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Gordon Brinton

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Re: Figuring my sub wattage based on FOH levels
« Reply #10 on: January 09, 2024, 10:00:02 PM »

I wouldn't want a limiter to be the deciding factor of how loud my show is.

No, I wouldn't either. It's not "loud" that I am seeking. I really just want to find a reliable way to know how much headroom I have left when I do start to get loud. I don't want to just keep pushing the system until I see red lights and then say, "I found it!". There must be easy to understand direct relationship between room level and watts delivered to the speakers without relying on lights. I want to feel more comfortable with knowing my sub's limits and behavior, and I want to feel confident that I have my limiters set correctly.
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Brian Jojade

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Re: Figuring my sub wattage based on FOH levels
« Reply #11 on: January 10, 2024, 12:37:14 AM »

Having limiters set 'correctly' is going to be different for what you expect the limiters to do.

Personally, I like to run rigs where I know I can run them into a PAIN threshold far faster than I can cause damage to any components.  If I EVER see a red clip light during show operation that means I didn't bring enough rig. Period.

As long as your amps and speakers are appropriately sized, limiters really aren't needed.  If you're trying to squeeze every last ounce of output from your rig you can use a limiter to keep you kinda safer, but not really.

If you're using a limiter to protect against things like someone dropping a hot mic on the floor, well, that's a good use scenario.

As far as 'room level' and watts, well, every room and placement of the speakers is going to be different.  Heck, in a room you can move a few feet and the levels can be drastically different.  And power fluctuations needed to get a little more output can be dramatically greater than you expect.

Case in point, I use Crown iTech 5000 amps which are 2500 watts per channel.  They have a power meter on them.  For MOST shows, the reading on the amp is well under 100 watts.  It's kind of amazing how loud even 20 watts can be!! Granted that's an average draw and peak draws can be much higher.  The point is, the average watts is what makes the average sound level you hear.  The short bursts for kick give you the dynamic range you want.  Put a limiter on and you can crank the volume up a LOT more. The difference is you start to lose the dynamic range needed for that extra when you need it.

Therefore, limiters really should never be hit on the mains on a properly designed rig.
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Chris Grimshaw

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Re: Figuring my sub wattage based on FOH levels
« Reply #12 on: January 11, 2024, 05:22:07 PM »

No, I wouldn't either. It's not "loud" that I am seeking. I really just want to find a reliable way to know how much headroom I have left when I do start to get loud. I don't want to just keep pushing the system until I see red lights and then say, "I found it!". There must be easy to understand direct relationship between room level and watts delivered to the speakers without relying on lights. I want to feel more comfortable with knowing my sub's limits and behavior, and I want to feel confident that I have my limiters set correctly.


The proper way to achieve this is to use amplifiers which have monitoring-over-network. I use the Powersoft T-series amps for this. With a little setting up, I can have a laptop at FOH displaying headroom for all the speakers in my system.


The problem you have is this: you can measure SPL, but you have no idea how hard each speaker is working. Is the 50Hz bottom end or the 100Hz punch louder? Where's your crossover point? Which cabinet is contributing more to the total SPL?


The best approximation I can find for you is this: subs on Aux. I don't particularly like the workflow, but it'll give you a level meter at FOH which will give you a reasonable idea of how hard the subs are working. NB - you'll need to think about where the crossover is applied to give sensible readings.



Chris
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Paul G. OBrien

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Re: Figuring my sub wattage based on FOH levels
« Reply #13 on: January 11, 2024, 09:43:49 PM »

I really just want to find a reliable way to know how much headroom I have left when I do start to get loud. I don't want to just keep pushing the system until I see red lights and then say, "I found it!".
Well.. that is the simple way to figure it out, push the rig until you see the clip indicators flash on the sub amp, note the mains level on the mixer when that happens.. both peak and continuous, and you can now work backwords to figure out the power levels the rig normally runs at. This assumes the system is configured the same each time.

There must be easy to understand direct relationship between room level and watts delivered to the speakers without relying on lights.
The lights are more accurate but some math will get you there too. The GXD8 has an input sensitivity of 3.9v rms, that means it needs an input signal of +14dbu (-10dB FS) to reach full output of 1200w rms into 4ohms and every 3dB down from that is 1/2 the power, so +11db is 600w, +8dB = 300w, etc. The one unknown here is your gain staging through the Ashly DSP, is it 1:1 input to output or do you have both inputs sumed to a mono output and/or the sub output boosted? If it is 1:1 then the main meters on your mixer will track as described above, but if you have the signal boosted in the DSP then it will be that amount lower on the mixer.
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Paul G. OBrien

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Re: Figuring my sub wattage based on FOH levels
« Reply #14 on: January 11, 2024, 10:03:24 PM »

If I EVER see a red clip light during show operation that means I didn't bring enough rig. Period.

As long as your amps and speakers are appropriately sized, limiters really aren't needed.  If you're trying to squeeze every last ounce of output from your rig you can use a limiter to keep you kinda safer, but not really.

LOL... you guys would hate doing the shows I do, the system is almost always undersized because of limited budgets and even when it's not the performers(DJs) will still attempt to wring every last watt out of it. Good limiting keeps me in business.
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Steve-White

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Re: Figuring my sub wattage based on FOH levels
« Reply #15 on: January 11, 2024, 10:29:43 PM »

LOL... you guys would hate doing the shows I do, the system is almost always undersized because of limited budgets and even when it's not the performers(DJs) will still attempt to wring every last watt out of it. Good limiting keeps me in business.

Sometime we need to compare notes.  Same here and I do lots of DJ shows.  Everything many times runs Full Tilt and I like to bring it home in still functional for the next job.

From back in the day, when we built up ground stacks with discrete components, splayed 90x40 2" fiberglass horns, and analog compressor/limiters it's been about squeezing every db out of the system that it could produce and hold together.

I got it down to life of drivers.  4-0way systems with 12" low mids, had things dialed up to the point that adding another db would shorten the lifespan of the 12's - they wouldn't burn out - well they would if ya really leaned on them, but pull them back from burning voice coils to the point were they literally mechanically fell apart.  Then, back 'em down another db and things held together.  Using Ashly SC-50/55's.

Carrying that forward to today, if using outboard processing nothing has changed if you want to push things, assuming the drivers are powered-up well amp wise.  You can still burn things down or tear things up without clipping amps.  Finding the sweet spot still takes trial end error - you can get close with test gear, but deploy it and see how it performs and if it holds together doing so is the final test.

This is what the OP is seeking to do, only with gear that has presets.  Then there's Gain Staging and overall system calibration so the meters on the console mean something beyond the signal condition of the console itself.
« Last Edit: January 14, 2024, 11:15:26 AM by Steve-White »
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Chris Grimshaw

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Re: Figuring my sub wattage based on FOH levels
« Reply #16 on: January 12, 2024, 12:06:11 PM »

Well.. that is the simple way to figure it out, push the rig until you see the clip indicators flash on the sub amp, note the mains level on the mixer when that happens.. both peak and continuous, and you can now work backwords to figure out the power levels the rig normally runs at. This assumes the system is configured the same each time.
 The lights are more accurate but some math will get you there too. The GXD8 has an input sensitivity of 3.9v rms, that means it needs an input signal of +14dbu (-10dB FS) to reach full output of 1200w rms into 4ohms and every 3dB down from that is 1/2 the power, so +11db is 600w, +8dB = 300w, etc. The one unknown here is your gain staging through the Ashly DSP, is it 1:1 input to output or do you have both inputs sumed to a mono output and/or the sub output boosted? If it is 1:1 then the main meters on your mixer will track as described above, but if you have the signal boosted in the DSP then it will be that amount lower on the mixer.


I'm afraid this won't work.
A vocal-heavy mix won't push the subs, but a bass-heavy mix will. They'll hit similar levels on the VU meters.


Chris
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Art Welter

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Re: Figuring my sub wattage based on FOH levels
« Reply #17 on: January 13, 2024, 08:07:30 PM »

Jeff from JTR gives details on both RMS and Peak limiter settings, but he doesn't say which one to use.

Thus, my quest is, (and what I should have asked to begin with,) which limiter to use and and where to set it? How do I discover how hard I am already pushing my subs?

Both RMS and separate peak limiter settings are required to protect for different program material.
Long term (RMS) power burns coils, short term (peak) can tear up cones/diaphragms.

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Gordon Brinton

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Re: Figuring my sub wattage based on FOH levels
« Reply #18 on: January 14, 2024, 08:29:11 AM »

Both RMS and separate peak limiter settings are required to protect for different program material.
Long term (RMS) power burns coils, short term (peak) can tear up cones/diaphragms.

Both!! I hadn't thought of that. Thanks for waking my ass up!
I've never used two limiters on one speaker before, but perhaps I will now.
The GXD8 has an RMS limiter in the DSP. I can set that to 800W slow.
The Ashly Protea speaker management has a peak limiter that I can set for fast attack and release near the max of the amp channels.

Hmm! I'll work out the details. Thanks again.

EDIT: Corrected spelling.
« Last Edit: January 14, 2024, 08:36:56 AM by Gordon Brinton »
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Ivan Beaver

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Re: Figuring my sub wattage based on FOH levels
« Reply #19 on: January 16, 2024, 08:31:44 AM »

No, I wouldn't either. It's not "loud" that I am seeking. I really just want to find a reliable way to know how much headroom I have left when I do start to get loud. I don't want to just keep pushing the system until I see red lights and then say, "I found it!". There must be easy to understand direct relationship between room level and watts delivered to the speakers without relying on lights. I want to feel more comfortable with knowing my sub's limits and behavior, and I want to feel confident that I have my limiters set correctly.
You could always put a little cable that has a connector )whatever type you want) to hook up a voltmeter to the output of the amp.

That way you can get an idea of the "average" (slow response) of the voltage delivered.  Then you use the "nominal impedance" to figure out the wattage.  wattage=voltage squared divided by impedance.

Yes the actual impedance will be different, and the meter response will be slow, but it will give you a decent idea of the power you are delivering.

Another idea is to simply put a "old school" wattmeter (ie refrigerator watt meter) in series with your speakers.  That way the impedance will be part of the meter deflection
 
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Re: Figuring my sub wattage based on FOH levels
« Reply #19 on: January 16, 2024, 08:31:44 AM »


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