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Figuring my sub wattage based on FOH levels

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Gordon Brinton:
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.

Art Welter:

--- Quote from: Gordon Brinton on January 08, 2024, 09:37:55 AM ---
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.

--- End quote ---
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



Gordon Brinton:
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.

Ivan Beaver:
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.

Gordon Brinton:

--- Quote from: Ivan Beaver 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.

--- End quote ---

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.

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