The lack of accuracy (or even existence) of real specifications has been around for 30-40 years. 20 years ago I decided to measure the performance of a box I was building to compare to the "big boys" (JBL, EAW, etc). After numerous phone calls to various departments I had very little luck in discovering how they came up with their numbers. I attempted (with a very crude set-up) to accurately measure the box in a whole-space environment using first noise and then a sine wave in increments from 30-120hz. Realized later that none of the published specs were done in a whole-space environment, and were averaged very broadly, including using the -10db trick in one case. I gave up after talking with another engineer who said they used formulas from some tests to extrapolate the published spec. Hope to actually test one again (or pay someone to do it) before I'm too old to build them.
Ivan has always voiced (in a much more educated manner) my frustrations with the blatant lack of true test data on most cabinets out there.
In my opinion whole space measurements of subs is pretty worthless. Because if either the sub or the mic (person) is on the ground you will get half space loading.
So you can fly the subs-but in most cases the audience will always be on the ground.
Well I will describe how Danley measures subs.
We measure outside with both the speaker and the mic on the ground. No you do not get additional "gain" by putting the mic on the ground. You only get the "boundary gain" once.
The reason the mic is on the ground is to eliminate ground bounce and the associated cancellation notch.
If you measure a relatively large box (like a sub) up close (at 1M) you will get a measurement that is HIGHER than what you would see if you used the numbers in trying to get an idea of the SPL out of a sub in a room (id designing the system).
So for that reason we measure subs at 10M (a 20dB loss of level over distance).
We apply a 28.3V signal (20dB rise in voltage over 2.83V)-so the increased drive level and distance cancel each other out.
This provides a number that can then be used in calculations for SPL over distance-even though the number is actually smaller than what would be actually measured at 1M.
The whole idea is to come up with numbers that make sense and that people can actually use/expect-not just what looks good on a spec sheet.
We calibrate an Earthworks M30 mic and use TEF as the measurement platform. The setup is the output of TEF into an amplifier and the voltage is measured via a HP400 analog meter.
No high or low pass filters or eq is used.
I use a sweep time that allows for the resolution to be 1/10th of the low corner of the cabinet.
So for 40hz this would be 4 Hz resolution.
This makes for some VERY long sweeps on some cabinets. But does give the accuracy needed.
When coming up with the numbers for the spec sheet I look for the AVERAGE SPL across the intended bandwidth. NOT a maximum number at some freq that would give an unusually high number-but is worthless at the freq of intended operation.
I know of one product that has a really high SPL number-it is a double 18 and that number comes from a peak around 1800 Hz. Not exactly a sub freq.
So yes the sub WILL produce that SPL-but NOT at the freq you will be using it.
So now that I have a sensitivity number I look for the freq on the curve that is 3dB down FROM THAT SENSITIVITY!
That is REALLY IMPORTANT. The SPL AND -3/-10 numbers MUST be tied together.
If they are not-then both are pretty much meaningless-just look at some measured specs and see for yourself.
If we were to rate the sensitivity higher (like 100Hz for example) then the SPL numbers would be higher-but if the low cutoff was linked to that number we would end up with low cutoff numbers that are much higher than what they actually are.
So you have to choose REALISTIC numbers that actually MEAN SOMETHING in terms of what the customer can expect when using it.
Or you can just "make up some numbers" and go with that because they "look good".
Who measures or understands anyway? is what I think many manufacturers think when doing spec sheets. "TRUST US" seems to be the name of the game in many cases.
I hope that helps some.