In this case, "it's only math". If the app learns what the input shows as 0dB relative (that's the key word) to the level it's putting out then you technically don't care about all the equipment in the middle. Short of the laptop mic completely failing under a high SPL, the RTA app should be able to discern an SPL from the electrical level that it sees at the mic.
As there is no way that the program would know what mic, speakers etc you are using, it has no way of knowing what SPL is actually outputted by the computer when playing its reference sound...
Even if the exact voltage coming out of the mic could be measured (which isn't possible because of the preamp etc..), it still wouldn't suffice as there is no way the program knows how efficient the mic is (and there are huge differences)!
The problem is that there are too many unknown factors for calibrating on unknown equipment without using a calibrated SPL meter as a reference.
An alternative could be that the software company tested their software on a calibrated sound source (independent of the measuring computer) with different laptops and different soundcards/different mic's. Then they saved the settings needed for correct measurement for each combination and provide these "profiles" so you can select them in case you have the same hardware.
This would give you a pretty correct calibration, although different versions of the same model mic or soundcard would have small differences...
Sorry to ruin you theory..
But most mic's can't physically cope with high spl, and will give totally inaccurate readings or possibly even collapse!
If you are competing you need a TermLab. end of story !
You need one of these.
It seems to be the least expensive and is a solid performer
You will also need one of these to run the MIC
makeitlouder.com. It can handle up to 186 db with no problem. The software you downloaded (I think) was designed to be used with this mic.
Sure, everyone would like to have termlab equipment but then again that is 650 bucks you could be spending somewhere else. That can buy a lot of MDF and at least one more sub.
You can look on e-bay for this mic as well. Search for 186 db SPL SOUND LEVEL METER. I just saw one for $75 and I think you may be able to get them for less on their site.
I just bought one a few minutes ago and they said it should be about four days before they ship it. I have been looking for something like term lab with out having to spend ridiculous amounts of money for it and this is the best thing I have come up with. Pretty much everything else will not handle the extreme levels of spl that are found in serious spl car audio environments. Those Radioshack meters only go to 130 db and you can find some other hand held models that will go to 140db but that is about it unless you go with something like this or something like term lab equipment.
If I remember where this site is, I will report back with how well (or not) it works.
The calibration stuff is for the mics they sell (I think). If I understand it correctly, every mic comes with some kind of calibration number and I think you are supposed to enter this number into the program so that it will measure correctly.
It seems to me that this should do everything I want it to do. I'll see in a few days.
If I don't write back here I will at least post something about it on my blog once I have tested it out and had time to sit down and write about it.
I am glad to see mention of a special mic for high SPL's. many mics cant handle much above 130db. they are just not designed that way. the internal preamp will clip, the diaphragm can distort or break etc. special mics for high spls is required to properly measure SPL above 120,130+++. you also then need a way to calibrate the mic. In the field of acoustics most purpose built mics are used with a pistonphone. which is a small device with an internal oscillator and a moving piston with a calibrated volume of air inside. when a mic tip is placed inside and sealed, the pistonphone will generate a calibrated SPL level. usually 104db something like that. then you have a known output level for a given input level.
This is the ONLY true method to calibrate a mic for SPL. Now the catch to this is that calibration is then accurate only at the frequency of the pistonphone. usually 1khz. Most mics don't have a flat response. some roll off on the lows. some have peaks on the highs. buying a known purpose built mic with proper calibration chart. and not just some artists rendering of a chart on the side of the box, can make all the difference in the world.
or you can buy a known calibrated mic.
also keep in mind most mics are temp sensitive. many mics use a mylar diaphragm that will get soft and or stretch with heat. and can get brittle when cold. keeping mics in a temp controlled environment, IE not the inside if your car, will ensure longevity and accurate results with time.