true rta spl meter program

kiaz69 · 09-05-2006, 05:15 PM

i recently downloaded this program and having got good readings.
in competition i hit 143 db spl and when i use this program on my lab top i only hit 120's i am only using the built in mic on computer.

does any one know what equipment u need to get good readings from this program? OR DOES ANYONE have this program and works good for them?

send any info to kiaz69@hotmail.com thanx

scott_fx · 09-05-2006, 05:32 PM

i'd say it's your mic

midiwall · 09-05-2006, 05:34 PM

I figure you're talking about this app?

http://www.trueaudio.com/

Are you using the free version? Did you do any calibration?

kiaz69 · 09-05-2006, 06:38 PM

yes the free version no i did not calibrate im new at this i just buy and install dont no the tecnicalitys my stero rips though just want to be able to meter it on my own so i can tweak it more currently hit 143 not bad for begginer

kiaz69 · 09-05-2006, 06:46 PM

Quote: Originally Posted by scott_fx

i'd say it's your mic

do i need a mic mixer or those special mics for lab tops

pate60 · 09-05-2006, 08:02 PM

Q: Besides TrueRTA, what else do I need to set up my own audio test lab?

A digital voltmeter to calibrate TrueRTA™ for your sound card
A collection of cables and adapters for connecting electronic gear to your sound card for testing. Most sound card jacks are 1/8 inch

If you want to do acoustic testing of loudspeakers in addition to the above you will need:
A calibrated microphone
A microphone preamplifier or mixer
An audio power amplifier to drive the speaker under test
Cables to connect the line out of the sound card to the input of the audio power amp

midiwall · 09-05-2006, 08:02 PM

Quote: Originally Posted by kiaz69

yes the free version no i did not calibrate im new at this i just buy and install dont no the tecnicalitys my stero rips though just want to be able to meter it on my own so i can tweak it more currently hit 143 not bad for begginer

Wow... ummm, the lack of calibration is a HUGE issue. Here's a crash course in what a "decibel" is and how it gets measured.

A "decibel" is a couple of different things, and can be confusing to measure. There are also a number of scales used to measure decibels which adds to the confusion.

A decibel can be an _electrical_ measurement (dBv, dBm), or a _pressure_ measurement (dB). What you're looking for is the pressure measurement... In order to measure this, you'll use the microphone in the laptop which will need to be "tuned" in the car at a specific physical location at a specific frequency. If you don't do this, then the program is only reading a voltage from the microphone input and "plotting" it again some internal scale. I will guarantee that TrueRTA will not be accurate out of the box - since it knows nothing about the acoustical environment that you're using it in, nor about the response characteristics of the microphone, mic amp (in the laptop), etc.

From what I see on the website, it looks like the free version of TrueRTA comes with "Microphone Calibration Files" which I would figure to mean that it can calibrate itself in a specific environment. You need to do this.

The process will usually entail having your laptop playback a file while the microphone listens. The file will probably be a "sweep", and will have been recorded at a known level (this is the electrical dBv). You'll be instructed to manipulate the level of the playback system to a certain point, then the level of the mic input on the laptop. Once that TrueRTA can "hear" the whole file, you'll probably be walked through setting levels for "0 dBa". This is the calibration point, you're teaching TrueRTA to recognize what it "looks like" when the SPL in the car is 0 dB.

Once this is done, you're pretty much ready to go.

To test the dB level in the car _as was tested at dB drags_ you'll need to know the "weighting" that the measurement was done on; and you'll need the file/audio clip they used to take the measurement. Both of these are very important if you're looking to match the numbers you got at the dB drags.

Once you have all this lined up, you should be able to test and get the same numbers you got at the drags. MIC PLACEMENT WILL BE VERY IMPORTANT. If they clamped a mic to a headrest, you can't expect to get the same numbers with a laptop on the seat.

NOTE: It is VERY VERY important that you not be in the vehicle when you're taking the measurements!!! You could easily blow an eardrum at the levels you're talking about!

ref:
http://www.phys.unsw.edu.au/~jw/dB.html
http://en.wikipedia.org/wiki/Decibel

midiwall · 09-05-2006, 08:11 PM

kiaz69;

I wanted to answer your PM in public 'cause I think it's worthwhile for the archives and other people on the thread. I hope you don't mind...

Quote: Originally Posted by kiaz69

do u know lots about this program if so please help me if its worth running this program

Hey man...

Specifically, I don't know anything about TrueRTA.... I have a lot of experience in setting up rooms and using RTAs, but I'd never seen this app before today.

Personally, I think it's overkill for what you're trying to do - that being just get an idea of the SPL in your car to (presumably) increase it in order to to better at the next dB drags.

If so, then I would NOT spend time with this app... you would do just as well dropping $50 on a simple dB meter from Radio Shack:

http://www.radioshack.com/product/in...ductId=2103667
http://www.radioshack.com/product/in...ductId=2103668

Those would be a LOT easier for you to deal with. Basically you turn it on, start a CD playback, grab your remote volume control and get out of the car.

T1NY W · 09-06-2006, 09:18 AM

I have one of these......

http://www.termpro.com/storefront/

www.termlab.com

Thats what you need.

Tiny

RedGTiVR6 · 09-06-2006, 09:41 AM

Quote: Originally Posted by kiaz69

currently hit 143 not bad for begginer

If you're planning on competing, there are beginners that will walk all over this, even in the lowest of classes.

I would recomend spending some money on a mic, and read the manual on how to calibrate the software. Otherwise, you're just shooting in the dark.

RedGTiVR6 · 09-06-2006, 09:42 AM

Quote: Originally Posted by T1NY W

I have one of these......

http://www.termpro.com/storefront/

www.termlab.com

Thats what you need.

Tiny

JC-S60 · 09-06-2006, 10:39 AM

Quote: Originally Posted by midiwall

From what I see on the website, it looks like the free version of TrueRTA comes with "Microphone Calibration Files" which I would figure to mean that it can calibrate itself in a specific environment. You need to do this.

The process will usually entail having your laptop playback a file while the microphone listens. The file will probably be a "sweep", and will have been recorded at a known level (this is the electrical dBv). You'll be instructed to manipulate the level of the playback system to a certain point, then the level of the mic input on the laptop. Once that TrueRTA can "hear" the whole file, you'll probably be walked through setting levels for "0 dBa". This is the calibration point, you're teaching TrueRTA to recognize what it "looks like" when the SPL in the car is 0 dB.

Personally I don't think you can calibrate without a calibrated soundsource. Not a file played back through a laptop with unknown speakers, amp, soundcard... Differences in frequency response can be measured this way, but I can't see how absolute SPL could be obtained...?

midiwall · 09-06-2006, 01:30 PM

Quote: Originally Posted by JC-S60

Personally I don't think you can calibrate without a calibrated soundsource. Not a file played back through a laptop with unknown speakers, amp, soundcard... Differences in frequency response can be measured this way, but I can't see how absolute SPL could be obtained...?

Understood, but if the software is worth it's price (figuring that the code in the free version is the same as the 1/24octave version) then it'll be able to get you close in terms of SPL. Frequency analysis is another issue, but we're only looking to get him an SPL at a fixed freq.

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.

JC-S60 · 09-07-2006, 02:18 AM

Quote: Originally Posted by midiwall

Understood, but if the software is worth it's price (figuring that the code in the free version is the same as the 1/24octave version) then it'll be able to get you close in terms of SPL. Frequency analysis is another issue, but we're only looking to get him an SPL at a fixed freq.

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.

But, if you put something over the mic (or put a resistor in between, or lower the level of the mic-preamp, or change your laptop's speakers volume or ...), 0dB will still be the same electrical level, but the reference sound (seen by the software as an absolute SPL) will be measured as lower SPL!

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...

T1NY W · 09-07-2006, 11:55 AM

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 !

Tiny

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