Thread: Live RTA of a car's transfer function

Are you talking about the types of software we use for live sound applications? Things like SmaartLive-$695 plus a decent measurement mic-Earthworks M-30 $475. (http://www.siasoft.com) This software has a 30 day free trial available.

If you didn’t want to spend that much, (keep in mind that price pretty closely = quality with these software’s) you could go with something like True RTA, but you'd need to get the Level 4 version for $99.95 (plus a descent microphone again. looking at a few hundred for a properly calibrated mic) to have any decent measurement ability. Anything lower only gives you up too a 1/6 octave RTA.
(http://www.trueaudio.com)

As far as active noise cancellation, what exactly do you mean? Are you talking about having the software decide how to eq your system to compensate for whatever noise there is? If so, I have never seen such software, one may exist, but a computer can't make things sound good. It can tell you what your spectrum looks like, and allow you to decide how to compensate, because in the end it is you who decides if it sounds good or not.

OOORRRRR you could just go all out and put a SIM3 machine in your car (http://www.meyersound.com/products/i...sim3/index.htm)

Also keep in mind that for an accurate reading, you will need to periodically have your microphone re-calibrated.

PS. If you did try something like this, I wouldn't try to run it on your carputer. These software’s take up system resources like none other. I'd recommend putting it (maybe the SmaartLive demo) on a laptop, making your measurements in the car, and creating a few presets for your eq based on these measurements. That would save you a lot of money. You don't have to use a calibrated microphone, but if you choose not too, you should try to find the most flat freq. response mic you can. Take a look at the freq. response charts, and compensate your measurements accordingly.

If you are feeling ambicious enough, please keep me updated on how it goes.

Originally Posted by Vacheron

You will probably initially need to borrow an audio CNE (constant noise emitter) to provide fixed values of the initial stages the sound will go through

The softwares i spoke of earlier all include a tone generator, pink/white noise generators, multiple sine wave generator, and custom sine wave sweeps.

haha, *races to the post button*

Ya, i say just go with the SIM machine. It'll only cost your house, car, computers, soul, and two animal sacrifices, but it's a great machine

Or better yet, just have the guys who did this setup come in and do your car

HAH.. I used to do a bit of production rigging, that looks like a nightmare to me.

I don't think i want the guys that broke the roof of that stadium to come anywhere near my truck

About the white/pink noise thing, i was going to try to get away from that.

What i want to do is much cooler, i want to replace the pink noise signal usually used for calibrating audio with real music, real sound.

Do an FFT of the source live out of the soundcard, and take the FFT of the measurement microphone. Apply the appropriate delay to the signals to compensate for latency, take the difference between them and apply it to the EQ, so while you're driving and rockin' out, the software is compensating by the DIFFERENCE between the source and the measured return.

And about noise cancellation, you all know about the noise cancelling headphones that measure the ambient noise, invert the phase and apply that signal to the headphones, using the property of destructive interference to "cancel" the noise.

Why not do the same thing in a car? measure the source waveform, measure what the microphone hears, weed out what isn't good signal from the measurement with a subtract operation after the appropriate delays, invert the extra noise and apply it to the signal.

The art is in calibrating and setting up the feedback loops, has this been done before? and if no, why the hell not?

You might need to buy a high end soundcard that can sample fast enough to do this, eg. if you're playing CD quality stuff you'll need a soundcard that can sample at 88.2khz else your result from the FFT won't be accurate enough to seperate noise from the signal and noise from sampling (nyquist).

I can imagine that you're going to need a lot of processing power to do this in realtime as well

Sure, but a lot of soundcards can do 24 bit at 96khz on 2 channels easially, and a measurement microphone is mono, so there's half the bandwidth right there.

The thing wouldn't need to write to HD, and i doubt it would take more than a P4 at 1ghz to do it if it was dedicated. This is the kind of thing i would need to build a little bit of specialized hardware for anyways, so why not just run it on a good fast PC running a stripped down nothing-but-audio linux build?

I can't see any issues with doing it in realtime on a modern PC if it's specialized and not running XP or anything bloaty like that.

just buffer the input. You can always compensate for speed in your control loop. 1gb of ram is a ****load of audio data, so if you don't plan on listening to any 2-hour long songs, it would be able to buffer the whole load and not flinch.