equalisation and measurement mics

starfox · 04-15-2004, 05:15 AM

Does anyone on these forums have experience with plotting frequency response graphs within a car for equalisation?

My current car audio 'experiment' is to use a car computer with a sb live in it, and kx drivers and to equalise the audio to get as flat a frequency response as possible in a car. This has the advantage where you (theoretically) don't have to worry so much about where your crossover frequencies, volumes and ranges, and you can equalise out frequencies that resonate badly.

I've recently built myself a measurement mic (http://sound.westhost.com/project93.htm), and it works quite well as a recording source on my turtle beach soundcard. Does anyone have recommendations on what software to use and any dos and don'ts? I hope i don't have to work all of this out on my own...

shakes · 04-15-2004, 09:47 AM

I was wanting to do this in my car as well. I have a decent ear but it's not great.

Unless I'm misunderstanding what you're wanting to do ... all you would need is a spectrum analyzer and some pink noise to watch the freq response ... correct? I'm sure there's some good spect analyzers you could run on your in car pc and have it only show the response of the mic in line. i've been meaning to look for one, but havent' gotten around to it.

Do you have any recommendations for a decently inexpensive measurement mics that can be purchased easily?

starfox · 04-15-2004, 10:28 AM

At the moment, i think measurement mic and inexpensive is almost an oxymoron.

I ended up purchasing a kit (url in first post) and building one after finding a company in Australia which sells electrets with a good frequency response. The circuit actually puts out a signal with enough voltage to be recorded using a line level input, that should help to minimise the amount of noise that would otherwise be picked up on the microphone jack on a PC..

The other thing about getting a real measurement mic is that you'll probably have to buy or build all the balanced line receiver equipment: phantom power supply and a balanced line to line level converter.. That would probably either be quite expensive or quite time consuming!

g'luck with your project though, i'll post if i make any progress on mine..

Banderon · 04-15-2004, 08:39 PM

http://www.audiocontrol.com/MobileAu...ers/index.html

SinnerG · 04-15-2004, 09:17 PM

Try this. You use your own computer to measure the sound output to adjust.

Quote:

PRAXIS is the next generation audio analysis and loudspeaker measurement tool from Liberty Instruments. PRAXIS performs convenient, sophisticated, and high quality audio measurements of loudspeakers, electronics, vibrating systems and rooms. PRAXIS utilizes higher quality Windows based digital soundcard hardware for versatility, compatibility and portability.

PRAXIS can be used free of charge (in its "free" mode) for viewing and processing audio data files, for performing basic audio measurements, or for exploring the features of the PRAXIS system.

The PRAXIS AudPod is a compact, portable audio interface and license key which enables the entire feature set and capabilities of PRAXIS for professional use. Because PRAXIS with the AudPod can calibrate for accurate measurements using PC Audio hardware, you can easily transfer the measuring system between computers without removing covers or uninstalling cards. Take it with your laptop when you're on the road. Connect it to lab or desk machines when you get back. Use the performance of better soundcards when higher sample rates (up to 192kHz) or 24bit measurements are needed.

PRAXIS provides unmatched flexibility with its script support and included Script Designer.

I haven't tried it yet, but it looks good.

shakes · 04-15-2004, 09:28 PM

good find sinnerg ... looks like they even have a little mic kit ... any idea how much it is? I couldn't find a price on there.

SinnerG · 04-15-2004, 11:48 PM

You've got to call for pricing here

david_594 · 04-16-2004, 12:09 AM

The software you are looking for would be something along the lines of SmartLive. Unfortunatley there is one major flaw in your plan. While it may seem all find and good that you can set up a mic, and play tones through your stereo and set all the level to be equal you are missing one major factor, that being the microphone isnt accurate.

To do what you want to do accurately, you first need to get a good high end microphone and bring it to a sound lab where you can get the frequency response of the microphone mapped. Because microphone DO NOT have a flat pickup curve, they have spikes and lows just like your stereo currently does. So if you dont get the mic mapped first you arnt helping yourself. Once you have that info you need good software, that can take into account the frequency response of the microphone so that you are then able to accurately set your levels.

It is a pretty big procedure to do it right, and the gains you get from doing it this way arnt that noticable. If your goal is to comptete in sound competitions, then something like this is a defenite must do, along with a 30 band EQ. Im not saying you shouldnt try it, im just saying dont expect it to be perfect.

starfox · 04-16-2004, 02:31 AM

I'm using an electret microphone with a preamplifier. By design, electret microphones will have the flattest pickup curve out of all sound sampling devices. Datasheet for the microphone is here: http://www.hy-q.com.au/pdf/10310008.pdf. I know that standard consumer microphones have crappy frequency response, that's why i built one myself (i can't afford to buy a pro level measurement microphone at the moment). The circuit i'm using is one of Rod Elliot's and the url for a page with info about the circuit is in my first post. The circuit is quite nice, it provides a higher voltage power supply to the electret and also has a preamp to reduce any noise pickup during signal transmission.

Might have to ask my university if they have a reference microphone somewhere. Calibrating the microphone may also prove to be difficult.. but this is all an experiment anyway.

My opinion is that you don't necessarily need a 30 band equaliser. I'm doing the equlisation in software, this should theoretically beat any powered equaliser hands down because the equalisation's done digitally and there shouldn't be any noise or distortion introduced (unless i'm retarded and soft-clip, or the interpolation routines in the software equaliser aren't done at high enough precision).

I'm running an SB Live with KX drivers with individual 5 band parametric software equalisers on front, rear and subwoofer channels. I can also run shibatch's equaliser plugin on winamp if i need to do more than the KX drivers will allow, which gives me an 18 band graphic equaliser and an unlimited number of parametric equalisation points (limited by cpu processing power).

I don't actually intend on entering any SQ competitions, because i haven't found one person here that actually cares about anything other than what SPL their car audio system can generate....

I'd just like an idea of what the frequency response in my car is like so i can equalise it and compare it with home theatre systems to see what discrepancies there are between audio produced in different environments.