Any reason why REW couldn't be used in a car?

**DaveNagy** · **07-13-06, 03:53 PM**

Hi folks,

I'm in the process of installing a computer in my car. It'll be used for lots of things, but one of its main tasks will be acting as a glorified MP3 player. Since there are all manner of software based EQs available, I figured that I would experiment with some of them to see if they could be used to tweak the (full range) response of my car stereo. (Which has no EQ of it's own, other than tone controls.) REW immediately came to mind as a measurement tool...

Is there any reason why this wouldn't work? I didn't see any discussion along these lines here, but then again, this is an HT forum. Is there a better venue for discussing non-HT applications of REW? The AVS forums, perhaps? This forum seems to contain the most "experts", but I don't want to annoy anyone by veering off topic.

**Sonnie** · **07-13-06, 05:16 PM**

No problem Dave and welcome to the Shack!

I see no reason why REW wouldn't work in your car computer as long as it has the proper sound card with inputs and outputs.

Look at the REW Online Help file connections section and see how it is connected. This might be the challenge in getting it setup. You need a mic, which in this situation would probably be the RS SPL meter since it's battery operated. You will have to connect the RS meter to the input of the computer soundcard and unless you have some way of connecting the output of your sound card back to the input of your headunit to play test tones you will probably have to use a CD with test tones and/or sweeps.

There could be more to it than I think though.

**DaveNagy** · **07-13-06, 06:10 PM**

Thanks, Sonnie. I just read through about a third of the REW online help files, and I'm starting to get a feel of how tricky this will be.

Here's a few comments and questions:

I'll be using a USB "soundcard" with the computer. I've already purchased it, so hopefully it will be adequate for use with REW. It has line outs, and "mic/line" inputs. When doing its normal music-playing duties, the line-outs of the USB device will be connected to the line-ins of a BlitzSafe adaptor box, which in turn feeds into the CD-changer input of my car's stock deck. Simple, huh?

I've hooked it up, and it seems to work fine for playing music.

I have the "old" analog RadioShack meter, which I believe could be directly connected to one of the line inputs on the USB doohicky. Question: Since I will want to do a "full range" EQ, will the mic in the SPL meter be accurate enough? I've read that they are not very accurate at higher frequencies. If this is the case, I may wish to purchase a cheap-ish mic and pre-amp.

The calibration process will be a more-or-less "one time" event, performed in the safety of my garage. So, there's no need to worry about having battery-powered everything.

One potential flaw in my plan that I see is: How do I "wire" the software-based EQ into the circuit, so it is downstream of the tone generator in REW? I'm not sure I can. The only parametric EQs I've found so far are implemented as Winamp "plugins". That means they only affect the signal coming out of the MP3 player, and would not affect any signal that REW is putting out.

I can think of two potential solutions: One, find a software EQ that is more general in design, and that can be configured to affect every signal the computer outputs. Two, use Winamp as the signal source during calibration. Use it to play MP3 test tones, while measuring the response in REW. (Is that even possible?)

The first option seems a lot more elegant. Do any of you folks know of a software EQ that can used with any and all sound sources on a computer? (For instance, one that could EQ both the output of WinDVD, and some game that's running on an HTPC.)

Many (most) of the EQ programs I've found so far are billed as "DirectX/VST plugins". They are designed to be used with "DAW" (Digital Audio Workstation) programs. You buy an EQ plugin, or a reverb plugin, or tube distortion plugin, and they will all magically become available to be inserted into the signal chain of one of these audio recording programs. Of course, I have no need for a fancy schmancy audio workstation, I just want the plugin!

I've heard rumors that there are programs that let you use these plugins more generally on your computer, but I haven't found anything concrete. Anyone know what I'm babbling about?

Well, enough questions. Thank you for your hospitality.

**brucek** · **07-14-06, 05:56 AM**

Quote:

Since I will want to do a "full range" EQ, will the mic in the SPL meter be accurate enough?

No, the RS meter is quite poor above 5K. It works well for low frequency duties when used with a calibration file (found here), but you may want to purchase a better mic for full range.

Quote:

One potential flaw in my plan that I see is: How do I "wire" the software-based EQ into the circuit

Yeah, that seems like a showstopper. You can use REW as the measurement and graphing tool (as you've suggested), but I don't know of any plug-ins that will allow the equalized modification of the REW's output during sweeps etc.

On a less sophisticated scale you could use REW to create an accurate frequency response of your car without equalization and then use this information to pinpoint the areas you want to tame with your equalizer. There would be no feedback to how well your settings worked though.

brucek

**JohnM** · **07-14-06, 10:13 AM**

Worth taking a look at Convolver and having a discussion with the author of that app.

**DaveNagy** · **07-14-06, 12:38 PM**

Quote:

brucek wrote:

No, the RS meter is quite poor above 5K. It works well for low frequency duties when used with a calibration file (found here), but you may want to purchase a better mic for full range.

Okay, I'll look into that. I'm sure there are plenty of threads here about what mics are good.

Quote:

brucek wrote:

Yeah, that seems like a showstopper. You can use REW as the measurement and graphing tool (as you've suggested), but I don't know of any plug-ins that will allow the equalized modification of the REW's output during sweeps etc.

I did some more googling, and I may have found a way to do it. Maybe. I found a "standalone" version of that nice looking EQ I had found. Unfortunately, it seems to be hardcoded to only take its input from the inputs of a soundcard. (I guess it's designed for use during recording.) Also, it appears to only be looking for installed soundcards that are running an ASIO driver. The nForce chipset on my home computer has such a driver, but God knows whether the USB doohicky on the car computer will. So, that's two strikes against this particular EQ, but there's still a faint hope I can make it work. How?

For the input issue, there's a product called "Virtual Audio Cable". In theory, one can use this utility to create "fake" inputs and outputs and "wire" them all together. So, I could create a fake output and tell REW to use that, and have that virtually connected to a fake input which I would then tell the EQ to use. There's a universal ASIO driver out there, and the Virtual Audio Cable guy claims it will work with his program.

Even if this scheme can be made to work, it seems like it will confuse the heck out of REW, since REW is going to be try to control the volume, etc., of an imaginary soundcard output. Hopefully, that functionality can be disabled if necessary.

Quote:

brucek wrote:

On a less sophisticated scale you could use REW to create an accurate frequency response of your car without equalization and then use this information to pinpoint the areas you want to tame with your equalizer. There would be no feedback to how well your settings worked though.

Yes, I was thinking that might be a good fallback plan. I could certainly use REW in the usual fashion to tweak the level and cutoff frequency of my "subs". (They're really just 6x9s, but they do have their own amp.) Then for the full range tweaks I could use REW's filter simulator thingy to "design" a correction curve. Then I could transfer those filter settings to the parametric plugin in Winamp. As you say, there'd be no way to test the resulting output, but if everyone has done their math correctly, it might be pretty **** close.

**DaveNagy** · **07-14-06, 12:41 PM**

Quote:

JohnM wrote:

Worth taking a look at Convolver and having a discussion with the author of that app.

I ran across that in my searches, but couldn't really discern what the heck it did.

I'll take another look. Thanks. Does the author post in this forum?

**brucek** · **07-14-06, 01:15 PM**

Quote:

As you say, there'd be no way to test the resulting output, but if everyone has done their math correctly, it might be pretty **** close.

Yeah, and your ears will be a decent tool to use in this case. If you've accurately identified the problem areas with REW and then modelled your frequency response to that (depending on the parametric capabilities of your equalization) and the resulting listening test feels right, I suspect you're fairly close.

brucek

**Sonnie** · **07-14-06, 04:39 PM**

Quote:

DaveNagy wrote:

Does the author post in this forum?

If he does he hasn't let us know.

**DaveNagy** · **07-19-06, 03:15 PM**

Well, I ordered a Behringer mic and mixer, and they've arrived already! I should be able to get this show on the road (heh) pretty soon.

Thanks again for the "Convolver" recommendation. I've been reading up on that, and it appears that impulse response processing is the new hotness. I'll try to get that working, rather than going with a software parametric.

**DaveNagy** · **07-19-06, 05:22 PM**

Okay, I have a question regarding my Behringer mic: Where and how should I position it in my car? I assume I should put the mic where my head typically is when I am driving. But, which direction should the mic be facing? Forward? Straight up? Somewhat downwards toward the steering wheel? Towards one of the tweeters?

The speaker positions are fairly standard. The tweeters are in the top-forward portion of the doors. The "mid-woofers" are in the lower-forward portion of the doors. The woofers (currently crossed over at ~100Hz) are on the side walls, aft of the doors, a couple feet behind the seats. (It's a two seater.)

Since everything (seats and speakers) is symmetrically positioned, I was hoping that both (L/R) channels could be equalized equally, and that if it sounded good for the driver, it would sound equally good for the passenger. Is that correct?

Hmmm, now that I think about it, maybe that isn't correct. If I go the Convolver/impulse-response route, I believe that will attempt to correct for phase differences. Since each listener is quite close to one tweeter, and relatively distant from the other, I suppose any correction would be pretty "lopsided" left-to-right, and what sounded good from the driver's side, might sound pretty bad from the passenger side. I'll definitely want to do a correction that's idea for the driver (me), since I'm the only one in the car 95% of the time, but how would I go about creating a happy medium correction for when I have a passenger? Place the mic halfway between the two listening positions?

**brucek** · **07-19-06, 05:33 PM**

The fact is, once REW is set up and ready to go, you can do a sweep every few seconds. So it doesn't hurt to move the mic around and see what the various spots reveal.

In a home theatre we usually place the mic in the listening position pointing up. Once the equalization is done I move the mic around to several areas to see the difference.

brucek

**Sonnie** · **07-19-06, 06:24 PM**

Dave... when I do my measurements, I sit in my listening position and place the tip of the mic right at the opening of my ear. After all, it's where I hear it at. I've compared left ear to right ear with no significant difference in my HT room.

I've tried the car thing before with an SPL meter and test tones... it was a challenge. I tried sitting behind the drivers seat and holding the mic about where my head would be, but moving it literally 2 inches caused a drastic variation in levels measured. I can actually move my head a few inches and notice quite a difference in response... and my ears are pretty bad. Someone that has good hearing would probably notice it more. You might try it and see if it is the same for you. This is why I gave up on eq'ing my car. The sub response was not quite as drastic, but the high end was impossible for me.

If I tried it again I'd sit in the drivers seat and place the mic right at my ear. I believe the mic is omnidirectional anyway, so how you have it pointed is not really a factor, I don't think.

It will be interesting to see your results if you can somehow graph them for us.

**DaveNagy** · **07-19-06, 07:07 PM**

Yes, I'll try to post some graphs here. I've ordered some new speakers for the car, so it will be a week or two before I can start this process "for real", but I thought I'd measure the old speakers to give me some practice with REW, and so I can compare the old to the new.

I tried to EQ my wife's car, using an SPL meter and some test tones. I also found it difficult. Moving the meter even slightly resulted in big differences in measured SPL. I'm hoping/wondering whether using sweep tones and REW will result in more "stable" results. Perhaps the quickly changing frequencies won't allow the standing waves to build up as strongly? (I'm probably deluding myself.)

Another question: Can someone recommend me a nice simple full-range room curve? I'm thinking it could just be a simple downwards slope, but I'm not sure how steep to make it. (Or indeed, if it should be straight at all.) If it was a straight line between (say) 40Hz and 15kHz, how many dB do you think it should drop between those two points? (Yes, I need to go read the the room curve sticky again.)

**Sonnie** · **07-19-06, 10:45 PM**

I've seen some recommended response curves for cars before and about all I remember is at 30hz it was +30db and at 20khz is was -18db. I wanna say it was gradually sloping and flat from about 100hz to 10khz, but don't hold me to it... it's been a while since I saw it.

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