REW (Room EQ Wizard)

[David Carter]

Has anyone used REW with just a mic but no SPL meter? I've downloaded and installed it, but all the instructions talk about using an SPL meter to calibrate to absolute SPL levels. I have a handheld meter, but it doesn't have any line out jacks. I have the ECM8000 mic. I don't care about absolute SPL levels. I just want to see the response curve to be able to EQ it flat. Can I use REW in this way?

I guess I should also mention that I would like to use REW instead of TrueRTA because they both have calibration files for the ECM8000, but I have to pay to get 1/3 octave resolution in TrueRTA while REW is completely free.

[Bill Fitzmaurice]

To calibrate the system simple play a sine wave tone at 1kHz/2.83v and measure the result. That's your base. Adjust your broadband SPL chart so that what it reads at 1kHz is the same as that measured SPL. No line out on the meter required.

[David Carter]

To calibrate the system simple play a sine wave tone at 1kHz/2.83v and measure the result. That's your base. Adjust your broadband SPL chart so that what it reads at 1kHz is the same as that measured SPL. No line out on the meter required.

Sorry for the newbie ignorance, but let me make sure I understand...

For example, I play a 1kHz tone/2.83v and the measured SPL on my meter is, let's say, 100dB ( I have no idea if that's a realistic number or not). Then, what you're saying is that after that I just need to run pink noise and EQ the whole thing flat so that my meter reads 100dB across the whole spectrum from the crossover point on up?

Is there a standard distance from the cabs I should be placing the mic or is this irrelevant if I'm just after a flat response curve regardless of the absolute SPL reading?

[Bill Fitzmaurice]

For example, I play a 1kHz tone/2.83v and the measured SPL on my meter is, let's say, 100dB ( I have no idea if that's a realistic number or not). Then, what you're saying is that after that I just need to run pink noise and EQ the whole thing flat so that my meter reads 100dB across the whole spectrum from the crossover point on up?

You don't EQ at all, you just know that the reference for 2.83v at 1kHz is 100 dB. So if you run a chart and 100 Hz measures 10dB down compared to 1kHz then the 100 Hz 2.83v sensitivity is 90dB.

Is there a standard distance from the cabs I should be placing the mic ?

1 meter, but you'll get a better result measuring from further away and compensating for it. 2 meters means you're down 6dB, 4 meters is down 12dB, 10 meters is down 10dB, add that to your result to arrive at the 1 meter reference figure.

[David Carter]

For example, I play a 1kHz tone/2.83v and the measured SPL on my meter is, let's say, 100dB ( I have no idea if that's a realistic number or not). Then, what you're saying is that after that I just need to run pink noise and EQ the whole thing flat so that my meter reads 100dB across the whole spectrum from the crossover point on up?

You don't EQ at all, you just know that the reference for 2.83v at 1kHz is 100 dB. So if you run a chart and 100 Hz measures 10dB down compared to 1kHz then the 100 Hz 2.83v sensitivity is 90dB.

OK. I think I get it. This is just the calibration phase you're talking about. The actual measurement and EQ comes next.

Is there a standard distance from the cabs I should be placing the mic ?

1 meter, but you'll get a better result measuring from further away and compensating for it. 2 meters means you're down 6dB, 4 meters is down 12dB, 10 meters is down 10dB, add that to your result to arrive at the 1 meter reference figure.

Assuming this is going to be done outdoors, at what distance do I need to start worrying about ambient noise like nearby traffic, etc. I don't have access to anything that is out in the middle of nowhere, but I do have a fairly large space nearby which will have minimal residential area traffic noise and possible some muffled noises from a nearby factory.

[Bill Fitzmaurice]

A typical backyard is fine. You'll get some reflections off buildings in the low end but nothing compared to those in a room. A 4 meter distance is adequate.

[David Carter]

A typical backyard is fine. You'll get some reflections off buildings in the low end but nothing compared to those in a room. A 4 meter distance is adequate.

Thanks, Bill. One last dumb question... When you say a 4-meter distance is fine, are you talking about the open area in all directions around the cabs or are you talking about the distance from mic to cabs? I'm assuming the latter, but I want to make sure. My back yard is fenced and is probably 50' deep x 70' wide with a 28' x 24' detached garage taking up one corner of the space.

Thanks for your patience in educating us newbies!

[Bill Fitzmaurice]

When you say a 4-meter distance is fine, are you talking about the open area in all directions around the cabs or are you talking about the distance from mic :

Distance to mic. 50 feet to the nearest building for precision measurements, but I doubt you need to be that fussy.

[David Carter]

Again, thanks for all your help!

[Tom Smit]

I think you might want to be at least 50' from any reflective surface (solid fence, etc.)

[jskrypek]

This may sound dumb but how do I get the voltage right for the 1 KHZ tone?

[David Carter]

This may sound dumb but how do I get the voltage right for the 1 KHZ tone?

Use a multimeter to measure the voltage directly from the output jacks of your amp. What I plan to do is get a couple of speakon connectors, connect some wire to the 1+ and 1- terminals and alligator clips on the other ends of the wires and attach to the meter. Play the tone, and set levels until you get the desired voltage reading on the meter. Disconnect the test leads and plug in the speakers.

[bgavin]

Bear in mind that a sine wave playing 1 KHz will have far more power (voltage) than typical music power.
The sine wave is uniform intensity across the spectrum, where music power concentration decreases with increasing frequency.

This is why you can have a 450w subwoofer running with a 75 watt tweeter.
However, if you do a swept sine test at 450w, you will most likely smoke your tweeter.

[myn]

You could also use the manual method, (not using REW), using 1/6 octave test tones and an excel spreadsheet. Tedious but I used this method prior using REW.

http://www.hometheatershack.com/forums/bfdmeasure/

One thing to note is that most of the cheapie SPL meters (< $150) aren't accurate and as such require a calibration ofset. If you have a popular SPL meter like the Radio shack digital or analog ones both REW and the Excel spreadsheet will account for it. Otherwise the results you'll get won't be too useful to you. Keep that in mind.

Also make sure you're using the LINE in and not the MIC inputs on your computer. My PC's and laptops didn't have a Line In so I was faced with buying an external USB one which works great in XP:

http://www.newegg.com/Product/Product.a ... -_-Product

If you need any more help, let me know. It took me a straight 8 hours of banging my head against the wall before I totally understood REW.

[myn]

As I am getting ready todo some measurements of my THT with REW this coming weekend I found a REW cheatsheet I've used in the past to get things up and running quicky.


Wayne A. Pflughaupt's REW Quick Instructions from www.hometheatershack.com
===================================
REW quick instructions

Here's a bit of a rough step by step of the setup once the soundcard.cal and meter.cal are loaded and the cables are all hooked up and ready to go..

To go through these steps only takes a second......

1. Run the Check Levels routine (Settings icon, Soundcard tab) selecting Check/Set Levels with Subwoofer in the Levels pull down box, with the SPL meter at the listening position and the dial of the meter at 80dB position (or whatever scale allows 75dB to be measured).

Ensure the REW output VU meter is at -12db and the Sweep Level at -12dB.

Set the Wave Volume at 1.000 and the Output Volume about ~0.500. (If those fields are blank, change Input Device Input and Output Device Output from Default to USB Audio, or the name of the soundcard, if that options available, and Speaker for output, and Line In for input. Change).

Adjust your receivers volume control so the actual standalone Radio Shack SPL meter reads ~75db at the listening position.

Then adjust REW Input Volume to end up with -12dB on the REW input VU meter (i.e., matches the Output meter).

2. Still in the Settings section, run the Calibrate SPL routine (in the Meter Tab) to match REW's SPL meter to the real SPL meter to ~75dBSPL. Make sure the “C Weighted SPL meter is checked.

3. Close Settings window. Run the Set Target Level routine (Target Settings icon to left of screen), which will set the target to ~75dB. Also, set the Cutoff, which is the crossover frequency you’re using

4. Run the Measure routine and set the End Frequency to 200Hz for subwoofers.

5. Measure.................... (use a larger number of sweeps and longer length for more accuracy and less noise)

6. Using the Graph Axis Limits icon, set the vertical scale to 45dB to 105dB and the horizontal scale for subs to 15Hz to 200Hz.

7. Select Filters icon at top of page, to call up windows to tweak EQ filters.

To get a waterfall graph, select the LF Waterfall tab at the top. Then press Generate LF Waterfall icon at the bottom of the screen.

8. To save graph, select Graph from Menu Bar, then Save Graph as JPG. Use 800 size setting.