How To Calibrate and Use REW
To Test and Tune Your Room Acoustics
To Test and Tune Your Room Acoustics
REW is a free acoustic analysis package, and is actually a lot better than some of the packages that cost money.
REW stands for "Room EQ Wizard", and you can find it here: https://www.roomeqwizard.com/
In short, it's amazing (and even more amazing that it is FREE!) I highly recommend it. It's what I use for pretty much all my acoustic testing these days.
But in order to get useful information from it, you need to first calibrate it, and use it correctly. Below is the series of steps that I use with my clients to set up and use REW in the most meaningful way. If you want me to analyze your data, then please follow these instructions carefully. If you post REW data here on the forum that shows you clearly didn't follow this procedure, then please don't expect a reply. It's not hard to do.
SOUNDMAN PROCEDURE FOR CALIBRATING R.E.W.
(prepared by Stuart Allsop - a.k.a "Soundman2020")
(updated July 2021)
(prepared by Stuart Allsop - a.k.a "Soundman2020")
(updated July 2021)
What you will need:
Computer and interface: You will need a computer capable of running REW, obviously! The computer will need a decent quality audio interface of some type for connecting your mic and speakers, and the REW manual refers to this as the "sound card". That might be a physical card inside your computer, or it might be an external box connected by USB, Firewire ( ), or some proprietary connection. It will have at least two output channels for connecting to your speakers (either directly, or via your DAW), and it will have at least one input channel, for connecting to your acoustic measurement mic (either directly, or via your DAW).
Acoustic measurement mic: Use only a proper acoustic measurement mic for these tests! If you don't have one, go buy one. There is no substitute. You cannot use a typical directional vocal or instrument mic, or a mic that does not have flat response across the spectrum. It must be an omnidirectional mic with flat response. So if you used an SM58 or U47 or C1000 for your tests already, throw the data away, get a proper mic, and do it again.
A typical acoustic measurement mic:
Which mic should you buy? I do NOT recommend the Behringer ECM8000 mic for this, as I've heard too many sad stories about people getting faulty mics, including my own personal experience. I have several mics, but the one I use for most testing is a simple, inexpensive PreSonus PRM-1. Other good ones are the dbx RTA-M, Audix TM-1, Nady CM-100, Beyerdynamic MM1, Dayton EMM-6, and if you really feel like blowing a lot of money, then get an Earthworks M30 (you can buy all of the others together for about the same price as a single M30...) There are some USB mics that supposedly don't need calibrating, but I'm still on the fence about those.... not convinced at all. I prefer a "real" mic with a "proper" XLR connector on the back... One reason for this is the need to move the mic around the room... if it is tethered to the computer with a USB cable, which can only be a few feet (meter or so) long, then you have a problem... How do you move the mic without also moving the DAW?
A good mic will cost you maybe around US$ 100 or so (2021 prices).
If you have more than one measurement mic, then always use the exact same one for all of your testing, otherwise you will not be able to validly compare tests.
Sound level meter: Yes you do need a proper, real, good quality hand-held sound level meter to do this. NOT an app on your iPhone. (Don't get me started on why that's a bad idea....) If you are building a studio, then the cost of a good sound level meter is peanuts, compared to what you spend on everything else. It costs less than half a dozen sheets of drywall! DO NOT GET A CHEAP CHINESE JUNK METER! Those go for under US$ 50, and they are nothing but toys. A good meter will cost you around the same as a good mic: in the region of US$ 100 (2021 prices). Good meters: Extech (eg. model 407730 or 407732), Galaxy, Phonic (Eg. PAA-2 or PAA-3), Reed Instruments, B&K, some Triplett.
A typical sound level meter:
Hearing Protection: The test signals can get pretty loud if there's a strong modal response in the room to some frequencies, and they don't sound very nice anyway, so it's a good idea to have something handy to protect your hearing. Typical "ear defender" headphone type protectors work very well, and so do the small "ear bud" type foam plugs. Whatever it is you get, make sure that it provides at least 20 dB of protection, and 30 dB would be better. And make sure you use it, every time you test.
Typical headphone type "ear defender" hearing protection:
Typical "ear bud" type hearing protection:
OK, so that's what you'll need (in addition to your speakers, cables, a mic stand, and some common sense!).
Now here's how to do the calibration and first test in your empty room.
1. Gear setup: Make sure that all EQ controls on your entire signal chain (including your speakers, console, DAW, mic pre-amp, speaker controller, etc.) are set totally flat. Also make sure that there is no compression, delay, or other effect applied anywhere, and that the gain structure is correctly set at the input and output of every piece of equipment in the complete signal chain, at –20 dbFS (for digital gear) or 0 dBVU (for analog gear).
2. Calibrate REW to your DAW sound card: There's a procedure in the REW manual for doing this: In REW, go to "help" --> "show help", click on the magnifying glass icon, and type in "Calibrating the Soundcard". Follow those instructions to generate a calibration file. Make sure you always use that calibration file, and that it is included in your MDAT file when you upload it.
(Be careful here: Some people who should no better might try to tell you that you can skip this step: that it is not necessary. Wrong! While it is true that the vast majority of modern sound-cards do not actually need this calibration file, since their response is almost always very flat, that does NOT mean that you can skip this. It is still important to do the procedure, because it will highlight any errors you might have in your setup. If the result is NOT flat, or very close to flat, then that indicates that you did something wrong.
When you are done with that, disconnect the loopback and connect your two main speakers to the output channels, and connect your measurement mic to the input channel.
3. Calibrate REW to the real sound levels: This is important, even though the REW manual says it isn't. If you don't calibrate REW, the absolute SPL levels on your graphs will be wrong, and I won't be very interested in helping you analyze them... (I have several standard settings that I use for viewing REW data. If I use those to look at your data, and if your data is in the wrong place, I won't be able to see it or analyze it. I would have to change my settings. And yes, that IS a big deal: I have thousands of REW data sets on my computer, from many, many dozens of studios, and if your data does not conform to that, then I'm not going to mess up my settings to try to match your data...)
Go to the PREFERENCES menu in REW, click on the "Soundcard" tab, and make sure you have the correct device and channel selected for both input and output. Just below that, there are several controls for setting levels. Make sure the "Invert" and "High Pass" boxes are NOT selected, and that the "sweep level" is set to -12.
About half way down the page, there's a heading that says "Levels". Make sure that is set to "Use main speakers to check/set levels". Click on the "Check Levels" button next to that, then click on "Next".
It should start playing pink noise through both speakers, and you should see the "Out" meter showing a level of -12 and the "In" meter showing about -18, like this:
Turn off the left speaker, so that ONLY the right speaker is playing. Get out your hand-held sound level meter (I'll call it the "HH meter" from here on), select "C" weighting and "Slow" response.
While REW is playing out that pink-noise test signal, use your HH meter (set to "C" and "Slow") to measure the level at the place where the measurement mic will be (ie, where you head will normally be while mixing), and turn up or down the volume control for the right speaker until your HH meter is showing 80 dB. Now turn off the right speaker, turn on the left speaker, and adjust that speaker until it is also showing 80 dB on the HH meter. It's hard to calibrate with pink noise, since it varies a bit, and there are also limitations on how accurately low frequencies can be measured, but do try to get this as accurate as you possibly can.
Yes, I know that the REW manual says to use 70 dB, but I prefer 80 dBC to make sure that there's a good signal to noise ratio, and that all of the room modes are triggered. If your level is too low, the data will be "dirty", and there might not be enough headroom to do decent analysis. Also, if your level is too low, it won't necessarily trigger all of the modes in the room at a decent level.
Click "Finish" and close the Preferences menu.
Now place your measurement mic at the mix position, in a good, firm, steady mic stand that won’t move during the tests. Set up the mic on its stand exactly where your head will be when you are mixing normally, on the room center line, directly above your chair, facing straight forwards and slanted a bit upwards, angled around 70° up. (Yes, I know that some people say to point it straight up, and others say to point it forwards, or at 45°, or at the speaker, but I'm not them, and there are very good reasons for me suggesting 70° for this test. I'm not going to go into them here, but if you want me to analyze your data, then please use 70°. (It doesn't need to be exact: just somewhere around 60° to 75°).
Click on the "SPL Meter" icon at the top of the REW screen: another window opens, with an SPL meter in it. Make sure that the buttons under the display are set to "SPL", and "C" and "S". Like this:
Click on "Calibrate". A window opens asking you to select the signal source: Set it to "Use REW speaker cal signal", and click "accept". Like this:
It starts playing pink noise again, and another window appears, titled "SPL Reading Calibration". Using your HH meter confirm that the level coming from your left speaker alone (right speaker turned off) is still producing 80 dB. Adjust the volume control on the speaker if necessary to get as close as possible to 80.0 dB on the hand-held meter, then double-click the number in the REW meter calibration box, change that number to "80.0", then click finished. Like this:
REW then tells you something about the maximum level that it can measure with this calibration setup:
Click "OK". Close the SPL meter window.
That's it. You have fully calibrated REW.
4. Make the measurements: With the same setup as for the previous step (do not change any settings at all from here on! Make a careful note of exactly where you have all the manual controls on your DAW, console, pre-amp, and everything else in the signal chain, so you can set them back to the exact same place in the future.), click on the "Measure" button in the top left corner of the REW screen. It opens another window, titled "Make a Measurement". Check that it is set to "SPL" (not "Impedance").
Makes sure that "Start Freq" is set to 10, "End Freq" is set to 22,000, "Level" is set to -12dB, "Length" is set to 256k, and "Repetitions" is set to 2
(NOTE: On older versions of REW, "Repetitions" was called "Sweeps". It's the same thing with a different name).
(NOTE: With some configurations, the "Sweeps" / "Repetitions" value will be grayed out and set to 1, so you will not be able to change it. For example, if you have a USB mic, or if you use "Acoustic timing reference").
That window looks like this, for newer versions of REW. (If you have an older versions the window for that is shown further down, below this one.)
Below is the old-style measurement window, from versions of REW prior to May 2019:
THE ABOVE IS IMPORTANT! If you post data that does NOT start at 10 Hz and end at 22 kHz, then don't expect me to help you analyze it.
Click on "Check Levels". It should play pink noise for a few seconds, and you should see the output meter jump to "-12", and the input meters should show a reading of somewhere between 0 and -20. After a few seconds, it stops playing the pink noise, then tells you what the level was, with a message saying if it was too high (clipping), too low, or OK to proceed. You might need to adjust the mic preamp gain or speaker levels to get that correct. If so you will have to repeat the above calibration procedure from step 3 onward.
For the actual measurement sweep, get out of the room while that runs! Yes, your body can affect the measurements, which is why REW has a feature for dealing with that.... Set the "Delay" option (called “Start Delay” in older versions) to several seconds, enough time for you to get out of the room and close the door, plus an additional 5 seconds for the room to settle down. In other words, if you need eight seconds to walk out of the room and close the door, then set the "start delay" number to 13 seconds.
Like this (RECENT VERSIONS OF REW):
OLDER VERSIONS OF REW:
Click "Start" at the bottom of the screen (which was called "Start Measuring" in older versions of REW), leave the room, close the door, and wait for the test to complete fully.
When you get back in, the first thing you need to do is to name the test! In the name box at the top of that test measurement (left hand column of the REW screen), type in the name: "L-- Baseline ", . . . . . LIKE THIS:
Do not name the measurements using the text box below the thumbnail! You can add extra information in the text box if you want, such as the treatment that you added to the room just before this test, but the name of the test MUST go in the tab at the top right of the thumbnail.
Then turn off the left speaker, turn on the right, and repeat the measurement step above. Do not change any settings! Once again, leave the room while the test runs. Name that measurement as "R-- Baseline ".
Then turn on the left speaker again, do one more measurement (with you outside the room), and name that one "LR- Baseline ".
Please makes sure that you arrange the tests in this exact order! Fist the "L--" test, then the "R--" test, then the "LR-" test. If you did the tests in the wrong order, you can fix that BEFORE you send me the file: REW allows you to drag each measurement up or down in that window. The reason I ask you to keep them in this order, is because that's the order I analyze them in, and also because I get many, many test results from many people: if I have to drag the tests around to put them in the order I want them especially for you, that just wastes time. It's only a few seconds, sure, but if I have to do that several times per day, it's boring, and annoying! Sometimes I get files from people that have dozens of tests, taken at different points in their studio build, and it can take several minutes to re-arrange those into a logical sequence. Make it easy for me to help you, and it's probable that you'll get better help!
If you also have a sub-woofer, then you will need to do a set of SEVEN tests, instead of just three, in the following order
--S Sub only
L-- Left speaker only
R-- Right speaker only
LR- Left plus Right speakers, but no sub
L-S Left plus Sub
R-S Right plus Sub
LRS All three on together
Save all of the measurements into one single MDAT file ("File" menu, "Save all measurements"), upload that MDAT file to your thread on this forum, not some other place. (Note. Post it in YOUR THREAD! Not this thread here. Your own thread, here on the Soundman Studio Design forum, where you are documenting your room build / tuning process. If you post links on THIS thread, I will remove them. I do not want this thread cluttered up with measurements or details about your own build. This thread is ONLY for instructions on how to use REW, and questions about that).
If you post your results on another forum or website, I probably wont see them, so you won't get a response from me. Post your results here, only, and on your own thread. You can post the actual MDAT file here on the forum, in addition to any graphs or pictures, and text messages. Some other forums don't allow you to post the MDAT file, as they can get very big, but there's no restriction here: post the actual data file here, where it will always be located right along with the rest of your thread.
5. Make a note of the mic location: LAST STEP: VERY IMPORTANT!!! When you have done all of the above, you need to measure EXACTLY where the mic is in the room, in all three dimensions, and using reference points around the room that wont change or end up hidden behind treatment! You need to be able to get the mic back to that precise location, every single time you do another set of tests, and it has to be accurate to within about 5mm (1/4"). The tip of the mic must always end up at that point, every time. If not, then you cannot compare the graphs, validly. Very important.
(One easy way of doing this is to get a small plumb-bob from a hardware store (not a big one! small and light weight), attach it to a piece of string, and hold the string exactly at the tip of the measurement mic such that the point of the plumb-bob is just above the floor. Wait for it to stop swinging, then mark that exact spot carefully on the floor (on masking tape stuck to the floor), and also mark the position where the mic is on the string. That way, you can always get the mic back to the exact same spot by holding the mark on the string on the mic tip, then adjusting the height and location of the mic stand until the plumb-bob is at the marked spot on the floor)
That's it! Not complicated.
I will add more information to this thread in the future, with instructions on how to look at your data (settings for REW), and basic tips on how to interpret your data, as well as other acoustic tests you can do in your room, to reveal more about it, and understand it better.
If you got this far and you are interested in doing a more complete and detailed analysis of your room, then here's another article I wrote that should help you with that: The "walking mic" test, using REW
- Stuart -