Using REW WITHOUT SPL metre, measuring with Mic

1984 Views 4 Replies 4 Participants Last post by Wayne A. Pflughaupt, Apr 23, 2010

unqlenol

Discussion Starter · Dec 11, 2009

Hi all,
This is my first post here and I am hoping some of you kind folk can help answer
A couple of questions of mine regarding REW.
I have looked at the manual and searched on the forum but admittedly I have not been exhaustive, so if the answers are already available I’d appreciate it someone could point me to a thread or suggest where to look.

My question is: Do I really REALLY need an SPL meter in order to measure my room? I can do without extra expenses right now and it is hard for me to get one down here in South Africa without paying a lot of money.

To expand on my scenario:
I have a small music studio which I am acoustically treating bit by bit.
The main thing I want out of REW is to get as flat a response in the room as possible.
Working out exact RT60’s and flutter echo are a bit less important to my plan.

I was hoping that I could use an omni-directional AKG 4050 microphone (not made for testing).
I thought maybe I could create a calibration file for the soundcard, a manually created calibration file for the Mic (I would want to punch in values based on the mics freq response chart) and then Roughly calibrate the SPL for testing by simply recording a test tone and adjusting the gains until my digital input metre reaches a certain relevant value dBFS.

So will this work? The primary concern is creating a mic calibration file and surviving without an SPL meter.
Please note that I am hoping to create as flat a freq response in my room as possible by adjusting speaker and listening positions and by using acoustic treatment.
I do not wish to introduce an equalizer if at all possible.

Sincere thanks to anyone who can advise.

Best
Nolan

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brucek

· #2 · Dec 11, 2009

Do I really REALLY need an SPL meter in order to measure my room?

No, you can simply set up a reasonable level during the Check Levels routine (that asks you to set the listening position to 75dB with an SPL meter). Once the Check Levels routine is complete, then run the Calibrate SPL routine and cheat by setting the value in the thumbwheel to 75dB. This now sets the internal REW SPL meter to match the level you set at the listening position (whether it was 70dB or 80dB doesn't matter). The graph will then place its traces at 75dB for you. Rerun Calibrate SPL every time you change a system level.

a manually created calibration file for the Mic (I would want to punch in values based on the mics freq response chart)

No problem. Just create a text file using Windows Notepad and save the file with a .cal extension. The files data is a list from low to high frequencies of the mics response you take from the graph. The interval spacing is not important as REW will fill in the blanks between readings. Download a calibration file of any mic from our download page and drop it into Notepad to get the feel for the layout. Once your file is created, load it into REW and take a look at the trace and see if it is a reasonable facsimile of the graph you have for the mic.

You can use Response, Waterfall and ETC to great advantage in your setup.

brucek

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unqlenol

Discussion Starter · #3 · Dec 11, 2009

Brucek,

Thank you so much for the expeditious and very generous response.
I suspected that that sort of approach would work..I needed to be sure.
I know it can be done now and you've even told me how...I couldn't ask for more!
I can't wait to get to the bottom of things.
Much appreciation and best wishes to you over the festive season!
Nolan

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Recordman

· #4 · Apr 23, 2010

wow how come i didnt think of that. i had to go through all the trouble of borrowing an SPL meter from a friend and then rushing all weekend to tweak and move things around. things still arent right. although... can you really correct for, not just nonlinear frequency response, nonexistent frequency response? i highly doubt an imac mic can pick up anything lower than like 100Hz...

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Wayne A. Pflughaupt

· #5 · Apr 23, 2010

Yeah, you really should use a mic with decent extension down to 20-30 Hz. Even if it's rolling out by that point, a calibration file can make up the difference. But only to a point. A calibration file isn't going to help if the mic is say, 30 dB down at 20 Hz. In that case, REW's compensation would only be boosting noise.

Regards,
Wayne

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