Comments on: Room EQ Wizard SPL Calibration – without an SLM

By: Craig

Craig — Mon, 12 Oct 2015 20:30:10 +0000

Hi John. In step three, whats the significance of the 100 x 94db. is it because the 11mv is lower than the range of your multimeter and the 100 is used to convert it to a higher value that works within the range of your multimeter. i.e. Im planning on buying a multimeter but the specs says it has an ac voltage range of 200mv – 750v. so i would have to times 94db to value that will work within my multimeter ac voltage range. Correct me if im wrong.

By: Peter

Peter — Wed, 28 Jan 2015 00:30:32 +0000

Hi John can this process be done with a XTZ Pro mic and soundcard.
The signal is unbalanced, but XLR because the microphone requires some low voltage (not really phantom power).

Thank you

By: John Reekie

John Reekie — Mon, 26 Jan 2015 03:12:24 +0000

Hi Marcin, it’s because in step 3 the voltage measured was 40 dB greater than the mic sensitivity. It’s just a convenient set of numbers that will (should) work with a cheap DVM. However, I’ve never been able to get it to give an accurate result, I don’t know why.

By: Marcin

Marcin — Mon, 19 Jan 2015 23:53:16 +0000

In step 4. Why we need to change the level of the signal generator to -43 dB. From where this number is came from? Why not 50 dB for example? Are You decreasing level of the signal generator and stop when DVM will show You 100x smaller value?

By: soundrex

soundrex — Thu, 08 Jan 2015 08:27:10 +0000

Hi all.
I quote from the net:
“Most cheap AC voltmeters read average of the rectified waveform, and the calibration of the scale changes that number to the equivalent RMS voltage.
In other words, most meters will read RMS voltage, but the number is accurate only for a pure sine wave input. For a non-sinusoidal input, or for a distorted sine wave, the reading will not be accurate.
Better meters will read “true RMS” and will be labeled as such. These will read the RMS voltage of a variety of waveforms”
As per the above 3dB error is not introduced.

By: NiToNi

NiToNi — Tue, 18 Nov 2014 20:57:08 +0000

Hi John,

Nice little trick!

However I am pretty sure that when microphone sensitivity is specified as ourput voltage it is in VRMS so you would introduce an error of 3dB unless you measure with a DVM capable of TrueRMS measurements or aim for a voltage 1.4142 times higher using a basic DVM.

By: rick

rick — Fri, 12 Sep 2014 12:22:50 +0000

I have started to run the setup documented here with the same microphone, a Scarlett 2i2 USB interface and a Lenovo W510 running Ubuntu 12.04. I have installed REW 5.01. My DMM is a Tenma 72-7720.

In order for me to get the 1.1 reading on the Tenma DMM I have to set the Output and Playback devices setting to 153% in the Pulse Audio Volume Control utility, then on the Scarlett 2i2 I set the Direct Monitor to On and rotate the dial to 95% of its rotation.

When I disconnect the clip leads and plug in the XLR loopback with the right channel gain control rotated to its minimum, the light-ring on the right channel gain control is red. The right channel is set to line, not instrument (inst). The Scarlett 2i2 manual indicated that when the light-rings turn red the signal is al least 0dBFS. That does not allow me to increase the input gain of the Scarlett 2i2 in step 6, at least without reducing the output through which seems to defeat the setup in step 3.

What should I do?

By: John Reekie

John Reekie — Wed, 24 Jul 2013 14:16:47 +0000

Hi Mirco, thank you for the comment. The calibration is done with the balanced output looped back to the input. Once you have done that, it doesn’t matter how the soundcard output is connected, as the calibration applies only to the input chain. Hope this helps – ?

By: Mirco

Mirco — Fri, 05 Jul 2013 17:39:50 +0000

John,

nice work, very well documented!

I was wondering if the same procedure would work with a mix of balanced and balanced ins and outs?

My “sound card” is basically a Native Instruments Komplete Audio 6 (http://www.native-instruments.com/en/products/komplete/audio-interfaces/komplete-audio-6/) USB audio I/F connected to a Emotiva UMC-200 (http://shop.emotiva.com/products/umc200) preamp via SPDIF coax cable. While the mic input on the audio I/F is balanced the line out on the preamp are not.

I could perform your calibration procedure with all unbalanced cables but then, will the EMM-6 have a different sensitivity when connected to the audio I/F with a balanced XLR cable?

Thanks,

—Mirco.

By: John Reekie

John Reekie — Sun, 07 Apr 2013 00:14:34 +0000

Hi Ergo, thank you for your comment.

I do prefer to use 1 kHz for calibration but it’s a simple matter to check whether the DVM reads differently at 1 kHz vs 50 or 100 Hz (just change the frequency in the REW signal generator).