SPL Cal procedure on ECM8000

**mgl_sh** · **09-22-08, 05:46 AM**

Hi guys,

I used to use Radio Shack SPL meter in the test. Each time before I start the tests, I will do the SPL calibration as instructed. Recently, I retired my Radio Shack SPL meter and replaced it with the ECM8000. The new setup works pretty well, but how should I run the SPL calibration procedure (the cal file has been installed)?

The mic itself does not show the SPL readings. How should I know that the MIC's reading is 75dB? Is the SPL readings shown in the REW the present SPL levels?

M

**brucek** · **09-22-08, 07:34 AM**

Quote:

The new setup works pretty well, but how should I run the SPL calibration procedure (the cal file has been installed)?

The standard procedure is to use your RS SPL meter hand held beside the connected ECM at the listening position to set the 75dB adjusted by your receiver, then put the RS SPL meter away. The listening position is now at 75dBSPL actual.

Then set the REW input level and run the Calibrate SPL routine and set to 75dBSPL. Now everything is calibrated.

brucek

**mgl_sh** · **09-22-08, 09:46 AM**

Thanks Brucek. How should I set the correct REW input level? Please let me know if below procedure is correct:

I am using the 802+ECM8000.

Set the mic trim and channel level on 802 to the middle position (as instructed in your old post).
Adjust the main output on the 802 to read 75dBSPL on the REW SPL panel. The position of the main output volume control determines the REW input level.

Max

**brucek** · **09-22-08, 10:11 AM**

Quote:

Adjust the main output on the 802 to read 75dBSPL on the REW SPL panel.

No, the 802 is set with the pots about middle position and left alone (unless you found you didn't have enough level).

1. The ECM and mixer and your receiver are all hooked up and ready to go, then the level at the listening position is set with your receiver volume control to 75dBSPL using the RS meter hand held with no connections.

2. The input level is set in REW to -12dB on the Input VU meter with the REW Input Volume control.

3. Run Calibrate and set to 75dBSPL.

brucek

**mgl_sh** · **09-22-08, 10:27 AM**

The "control output mixer/volume" and "control input mixer/volume" in my REW is grey and unable to select.

How can I activate these functions? I am using the Creative SB-live! 24 bit external USB soundcard.

**brucek** · **09-22-08, 10:42 AM**

Select the card and the line-in and line-out port in the Settings screen Input and Output Device pull downs, then check the Control Input Mixer Volume box.

Unless of course you're using Vista, then its selected in the Vista devices playback/record mixers.

brucek

**mgl_sh** · **09-22-08, 11:16 AM**

Got it.

I just need to adjust the input volume control to let the SPL meter in the REW read 75dB, right? There is no need to adjust the SPL figure (a pops up window which starts with 75dB)?

**brucek** · **09-22-08, 11:23 AM**

Quote:

I just need to adjust the input volume control to let the SPL meter in the REW read 75dB, right? There is no need to adjust the SPL figure (a pops up window which starts with 75dB)?

No, that's not correct.

The Input Volume setting is adjusted to get a proper level on the Input VU meter.

Then the CALIBRATE routine is run. This generates a pink noise signal that we know will be 75DBSPL at the listening position because you already set it with the RS meter. Then while the test is running we adjust the thumbwheel to set 75dBSPL in the popup to match REW to that level. For convenience only, the default starting value in the popup is 75dBSPL. You may have set the value at your listening position to 76dBSPL, so you would adjust the popup to 76 at this time.

brucek

**mgl_sh** · **09-22-08, 11:35 AM**

Thanks Brucek. I finally know how to do this.

The proper level on the Input VU meter is within 3dB, right?

By the way, you said that the use of the RS meter should be limited to 3KHz. How about the ECM8000+802? Will the limit be extended to 10KHz?

MAX

**brucek** · **09-22-08, 01:02 PM**

Quote:

The proper level on the Input VU meter is within 3dB, right?

Yep...

Quote:

How about the ECM8000+802?

Up to 20KHz.. It's a full range measurment mic. Be sure to download the calibration file for it.

brucek

**mgl_sh** · **09-23-08, 12:23 AM**

Hi brucek,

I found the calibration file for the ECM8000 has severe high frequency roll-off after the 10KHz, is that normal? The roll off was also found in the low frequency portion as well, does this mean the ECM8000 has low-frequency boost problem?

MAX

**brucek** · **09-23-08, 07:17 AM**

The ECM is much like any other microphone, and has a limited bandwidth that requires correction at upper and lower limits. The calibration file compensates for those inadequacies and renders the microphone flat over the audio range from 5Hz-20KHz.

brucek

**mgl_sh** · **09-23-08, 10:48 AM**

so you mean the severe high frequency attenuation compensates for the inadequacies of the ECM8000? Does that sound a bit strange?

**brucek** · **09-23-08, 10:59 AM**

Quote:

so you mean the severe high frequency attenuation compensates for the inadequacies of the ECM8000? Does that sound a bit strange?

You may be confused. The calibration file entries reflect the actual response of the ECM8000 microphone as tested by a calibration company.

REW examines each entry and provides the required offset in your graph. So, if an entry at 10KHZ was -5dB, then REW would add 5dB along with proper interpolating values between frequency points.

brucek

**mgl_sh** · **09-23-08, 11:06 AM**

I re-checked the ECM8000 cal file and found it only listed the frequencies that need to be corrected. That's why I would see high frequency roll off in the Filter Adjust page. Because the high frequency portion after the 6KHz does not need to be corrected. Is that right, Brucek?

**mgl_sh** · **09-23-08, 11:12 AM**

I opened the cal file with excel and now I understand what you mean. Thanks for the help, Brucek.

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