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Hello - I'm getting ready in the next few days to measure my HT room with REQW. I currently have temporary use of a B&K model 2230 SLM with B&K 4155 mic & preamp that has been ISO calibrated and certified. Basically it is flat from 20Hz to 2K. I'm assuming a cal. file is not needed, but need opinions? Also, the B&K SLM has a 2.5VAC output. Does this sound compatible? I will be using a laptop with a Creative Audigy NX 24bit USB card. I plan on reading all the help files for REQW before I start. Any help is appreciated!
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Basically it is flat from 20Hz to 2K. I'm assuming a cal. file is not needed, but need opinions?
If its been calibrated, that usually means it's not flat, but has been tested and a calibration file or graph produced to show its actual response in relation to a flat standard. If you have this file or graph, you can easily create your own calibration file for use with REW (I can show you that if necessary). This would still leave your bottom end at 20Hz for measuring accurately. That should be fine unless you're determined to go down lower.

Also, the B&K SLM has a 2.5VAC output. Does this sound compatible? I will be using a laptop with a Creative Audigy NX 24bit USB card.
By this I assume you mean the output of the mic preamp is 2.5vAC. Often, a mic preamp would have some sort of volume adjust that allows you to set the output level. If not, then you have to hope that the output level (which would be stated as maximum output) would not clip the input of the soundcard. The soundcard (which is a standard consumer device) would likely have a maximum input level of around 1.25vAC (which is +2dBV). You'll just have to see how that goes...

Any help is appreciated!
When setting up REW to test the response in a room, you require some method of calibrating the actual level in the room at the listening position (at the microphone) to REW's internal SPL meter. This involves using an SPL meter for the setup. Then the SPL meter is put away and the microphone does the acual measurement. Most people use a cheapy Radio Shack meter for this duty. You would need some method of knowing actual SPL levels. :)

brucek
 

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If its been calibrated, that usually means it's not flat, but has been tested and a calibration file or graph produced to show its actual response in relation to a flat standard. If you have this file or graph, you can easily create your own calibration file for use with REW (I can show you that if necessary). This would still leave your bottom end at 20Hz for measuring accurately. That should be fine unless you're determined to go down lower.
Brucek, thank you for replying and information. I do have a calibration sheet, I'm attaching it. Can a cal file be made from it?


By this I assume you mean the output of the mic preamp is 2.5vAC. Often, a mic preamp would have some sort of volume adjust that allows you to set the output level. If not, then you have to hope that the output level (which would be stated as maximum output) would not clip the input of the soundcard. The soundcard (which is a standard consumer device) would likely have a maximum input level of around 1.25vAC (which is +2dBV). You'll just have to see how that goes...
The microphone is on a three foot cable attached to the SPL meter through a dedicated connector. On the side of the SPL meter is a 1/8" mini jack which is the output and it has a sticker that someone put on that says 2.5VAC


When setting up REW to test the response in a room, you require some method of calibrating the actual level in the room at the listening position (at the microphone) to REW's internal SPL meter. This involves using an SPL meter for the setup. Then the SPL meter is put away and the microphone does the acual measurement. Most people use a cheapy Radio Shack meter for this duty. You would need some method of knowing actual SPL levels. :)
brucek
In this B&K SPL meter the microhone stays with and works in conjunction as a system. The meter has a digital readout and can select A, C, L weighted if needed, and these settings output at the SPL meter 1/8" jack. I hope what I have is not too confusing and we can get it to work ok?
 

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Yeah, looks like a real good mic. It's handy to have the SPL readout so you won't need to get an SPL meter in addition.

The mic is very flat and quite good. If you intend to measure up to 20KHz, then you could create a file, but the fact is, even then, it only has a small 1.5dB peak at ~15KHz, so I wouldn't bother.

Where we usually are concerned with our measuring and mic calibration files is the area from ~15Hz to 200Hz for subwoofers. This is because it's the range where equalization is applied.

Anyway, if you read the small print about your microphone it says that its accurate from 6Hz to 5KHz flat within +/-1dB or alternately from 4Hz to 16KHz flat within +/-2dB and the lower limit is 1Hz to 3Hz down -3dB

OK, that's so good, you don't need a calibration file. I suspect the microphone is always a flat output and the weighting selection only applies to the meter, but if not, select the flat output, and be sure to uncheck the C-Weight box in REW. The SPL meter will only be used during initial REW calibration and then not used again.

The only thing you need to calibrate is your soundcard. Read the REW Help files about that.

On the side of the SPL meter is a 1/8" mini jack which is the output and it has a sticker that someone put on that says 2.5VAC
Remember when you create a cable from the meter to the soundcard, that the meter is a mono 1/8" connector, but it must connect to a stereo 1/8" connector at the soundcard, except you only use one channel (i.e. right channel). This is because the soundcard is stereo.

brucek
 

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Discussion Starter #5
brucek - Thank you so much for the information. I printed all the help files for REW and going to read them before I start.
Thanks!
Finalrinse
 

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I'm just going by memory here so don't take it as gospel. The AC output is run through some weighting parameter. It think it tells you which one in the SLM manual. I don't think you can change it with the buttons on the unit itself.

Regarding calibration. I used to calibrate the SLM before and after each use. Our company had the calibrator. It put out a steady tone at say 98dB. You are in effect telling the meter what 98dB is. The meter in turn tells you what it thought 98 was, (say 97.2). At the end of your work you do it again. It will say it thought 98 was 98. If it thought it was 50 you redo your work. If it keeps saying it is 50 or 90 instead of 98, then you can not calibrate. This means there is something wrong, likely microphone. It could however be your calibrator so that gets sent in on a regular basis to get calibrated. I kept paper records of each result ever done. If your other 3 SLMs all calibrate properly and one doesn't then likely your calibrator is fine.

The sheet you have is from the factory calibration. In a way it is like a fingerprint. No two mikes would be exactly the same. Along the same vein, a bit of dust or hard knock could change your mikes curve a bit. Most people would need Bruel & Kjaer to confirm this at $300.00 a pop. You may know how to do it yourself, maybe to their accuracy, maybe 10 times less accurate.

I'm not sure you needed or wanted to know any of the above. I'm just putting it out there. If your mike is clean, dry, not corroded inside, and hasn't taken a bad knock, since B&K made that graph, you can assume it is repeating that curve sensitivity if it calibrates with the hand calibrator and the hand calibrator is known to be okay.

Again I am not sure, but when they test the mike for that graph - they take it off the SLM and put it on their own machine. They do not get their results using the SLM dc amplifier or the readouts from the screen. If you must know for sure just phone up B&K. They have product specialists around that can answer most queries. :scratchhead:
 

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I'm just going by memory here so don't take it as gospel. The AC output is run through some weighting parameter. It think it tells you which one in the SLM manual. I don't think you can change it with the buttons on the unit itself.

Regarding calibration. I used to calibrate the SLM before and after each use. Our company had the calibrator. It put out a steady tone at say 98dB. You are in effect telling the meter what 98dB is. The meter in turn tells you what it thought 98 was, (say 97.2). At the end of your work you do it again. It will say it thought 98 was 98. If it thought it was 50 you redo your work. If it keeps saying it is 50 or 90 instead of 98, then you can not calibrate. This means there is something wrong, likely microphone. It could however be your calibrator so that gets sent in on a regular basis to get calibrated. I kept paper records of each result ever done. If your other 3 SLMs all calibrate properly and one doesn't then likely your calibrator is fine.

The sheet you have is from the factory calibration. In a way it is like a fingerprint. No two mikes would be exactly the same. Along the same vein, a bit of dust or hard knock could change your mikes curve a bit. Most people would need Bruel & Kjaer to confirm this at $300.00 a pop. You may know how to do it yourself, maybe to their accuracy, maybe 10 times less accurate.

I'm not sure you needed or wanted to know any of the above. I'm just putting it out there. If your mike is clean, dry, not corroded inside, and hasn't taken a bad knock, since B&K made that graph, you can assume it is repeating that curve sensitivity if it calibrates with the hand calibrator and the hand calibrator is known to be okay.

Again I am not sure, but when they test the mike for that graph - they take it off the SLM and put it on their own machine. They do not get their results using the SLM dc amplifier or the readouts from the screen. If you must know for sure just phone up B&K. They have product specialists around that can answer most queries. :scratchhead:
Thanks for the reply scratchhead. Yes, this mic and SLM were ISO calibrated recently, they were still at specification. I have a 94dB/114dB - 1Kz/250Hz calibrator that I feed this mic into. I'm going to try and use this to set the cal number in REW instead of using my speakers, I'll let you know if it works. Thanks for all the information.
 

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Thanks for the reply scratchhead. Yes, this mic and SLM were ISO calibrated recently, they were still at specification. I have a 94dB/114dB - 1Kz/250Hz calibrator that I feed this mic into. I'm going to try and use this to set the cal number in REW instead of using my speakers, I'll let you know if it works. Thanks for all the information.
The 94dB calibrator worked great for matching and setting SPL meter in REW.
Tom
 
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