AKG 451EB as REW mic?

[Choots]

Hey there, I'm a new member and first time poster... I have a DIY 12" Dayton RS HF sub in a sealed 2 cu ft. box with the Dayton 500W amp, and an FBQ 2396 just waiting to do filter duty...no room treatment yet, but I am planning for a little first reflection and possibly a couple corner bass traps.

Here's my question.

I have two AKG 451EB microphones with CK1 capsules, and they came with frequency response charts. I'll try to attach a picture of one of the charts. Can I use the frequency response chart to develop my own calibration file manually and use that to run REW? How much accuracy is generally good enough for general use in tuning a sub?

I've read as much as I can, but I can't seem to find anything that describes the calibration file format. I've looked at the ECM 8000 file, and it appears the first value is the frequency, and the second value in the file is the y-axis deviation in dB, but what is the third value in the file?

Also, I've read an SPL meter is needed to provide an absolute reference value on the y-axis scale, but won't I still be able to get some use out of REW without it? Can you describe the impact of playing with REW and tweaking without an SPL meter? This looks like a great forum, thanks for any help you can provide!

[I can't seem to shrink the image I have of the freq. response chart to fit within the required image size - no software on this machine - so I'll post it as soon as I can.]

Thanks!

[brucek]

Can I use the frequency response chart to develop my own calibration file manually and use that to run REW? How much accuracy is generally good enough for general use in tuning a sub?

Yep, you can make the cal file up in notepad and then change the extension to .cal from .txt

I've looked at the ECM 8000 file, and it appears the first value is the frequency, and the second value in the file is the y-axis deviation in dB, but what is the third value in the file?

The values are frequency, deviation, phase. The phase is not needed. I don't think we include phase in any of our files. You can set it to zero or omit it.

Also, I've read an SPL meter is needed to provide an absolute reference value on the y-axis scale, but won't I still be able to get some use out of REW without it? Can you describe the impact of playing with REW and tweaking without an SPL meter?

The SPL meter (when use with a microphone as the measuring device) is used to set a known level at the listening position, so that the REW meter can then be set to match it (75dB is mostly used). REW can be used without the SPL meter by quessing the SPL level of 75dB and matching REW to it. It doesn't change the relative accuracy of REW.. The Radio SHack SPL meters are quite cheap though and handy for this duty...


brucek

[Choots]

Thanks brucek.

I mostly just want to play around with the FBQ2496 and see what difference it makes. Having a tool like REW is great.

My freq response graph might be a little hard to read, but I'll give a it a try... On the graph, the curve is centered vertically in the middle, and units appear to place it around 30 or 15 dB depending on which scale you use (there's two side by side)... I'll try to post it to show, but I have to edit the picture or upload it to my site and make a link, neither of which I can do right now.

How many data points should one try to enter? Do I treat the "average" position of the curve as 0 db and then determine the deviation above and below that point?

choots

[brucek]

How many data points should one try to enter? Do I treat the "average" position of the curve as 0 db and then determine the deviation above and below that point?

Yeah, that would be reasonable. Remember to interpret correctly that if at (for example) the mic outputs low at 30Hz by -5db, then the entry in the cal file is [ 30.0 -5.0 ]. Seems obvious, but I've seen people decide that they need to add 5dB to compensate for the -5dB deficiency. Not so, the cal file simply tells REW what the response of the mic is, and it compensates correctly.

How many data points should one try to enter?

That's up to you. REW will 'connect the dots' of your file. The last and first entry will be where REW continues on at that last reading.
Dump our cal files from the download page into notepad and take a look at them and load them into REW and get a feel for it.....

brucek