sound calibration and beyond, a few questions

[jaspal kallar]

Hello

I'm a using a Radio Shack SPL digital meter, Model 33-2055. I have no sub just 2 speakers connected my integrated amplifier.

I'm using a external usb creative mp3+ soundcard on windows xp. I used

Does attached picture of the soundcard calibration below look ok?

Anyway, I went on to do the "check levels". Here I increased/decreased my amplifiers volume to get a reading of 75db on the SPL. I'm using the right channel as Input Channel, btw. I'm told as part of "check levels" to next change the input volume. I do this via the USB soundcard mixer. My question . do I change this by viewing the "Right" meter until the green bars read around -18 and adjusting the value via the USB soundcard mixer? I did this anyway and I noticed the peak for the right was around -6. Is this normal?


Next part, "Calibrating the SPL reading". I loaded the given calibration file 'RADIOSHACK-33-2055_4050.cal'. Question do I have to do the "Calibrate SPL" (where you enter the SPL reading from your meter) also since I've loaded the calibration file? I did this anyway since I wanted to go on.


I did try a quick measurement for the right speaker only (since I'm awaiting the correct y rca-adapter for the line out rca cable so that I can use both right and left channels). Start freq was 0, End Freq was 20000. level -12db. However, it said I got clipping when I performed the measurement and suggested lowering my amplifiers (AV) volume setting. I do have one question here. If I did adjust the volume on mu amp and did another measurement, don't I need to go back and do the "Check Levels" or "Calibrate the SPL reading" parts again is it enough just to decrease the volume on my amp and do the measurement?

Ideally, I should go one step at a time since one can effect the next step but I wanted to go through the whole process to get see what I didn't understand.

jaspal.

[brucek]

Does attached picture of the soundcard calibration below look ok?

Yes.... hopefully you did a quick measure of your loopback cable (before removing it) to prove that it measured flat and proving that your soundcard file was correct.

do I change this by viewing the "Right" meter until the green bars read around -18 and adjusting the value via the USB soundcard mixer?

Yes, but I'd like to see an RMS reading of-12dB as long as the peak doesn't go too high and go over 0dB. The bottom reading (below the VU meter) is the RMS and the top reading (above the VU meter) is the peak. You can certainly adjust the USB card mixer, or if REW recognizes and allows you to check the REW control box, you can adjust using REW - either way.

Question do I have to do the "Calibrate SPL" (where you enter the SPL reading from your meter) also since I've loaded the calibration file? I did this anyway since I wanted to go on.

Mmmm, interesting that so many people appear confused about this - I haven't figured out why that is yet.
Anyway, the calibration file is used to compensate for the frequency response inadequacies of the meter or microphone you're using, such that it results in a flat response.
The Calibrate SPL routine is used immediately after you've set up your input and output levels and have your real SPL meter reading 75dB. The routine simply tells the REW internal SPL readout that the level is now 75dB. How else would REW know? It simply matches / syncs the real and the REW meter readings..

However, it said I got clipping when I performed the measurement and suggested lowering my amplifiers (AV) volume setting. I do have one question here. If I did adjust the volume on mu amp and did another measurement, don't I need to go back and do the "Check Levels

You could, but the Measurement routines Check Level routine is simply telling you that the initial Check Levels routine didn't uncover all the peaks in the overall signal you're measuring with its band limited pink noise signal. You will only have to turn down the volume a bit to clear the clipping.

brucek

[jaspal kallar]

brucek wrote:

Yes, but I'd like to see an RMS reading of-12dB as long as the peak doesn't go too high and go over 0dB. The bottom reading (below the VU meter) is the RMS and the top reading (above the VU meter) is the peak. You can certainly adjust the USB card mixer, or if REW recognizes and allows you to check the REW control box, you can adjust using REW - either way.

I'm still a little confused here when doing the "Check Levels Procedure". In essence you firstly adjust your amps (or AV's) volume pot to get a reading of 75db on your SPL meter.

Then secondly you adjust, as you've suggested above Bruce, to get a RMS reading -12 (or -18 as suggested in the Help section) via, in my case, the soundcard mixer. Now when I do change the value via the mixer, to get near -12 (or -18), I notice the output from the speakers is getting higher or lower.and thus results in an higher or lower SPL reading i.e away from 75db.

So what is the goal here... is it in the end to get a reading of 75db on teh SPL meter by virtue of combining changing the RMS value (via the soundcard mxier in my case) and my amp's volume pot? To me the one effects the other.

The reason behind this query is that I added some room acoustic treatments (in each corner) and now when doing the "Check Levels Procedure", I first attain a 75 db reading on the SPL via my amp then when I try to change to get a value of -12 via my soundcards mixer I can only get as far as -21 or -24 (I believe, I have to check this again since I did it late Sunday evening) since the peak value is over 0. At this stage the soundcards mixer volum is at the maximum value. So clearly something is either wrong on my setup or what I'm doing or the corner treatments are having an effect here.

So it is vital I get a RMS reading between around -18 ?

Thanks

jaspal.

[brucek]

when I do change the value via the mixer, to get near -12 (or -18), I notice the output from the speakers is getting higher or lower.and thus results in an higher or lower SPL reading i.e away from 75db.

No, the speaker level shouldn't change here at all. You likely have the "monitor" mode enabled in your soundcard. Shut that off.

brucek

[jaspal kallar]

brucek wrote:

No, the speaker level shouldn't change here at all. You likely have the "monitor" mode enabled in your soundcard. Shut that off.

brucek

Opps, I'll have to look into that.

Another (minor) question, In the *peaks found" under Filter Tasks, are these the frequencies that are causing problems (in the given range 0-200Hz)

I know some people use an SPL meter and a test tone CD split up into various tracks at various frequencies to find what frequencies that are effecting their room. Are the "peaks found " in essence these frequencies.

Someone has suggested I use a test tone cd and the SPL meter to find out what frequencies are causing problems between 0-200 Hz. Hence my question.

jaspal

[brucek]

In the *peaks found" under Filter Tasks, are these the frequencies that are causing problems (in the given range 0-200Hz)

Yes, given the target that you set before you "Find Peaks", and then only the ones that meet the criteria of a modal peak set by REW's software.

I know some people use an SPL meter and a test tone CD split up into various tracks at various frequencies to find what frequencies that are effecting their room. Are the "peaks found " in essence these frequencies.

The CD tones method is quite crude and only relying on amplitude. REW is light years ahead of this in sophistication.

brucek