RS meter woes II

[RonS1958]

Hello to everyone. I have been trying very hard to figure this REW thing out. I have read everything many times. (Don't understand most of it but I have come a long way!) I have been on and off it for about a month now. I am mainly interested in getting an analysis of my room acoustics to help make an informed decision on what acoustic treatments to add to the room, if any.
I am using a digital RS Meter, HP Laptop dv9410us(Vista 32 bit), with external USB Blaster X-FI Surround 5.1 soundcard, running to a Yamaha HTR5750 receiver(set to 2 speakers) with Klipsch Synergy Sub 10. I now believe that I have all connections, card settings(all "extras" turned off, etc), input levels, sound card cal and RS meter cal correct now, or at least as far as I can tell. I do the SPL calibrate routine and it seems to work correctly. (I am doing the steps in order).
However, when I go into check levels before measuring, I can only get "level OK" by lowering the RS meter's range down to 60db. Of course, at this level the RS meter doesn't even regisiter the 75 db and just tops out at a blinking 70 db.(It appears that the REW SPL reading comes out close to what the RS meter would be had the range not been so low). When I do run at this level I get a measurement that I think looks OK, but it just doesn't seem like it can be since the RS meter didn't even get up to the 75 db.
I guess my only question for now is am I fighting a losing battle with this setup? If I am then I have until Saturday to return the soundcard to Staples. I really don't need it except to run REW!
Thanks,
Ron

[RonS1958]

I see that I am getting no replies. Is my question not appropriate, too vague,.....?

[brucek]

I see that I am getting no replies. Is my question not appropriate, too vague,.....?

This isn't instant messaging. It's a forum, and takes time for people to see the message and respond.

when I go into check levels before measuring, I can only get "level OK" by lowering the RS meter's range down to 60db.

The output of the RS meter (like other SPL meters) outputs more signal as the range is reduced. Usually the signal from the meter at 80dB setting is suffice for most soundcards. Some soundcards require the input level control to be at maximum to get enough level.

Be sure you are using the appropriate pink noise test signal for the range you are testing. (i.e. subwoofer test signal for 0-200Hz measurements).

Are you indeed able to control the input level with the soundcard application (or the windows mixer)?

brucek

[RonS1958]

Bruce, Didn't mean to come off that way.
I believe that I am able to control the input. I am running the line in at the 100 level. If I mute the level the REW meter drops to -27.4 db. If I put the level at 50 REW meter reads 35.4 db.
If I am "sitting idle" the REW meter reads about 50 db. I can tap on the RS meter and the REW meter reading responds.
Previous attempts I used the speaker test signal because I thought that's what I was supposed to do. I was not running the subwoofer test signal, but I did try it last night after posting and still did not seem to act right. I did have it set to 0-200 hz.
I just tried again with the subwoofer signal and I was able to get the REW to say level OK with the range selected at 70db instead of 80db. Is that acceptable?
If it is I will keep pounding to learn more.and keep the soundcard. I have only worked on making sure that I am getting reliable measurements with this setup before proceeding.
I have had a lot of fun with this.
Thanks,
Ron

[brucek]

range selected at 70db instead of 80db. Is that acceptable?

The problem with do that is the very good chance of clipping the signal from the meter when running the measurement. When you select the 80dB scale, and then set the receiver level such that the listening position is at 75dB, it gives sufficient room for the signal to go quite a bit higher before clipping. When you select the 70dB scale, (and then set the receiver level such that the listening position is at 75dB), you run the risk of clipping the signal.

You can certainly check that the signal isn't clipped by examining the SCOPE tab (directly after doing a measure) for any square tops on the input signal (the darker red is the input). Expand the scopes horizontal scale and examine the signal.

You could also try using the 80dB scale and setting the level at the listening position a little higher (such as 78-80dB) and see if that helps with the input level somewhat (careful to not go over 80dB or you may be in the same boat with a chance of clipping).

Don't forget to run the Calibrate routine directly after the Check Levels routine, and set it to 75dB (even if you cheat and have set the listening position to ~80dB).

brucek

[RonS1958]

Bruce,
I hope that I now have it. I never changed the volume on the receiver before once it was set because I thought that you should not do that. But "cheating " seems to have gotten it. I did go a little higher on the dbs on the RS meter but I figure that with its inherent inaccuracies that it is probably OK. The check level comes back with level OK with the RS meter set at the 80 db range and my receiver setting at -16db. I looked at the scope not really knowing what I was looking at but I expanded it and don't see anything that looks flat. (It looks pretty cool, maybe I can put it on T shirt) Please tell me if my results that I have gathered look reliable. I have attached in this post.
Got to mow the grass and go to 8 year old's baseball game.
I really appreciate your guidance.
Thanks, Ron

[brucek]

Please tell me if my results that I have gathered look reliable

Yep, you've got it now.

Now let me give some graph tips and advice on measuring.

For subwoofers, always use the standard Vertical graph axis of (45dB - 105dB) and the Horizontal graph axis of (15Hz - 200Hz) using the Graph Limits button in the top right corner of REW.
Use a measurement start frequency of 0Hz and an end frequency of 200Hz.

For full range (you can only measure reliably up to 3KHz with a Radio Shack meter), use the standard Vertical graph axis of (45dB - 105dB) and the Horizontal graph axis of (15Hz - 3KHz for an RS meter).
For full range only, enable smoothing to eliminate the comb filtering. Use a 1/3 octave smoothing.

When attempting to get the best results from your sub and mains, be sure to start by measuring the sub only (simply have the mains speakers disconnected - be sure the power is off when you disconnect. The receiver is always in stereo mode for REW, so the other speakers won't be on). This allows you to see the sub by itself without any other influences. Try to find the best position for the sub, with the smoothest response (note your undesirable peak at 30Hz). If you have a sub Equalizer, apply the EQ after the positioning is complete.

Then add the mains and measure again from 0-200Hz. (Continue to use the sub test signal for this test). Adjust the subs phase control to achieve the smoothest crossover transition.

brucek