Question, some graphs

**fredk** · **09-08-08, 02:05 AM**

Is it normal to have to adjust levels before doing a measurement?

I went through all the cal file setup and meter calibration stuff. before measuring I checked levels only to find that they were about 7 or 8db low. I turned up the volume on the receiver until levels checked OK. Is this correct? In the help files, it dosn't really say what you should do to adjust levels if they are high or low.

I will post graphs tomorrow. I need to resize them, but I'm getting a little tired.

Fred

**brucek** · **09-08-08, 08:19 AM**

Quote:

Is it normal to have to adjust levels before doing a measurement?

Yes, if you are referring to re-adjusting levels when the Measuring screen Check Levels routine tells you they're high, even though you've already set levels with the Settings screen Check Levels routine.

The Check Levels routine in the Measurement screen and the Check Levels routine in the Settings screen use different values to generate the pink noise level setup test signal.

The Check Levels routine in the the Settings screen uses the fact that you have the Check/Set levels with Subwoofer pull-down selected to generate its test noise (this a band limited signal with a low and high cut of 30Hz - 80Hz).

REW double checks and verifies that the end frequency of the sweep that you have selected in the Measurement panel roughly matches the test signal setting for sanity, and so its Check Level routine uses the end frequency as the test pink noise hi-limit cutoff. So, if you had 20KHz as the end frequency to measure for your sweep, you can see how the pink noise would be different (between the two Check Levels) and as such the low level signal you're experiencing.

Often, even if you have the levels set correctly, and you're using an appropriate end frequency, a peak in the response of the speaker being tested will have energy at a frequency that trips the sanity check and announces that levels are too high.

Simply return to the Settings screen and adjust the input level of the soundcard a bit lower and then run Calibrate SPL and carry on.

brucek

**fredk** · **09-08-08, 02:25 PM**

Thanks Bruce. I think I understand now.

If I understand correctly, I want to leave the receiver volume as set when calibrating to the meter call file so that I measure at what I have set as my reference level (nn on the receiver = 75db).

Something else just clicked. I can adjust both line in and out volumes and had been doing so in the soundcard tab. So I need to bump up my line in and drop my line out so that the level stays at around -18, but the input level is higher for the measurement stage.

Thanks again for your help.

**brucek** · **09-08-08, 02:34 PM**

Usually the Wave Volume is set at 1.000 and the Output Volume set about 0.500, and then the sweep level (which actually changes the output VU level of the soundcard) is set about -12dBFS.

Then your receiver volume is adjusted to set a real 75dBSPL at the listening position using your SPL meter. Once that's set, don't change it.

Then the Input level is set on the soundcard to get the right level ~-12dB.

Then you run the Calibrate SPL routine to tell REW that the level at the listening position meter is 75dB (how else would it know unless you told it). Now the REW meter will track the actual level at the listening position.

So, if the Check Levels tells you that the level is too high for the full sweep its about to run, you need to lower the level. Don't touch anything except the input level to the soundcard. Then when that's done, you need to run Calibrate SPL again and set to 75dB.

Then you're ready to try the Measurement Check Level again to be sure it's OK

brucek

**fredk** · **09-08-08, 04:40 PM**

Thank you for your patience Bruce.

I should have mentioned in my first post I am running Vista. When I bring up the sound control window I only get 3 controls and Wave is not one of them. I do not know if this is a Vista thing or if it is somehow disabled on my install. I looked around and did not see anything to indicate the latter.

The audio is the onboard chipset on my Gigabyte mobo, by far the best onboard solution when it was released.

My line in volume is now set at 1 and line out is around .5 and the sweep level was left at the default -12 dbfs.

This gives me a line in level of -12 db. The help suggests -18 so this is where I set the level originally.

Reran calibrate spl to set 75db reference level.

Sweep was set to start at 15hz and end at 100 (for the sub)

Doing the check level routine in the measure window gives me a level of -16 when both the sub and speakers are connected.

From what you keep writing, if anything, I should get levels too high for measuring speakers + sub. That dosn't seem to be the case for me.

Disconnecting the speakers gives me a level of -30, just a tad low.

My levels are set between

and

giving me a combined output of

I tried running a sweep with just the sub (no further adjustments) and get a popup telling me to turn up the volume or adjust the sweep level. That puts me right back where I was last night when I rand sweeps with the volume turned up.

Gotta go to work now. I will check back and run some more sweeps when I get home. At least its nice and quiet at 3 in the am.

Fred

**brucek** · **09-08-08, 06:01 PM**

Quote:

Sweep was set to start at 15hz and end at 100 (for the sub)

For a sub use 0Hz-200Hz in the measurement screen (not 15 and 100).

Quote:

Disconnecting the speakers gives me a level of -30, just a tad low

I'm confused by your discussion of the Input Levels you're getting, as if you have no control of the Input Volume. Simply turn up the input volume in the Windows Recording Devices screen until it's correct.

Although some of this will not interest you, please take a look at this thread. I went to extra lengths in showing how to adjust levels in VISTA for a different soundcard than you have, but it still applies.

brucek

**thewire** · **09-08-08, 06:10 PM**

Quote:

fredk wrote:

Thank you for your patience Bruce.

I should have mentioned in my first post I am running Vista. When I bring up the sound control window I only get 3 controls and Wave is not one of them.

I have a similar problem running Vista with a different soundcard. The highest I can set my input now is shown bellow. It can read 1 while I do a check levels but goes back to the same setting when ready for a measurement or goes back to that after a measurement. I did not have any issue prior to using Vista. Also the name of my card changed last week. Then the input output devices selections started changing on me, now the input level issue. I recommend to keep a close eye on the settings window. Also.. begin the check levels before opening the mixer in Vista.

Quote:

Sweep was set to start at 15hz and end at 100 (for the sub)

For subwoofers I think generally we use 0Hz as a starting point, even if it is not to do with a measurement, unless there is some kind of a filter envolved making the 15Hz or lower non-relevant. Most of the mics/soundcards commonly used here are seemingly good to 10Hz-15Hz. A 200Hz ending point is critical because this is the range of frequency your room will at around for lower frequencies. If we ended at 100Hz there is alot missing to look at.

**fredk** · **09-09-08, 04:47 AM**

Quote:

I'm confused by your discussion of the Input Levels you're getting

I read the link you posted, thanks. I guess I should have mentioned that I am controlling input levels manually using the Vista controls. It seems my levelsa are all where they should be.

I reset the sweep for 0-200Hz.

I had to boost the volume on my receiver considerably for the sub measurement. I am still not clear on whether or not this is normal.

thewire. I am not running into the same issues as you on Vista. My levels on the soundcard tab are fine. They just seem very low when checking the levels before measurement with just the sub connected.

**fredk** · **09-09-08, 05:01 AM**

Now some measurements. Sub sweep:

Sub Waterfall:

I am in a concrete construction apartment, so I expected reflections to be a problem.

So, I guess my next step is moving the sub around to see what that does to the graphs. I unfortunately have very limited choices on where I can put the sub. At the moment it is on a side wall about half way between the front and back of the room. I can move it a couple of feet along that wall or put it in a back corner.

**fredk** · **09-09-08, 05:14 AM**

Out of curiosity, I measured the sub + mains to see what I am listening to right now.

I had to set the volume on the receiver back down quite a bit for this measurement and ended up with only 2db of headroom.

I am surprised that adding in the mains affected the 20-40Hz region. The -3db point for the mains is 34Hz, so I guess they are still adding something in between 35 and 40Hz.

Perhaps the positioning of the mains causes them to excite a room mode/node (still shaky on the terminology) at around 37Hz?

Thats it for now. Gotta sleep.

**brucek** · **09-09-08, 07:59 AM**

Quote:

So, I guess my next step is moving the sub around to see what that does to the graphs.

Yep, hopefully you can clear up those rather severe dips..... Positioning is really your only option.

brucek

**fredk** · **09-09-08, 02:43 PM**

I decided to measure a few other locations with the current sub position.

The left seating position, 2' to the left:

This seems to be closer to what I am targeting, though there is still a serious drop at 70Hz. Edit: Pink trace is the left seating position, green is the center seat.

I also tried measuring 1.5 ft forward of the current couch position, but there was not much of an improvement

Fred

**salvasol** · **09-09-08, 08:26 PM**

Quote:

fredk wrote:

I decided to measure a few other locations with the current sub position.

The left seating position, 2' to the left:

I suggest you to move your sub to find a better position ... (I know that you want to play with REW

, but the sooner you find the right spot the sooner you'll be enjoying movies

)

**fredk** · **09-10-08, 12:18 AM**

I watched a movie yeaterday. Must... play...

**fredk** · **09-14-08, 12:50 AM**

OK, I'm seriously depressed now. After moving the sub to the two other positions, my best measurement is from the left seating position with the sub in its original position.

The best measurements for the main seating position are with the sub in its original position, but turned toward the front of the room.

I'm gonna go sit in my left seat, watch a movie and sulk now. I'll post a graph or two tomorrow.

Fred

**fredk** · **09-15-08, 01:12 PM**

After doing additional measurements (remembering to check phase this time) here are the two best positions:

Side wall facing forward:

In this position the sub is 2.5 feet from and firing directly into the front left main speaker. I don't know if this is bad or not.

Rear Corner:

Does anyone have any opinions on which of these is better to work with? Either way it looks like I would benefit from EQing via bfd.

**salvasol** · **09-15-08, 01:47 PM**

Quote:

fredk wrote:

... After moving the sub to the two other positions, my best measurement is from the left seating position with the sub in its original position.

The best measurements for the main seating position are with the sub in its original position, but turned toward the front of the room.

Are you limited on the sub positions to use??? or Can it be placed anywhere???

If it can be placed anywhere, do the crawling test (place the sub in your main position, then crawl around the room to find the best position to place the sub)

It seems to me that you don't have a lot of options to place the sub, Right???

Have you used the autocalibration on your AVR (if it has one)??? ... Can you EQ the sub with your AVR??? ... Do you have accoustic treatments on your room??? ... all this will help you before getting a BFD

**fredk** · **09-15-08, 03:16 PM**

Thanks for the reply salvasol. My AVR has no calibration or EQ.

My placement options are limited by furniture and HT placement. I have an old aquarium I need to get rid of and that may free up a few other options.

The placements I have tried are the only side wall available, the only corner avialable and between the right speaker and the display.

I have done quite a bit of reading in on room treatments in the last few days. There is not much available that will treat below 80Hz unless I get into something like a Helmholtz or pannel resonator. Again, I have extremely limited options for placement. I will start a thread in the HT acoustics forum to explore my options.

I am still looking for opinions on which of the two proposed locations are going to be better to work with. I am inclined to go with the side location and just accept that I am going to have a nasty dip around 76Hz.

**brucek** · **09-15-08, 04:10 PM**

Quote:

I am inclined to go with the side location and just accept that I am going to have a nasty dip around 76Hz.

The dip at 76Hz is not that significant in those last plots and will likely be somewhat ameliorated by adding the mains and adjusting the subs phase.

The real problem is the peak at ~35hz which needs to be dealt with equalization. You need a BFD. Cheap and effective.

brucek

**fredk** · **09-15-08, 08:41 PM**

Quote:

The real problem is the peak at ~35hz which needs to be dealt with equalization. You need a BFD. Cheap and effective.

Thats what I thought. Even if I end up with a hole at 76Hz, the overall response will be much better than what I currently have.

I think I will have a challenge with the mains as well, but will be able to use room treatments to tame issues higher in frequency.

Thanks again Bruce.

Advertisement