Basic Questions...

[Icnognito]

Howdy guys...first post...sorry for stupid questions.

I've just started using REW to get measurements, since I've got my new subs, and I was curious about the "target" level. The target curve starts rolling off around 30hz on down to around 90hz. I was curious if the test tone that REW generates to take the measurement, follows that trend in the target? I've read that the goal is to get your sub to match that target (tracking), so I thought the tone that was generated (pink noise for sub calibration, I think) followed the target curve.

Thanks for you help, in advance!

Rick

[Otto]

Nope, the noise generate should have equal power, not following the curve. The target curve represents your crossover, and should be the same as your crossover point (30 Hz sounds a little low).

You want the input signal to the system to be consistent. In the end, the curve will be determined by the crossover of your bass management system and any EQ applied by a device such as the BFD.

[Icnognito]

Hi Otto...

Thanks for the response.

The "target" that I was trying to describe is shown in this picture that brucek posted in another thread:



It seemed like my subs response was dieing off after 80hz, but I was plugging the soundcard output into the 6-channel analog inputs on the back of the preprocessor (right channel from sound card into sub input on the analog connections). I thought that bypassed any bass management or crossover settings inside the preprocessor. The reason I did it that way, instead of plugging directly into the sub was that I have two subs running off a mono output on the back of my pre.

Thanks again!
Rick

[brucek]

The reason I did it that way, instead of plugging directly into the sub was that I have two subs running off a mono output on the back of my pre

I don't quite follow, but anyway, generally you will ensure the PC's soundcard is in standard stereo mode with no effects, etc. Then you plug the line-out of the soundcard into the CD or AUX input of your receiver (some use a Y-adapter here, or just feed left and right from the soundcard). Select the standard crossover you use in the receiver and ensure your receiver is in stereo mode. Now the signal to the sub(s) will have your normal bass management engaged without effects. Also ensure that your subwoofer(s) themelves don't have their own low pass filters engaged in the frequencies of interest. Turn them fully clockwise to eliminate them from the mix or they'll fight the receivers bass management.......

Select the same crossover in REW, so that hopefully the response you receive (after a sweep) tracks that target. It should. Apply filters to make it so...........

Yes, an 80Hz crossover will begin dropping off way before 80Hz. That's how they work. The signal will be down -6dB by the time it hits the crossover corner frequency.

brucek

[Icnognito]

Thanks Bruce...

What I was trying to do was get a full response of the sub(s) without any crossovers first. They are suppose to go up to 200hz via a ground plane measurement posted (they are Mark Seaton Submersives). So, to bypass all of the crossover settings in the preprocessor, I plugged the soundcards output into the analog sub input on the back of the processor (that we would use for sacd and dvd-a playback). It really wasn't that I was trying to do anything fancy. I was only hoping for a full frequency response of the sub through it's posted range.

What I thought might be killing it's response, given the way I hooked it up, was if the pink noise generated started tailing off like the target line on the plot above. If it doesn't...then I don't know why my subs response is so horrible when it gets out past 100hz. Here is a plot of what I'm talking about....please disregard the red line. The green line is the response of both subs from REW before I did anything with eq.

[brucek]

I don't know why my subs response is so horrible when it gets out past 100hz

OK, I get what you are doing, but I don't really think you need to be too worried about a subs response past 100Hz. If you are using an 80Hz crossover in your normal configuration, then the sub doesn't add much to the mix in relation to the mains at 100Hz. It's down about 12dB by that time and continues lower after that. See the pic below with the mains and subs outputs with an 80Hz cross.....

80crossover sub plus main.jpg

brucek

[Icnognito]

Bruce,

Thanks for the plot. What do you typically do to avoid that dip you get at 80hz in the response, due to the transition from subs to main?

Also, what do you think might be causing that death in the response? Could it be something in REW, the room acoustics, or should I be worried about the sub? I know this isn't even comparable to ground plane...but it seems like I should be getting something out to it's quoted frequency range. I agree, however, that it won't matter for how I'm going to setup the system.

Thanks,
Rick

[brucek]

What do you typically do to avoid that dip you get at 80hz in the response, due to the transition from subs to main?

See the resultant red line in the picture above? That's the result of the mains and sub adding together when the signal 'leaves the speakers'.... It combines for a flat overall response. There is no dip......

That's in a perfect world. In reality, the phase of the sub and mains signal (as a result of distance apart and time settings on the receiver), sometimes cause an other than perfect addition of the signals. This is rectified by adjusting the phase control of the sub and also the distance setting in the receiver. Use REW sweep up to 200Hz with the sub and mains playing together (with normal crossover settings) and examine the crossover region. Then tweak to get it smooth........

This is why we first take a measure of the sub though the receiver alone and equalize up to about 100Hz. Then add the mains and examine the crossover region and adjust the level of the mains and tweak to get the best transition of the crossover using phase and distance and sometimes a bit of filtering.

brucek

[Icnognito]

Cool. Thanks Bruce. That makes sense.

[Icnognito]

Bruce,

I ran into another weird issue (that's probably entirely my fault) where after a few measurements, the SPL meter (in REW) starts reading ambient noise a lot higher than is actually in the room. Most of the time, the noise level in my room when taking measurements is aroudn 45db. On occasion that REW meter will read 90db with no tones playing. I've also had it tell me that the pink noise it generates for measurements (SPL level setting process) is at 75db but then the tones are so faint you can barely hear them. REW somehow measures the response to be around 80db on the graph from that tone that was so faint though. Any ideas what might be causing those issues? Before you answer though, I need to mention that I think I have been doing a few things wrong. I haven't been "checking levels". I was using the SPL meter in REW to set the volume on my amp to the desired "target" level. So, I would play the SPL calibration tone until the SPL meter read 80db by turning up the main amp volume...and then type in 80db on the calibration box that is open.

Sorry for the dumb questions...this is all still new to me.

[brucek]

Yeah, you really need to go through the setup properly to ensure it's all correct. If you change the End Frequency or any other levels, it only takes a second to go back through and set it up.

1. Check Levels routine in Settings Soundcard tab (using Check/Set Levels with subwoofer pull down selected).

2. Calibrate SPL in Settings Mic/Meter tab.

3. Set Target Level.

4. Set End Frequency in Measure routine.

5. Measure.............

brucek

[Icnognito]

Bruce,

What does the "check levels" do that I can't do in the "calibrate SPL" tab? (Just curious...not being a smart butt). If in the check levels you only set the main volume to get a desired SPL on your meter, can't you do that while your in the cal SPL tab and then just type in the corresponding number? Also, the SPL meter that pops up in REW, does that show the SPL with the meter calibration file, or the raw spl from the Radio shack meter itself?

Thanks again for your help!

Rick

[brucek]

What does the "check levels" do that I can't do in the "calibrate SPL" tab?

Set the Input Level to the soundcard.................. very important.

Also, the SPL meter that pops up in REW, does that show the SPL with the meter calibration file, or the raw spl from the Radio shack meter itself?

Shows the raw result from a band limited pink noise from 30Hz to 80Hz.....

brucek

[Icnognito]

Does it use the mic correction file then when it plots it for you then?

[brucek]

The results of a measurement is the impulse response. Everything is derived from that along with the soundcard cal and mic cal used to correct the graphs.

brucek