Home Theater Forum and Systems banner
21 - 40 of 98 Posts

·
Premium Member
Joined
·
134 Posts
Discussion Starter · #21 ·
weverb, cleared Soundcard cal, unchecked c weight and cleared spl cal.

Here is a screenshot of the settings page just before soundcard cal and the graph. Looks as it should compared to Brucek's, I have the TB SRM card like brucek.

See anything wrong?

Thanks,
 

Attachments

·
Registered
Joined
·
486 Posts
Looks good. Now take a measurement of it.
 

·
Registered
Joined
·
7,514 Posts
With C weight unchecked and SPL cal still unloaded ?
Yes, it should be a flat line.

See anything wrong?
No, everything looks good.

sweep was from 15hz to 135hz
Measure subs with a sweep from 0Hz-200Hz.

but the MBM was design to fall off after 50Hz. It should not continue down to 15Hz.
The response looks about like I would suspect. It does drop off at the assigned 50Hz. Below that the remaining signal is likely enjoying room gain. There also may be some low frequency noise in the room. I don't see a probelm with the MBM response. Take a near field with the sub in the middle of the room (or outside even better) to get a clearer picture.

I see what you mean about the sub and MBMs not playing well together
Ever considered an 80Hz crossover and the 3 3 T as your only sub and not use the MBM's. The 3 3 T is certainly capable enough. What is your intension/goal with the MBM's?

You do need to turn down the level of the MBM's in relation to the low sub. They're too high to integrate well with the low sub from the graphs you've posted so far. They're both peaking over 80dB. Turn them down.

As a side note on REW, it's a lot easier to evalute responses of each sub if you adjust the REW cutoff target line to the desired crossover. So, if you are testing the low sub, set the target line at 50Hz or 60Hz (there is no 55Hz) and when testing the MBM set the target at 120Hz. It has no effect (except when creating filters), but it helps you visualize how close you are to the desired target.

brucek
 

·
Registered
Joined
·
486 Posts
The response looks about like I would suspect. It does drop off at the assigned 50Hz. Below that the remaining signal is likely enjoying room gain. There also may be some low frequency noise in the room. I don't see a probelm with the MBM response. Take a near field with the sub in the middle of the room (or outside even better) to get a clearer picture.
I understand having some room gain, but 70-72dB at 18Hz!?! :huh: Is there a way to test for "low frequency noise in the room"? Can you turn the highs and lows off and run a measurement? Would that show you anything?
 

·
Premium Member
Joined
·
134 Posts
Discussion Starter · #26 ·
Thank Brucek, all input and ideas are welcome and I will try your suggestions.

I am not sure what my intent was with the MBMs now, I think I just started aquiring toys which is not good. I should have apllied science first like learning to use REW. No harm done if I find I do not need them, the used market for MBMs look good.

I like your idea of focusing on the sub and may go back to near field with one of the MBMs if needed and see how REW shows it integrating.

I do have a question for you, I had the 3 3 T and MBMs balanced using this method, any issues with this approach?

Using AVR test tone I matched 3 3 T and MBM
Ref test tone was 80dB and measured one MBM to 78dB, then turned off and did the same for second MBM. With both MBMs on it gave me 80dB.

With both MBMs off I measured the 3 3 T to 80dB with the XO out(normally in).

With the 3 3 T and MBMs on I lowered the AVR sub level to 80dB. Then set 3 3 T XO back to in.
 

·
Registered
Joined
·
7,514 Posts
Using AVR test tone I matched 3 3 T and MBM
A poor way to do it if there are peaks in a subs response. There is a difference in energy returned when using band limited pink noise if there are peaks, compared to a sub that has a flat response across that band. Peaks will always upset a simple SPL meter level check. In your case I would use REW as the best way to tweak the levels. I would first use the receiver test tones to set them all equal (subs, mains, etc) and then use REW to tweak the level of the MTM's (down in your case).

brucek
 

·
Registered
Joined
·
7,514 Posts
Is there a way to test for "low frequency noise in the room"?
Yes, use the REW Spectrum Analyzer.

Once your levels are setup for a normal response measure, then select the Spectrum tab and the following settings.
Mode = Spectrum, FFT Length = 65536, Averages = 2 or 4, Window = Rectangular, Y Axis = dBFS

Select the Signal Generator and select WhitePN (periodic White Noise).

Press the RED record button on the Spectrum page.

It lets you examine the room for noise energy that you may not be aware of.

Below is a pic of my office with the Spectrum analyzer running with no signal being generated.

You can see my furnace is introducing some low frequency noise. If I shut it off, the noise is eliminated.

You can see I have the typical poor PC computer 60Hz hum and its related harmonics at 120Hz, 180Hz, 220Hz etc.

You can also see (amazingly), my NTSC RPTV CRT's horizontal oscillator frequency of 15.750Khz present in the room.

The TV is several rooms away and happened to be turned on, but the oscillator noise is present in my office (not that I can hear it, but there it is). Shut the TV off and it goes away.

The Spectrum analyzer can tell you a lot about your room and system before you even do a measure.

Sometimes you'll see a waterfall with a strange signal that rings out for many hundred of milliseconds. If you looked at the spectrum, you might see the source, rather than incorrectly deciding it comes from a modal resonance.... My furnace is a case in point on that issue.



The RTA is also quite useful in REW for dynamic measurements, rather than a simple static measure.

Below is a response measure and then a quick RTA. Once this is running, I can then move the microphone around to see what different listening positions look like (all in real time). It is revealing to see the difference a few feet make when you move the mic, and to compare against the simple response measure on the screen at the same time for comparison.

The RTA also allows you to change filters or move equipment and watch the RTA screen for the real time changes. It's also really useful to adjust phase on a subwoofer for the best crossover response. You simply watch the RTA screen as you dial the phase control. A lot better than taking a bunch of measures to accomplish this task.

Here's a pic of RTA and a response measurement on the same screen (measurements are selectable to be placed on the RTA screen for comparison).
I was sure not to move the mic when I took the measure and then started the RTA.

Pay attention to set the RTA up properly. See the setup values on the plot below. (note that the RTA requires switching the Signal Generator to PinkPN (periodic Pink Noise)




I understand having some room gain, but 70-72dB at 18Hz!?!
Well it's about 15dB above the noise floor. Remember, if there's a high noise floor (typical 45dBSPL, but can be much higher), then the meter calibration file will raise the signal level as the frequency drops. A response chart signal can be mis-interpreted as actual signal, when it's really just noise being raised by the meter calibration file. Consider an old analog Radio Shack meter is adding +30dB offset at 10Hz from the cal file. A high noise floor will show as signal. This is the case with the MBM plots on page 1 of this thread. See how the signal drops nicely until about 30-35Hz and then begins to rise as the frequency goes lower than that. The signal below the drop is noise......... ignore it.

brucek
 

·
Premium Member
Joined
·
134 Posts
Discussion Starter · #29 ·
WOW BRUCEK, excellent information ! When I was calibrating my SPL before performing a measure test I was seeing 45 or so dB when no one was in the room and no tones being generated. I do think my AC was running (92 degrees hear in Dallas yesterday). Looks like I now have a way to measure the room before taking measurements.

THANK YOU !
 

·
Premium Member
Joined
·
134 Posts
Discussion Starter · #30 · (Edited)
Weverb, ok here is the graph with Soundcard calibrated, SPL cal unloaded and c weigh unchecked, then with loop in place measured like I would normally.

Looks good to me but would appriciate confirmation.

I think I am done measuring until the weekend, seems I have come down with a cold:sad:
 

Attachments

·
Premium Member
Joined
·
134 Posts
Discussion Starter · #32 ·
Thanks weverb for confirming the soundcard calibration.

Soundcard and RS analog cal file loaded and c weight checked.

RS SPL meter at LP set to slow and C weighting.

The graph below represents my starting point. MBMs powered down. HSU VTF3-MK3 turbo off, XO out, balanced with mains and located in left front corner (ceiling height is 8' vs opposing corner with ceiling 12'). AVR XO 80hz, Auddessy off.

I have moved and mesaured the sub within the limits I can move it in front left corner, chart represents best location.

Any tips on next steps would be great !
 

Attachments

·
Registered
Joined
·
486 Posts
Take some comparison graphs of the 3.3 in 0 versus 180 phase like brucek mentioned. Also try max extension versus max output mode. I take it, this last graph was with the mains turned on? Keep the turbo off for now. We will add that variable later.
 

·
Premium Member
Joined
·
134 Posts
Discussion Starter · #35 ·
Thanks Bruce. I have added a pic of my room in case that helps.

I will move the sub tomorrow to see if I can tackle the 32hz dip. Unfortunately the sub only has 0 and 180 degree phase adjustments but I will try it as I move the sub.

I also want to put one of the MBMs near field and see if it will integrate. Looks like the only place it might help is the 70hz dip.

Are there other graphs I need to look at that may reveal a need for BFD?
 

Attachments

·
Premium Member
Joined
·
134 Posts
Discussion Starter · #36 ·
Take some comparison graphs of the 3.3 in 0 versus 180 phase like brucek mentioned. Also try max extension versus max output mode. I take it, this last graph was with the mains turned on? Keep the turbo off for now. We will add that variable later.
OK, I did try MO, but did not save the measurement, I will re-measure and post tomorrow.

Yes the mains were both on.

Thank you !
 

·
Registered
Joined
·
7,514 Posts
Unfortunately the sub only has 0 and 180 degree phase adjustments but I will try it as I move the sub.
You may also use the subwoofer distance setting in the receiver. It is a close proxy for the phase adjustment.

Are there other graphs I need to look at that may reveal a need for BFD?
Waterfall plots will reveal the time domain. Use the same scaling as response graphs.

brucek
 

·
Premium Member
Joined
·
134 Posts
Discussion Starter · #38 ·
Here are my latest measurements with just the 3 3, no turbo or MBMs. I used different phases combined with AVR distance. Audessy has the sub at 20' but real distance is 14'. I made test on distance focused primarly on these 2 ranges as either side of 14' and 20' had worst results.

Looks like second graph is the best.

1st graph is phase 0 AVR distance 14'
2nd graph phase 0 AVR distance is 20'
3rd graph is phase 180 distance 14'
4th graph is phase 180 distance 20'
 

Attachments

·
Registered
Joined
·
486 Posts
I would agree on the second also. How does it sound? Are you planning on a BFD? Have you read about making a house curve? Depending on your answers to these questions will help decide what your next steps are.
 
21 - 40 of 98 Posts
Top