First bass measurement question...

Hello!

My new dedicated HT is so close to being finished I can hardly stand it. Four months in the making!

I have started calibration and have taken some initial measurements using REW and I am pretty pleased with the results. The bass traps must be working!

(Here is where I would input the graph, but there is no button for Manage Attachments!)

Here's the only issue:
In my HT I currently have two subs in the front corners of the room (JBL 12" and JBL 10"). The frequency response is extremely flat at approx 75dB from 25Hz to 60Hz, then makes a 5dB minor spike at about 68dB, then suddenly plumets about 30dB at 73Hz returning to normal at 93Hz. My crossover is set at 80Hz- does this sound like a phase problem to anyone?

I will certainly experiment tonight with phase settings, moving the subs, and trying one without the other- but it's such an odd drop RIGHT at the crossover- I thought maybe you all would have some ideas.

Also- not sure if it matters, but the front speaker are relatively new and haven't been completely broken in. The subs are definitely broken in.

Thank you!

Ryan

[Wayne A. Pflughaupt]

Hi Ryan,

(Here is where I would input the graph, but there is no button for Manage Attachments!)

We’ve been having trouble with spammers, so we had to institute a 10-post minimum before attachments for new members, to discourage them (the spammers, not the members!). There is a way for administrators to lift the minimum requirement, and hopefully I was able to do that for you (you’re the first one I’ve tried it for). See if you have the Manage Attachments button now.

Regards,
Wayne

Thanks, Wayne! It looks like it worked:

(Replaced Below)

What do you think? I feel like it could be a phase issue...

[brucek]

What do you think?

Can you change your vertical scale to 45DB-105dB.. (that's the standard that we use)

brucek

There we go. Doesn't look as flat at this range, does it?



Any suggestions on taking better measurements, by the way?

[maxcooper]

Can you change the phase on the sub (switch/knob on sub, does BFD have this feature?, wire main speakers with reverse polarity just to test , etc. -- there are many ways to change the phase) and do another measurement? My sub has continuously-variable phase. I just did measurements at 0, 90, and 180 and picked the flattest one. Comparing similar plots for your system would help to identify if the drop is a phase issue or not.

Or maybe your receiver has a delay or distance setting for the sub that may help eliminate the void?

-Max

[brucek]

Any suggestions on taking better measurements, by the way?

Two subs is usually tricky, especially when the two aren't exactly matched in their responses. The overall response ends up reverting to the poorer, more challenged of the two. On the other hand, headroom is a positive result when using two subs. Often, when someone decides to get a new more powerful sub, they decide it would be a good idea to still use the old one, and it ends up a worse situation.

Either way, your response is quite good and I agree it would appear that you have a crossover cancellation problem. It's always best to begin measurements with a single sub, with the mains off, and the receiver in stereo mode (a non surround mode). This then allows you to see how the sub itself is acting in the room and enables you to set filters in a BFD (if that's your intent), or move the sub around the room to get the best location (front corner usually being the best).

Then in your case, you would add the second lesser sub and do a response to ensure there are the least cancellations and response changes (especially in the very bottom extension where mismatched subs are a problem).

Then once that conundrum is solved, add the mains and work on any integration problems around the crossover frequency.

REW makes all this fairly easy.

brucek

Thanks for the two-sub tutorial, brucek.

I did several experiments last night with the two subs, including starting with the big one as you suggested. There was and still is a peak at about 70Hz that would be nice to eliminate, but I'd need the Behringer to do so. That may be an option in the future. There was a drop just after 80Hz, but adding the lesser-powered sub defeated the null and really flattened the frequency response across the board, but maintaining the peak at 70Hz.

Integration with the fronts went very well. Slightly moving the subs helped quite a bit. Frequency response is quite good now.

Experimenting with the levels on the subs had more impact than I would have expected- very interesting.

I left my other computer at home so I can't post graphs, but perhaps tomorrow.

Ryan

[DrWho]

You might find this article helpful:
klipschcorner.com/Articles.aspx?guid=feb5d669-1513-426d-a8fc-29f8aa90869f

It makes a lot more sense when you can see the frequency response change too.

It would also be informative to provide a plot of your subwoofer's response without the mains (just unplug the speaker wires from the mains). This will tell if you the dip is the result of acoustical interference with the mains, or if it's room related. Try measuring each sub independantly too.

You can show these all at once if you use the tabs built into REW. Ignore that they say Left, Center, Right, etc.... and manually set the "speaker type" (the dropdown box that says "Fullrange, Bass Limited, etc...). Then you can click on "Measured Responses" in the drop down menu at the top right of the screen and view everything at once.

If each of your subs has a dip with the mains off, then your issue is going to be room related. EQ will only work if it's a minimum phase situation, but most acoustical issues aren't. You will want to look for boundaries (walls) that are about 4 feet away from the speakers. Reflections off these boundaries will cause cancellations at the listening position because they are 180 degrees out of phase. Peavey has a good article that describes 1/4 Wavelength Boundary Cancellaton:
peavey.com/support/technotes/soundsystems/boundarycancellation.cfm

hmmm, seems I haven't hit 10 posts yet either (so can't post clickable links)

Thanks for the link, I'll check it out!