A question about low freq responses... [Archive] - Audio & Video Forums


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karl k
01-12-2004, 04:51 PM
Over the last several months, I've been trying to put together some main L/R speakers. They, for the most part, have been progressing nicely but I've run into a snag on the low end. Up til last weekend, I've been limited to testing with Speaker Workshop in my living room and while useful only down to 1KHz or so due to reflections corrupting the recorded signal, it did alow the mid high x-over point to be worked on with a fair amount of success. I took nearfield measurements for the low end and the port, cut and spliced everything together to make a nice graph. Well, to make a long story short, I went to my workplace to do some testing in a large enough room to measure the low end without reflections being a problem. I also wanted to measure from a distance that was more compairable to the actual listening distance in my livingroom. So I get there, unload and setup everything, put the speaker on one forklift and raise it 10' in the air. I put the mic(Behringer ECM 8000 test mic w/o calib.) on a stand, on another forklift 9' away and raised it to be level with the midrange on the speaker. I get all the volumes set and begin testing.

Now for the speaker...

2ea Peerless CSX 10" woofer(wired in series), in a vented enclosure 130L tuned to 26Hz

1ea 250wt plate amp, variable active x-over 40-160Hz, 2nd order Q=.275, x-over bypass switch.

1ea Peerless CSX 6 1/2" midwoofer in sealed enclosure 6.55L with stuffing. No high pass, 2nd order L/R low pass at 3KHz.

1ea HiVi RT2E-A planar tweeter angled vertically down Approx 10deg. 2nd order Butterworth at 3KHz

The drivers are offset from each other to facilitate time alignment. Woofer to mid = 1.5" and mid to tweeter = 2.25".

Here's a pic of the speaker in question...(see IM000052)

And now for the question...(see farfield)

If you look at the response from 300Hz and down, you'll notice a series of peaks and dips. Normally, I would attribute these to reflections in the room. But in studying the graph today, I noticed a pattern that cant be mear coincidence. I know the dip at 45Hz is most likely an impedence spike as it shows up on another graph. You'll notice the rest of the dips are spaced about every 25Hz apart. There's one at 22, 45, 70, 95, and 150. Can anyone explain what this might be? I thought about box resonance, have ruled out standing waves in the room(too large and too short of play/rec time for waves to build), interference from the midbass(peaks still exist in the same place and magnitude while only woofers are playing) and have just about ruled out reflections as the reflections would have to be very large and opposite in phase to create dips of this magnitude(I think). I just haven't learned enough about figuring resonances in box's yet to know if that's the cause or not.

I'll take any help I can get on this matter.

karl k
01-12-2004, 08:36 PM
The woofers are in a "common" air space in the box. Is it possable that the 2 woofers are creating a "beat" frequency response? The idea that if one woofer is "lagging" in time at certain/all frequencies relative to the other might cause this type of constructive/destructive summation in the frequency response? Could it be due to the "common" box volume causing the 2 woofers to fight for the same air, or the differences in speaker characteristics(parameters)?

karl k
01-15-2004, 07:04 PM
Never mind, I figured it out.

01-15-2004, 10:32 PM
Well genius what was it????

Internal enclosure standing waves??? (my first guess) but only if there aren't any internal baffles in the enclosure.

What's the Near field graph look like?

Or crossover anomalies?

What is the impedance doing.

It would be interesting to track the RMS voltage on the driver voice coils when you sweep them.

C'mon out with it.

What happened to Hertz???

karl k
01-15-2004, 10:59 PM
Reflections. It hit me today as I was comparing the ratio between the distance to the mic(directly) and the distance to the mic via the floor. It occurred to me that the reflection had to be out of phase close to 180deg for those kinds of dips to happen, and the ratio of the distance's was about 1:1.5. So I started to figure the wave lengths of the frequencies in question and discovered that most(not all) were a function of the reflective distance to/from the floor. This would also explain why I didn't see these dips when testing farfield in the house. The distance to the floor was next to nothing and therefore no cancellation was occurring below about 200Hz. Instead, the cancellations were occurring between 200Hz and 1KHz because of the midrange reflecting off the floor.

So what did I get out of all this?

Essentually, The midrange will have to be boosted in the house about 3-5db from 200Hz to 1KHz and the bass will have to be cut 8-10db at about 50Hz or so for the speakers to play "flat". If that's what I want. I still have an impedance spike at 45Hz, but that may not be a bad thing as I'm peaking there anyway(in the house). If I try to correct it, it may do more harm than good.

Overall, I'm pretty happy with the way it sounds and am anxious to get a pair going to truely listen. Thinking real hard about a Behringer Eq, just trying to figure the best way to hook one up for all of what I do(no tape monitor on the receiver).

I don't know about Hertzy, must be cuttin down the forest's in Calif. for the time he's been gone! LOL
I tryed to search the forums for him but no dice. I'm sure he'll be back with a note from his mother!(dear Mr. Kotter... signed Hertzies mother)LMAO!

01-16-2004, 07:36 AM
"Overall, I'm pretty happy with the way it sounds and am anxious to get a pair going to truely listen. Thinking real hard about a Behringer Eq, just trying to figure the best way to hook one up for all of what I do(no tape monitor on the receiver)."

The Behringer Feedback Destroyer is a great piece of equipment, and IMO a must for low end. I hook mine up between the LFE output of my preamp and the Sub amp, I've also tried it between the output of my Marchand XM44 low pass output and the sub amp.

Nice project BTW, very ambitious, I like it. I would recommend simple SPL in room measurements for the 100Hz and down region, just use Anthony Gomez's excellent PEQ.xls program for plotting in room graphs then use the BFD to flatten the curve.


01-16-2004, 08:53 AM
KDkeith - I read throught your thread. The project is awesome in terms of build - beautiful work.

I didn't and don't want to throw any cold water on things so I just didn't post. Things were off pretty solid on the Behringer direction as a solution.

Here's some food for thought. Without seeing the layout or your room or placement of the LF enclosures - IMO 70-100% of the problem could be solved with placement.

That being said, let me also add that a compromise in optimal placement for aestetic purposes is an understandable/acceptable situation.

What drivers are in your subs - 15" or 18"?

With a 700-1000 watt amp and that kind of firepower, the system should take the room by the ballz without much effort. The only way I can fathom that system not haveing the "Near Field" grunt to control the room would be from placement issues.

01-16-2004, 10:17 AM
"That being said, let me also add that a compromise in optimal placement for aestetic purposes is an understandable/acceptable situation.

What drivers are in your subs - 15" or 18"?"


Yes I agree, and as you stated it was a compromise for placement, the right Sub was not in a good acoustical place but for aesthetic and logistical reasons that was the only place I could put a 120Lb 24" diameter missile silo. I update my thread with some new pictures of the entire room and my final details of the port valve knob that I just finished.

There using 18" Maelstroms powered by a Mackie 1400i @ 700W per sub. I wanted to point out that my recent experience with a digital PEQ like the BFD was a very enlightening learning experience, and I am amazed just how much better the quality of bass I can now get after equalization.

Sorry Karl, not trying to hi-jack your thread.

01-16-2004, 10:25 AM
Thanks KD. I don't think Karl will mind. There's nothing going on around here these days anyway.

Thanks Karl!!!

Back to the KD bass rig. Beautiful work. I guess you did the plugs on a lathe???

Glad you didn't take my comments as just some jerk throwing cold water on you project.

I am moving my 2ch system into the living room - converting it into a dedicated 2ch only room. The den will be the HT only room.

Anyway, when I get it done - the next 12-24 months, I'll email you an invitation to come on by and have a listen sometime - as I remember you are in Dallas, I'm living in West Fort Worth.

karl k
01-16-2004, 03:19 PM
Steve was right about the activity here(not that I'm complaining) and as far as I'm concerned, we can talk anywhere on this board we choose until someone speaks otherwise. It is hard sometimes to jump around and try to catch someone in the "Politically Correct" zone. I'm just glad and humbled to be a part of the subject matter! Besides, my question got answered so this is really free space!

Your welcome Steve!

I can appreciate the difficulties involved in locating those subs. I think your graphs look pretty respectable and I'm sure the sound would be more than acceptable for my tastes. As you might remember, I(with Steves help) chose a less than practical mounting method for my subs. You do what you can with what you have! I will have to post some more graphs and pics this weekend for your entertainment. Any questions or comments are welcome whether positive or not.