Help with 4312's

[old_4312]

Hello Zilch!
Thank you very much for the tip!
I'm off to Amazon, then on to JBL consumer.
It's nice to know I'm not the only one who has wrestled with this!
I sure wish I had conversed with you about all this 20 years ago!
I'd probably be blasting instead of typing..
Thank you for all the help!

[Zilch]

It's nice to know I'm not the only one who has wrestled with this!

Reviewing the thread, your nearfield LF measurement between woofer and port (Post #26) seems to provide the best characterization thus far. Next time you're set up measuring, repeat that, but then close the port with a plumbers' test plug and measure again. I believe you'll find the difference to be, well, "dramatic."

When you work with groundplane, please take pics for the forum of your test setup. I don't believe anyone has ever show how to do it here. It's not very difficult; outside away from walls or structures will give the best results, as long as there's no railroads, freeways, garbage trucks, or earthquakes to interfere....

[old_4312]

Hello!
Is this in the neighborhood of what we are talking about?
http://stereophile.com/reference/405time/index1.html
http://www.svconline.com/mag/avinsta...ing/index.html
I can handle that.

Originally posted by Zilch:
It's not very difficult; outside away from walls or structures will give the best results, as long as there's no railroads, freeways, garbage trucks, or earthquakes to interfere....

Naw..Maybe the sound of a few freaked out squirrels hitting the ground--but I can always retest around the thuds.
To do it on a fairly flat surface, I'm gonna need more cables.
Also, I don't have a camera. I don't know if those disposable cameras would work for these kind of photos.
I'll look into it...
Later everyone!

[lauries2]

G'day! I may be taking this sideways... but I something I saw in an earlier post resonated() with me. With my 4410s, I went through about 4 amplifiers until I found one that would drive them "properly". By properly, I mean controlling bass that was audibly unpleasant (I never did measure the response). I mean really unpleasant - resonating and floppy.

I finally found a build-it-yourself kit MOSFET power amp (185W per channel) that sounds awesome. It's defining characteristic is that it has a HUGE damping factor (over 1000 as I recall). After checking the other amplifiers that didn't sound good, their damping factors were in the low 100s. This seems to be a characteristic of JBLs - great bass but only if you use the right amp.

Ever since this experience, I've used amplifiers that have very high transient current output devices which give them high damping factors - this is the only way I've found to control the bass response. I've been using this approach in my live sound production as well - I drive an enclosure with two 2241H drivers with a bridged Crown CE2000 amp and it controls the bass quite tightly (the damping factor is rated at 400 but seems to be higher to my ear).

Hope this hasn't been too far off topic...

[boputnam]

...it has a HUGE damping factor (over 1000 as I recall). After checking the other amplifiers that didn't sound good, their damping factors were in the low 100s. This seems to be a characteristic of JBLs - great bass but only if you use the right amp.

Not really off-topic, at all. It is certainly worth discussing.

I too, long-ago, awoke to the importance of output impedance / damping factor in bass response. Ironically, the cabinets I most struggled with this were my 4312's... That is, before I swapped-in the 128H's and got a real amp into the signal path.

If the amp is struggling with "back EMF", it is woefully less efficient. JBL certainly knows this as evidenced by their new line of differential-drive woofer motors which improve the amp's chance of efficient operation.

Hey "old_4312" - have you tried different amps, and what is the damping factor of the amp you are currently using?

[Zilch]

I finally found a build-it-yourself kit MOSFET power amp (185W per channel) that sounds awesome. It's defining characteristic is that it has a HUGE damping factor (over 1000 as I recall).

Please start a thread on this amp over in "Lansing Related Gear."

It's likely members want to know more about it....

Is this in the neighborhood of what we are talking about?

Yup.

I don't know how much "flat" is actually required outside of the immediate measurement area.

I'm thinkin' the driveway may be good enough for this, as long as no air-conditioning compressors are running nearby. You'll be able to read the ambient noise baseline immediately.

I doubt turf is gonna reflect much in the area of interest....

[old_4312]

Hello everyone!

Originally posted by BOPUTNAM:
Hey "old_4312" - have you tried different amps, and what is the damping factor of the amp you are currently using?

The current amp is a Yamaha P2250.It has a damping factor of >110 up to 1kHz.
I've tried listening evaluations with a Crown Microtech,UREI 6290 and a Mackie M1400 as well.
The better amps with higher DF, made the low end sound cleaner and more punchy, but thinner.
If I ever fix this problem, I suspect I will do another A/B test with a Mackie to see what the end result is. Without the canyon in bass response, I will most likely prefer the Mackie. There is some resonance I hear with the Yamaha, that I don't hear with the Mackie.There is also a resonance I hear around 270Hz that doesn't go away regardless of the amp.
It seems to be more pronounced on lower register female vocals than anything else. For anyone who's curious, listen to track 7 on Trisha Yearwood's "Where Your Road Leads". When she sings the words, "you mean to me", in the first line, that's the range where my 4312's howl a little bit.It doesn't seem as noticeable with male vocals or music beds.
I'll deal with that after I resolve the bigger problem!
If the groundplane tests reveal the 4312's are ok, and hopefully they will,then I'm really gonna have a tiger by the tail.
The whole front end of my listening room will have to be overhauled!
I've never built bass traps for the floor.
I'll post the groundplane test results as soon as I can run them.
If I can find a camera that will work, I'll include photos as well.
Question for Zilch:Should I run the nearfield test at the time of the groundplane test for comparison, or should I elevate or position the speaker skyward for the nearfield test?
Thanks everyone!

[Zilch]

Question for Zilch:Should I run the nearfield test at the time of the groundplane test for comparison, or should I elevate or position the speaker skyward for the nearfield test?

Just elevate it off the ground. That's the virtue of nearfield: the mic's so close that no reflections from anything else have opportunity to appear, so skyward is not necessary. Just get it up off the ground.

Groundplane actually USES the reflection to create a virtual second driver below the ground and acoustically sums the two.

FUN, huh?

[old_4312]

Hello!

Originally posted by Zilch:
Groundplane actually USES the reflection to create a virtual second driver below the ground and acoustically sums the two.

Thanks Zilch for explaining that!
I was wondering why the mic had to placed 2X the original free field mic distance.
If I understand this right,the virtual driver like a real driver, doubles the SPL, so the mic must be placed back twice the free field reference position to compensate for the 3db rise in pressure.
Cool!
Just for grins, in addition to the nearfield measurements, I'm also going to repeat the elevated 1 meter away test with the ground boundary only--with no rear wall interference.That will show exactly what the floor/ground boundary alone is doing to the acoustic output. The on the ground skyward test is showing the effects of the rear boundary.
I hope the next set of tests finally solves this mystery--and my neighbors won't stop talking to me!
I'm ordering the book Zilch mentioned in a few days.
For anyone who's interested, it's available at Amazon.
Thanks everyone!

[old_4312]

Hello everyone!
Ran tests this morning. Per Zilch's request, I purchased a disposable zoom camera and took photos of the tests.
Groundplane results weren't the same as the JBL specs, but they looked much better.
As soon as I can get these developed, I'll post pics and all.
I'm very interested in getting the group's opinions on the test results.
Later everyone!

[Zilch]

Groundplane results weren't the same as the JBL specs, but they looked much better.

Yup, each method gives different results. See D'Appolito Fig. 4.23.

Ground plane should approximate the rising response (above 100 Hz) of anechoic. Halfspace per JBL would be flatter, according to that. Most importantly, the boundary effects should be gone.

Thank you for doing these experiments. I look forwared to seeing your groundplane results, and what happened when you closed the port(s).

[old_4312]

Hello!
Originally posted by Zilch:

Yup, each method gives different results. See D'Appolito Fig. 4.23.

Ground plane should approximate the rising response (above 100 Hz) of anechoic. Halfspace per JBL would be flatter, according to that. Most importantly, the boundary effects should be gone.

Thank you for doing these experiments. I look forwared to seeing your groundplane results, and what happened when you closed the port(s).

No problem! I'd like to thank everyone here for their help in diagnosing this problem.A special thanks to Zilch for explaining groundplane testing and for the help on isolating the different effects of boundary interference.
It's been a real learning experience for me!
The response graphs of each isolated boundary makes it all pretty clear.
I hope others will find this information useful as well.
Providing the photo's are useable,would the group prefer the photos and tests in pairs, or posted seperately?
I should be posting them late Wednesday night.
Later everyone!

[old_4312]

Hello!
Well the photos are not great, but that's the cost of using a disposable camera,I guess.
I've tweaked them as best I could.
There should be enough image on each one,to see what I was doing.
I'll post the photos and tests in groups.
I believe the mystery is solved, finally after 20+ years,but I would like to hear any comments anyone might like to add.
Thanks everyone!

[old_4312]

Acoustic tests continued..

[old_4312]

Last page.
Two reposts:
4312 flush on ground, mic'ed @ 1 meter, skyward measurement, rear boundary interference only,reposted for comparison.
4312,elevated 29", mic'ed @ 1 meter, flush against brick wall. Rear and ground boundaries effecting acoustic output, reposted for comparison.