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-   -   JX92S and baffle step compensation (http://www.diyaudio.com/forums/full-range/91385-jx92s-baffle-step-compensation.html)

neil_kaye 1st December 2006 03:00 PM

JX92S and baffle step compensation
 
Hi,
Would someone mind explaining to me what baffle step compensation for a full range driver is and why one might want to implement it in a Jodran TL enclosure?

Thanks

Dumbass 1st December 2006 03:52 PM

In a normal-shaped speaker box, high frequencies behave as if radiating into half-space, and low frequencies behave as if radiating into full-space. Low frequencies have such long wavelengths that they don't "see" the speaker's front baffle and act like a point source. High frequencies have such short wavelengths that the front baffle acts like an infinite plane, so high freqs act like a point source radiating into only one side of that plane. [Actually, the radiation pattern at mid/high frequencies is more complicated because of beaming and diffraction effects, but put that aside for the moment.]

So if you placed this speaker box into an anechoic chamber, high frequencies would have a 6dB boost compared to low frequencies.

Here's a longer discussion:
http://www.quarter-wave.com/General/BSC_Sizing.pdf

One way to eliminate baffle step effects is to place speakers directly against a wall, because the wall extends the speaker's baffle. Here is what Ted Jordan has to say on the matter (although he doesn't specifically address baffle step effects).

"[D]espite convention, there is strong argument in favour of placing the loudspeakers as close to the wall as possible. This reduces the time lag between the direct sound and the reflection from the 'virtual images' and substantially improves coherence and spatiality. (The ideal, but usually impractical, solution, is to mount the drivers in the wall where there would be no virtual image)."
http://www.ejjordan.co.uk/basics.html

BTW baffle step applies to multi-way speakers as well as single-drivers. Most commercial speakers have some sort of baffle step compensation built into the crossover circuit. The speakers are "voiced" for an assumed listening environment; note how higher-end speakers have some sort of recommendation for where they should be placed, i.e. "at least three feet from any wall" or what have you.

Dumbass 1st December 2006 03:57 PM

P.S. Note that Ted Jordan's favored design for the JX92S, the VTL, has a very wide front baffle. By having a larger baffle, the baffle step cutoff frequency is lowered, to the extent that room boundaries start coming into play.

I believe the "frequency compensation" circuits he recommends do help compensate for baffle step losses:
http://www.ejjordan.co.uk/systems/jx92_system.html

neil_kaye 1st December 2006 05:14 PM

dumbass thanks for the brilliant response.
Looking at teds site i would need the following circuit:
Resistor: 4.0 ohms
Inductor: 2mH

Where would i find these parts and how are tehy wired?

Dumbass 1st December 2006 05:24 PM

Quote:

Originally posted by neil_kaye
dumbass thanks for the brilliant response.
Looking at teds site i would need the following circuit:
Resistor: 4.0 ohms
Inductor: 2mH

Where would i find these parts and how are tehy wired?

http://madisound.com/pdf/capcoil.pdf

I am 95% certain that the resistor and coil are in parallel to each other, but in series w.r.t. the speaker terminals, like L_bsc and R_parallel in figure 3 from the whitepaper I linked. The copy on the Jordan website is a bit confusing to me.

planet10 1st December 2006 05:31 PM

to learn about bafflestep start here:

http://www.t-linespeakers.org/tech/b...intro-bds.html

dave

Jim Griffin 1st December 2006 09:08 PM

Jordan Mini-monitor Baffle Step
 
At the 2001 Washington, DC DIY Loudspeaker event, Dennis Murphy measured the performance of my Jordan JX92S mini-monitors which had won the Atlanta DIY Loudspeaker event a month or so earlier. Note the in-room response with and without baffle step compensation at the link. The 'with BSC' data yields a much flatter and better sounding response vs. the 'without BSC' case.

The BSC for the mini-monitor speakers was a 1.5 mH inductor and a 4 ohms resistor connected in parallel and then that network placed in series with the positive lead of the speaker. While this type of BSC network does decrease the sensitivity of the speaker by 3 dB, the better balanced frequency response are well worth it. Other two-way designs that I have used (for instance with the JX92S with a ribbon tweeter) are designed to achieve BSC inherently within the crossover circuit so no additional components are needed.

Scroll down the DC DIY2001 results and measurements to see the Jordan mini-monitor data near the end of the page.

http://murphyblaster.com/content.php?f=DIY_2001.html

neil_kaye 1st December 2006 09:08 PM

Planet10,
Thanks for the link, but i am still not sure how to wire a passive BS compensation circuit. Do you have a link to a circuit diagram?
What manufactueres would you recommend to go with the Jordans?
How difficult would it be to fabricate a passive line level circuit for a 3886 chip amp? Would this even be recommended?

planet10 1st December 2006 10:20 PM

Quote:

Originally posted by neil_kaye
Thanks for the link, but i am still not sure how to wire a passive BS compensation circuit. Do you have a link to a circuit diagram?
http://www.trueaudio.com/st_diff1.htm

dave

neil_kaye 2nd December 2006 12:48 AM

Jim,
The resistor that you used, what make and wattage was it?

Did you find a sound difference between using an air-core or a copper coil inductor and if so how does the gage of the copper foil play into the results assuming the same mH across the board?

You actually mentioned to me in a recent email about a BCS and i just glossed over it, now with more research it is like "duhhh" of course
Thanks for the help


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