Dispersion characteristics vs driver size

[Beowulf57]

A problem I noted when switching from the LX5 (500Hz LF crossover point) to the N1200S (1200Hz LF crossover point) with my D130 (15") / D131 (12") bass drivers and LE85 midrange drivers. The sound became excessively peaked in the midrange with the higher crossover point. might this have been due in part to "beaming," i.e., a more narrow dispersion pattern of those frequencies by the larger cone drivers?

I know I can knock the HF level down further with the addition of an L-pad, but wondered if anyone has done any measurements of frequency dispersion characteristics of large driver cones?

[boputnam]

I doubt "beaming" was at play.

This is a weird comparison as you are changing crossover points and woofers. Lacking individual measurements of each transducer's performance in the different configurations, your symptoms could be do to:

- woofer resonance. This would have been above the lower HPF setting and thus not excited.

- different EQ response of the cabinet (related to the above).

- other. :)

[Robh3606]

Just curious but why do you think it's a dispersion issue?? I would be more concerned about the on axis response above the higher crossover point. You may simply be hearing cone break-up modes that were not audible with the lower 500Hz crossover. You may be getting a rising response with the aluminum dome VC covers like in the E series drivers.




Rob:)

[Beowulf57]

I doubt "beaming" was at play.

This is a weird comparison as you are changing crossover points and woofers. Lacking individual measurements of each transducer's performance in the different configurations, your symptoms could be do to:

- woofer resonance. This would have been above the lower HPF setting and thus not excited.

- different EQ response of the cabinet (related to the above).

- other. :)

Sorry...needed to be a bit more precise. I'm not changing woofers: I have two folded horn cabinets of slightly differing sizes; one optimized for the D130, the other for the D131. Built at different times by different people, but I have been able to use them as a stereo pair without any real problems. So, I'm really comparing the "sound" of my speaker system using the two different crossovers.

Thanks for the thoughts.

[Beowulf57]

Just curious but why do you think it's a dispersion issue?? I would be more concerned about the on axis response above the higher crossover point. You may simply be hearing cone break-up modes that were not audible with the lower 500Hz crossover. You may be getting a rising response with the aluminum dome VC covers like in the E series drivers.




Rob:)

Yes, I agree with the rising response (this was actually designed into the D131/D130 characteristic)...just wondered if beaming due to the cone diameter approaching the wavelength at the higher crossover point could contribute to the observed sonic signature change.

[boputnam]

So, I'm really comparing the "sound" of my speaker system using the two different crossovers.That's actually what I first guessed, but then I wasn't so sure - syntax can be a tricky thing as we are about to discover... ;)


Yes, I agree with the rising response (this was actually designed into the D131/D130 characteristic)...just wondered if beaming due to the cone diameter approaching the wavelength at the higher crossover point could contribute to the observed sonic signature change.I woulda said that, thusly:

"...due to the wavelength at the higher crossover point approaching the cone diameter...". The cone diameter is not changing, unless you have some sort of shape-shifter involved here. :)

Like I think I said: cone resonance issues which were obviated by the lower HPF.

[Beowulf57]

That's actually what I first guessed, but then I wasn't so sure - syntax can be a tricky thing as we are about to discover... ;)

I woulda said that, thusly:

"...due to the wavelength at the higher crossover point approaching the cone diameter...". The cone diameter is not changing, unless you have some sort of shape-shifter involved here. :)

Like I think I said: cone resonance issues which were obviated by the lower HPF.

LOL...good syntax point ;)...though not in my original, rather hasty, off the cuff response: the cone diameter "varies" from 12" to 15" in one discrete step between the two bass drivers. Aw, WTH...no need to :banghead: home rejoinders with :blah: that would just be :bs:!

Thanks for your note concerning cone resonances. BTW...what does HPF indicate? High Pass Filter? If yes, wouldn't it be the lower LPF that would obviate the cone resonance issues, as that is the section that rolls off the woofer response?:D

[boputnam]

BTW...what does HPF indicate? High Pass Filter? If yes, wouldn't it be the lower LPF that would obviate the cone resonance issues, as that is the section that rolls off the woofer response?:DYes, HPF = high pass filter, the equivalent of low cut filter. Sorry - I was looking at this from the entire crossover point.

So, yes, it would be the "lower" LPF that is most critical here.

[speakerdave]

Study the response graphs of those two extended range drivers available under the 2135 and 2130 pro numbers here:

http://www.lansingheritage.org/html/jbl/specs/pro-comp/pro-comp.htm

In a bass reflex or horn enclosure I believe they both will give a fairly flat system response up to 500 Hz, and the efficiency jump they both take from 1000 Hz up to about 4000 will begin 12 dB down using that crossover. Although this will have an effect on the response of the whole system, I do not think it will be as pronounced as when using the 1200 Hz crossover. With that crossover the fast rise in response of both those drivers is happening both at the crossover point and in the first two octaves above it. I think this would be much more audible and may be what you are hearing.

David

[Beowulf57]

Study the response graphs of those two extended range drivers available under the 2135 and 2130 pro numbers here:

http://www.lansingheritage.org/html/jbl/specs/pro-comp/pro-comp.htm

In a bass reflex or horn enclosure I believe they both will give a fairly flat system response up to 500 Hz, and the efficiency jump they both take from 1000 Hz up to about 4000 will begin 12 dB down using that crossover. Although this will have an effect on the response of the whole system, I do not think it will be as pronounced as when using the 1200 Hz crossover. With that crossover the fast rise in response of both those drivers is happening both at the crossover point and in the first two octaves above it. I think this would be much more audible and may be what you are hearing.

David

Thank you David...this was my impression from studying the specifications. It leads me to suspect that using these LF drivers with the lower (500Hz) crossover is a better choice for a balanced sound in my setup. Using the higher crossover point would probably make crossover modifications necessary to reduce the woofer response in the midrange. Also, better midrange dispersion would be available by using the LE85/HL92 in that region.