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double woofer heresy question


dhsettim
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Hi Ya All, someone mentioned recently adding a 2nd woofer to the rear panel of a Heresy. They said to wire it out of phase but I would like to try it in phase to help cancel mechanical forces. Has anyone actually done this mod, and what changes need to be done to my Type E crossover to make it work. Thanks, tim

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That would have been me. I have been tooling around with the modified heresy center.

Yes, the placement of the woofer installed on the back cover would be an easy mod that is reverseable if you don't like it. Just cut out a new back cover and try it. Supposedly, this will cancel out odd-order nonlinearities thus reducing driver distortion. I believe you drive them electrically out of phase. Driving in phase shouldn't hurt anything. It would be easy to change that at anytime on the "back" woofer. Not sure which way would "cancel" mechanical forces.

You would need one more 2.5 mH inductor if you are using a Standard Type E Heresy network. Just run the positive lead from the network input to the positive of the woofer. Then ground the woofer with the network. Now this is in parallel and in phase if you want to try it. Some of your low end will go down into 4 ohms but this shouldn't be too much of a problem for traditional amplifiers.

The push/pull will raise your dB by 3 db. I think to get this you will need to be out of phase. Someone help me with that one.

Also, not sure of the effects of comb filtering? Dr. Who is the man on this one.

post-16499-13819271597666_thumb.jpg

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I was unsure what the goal was here. If using the same sized box, then Tbrennan is right, the -3db point would move up.

If you build a new cabinet with twice the internal volume, using 2 woofers, this will get you somewhere. You should couple them as closely as possible in the vertical axis. In some respects, you would be better off with a single 15" in a larger cabinet, then sticking with 2 k-22 or 2 k-24 woofers in each cabinet.

Then you could debate the advantages of moving to a cornwall type vented cabinet. The point is that there are so many different aspects to consider.

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I agree with Mike here.

At first I thought perhaps you were considering an isobaric loading, but it soon became apparant that this was not the case. And adding a woofer to the back of the enclosure would essentially make this design a dipole (all phasing issues ignored for the time being) which would radically change it's potential applications and placement. And personally I would oppose this approach.

Are we simply seeking increased bass response? That would tend to take use to a Cornwall style unit.

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First of all, remember, I said this is a reverseable mod you could try. No I don't think this would increase bass response.

The only reason the idea was mentioned in another thread by me was to improve dB.

Guys, you are gonna have to prove to me what this will do to the frequency response or low end. Basically expain it to me again. Not sure I follow. Is it because you didn't change the box volume?

No this isn't a Isobarik Idea. Although I have come with a couple of options. Would require a new box.

Oops...Gotta run....will finish this post later.

jc

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http://www.eaw.com/technology/nonproprietary/pushpull.html

<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

each pair of LF transducers is arranged so that one is inverted and facing the opposite direction of the other. The transducers are then connected with opposite polarities so that both cones move in the same direction in response to the input signals. At high output levels where the asymmetries in piston action might occur, each LF cone "loads" the other cone in the opposite direction to smooth out any mechanical asymmetries. The outcome of this action is a significant reduction in distortion.

The advantage in using the push-pull arrangement is that it helps to cancel 2nd order harmonics of the drivers, thus producing a cleaner bass.

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"The advantage in using the push-pull arrangement is that it helps to cancel 2nd order harmonics of the drivers, thus producing a cleaner bass."

I am agreeing with everything you just said. Push/pull arrangement won't work on an existing heresy cabinet. Unless however, you remount the front woofer in the opposite direction and mount another woofer on the back of the cabinet to make them push/pull. Now this would look rediculous with two woofer magnents sticking out.

I think the originally idea of the suggestion has been lost. To get a the mods you all are suggesting, a new cabinet would be needed. Hey, I love new cabinets. I have been contimplating (sp) an isobaric setup for a modified heresy for months. If I were to do it, it would be a push/pull in a small cabinet (back to back) placed inside another cabinet. Electrically in phase and wired in parallel.

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Here would be my choice if building a cabinet from scratch. The cabinet would remain small for a center channel. However, now you have a cabinet with 2 12" woofers and still have the low end of a Heresy. I believe that your dB would increase.

post-16499-13819271640916_thumb.jpg

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JW----That's an isobarik. You will get a DECREASE in efficiency of 3db with such an arrangement. What this arrangement gives in effect is a single driver having twice the moving mass, half the compliance and half the impedance of a single driver.

What you gain is deeper bass, what you lose is efficiency.

I suggest you get some books on this subject. "Loudspeaker Handbook" by John Eargle is in my lap as I type this. Nearby is "Acoustical Engineering" by Olson. Also very good are "Loudspeaker Cookbook" by Dickenson and "High Performance Loudspeakers" by Collums.

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I am going to have to disagree with you (nicely). Isobarik designs in general do exactly what you just said. That information is available all over the net and in books.

I have one of the books you listed above. From what I understand, with the particular arrangement listed above, there will be an increase in 3 dB.

This entire concept was discussed recently on another thread. There is a link to some information I provided from "Loudspeaker Design". Note: I quoted the source so I don't want to be sued. Let me know if you have a hard time downloading it.

Again, I agree with your quoted concept of Isobarik designs but this particular configuration may increase your dB.

I would love to be proven wrong here so I can get this "compound loading" and "Isobarik" designs out of my head.

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Modelled in WinISD:

Graph 1 = Transfer Function

Graph 2 = Max SPL

Green = Regular Heresy

Blue = Isobarik Heresy

Modelled using the same cabinet volumes...you can achieve the same

slope as the heresy with the isobarik and half the cabient size (which

might be of benefit in other applications...though the HF/MF section

inhibits this). Btw, with an isobarik enclosure, only one side of one

woofer is open to the environment outside of the cabinet (aka, the

second woofer is entirely enclosed inside the cabinet). There were a

few comments that led me to believe this was unclear. If you think

about it, it's basically a single speaker with two voicecoils and twice

the diaphragm mass (note the HF roll off).

I think the graph speaks for itself...(Isobarik = waste of time and money)

Just one more comment...you can achieve the same SPL's if you double

the amount of power to the woofer. So that would be 420 watts into a 4

ohm load, but then you would have to go with the smaller cabinet to

control excursion. I think an isobaric design would be interesting on a

cornwall....throw in four 15" woofers and keep the cabinet volume the

same (might need to make it narrower and a bit taller), then throw in

the MF/HF section of a lascala (basically reducing the distortion of

the cornscala dbb while also making it a smaller speaker). I've got

lots of watts available so padding down the HF/MF section won't be a

problem. Btw, I only suggest it as a way to reduce distortion...not

sure if it's worth all the extra cost (or if it'd even be noticeable).

post-10350-13819271643136_thumb.gif

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Ok...it's official. Isobarik designs for this application sux.

Doc, I've been using that same software. Got the same type curves...however, how does it account for wiring in parallel?

I am puttting together a center for my setup. I actually already dropped the Isobarik idea a few weeks ago due to the weight of two 12" woofers on my TV stand.

Will start a new thread soon of my idea. YES, it will probably involve compound woofer loading.

DOC, If one were to chose woofers that are not shielded or not alnico, how can I fix that. From what I understand, no "PRO" drivers are shielded. What can a man do for a custom center project?

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Just to provide a bit of balance and to play devil's advocate, isobaric loading does not "suck"!

This loading configuration addresses certain specific issues. Namely allowing a smaller volume cabinet and reducing distortion, which in a sub can be quite a substantial issue! It is but one tool in an arsenal. And the tool is valid and works!

Is the cost of an additional woofer, the weight, and the additional complexity of the cabinet an acceptable trade off for the advantages offered. Perhaps not. It is a special purpose "tool".

The proper criterion is "what are you trying to achieve and what parameters are limiting to you"!?

If you simply want greater gain, it is not the proper tool!

Just as if your desired task is to cut a board, it makes NO sense to deride your hammer! Your hammer works fine! And to deride it simply tells me that you have an improper understanding of the purpose for the hammer! (Since some missed this! If your goal is additional gain and cabinet size is not limiting, then the isobaric is the wrong design for the job - its the wrong tool!, but it makes no sense to slight it - instead just use a better option (the 'correct tool') for the situation! But given restrictions in cabinet size and a desire for lower 2nd order harmonics, the isobaric just might fit the bill!)

If there is indeed something wrong, it is the operator who chooses the wrong tool for the application!

But given the the concern by many for low distortion subwoofers, and the preoccupation of many over subwoofer size, isobaric loading may be just the solution you are looking for!

So evaluate your needs and choose your tools appropriately and wisely![:D][:P][6]

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Ok...it's official. Isobarik designs for this application sux.

Doc, I've been using that same software. Got the same type curves...however, how does it account for wiring in parallel?

I

am puttting together a center for my setup. I actually already

dropped the Isobarik idea a few weeks ago due to the weight of two 12"

woofers on my TV stand.

Will start a new thread soon of my idea. YES, it will probably involve compound woofer loading.

DOC,

If one were to chose woofers that are not shielded or not alnico, how

can I fix that. From what I understand, no "PRO" drivers are

shielded. What can a man do for a custom center project?

Depends on how you define wired in parallel... [;)]

For the same voltage coming out of your amplifier (which afterall your

amplifier is theoretically a voltage source): 2 watts into an 8 ohm

load is the same as 4 watts into a 4 ohm load...so if you have two 8

ohm speakers in parallel, you need to enter 4 watts into WinISD when

comparing against a single 8 ohm speaker running at 2 watts on the same

amp. Likewise, two 8 ohm speakers wired in series should be modelled

with 1 watt entered into WinISD.

If you want to think about it another way, the power you enter into

WinISD refers to the actual power output of your amplifier. So for the

same voltage your amplifier is putting out twice the power when the

impedance drops in half (V=IR....cut the resistance in half and keep

the voltage the same, the current is going to double). And again with

series....if you double the resistance and keep the voltage the same,

the current is going to be cut in half.

WinISD does this because it allows you to figure out for yourself how

you want to wire it (do you want to have those 4 speakers wired in 2

parallel sets wire in series for an 8ohm load, or do you want to model

those 4 speakers all wired in parallel for a 2 ohm load). Depending on

what you pick will change what you enter into WinISD.

Also, not all amplifiers are capable of doubling their power output

everytime you cut the impedance in half. One great way to tell how much

brute force an amplifier has is to look at how many times the impedance

can be cut in half and the amplifier still maintaing a doubling of

power (some of the crazy crown amps can double twice from an 8 ohm to 4

ohm and then to 2 ohm....and then some come close to doubling down to 1

ohm too!).

Btw, do you ever use your fireplace or is it just for show?

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This is the first time someone has said something good of Isobarik. Alright.

Yes, most of what I read used the Isobarik for Subwoofer applications.

You lost me on the hammer analogy. Just as well. Point well made.

Again, I dropped the isobarik idea a week or so ago. Will go to simple compound woofer loading as I did with the dbb project. I won't throw away those sketches for Isobarics. May need a low volume cabinet for a sub one day. The measurements I made would work for a K22 sub.

jc

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Doc, I was typing as you posted.

I will try this on the software.

That fireplace has never been used. Very much for show. It is gas and does work. That room is my "Zone B" in my area that we entertain guest. The other side of that room is my HT which right now has my two Cornscalas (non dbb) as the fronts. In process of a custom center channel.

Did you see my comment/question on shielding pro woofers?

Well, the Heresy sub would be nice. Could bet Low...also get in alnico woofers. Bob Crites has a pair rebuilt.

jc

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JC, this is an interesting post.

I think you are trying to come up with a center speaker with efficiency similar to your double Cornscala's. You could try making a speaker with 2-12" woofers in a horizontal arrangement

with the mid and high horns in between them, similar to the new RF series. You would need to double the volume of the Heresy enclosure, since the VAS would be twice as large with two woofers.

In order to shield the woofers you could try using a bucking magnet available from either Madisound or Parts Express, but I'm not sure if they are large enough to capture the stray magnetic field. Tannoy lines the inside of their cabinets with metal to further shield the enclosure. These bucking magnets might also effect the QTS a little, but since this is a sealed enclosure the slight change in QTS will not effect the alignment that much.

I also experimented with Isobaric enclosures in the past and you can achieve low bass in a small enclosure by giving up some efficiency. The only down side is, if they are not crossed over low enough, (i.e. a subwoofer application), you can sometimes hear some blurring in the midrange caused by the rear woofer.

Keep up the good work, I enjoy your posts!!

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Thank You for your support and Don't forget to tip your waitress.

Wow, you must have the same dreams as me.

When I gave up on the isobarik stuff, I have been plotting a dual woofer setup. The two K22's is a bit much for space for be. To put a "heritage" horn between two 12 inch woofer would be too wide for my setup.

Then have been flirtin with other woofers....mostly by Eminence. I don't need a bass machine as I would have the MC Bass mode going. I also don't want a bunch of vibration around my TV.

GOAL: EFFICIENT custom speaker that would timbre match (and match in other ways) with Cornscalas. Therefore, i was seeking a 97-98 dB for the low end. I could match this with a basic network setup to attenuate the K77 and k55 at the same dB.

For some ridiculous reason, I couldn't locate the bucking magnets on Parts Express untill now.

Take a look at the attached image.

post-16499-13819271657654_thumb.jpg

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