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Old 21st December 2010, 06:58 PM   #1
Soulman is offline Soulman  Sweden
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Question Magneplanar with Helmholtz resonators acting as monopole/dipole

Magnepan MMG has a frequency response down to 50Hz if placed at optimal distance from the wall behind the speaker. This is too high for my taste since I want to hear the lowest tunes from a bass guitar. 40 Hz will be enough for almost all of the records that I play. Iím not a big fan of subwoofers and think itís more for movies than music.

Magnepan MMG is dipole speakers that are rather slim so I think that there will be a reduction with 6 dB per octave from around 200 Hz. This means that there is a 12 dB reduction down to 50Hz when the speaker stands in the middle of the room.

Magneplanar speakers use the wall behind the speaker to increase the bass response but this leads to a bass response that goes like a ďrollercoasterĒ in the bass region depending on if the back wave is in phase or not with the front wave. If the left and right speakers are placed at different distance from the wall is this maybe not such a big problem since most records have the low bass in the in the middle between the channels and with two speakers will the response be rather flat.

In my listening room is my record shelf with over 6000 vinyl records on the wall behind the speakers. I think that this will absorb most of the bass behind the speaker. I think that Magnepan MMG acting as dipoles will have hard to give bass response under 100 Hz under these circumstances.

The normal way of increase bass from a dipole loudspeaker is to make the baffle bigger. I donít like the idea since it will increase the difference in time between the front and back sound wave.

I think that I have found out a way to increase the bass and still make the speaker act as a dipole in the mid and treble, and it will also be more independent of the wall behind the speaker.

The solution is to use Helmholtz resonators to take away low bass that are created on the back side of the speaker. One of the good things with Helmholtz resonators is that they can be tuned to reduce sound pressure at a specific frequency. Formula for Helmholtz resonators is:

f = c / (2π) * sqr ( S/VL )

f = frequency
c = speed of sound
S = port area
L = port length
V = volume

A 1200 mm long tube with a diameter of 200 mm with some mineral wool inside and six ports with diameter 25 mm, length 45 mm will give a resonance frequency of 50 Hz. Tubes can be made of that type of paper tubes that are used for concrete casting. Both ends closed of cause. I donít know if this is the optimal design but it is a start. Maybe it is needed double tubes tuned to 50 Hz and 100 Hz. Only more detailed calculations and tests can tell.

I think that one tube on each side placed maybe 100-150 mm behind each edges of the speaker will work. Ports should be faced against the edge of the speaker and act as a high pass filter for the sound wave that travel in the gap between the speaker edge and the tube. The tube resonators will reduce inverted low frequency sound waves from the back of the speaker but will have little impact on the higher frequency where the speaker still will act as a dipole. Maybe the tubes need to be dressed with some 10-20mm thick isolation to prevent reflections back to the speaker.

A third tube placed 500-600 mm behind the speaker might also help.

As complement to can the speaker be placed on a box that also act as a Helmholtz resonator. It is written on a lot of sites that Magnepan MMG sounds better if it comes up from the floor 300 mm.

If everything works will the speaker act as a monopole in the bass area and as a dipole from maybe 200 Hz and up. Since the speaker is not dependent on the reflections of the bass from the wall behind will maybe the bass be much faster and dynamic compare to the normal setup. Who knows?

I havenít found anything written on this kind of monopole/dipole speaker. Is it a new type of speaker? It is maybe too good to be true so I have maybe missed something, or ÖÖ

The principle can also be used for dipole bass speakers but then is there no need for the gap between the baffle and the tube. Just remember that the pressure is zero at the edge of a dipole baffle so the Helmholtz ports must be placed at one third of the distance from the back of the speaker to the front of the speaker.

It should be nice to some day try if my theory about monopole/dipole speakers works with a pair of Magnepan MMG or better with a pair of Magnepan 1.7. But until I know if it works will it just be theories.

If there is anybody out there that owns a pair of Magneplanar speakers and try this please let the rest of the world know if it works.

Tomas Eriksson, Sweden
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Old 21st December 2010, 07:58 PM   #2
nac134 is offline nac134  Canada
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You're going to lose a great deal of volume, and your soundstaging will go out the window. I had a pair of MMGs and I was happy with the bass extension, even for rock. Adding a subwoofer would be a challenge. The simple dipole design is what makes magneplanars special, and its best not to change that aspect of the design. If you must have more bass extension, go a model up.
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Old 22nd December 2010, 01:31 PM   #3
a.wayne is offline a.wayne  United States
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I would add subwoofers , 1/ch make the boxes vented rear firing ports, run them slightly behind the Maggies...

Last edited by a.wayne; 22nd December 2010 at 01:34 PM.
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Old 24th December 2010, 12:22 PM   #4
BaMorin is offline BaMorin  United States
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Quote:
Originally Posted by Soulman View Post
Magnepan MMG has a frequency response down to 50Hz if placed at optimal distance from the wall behind the speaker. This is too high for my taste since I want to hear the lowest tunes from a bass guitar. 40 Hz will be enough for almost all of the records that I play. Iím not a big fan of subwoofers and think itís more for movies than music.

Tomas Eriksson, Sweden
I think the biggest issue you are actually having with your bass response is your back refletive wall.

My pans (3.6Rs) reflect off of brick and stone. I set them up by standing between them and listening to the wall. When the stereo image sounds correct to me, I cannot hear the speakers beside me. I have zero phasing problems then when sitting in my standard position. I have done this with MG1's and up to my current 3.6's
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Old 24th December 2010, 01:35 PM   #5
kevinkr is offline kevinkr  United States
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Soulman, your comments sound a bit like they are based on conjecture - if you have not got your maggies yet I would wait to see how they perform in room before doing anything..

I ran MG1.4, and then MG1.6QR for a number of years and whilst they are significantly larger than the MMG I would at least expect the MMG bass performance to be adequate for most uses.

If the walls are very reflective you might want to treat them with something. I never found this to be an issue with standard drywall in any room in which they were installed.. (4 different rooms)

FWIW I doubt your shelving/records will absorb that much bass, depending on where the speakers are situated relative to the shelving you may have some other problems..
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Last edited by kevinkr; 24th December 2010 at 01:37 PM.
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Old 30th December 2010, 07:42 PM   #6
Soulman is offline Soulman  Sweden
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Thanks for all feedback.

Dear kevinkr, you are right that my comments are based on conjecture.

With more than two meter to the wall behind the speakers and all records acting as a bass trap behind the speakers would I be surprised if the Magnepan MMG will give the amount of bass that that I need. So a pair of subwoofers is probably necessary in my room or more expensive Magnepan speakers.

I leave the idea of add bass to magneplanar with Helmholtz resonators to the future…..and try if the theory works with a pair of Markaudio Alpair 6 that I plan to use as computer speakers. It can be worth 10$ to buy some smaller tubes and try if it works.

If you read this and have experience from reducing bass on the rear side of dipole speakers with Helmholtz resonators please write a comment.
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Old 14th January 2011, 07:26 PM   #7
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Hello,

http://www.pvconsultants.com/audio/reflection/rrc.htm

have you tried to optimize speaker/listening position
due to low bass response ?

In some rooms it may help to bring a little asymmetry
into the speaker's positions

- slight difference in distance to the side wall for L and R speaker

- have sufficient distance to the side wall

- and place the listening seat offset from the middle of
the room by 40cm at least.

If your room is narrow you can also try a very small
distance of the speakers to the side walls, which
increases the effective dipole path length.

Many dipoles can handle that without serious "damage"
to imaging.

If you have tried all those tricks/combinations
in placement and you are still unstatisfied, you
may indeed think about a subwoofer.

Kind Regards
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Old 14th January 2011, 07:38 PM   #8
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you might also try positioning the speakers directly in the corners of the rooms. I did this with good results with 0ne plus 0ne Acoustats. The panel's rear wave is directed away from the speaker not reflected back to the diaphragm. Bass loading was adjustable depending upon how closely the speakers are placed into the corner of the room. Have fun good luck best regards Moray James.
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Old 19th January 2011, 01:29 AM   #9
Josh358 is offline Josh358  United States
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Helmholtz resonators aren't necessary. The MMG's are underdamped and produce prodigious bass, most of which is lost to the 6 dB/octave dipole cancellation. An enclosure or infinite baffle alignment will do the trick, but if you use an enclosure, you'll lose the benefit of the dipole radiation pattern and have to deal with box resonances. You might have better luck with a wing on the woofer side. Don't worry about the time delay, if they're EQ'd right it should all be minimum phase. But be prepared to experiment first with cardboard and to equalize the bass response, since it will throw the dipole compensation out of alignment.
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Old 29th January 2011, 03:21 PM   #10
Soulman is offline Soulman  Sweden
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Thanks for all comments.

Dear LineArray and moray james:
My problem is that speaker position is more or less fixed in my room. Distance to back wall is more than 2 meter and the back wall will absorb a lot of the bass that goes backward. Your comments are good recommendations but it canít be used in my setup.

I found a dealer in my hometown that have Magnepan 1.7 so I will go there and find out what happens when the speakers are placed at a big distance from the back wall. I this moment I think that 1.7 or second hand 1.6 are more what I looking for than the MMG.

Dear Josh358:
The reason for Helmholtz resonators is to handle the 6dB/octave dipole cancellation at lower frequencies without changing the way the speaker acts above 100 Hz. It is still a lot of air that can pass between the speakers and the resonator. The question is of cause if itís enough air and if it will give reflections but I believe that there is only way to find out and that is to build and listen. Another question is if the Helmholtz resonators will reduce enough of the low frequencies.
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