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Old 24th June 2010, 08:11 PM   #1
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Default Padding the rear wave of a dipole

I'm looking for a way to attenuate only part of a dipole's rear wave w/o greatly affecting the power response. I'm thinking of experimenting with varying densities of fiberglass batting placed behind the driver at different distances. Can anyone speculate on the relationship between material density, distance, location, shape, etc. and how that might pad a range of frequencies.

My goal is to pad only a small (undesireable) part of reflection produced by the rear wave, w/o impacting the front directly.

Ed
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Old 25th June 2010, 12:07 AM   #2
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Quote:
Originally Posted by matevana View Post
My goal is to pad only a small (undesireable) part of reflection produced by the rear wave, w/o impacting the front directly.
If the undesired reflection is coming from a certain area of the rear wall, I'd say best results would be had by placing 4" thick rigid fiberglass(or your absorbing material of choice) on the wall to absorb the reflection.
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Old 25th June 2010, 02:24 AM   #3
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M,

Here is an idea that might work for you:

Martin Leach tweak

Think generally. You wouldnt have to do the whole panel top to bottom...maybe just were the reflection originates.
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Old 25th June 2010, 03:25 AM   #4
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Quote:
Originally Posted by matevana View Post
snip
My goal is to pad only a small (undesireable) part of reflection produced by the rear wave, w/o impacting the front directly.

Ed
I'm having trouble understanding what you are trying to do or why?

Is it something you hear or some theoretical shortcoming you want to address?
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Old 25th June 2010, 08:41 AM   #5
Calvin is offline Calvin  Germany
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Hi,

padding in close proximity of the panel itself is rather helpful in controlling and reducing the high Q of the base resonance. Of course the padding influences the panel´s behaviour over its complete working range. Besides introducing non-linear damping behaviour there will be early reflections which can be quite annoying. I´d rather opt for electronic means to control the Q and leave the panel mechanically open otherwise. If You want to control the amount of reflections stemming from reflected soundwaves from the room´s walls You can experiment with reflectors and/or dampers situated at a greater distance behind the panel. Soundlab sells as accessoire, the Sally, which is basically a foam covered stick -and terribly ugly I may add- which can be freely positioned anywhwere behind the panel. Since the amount and distribution of the rearside soundwaves defines the ´size´ of the acoustical image impression You only can try where to place dampers or reflectors till it fits Your taste. You can easily and cheaply evaluate the effects by using a plant or pieces of cardboard or pillows etc that You place behind the panel.

jauu
Calvin
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Old 25th June 2010, 09:50 AM   #6
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Quote:
Originally Posted by dawnrazor View Post
M,

Here is an idea that might work for you:

Martin Leach tweak

Think generally. You wouldnt have to do the whole panel top to bottom...maybe just were the reflection originates.

Very interesting! Exactly what I had in mind. Thank you.
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Old 25th June 2010, 11:33 AM   #7
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Quote:
Originally Posted by Calvin View Post
Hi,

padding in close proximity of the panel itself is rather helpful in controlling and reducing the high Q of the base resonance. Of course the padding influences the panel´s behaviour over its complete working range. Besides introducing non-linear damping behaviour there will be early reflections which can be quite annoying. I´d rather opt for electronic means to control the Q and leave the panel mechanically open otherwise. If You want to control the amount of reflections stemming from reflected soundwaves from the room´s walls You can experiment with reflectors and/or dampers situated at a greater distance behind the panel. Soundlab sells as accessoire, the Sally, which is basically a foam covered stick -and terribly ugly I may add- which can be freely positioned anywhwere behind the panel. Since the amount and distribution of the rearside soundwaves defines the ´size´ of the acoustical image impression You only can try where to place dampers or reflectors till it fits Your taste. You can easily and cheaply evaluate the effects by using a plant or pieces of cardboard or pillows etc that You place behind the panel.

jauu
Calvin

Calvin,

That seems like very good, common sense advice.

Thank you.
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Old 25th June 2010, 01:15 PM   #8
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I take it you are interested in the rear wave for reasons of bass cancellation.
Soundlab made wings for theirs. They were wooden, as tall as the speaker, and quite wide. They were on hinges so you could adjust the angle.
When I first got my 2+2's I tried putting absorbing material behind them. I thought it affected higher freq. more than lower. In my basement with cement floors my acoustats have too much bass, possibly explaining my problem with this solution.
There are room treatment type absorbing devices (room tunes?) that are made for electrostatic speakers. They are a cylender about foot in diameter, tall and filled with stuffing. They sometimes are seen with a reflecting side that you would face the speaker with so that it will reflect the mids on up, but would still be effective for absorbing the lows. The larger in diameter the tube is the more effective it will be.
Place it "about" a foot behind the speaker.
I made mine with chicken wire from home depot. I got a
5' by 5' piece sewed the two ends together with sewing thread and a needle, forced the result into a tube shape.
Put in a piece of steel flashing material 4" wide and slightly dished it, place it against the inside of the chiken wire. I filled the rest with fiberglass stuffing. Actually there is blown in stuffing, that is stuffed and compacted in the center. Cover it with fabric and its quite effective.
Except for the sewing, It took me an hour to do mine.
Cost: done years ago? probably less than the cost of shipping if you bought them.
Paul
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Old 25th June 2010, 04:57 PM   #9
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Quote:
Originally Posted by pforeman View Post
I take it you are interested in the rear wave for reasons of bass cancellation.
Soundlab made wings for theirs. They were wooden, as tall as the speaker, and quite wide. They were on hinges so you could adjust the angle.
When I first got my 2+2's I tried putting absorbing material behind them. I thought it affected higher freq. more than lower. In my basement with cement floors my acoustats have too much bass, possibly explaining my problem with this solution.
There are room treatment type absorbing devices (room tunes?) that are made for electrostatic speakers. They are a cylender about foot in diameter, tall and filled with stuffing. They sometimes are seen with a reflecting side that you would face the speaker with so that it will reflect the mids on up, but would still be effective for absorbing the lows. The larger in diameter the tube is the more effective it will be.
Place it "about" a foot behind the speaker.
I made mine with chicken wire from home depot. I got a
5' by 5' piece sewed the two ends together with sewing thread and a needle, forced the result into a tube shape.
Put in a piece of steel flashing material 4" wide and slightly dished it, place it against the inside of the chiken wire. I filled the rest with fiberglass stuffing. Actually there is blown in stuffing, that is stuffed and compacted in the center. Cover it with fabric and its quite effective.
Except for the sewing, It took me an hour to do mine.
Cost: done years ago? probably less than the cost of shipping if you bought them.
Paul

Thanks for your helpful suggestions. I am mostly interested in the science of surgically striking only a small part of the rear wave that might otherwise become adverse reflections off the rear wall. My thought was to place absorbant material within a few inches of the driver's back, before the wave had a chance to disperse much. As an example, I might only be interested in padding frequencies between 300 and 600Hz, depending on what the environment called for.
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Old 25th June 2010, 10:28 PM   #10
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A porous absorber near the membrane will always have a wideband
absorption, regardless what type of speaker you treat this way,
and it will eat your bass. If your bass is too fat and boomy ...

In the Martin Leach tweak, the porous absorber has distance
to the membrane and may let the bass breathe this way. Not a bad
idea but i doubt that using an ESL Panel there will be no
reflections heard through the thin membrane from the egg crate
foam, as Calvin pointed out.

A narrow ribbon which sits off center may be not be affected
that much from the reflections.

From my experience with ribbons i would say that foam should
not be the material facing the membrane from the rear, there is
a less reflecting material needed (sheep wool, fine and soft
polyester wool e.g.) which should be the first layer.

Since a porous absorber has to be positioned in the velocity
maximum of a wave to be absorbed, for selective absorbtion i
would choose half a wavelength distance to the rear side of the
panel which would be about 50cm for 340Hz e.g.

Dipoles excite the room at velocity maxima of the standing waves,
so the next maximum which can be used for absorption is half a
wavelength away from the membrane.

But i wonder if optimizing the distance to the rear wall would not
be the first step. If there is a pronounced frequency range with a dipole
often the comb filter effect from the rear wall interacts in a bad
way with room modes. A narrow peak in the mentioned frequency range
can often be healed by moving the speaker a few decimeters.

Having the dipole membranes parallel to the rear wall mostly yields
no good result. Before placing absorbers i would ensure that everything
which can be achieved by turning the speakers some degrees and
moving them forward/backward has been tried before.

Sometimes also the listening position can move a bit.

Kind Regards
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Last edited by LineArray; 25th June 2010 at 10:32 PM.
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