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-   -   OB bass with bass trap (http://www.diyaudio.com/forums/multi-way/219595-ob-bass-bass-trap.html)

oon_the_kid 12th September 2012 05:03 PM

OB bass with bass trap
 
Hi,

A thought came to me.

The reason we want to have open baffles is the mids that are reflected off the back wall enriches the sound. However for bass the reflected bass will cancel out the bass from the front. The purpose of using OB for bass is that it does not have cabinet resonance and no back reflection to colour the sound.

One way to increase bass output is the use of slot loaded OB bass. To create an imbalance between the front and the back.

My thoughts are, what if we put a bass trap like this wedge bass trap directly behind the woofer, placed directly behind the magnet.

Super Wedge Acoustic Wall Tile 5" x 12" x 12" (UL 94) Each 300-895

This would absorb a significant amount of bass radiating from the back and create the imbalance with the front. Reducing the canceling power of the bass from the back.
This would increase the bass output from the front.
I would probably try it out one of these days.

Does somebody have any thoughts on this?

Oon

Melon Head 12th September 2012 05:16 PM

I had the exact same idea, and tried it out, but it did not produce the results that I hoped for.
Give it a go anyway, you might have more luck

djn 12th September 2012 05:41 PM

If the woof is in an H or U frame, why not cover the back of the frame with it. Might look cleaner.

StigErik 12th September 2012 05:52 PM

Quote:

Originally Posted by oon_the_kid (Post 3162101)
However for bass the reflected bass will cancel out the bass from the front.

No, its not the reflected sound that cancel out the bass, its the rear wave that diffracts around the baffle.

Elias 12th September 2012 05:59 PM

Quote:

Originally Posted by oon_the_kid (Post 3162101)
The purpose of using OB for bass is that it does not have cabinet resonance and no back reflection to colour the sound.

I think the main purpose of using dipole bass in a small room is due to the loudspeaker-room interaction.


Quote:

Originally Posted by StigErik (Post 3162179)
No, its not the reflected sound that cancel out the bass, its the rear wave that diffracts around the baffle.

It's both, diffracted and reflected sound, forming a second order system. Of course it depends on the distance to the front wall.


- Elias

Melon Head 12th September 2012 06:03 PM

Quote:

Originally Posted by djn (Post 3162165)
If the woof is in an H or U frame, why not cover the back of the frame with it. Might look cleaner.

Also tried that witnout success

tvrgeek 12th September 2012 10:02 PM

Real bass traps are big. Really big. A stack of home insulation rolls floor to ceiling is a good example.

I find my well made sealed and stuffed boxes do just fine for the subs. The only way to really eliminate the backwave is the true IB design. If I had a place, it is a most attractive idea. Basically you are using another room as the box. The "box" is large relative to the drivers resonance. If that room is so large as not to cause reflection problems, so the better.

oon_the_kid 13th September 2012 01:42 AM

Hi Melon Head,

Just curious what did you use to absorb the backwave? The idea behind a wedge absorption system is the sound that is reflected of the triangle reflects into the valley of the wedges and it is this multiple reflection that actually absorbs the sound. For it to work on bass it has to be very tall structures because bass frequencies have long wavelengths. Hence the one i have chosen is 5 inch high. Normal sound absorption wedge foam is only 2 inch. Did some reading, trying to control the bass nodes of my room..

And as for the bass trapping. I intended for it to be placed immediately behind the magnet. That would allow it to absorb at the source rather than after the bass have left the speakers.


I suppose if one were to build it in a u frame then the sound will be somewhat halfway between a sealed box and OB. Which could sound pretty good, getting the best of both worlds..

Oon

Melon Head 13th September 2012 02:17 AM

I tried convoluted accoustic foam, fibreglass insulation, and polyester insulation, but please try because you might have more success.

john k... 13th September 2012 03:36 AM

The reason to go with dipole bass is to reduce the excitation of many room modes. However, when a dipole woofer is placed close to a wall the reflected wave cancel the low bass response. This is in contrast to a monopole woofer which has the bass reinforced by placing the woofer close to a wall. A correctly damped U-frame or cardioid is not affected by reflections from the wall behind the woofer.

Damping the rear wave of a dipole to produce asymmetry between front and rear will make the woofer behave more like a monopole. While it will tend to increase bass response it will also increase room mode excitation making the response potentially more irregular.

Also, slot loading doesn't produce much in the way of asymmetry at low frequency, as I have shown through measurements several times.

Here is a link to a discussion of woofer ype and rear wall reflections.


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