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-   -   Rear port close to wall dilema (http://www.diyaudio.com/forums/multi-way/194258-rear-port-close-wall-dilema.html)

Aleph 5 8th August 2011 07:55 PM

Rear port close to wall dilema
 
I have these fine speakers that are not working in my room because of the small distance from the back wall (distance to side walls is huge).
I am getting excessive boominess and congestion. :(

My quest is to solve this poblem without increasing the distance from speakers to wall. Do you have any suggestions? :cool:
If necessary I can change the port, use accoustic materials on the wall behind the speakers but I cannot increase the distance from wall.

Data:
- Distance from wall is 13" and I've discovered that at 40" the problem is solved (but I must keep them at 13").
- Rear port is vertically at the center of the speaker, from my calculations Fb=46Hz.
- The design of these speakers has to be outstanding because they have fabulous integration and magic midband while using modest drivers! From the looks I would guess they are:
-- Tweeter: Scan-Speak Revelator D2905/990000 (off-center)
-- Woofers: 2x Vifa C17WH0908 (possibly significantly modified by the manufacturer) in MTM configuration.

LineSource 8th August 2011 10:47 PM

1 Attachment(s)
Can you put the port on the bottom of the cabinet?

B&W wrote a white paper discussing the advantages of putting the port on bottom of the cabinet, and shaping a base plate to block the port exit air from the rear of the cabinet. The attached picture shows the triangular space of the B&W bottom area that pushes the port air mainly to the front, with some side output, but a blocked rear output. The solid base plate assures constant output on hard floors or on carpet.

B&W claims a bottom port:
1) minimizes the amount of midrange energy that escapes from the port into the room. This helps the soundstage clarity.
2) gives a uniform and maximum floor gain
3) allows a long port tube based upon the cabinet height and not depth
4) avoids an "ugly" port hole on the front of a cabinet
5) allows a large radius diameter on the port edges
6) on some speakers, B&W puts the xover components in this base wedge

I have seen several new speakers that use the B&W bottom port ideas.

bbggg 8th August 2011 11:41 PM

At port resonance, wavelengths are too long to vary with port position on a box, unless your speaker is 40-13=27 inches deep. My hunch is that your problem has to do with room modes.

vacuphile 9th August 2011 12:01 AM

Quote Aleph5: Distance from wall is 13" and I've discovered that at 40" the problem is solved (but I must keep them at 13").

Why not have the vent on the front?

DaveG 9th August 2011 12:19 AM

what happens if you put a couple of sofa cushions behind the speakers?

Cal Weldon 9th August 2011 12:56 AM

Or a loosely/slightly snug sock in the ports if the cushions aren't enough?

bbggg 9th August 2011 02:12 AM

Or a rolled towel for that matter, although the alignment (Q and rolloff) will change drastically.

boris81 9th August 2011 02:56 AM

Quote:

Originally Posted by bbggg (Post 2665352)
At port resonance, wavelengths are too long to vary with port position on a box, unless your speaker is 40-13=27 inches deep. My hunch is that your problem has to do with room modes.

I suspect bbggg is right, your problem is probably caused by the wave going to the wall, then being reflected and traveling back in time to combine (or cancel) with the wave coming out of the speaker. The location of the port doesn't matter because bass frequencies spread out uniformly in all directions.

It's not an easy problem to solve but well placed EQ might be the most practical solution.

Cal Weldon 9th August 2011 03:40 AM

Quote:

Originally Posted by bbggg (Post 2665459)
Or a rolled towel for that matter, although the alignment (Q and rolloff) will change drastically.

Loosely rolled sock so as to be considered aperiodic. Nothing drastic like blocking the port.

whgeiger 9th August 2011 08:00 AM

Wall Deflector
 
1 Attachment(s)
Quote:

Originally Posted by Aleph 5 (Post 2665123)
I have these fine speakers that are not working in my room because of the small distance from the back wall (distance to side walls is huge).
I am getting excessive boominess and congestion. :(

My quest is to solve this poblem without increasing the distance from speakers to wall. Do you have any suggestions? :cool:
If necessary I can change the port, use accoustic materials on the wall behind the speakers but I cannot increase the distance from wall.

Data:
- Distance from wall is 13" and I've discovered that at 40" the problem is solved (but I must keep them at 13").
- Rear port is vertically at the center of the speaker, from my calculations Fb=46Hz.
- The design of these speakers has to be outstanding because they have fabulous integration and magic midband while using modest drivers! From the looks I would guess they are:
-- Tweeter: Scan-Speak Revelator D2905/990000 (off-center)
-- Woofers: 2x Vifa C17WH0908 (possibly significantly modified by the manufacturer) in MTM configuration.


Try this. What comes out of the port are 'bullets of air'; so, just deflect them!

See attached sketch.

Regards,

WHG


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