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Old 9th August 2011, 10:44 AM   #11
bbggg is offline bbggg  United States
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One of the issues in such cases is that port output takes a while to decay. If it coincides with a room mode, this mode is more likely to be excited by the port's resonant tail. In such cases, placement at odd fractions along the mode axis (1/3, 1/5) can ameliorate the problem; even fractions (1/2, 1/4) will exacerbate it.
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Old 9th August 2011, 10:49 AM   #12
bbggg is offline bbggg  United States
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Quote:
Originally Posted by Aleph 5 View Post
Rear port is vertically at the center of the speaker
I'm not quite sure about this - see above.
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Old 9th August 2011, 11:33 AM   #13
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Quote:
Originally Posted by Aleph 5 View Post
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?
If necessary I can change the port, use accoustic materials on the wall behind the speakers but I cannot increase the distance from wall.
This is a very typical interaction between speaker location and wall boundaries. As you get closer to the wall the bass is augmented by the boundaries to a higher (midbass) frequency. Location of the port on your cabinet is not the issue. Proximity of the system to the back wall is.

Damping the wall will not help you because you won't want to take the measures required to absorb the lowest frequencies. This would require severel feet deep treatment over a large area behind the systems. Any less than this won't make neough difference to notice.

The easiest approach is to buy an equalizer and reduce the level of the boom frequencies.

David S.
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Old 9th August 2011, 10:09 PM   #14
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First some questions that haven't been asked -

This is a DIY Forum, are you building these speakers, or did you buy them? We will still help either way, but it is nice to know.

Next, give us the details of the design - Drivers, Cabinet Volume, cabinet shape and dimensions, cabinet/port tuning, how many ports and what size, etc...

Next, if the cabinet still in the 'test' wood/prototype stage where modifications can be made, or is it already in the final finished wood stage?

You can play with the cabinet damping material, adding more or less to tune the bass. You can also play with the cabinet resonance tuning, moving it up or down; though likely up would be best. Both are easy enough to do.

Also, you could reduce the interior volume of the speaker, and re-tune the cabinets to match the new volume. That should lower the efficiency of the bass section of the speaker.

You can also consider not having a port. Perhaps you will get sufficient bass response with a sealed cabinet.

Again, so far we have "speaker", it would be nice to know all the details to see where the design could be improved or changed. Also, a description of the room itself, and the placement of the speaker in the room, and the relative location of the prime seating area.

Finally, what equipment do you have to test the speakers? Computer? SPL Meter? What????

In all honesty, I think we have far far far too few details to be speculating on anything.

Steve/bluewizard

Last edited by BlueWizard; 9th August 2011 at 10:13 PM.
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Old 11th August 2011, 05:52 PM   #15
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Thanks for all replies. Meanwhile I have implemented some of the easiest ideas.

I have bought these great speakers. The manufacturer is Living Voice. They played perfeclty in the audition room, still they were placed at about 1.5 meters from the rear wall. They were not boomy and not congested.

So, the challenge is to reach the same effect at a distance of only 13" to the wall. Maybe it's not possible, due to the nature of the speaker... I mean they may have not been designed to play near a wall.

All the data:
- 2-way MTM design in regular box vented with reat port.
- Enclosure: should be about 30L internal volume (may be slighty different depending on internal bracing).
- Rear port (mid located) is approx. 7cm diameter x 12cm long. Using WinISD I figure the port should be should be tuned at about 45-48Hz.
- Tweeter: Scan-Speak Revelator D2905/990000
- Woofers: Seem like 2x Vifa C17WH0908. Probably they were significantly modified by the manufacturer.
- Must open them to confirm, but by looking through the port they do not seem to have internal acoustic materials. So this means there is a margin to modify them here.

- Room is a perfect square (25x25ft) and listening position is at the exact center of the room, while speakers are close to one of the walls (at 13" distance). I use an absorver (a carpet) on the middle of the wall opposite to the speakers. All corners are clear.

- Port location -
To modify the port location is not an option. I agree with the advantages of using a port like B&W but if the speakers do not work in my room I will have to sell them and that modification would affect their value.

- Sofa cushions/deflector -
If placed near the wall, effect is almost none. If placed close to the speaker, working as a deflector, it improves a little but it's far from perfect.

- Roll towel/sbug snock -
If tight (like almost eliminating the port) the problem improves a lot but at two costs: lower dynamics and an apparent change of the overall nature of the sound. If loose, it's still boomy/congested but better than without it. Do you know of other materials I could try here?

- Sealed -
In this mode I feel an important lack of bass and dynamics, although the problem is gone. Assuming the drivers stay close to the Vifa's specs, according to WinISD, EBP ~122/recommended for vented. Do you think a sealed solution could work if I added a subwoofer?

- Accoustic Material -
I have another pair of speakers with rear port that work well near the wall that use foam (sponge) on the internal surfaces plus wadding to fill the rest of the volume. Do you think this is worth the try? Which should I buy - foam, wadding or both?

I will make more tests and plug a Behringer DEQ to do simulations and use RTA.
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Old 11th August 2011, 06:21 PM   #16
bbggg is offline bbggg  United States
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Quote:
Originally Posted by Aleph 5 View Post
Room is a perfect square (25x25ft) and listening position is at the exact center of the room
This is too bad. You had best invest in a room analyzer.
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Old 11th August 2011, 06:51 PM   #17
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Originally Posted by bbggg View Post
This is too bad. You had best invest in a room analyzer.
Which one is bad? The fact of being a square shape room (and not a rectangle) or the fact of the listening position is in the center of the room? (or both )

As a room analyzer, do you mean something like this:

Room Analyzer - XTZ | Sound in balance
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Old 11th August 2011, 07:19 PM   #18
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Quote:
Originally Posted by Aleph 5 View Post
- Roll towel/sbug snock -
If tight (like almost eliminating the port) the problem improves a lot but at two costs: lower dynamics and an apparent change of the overall nature of the sound. If loose, it's still boomy/congested but better than without it. Do you know of other materials I could try here?
Not really. It was worth a try as it was so easy to implement. If you are not happy with either tight or loose then I will jump on the speaker dave band wagon and suggest the next easiest is to EQ the boom out of it.
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Old 11th August 2011, 11:39 PM   #19
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As an experiment, try moving your speaker forward to 18", then at 24", then at 36".

See if you can find a location where the sound is tolerable, even if it is not practical in everyday use. That is a least a starting point.

In a 25 ft room, I have a hard time believing that you are forced to keep your speaker 13" from the wall.

Next, I'm curious about the midrange. Is this clear and distinct, and does it improve when you plug the ports. I had some bass heavy JBL, and at 12" the rear reflections totally messes up the not that great midrange of the JBL Stadium. Plugging the ports improved this dramatically.

I think these JBL, which were already bass heavy speaker would need a minimum of 24" to sound right. Certainly 18" was not nearly enough. And I question where 24" would have been ideal or just tolerable.

Steve/bluewizard
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Old 12th August 2011, 12:46 AM   #20
godfrey is offline godfrey  South Africa
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How are the speakers arranged?
e.g. Green may be better than blue (or not)
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