Soundstage

[JohnA]

Just a thought here...I thought part of the magic of the ML was its dipole design..

That's what I thought as well

By deadening the front wall - aren't we sort of defeating that?

That's what I thought as well

I guess what I am asking is - those of you that have done this - have you gained image - but lost transparency?

I've tried different angles on all planes, and various thickness of absorbing matterial behind them: The more I added, the better the image AND the transparency.
I've now got curtains to hide the 4" thick acoustic foam behind the speakers.
Aside from the loss of efficiency (the power amps work harder for the same volume) it is all positive acoustically.

 

[JonFo]

… With the bi folds in place , the rear wave is dispersed into the room instead of hitting the wall behind the stator and bouncing back into the stator mucking up the sound, and beleive me muck up the sound it does without the bi folds …

Moon, that’s an elegant trick, and as you note very effective at ensuring the rear wave does not propagate back through the panel.

So for those who want to preserve the volume and general ‘sound’ of dipoles, then this is a great way to mitigate some of the downsides of the rear wave.
It still does not deliver the smoother frequency response of rear wave absorption, but sure fixes the worst problems of wall bounce.

Also, since part of the bi-fold reflects some of the sound back between the speakers, it creates a different type of ‘soundstage’ as it’s as if you had an additional set of speakers sitting behind the ML’s (at the bifold surface) running a slightly delayed, out-of-phase signal. This, interestingly enough, is part of what some ‘ambience’ sound fields do to play their tricks on regular speakers.

 

[MOON]

Jon Fo,

That is a good explanation of the benefits of the bifolds. The original article is in the March 2001 Stereophile in the fine tunes section. The editor noted at the time that he had slatted blinds behind his Prodigy's from floor to ceiling angled back as they went up. If anyone wants to read the original article, I can post it here. It is interesting reading.

The bi folds are not expensive, available at any home center. They come in various widths. I am using the largest, with each side measuring 18 inches wide. the bi fold tranversing the corner on a 45% is not as wide.

 

[JohnA]

The bifolds are a neat trick (if the room can accomodate them of course)

The ones in the picture, are they see-through? I mean can air get through, or are they just a grooved panel?
I might prefer them to allow some air to pass, so some energy could get through and then after reflecting off the wall get trapped between blinds and wall, taming the sound a bit more (bit how some bass traps work)

 

[MOON]

John,

The slatted bi fold doors do allow some amount of sound to pass through them, although the gaps are small between the slats. The slats are on about a 45% angle. If you go to the home depot they have them in the door section.

It will speed the finishing process up if you use white spray primmer. This is in an arisol can which you can spray directly on and then paint them your wall color afterwards after the primmer coat has dried.I have thought also as you did about putting some absorbtion behind the slatted doors. I have treatments for 1st and 2nd reflection points as well as ASC corner traps, 2x4 ft absorbtion panels for the rear wall behind the listening position and Micheal Green room tunes as well going into the new room. I want to see what all the new treatment does first before considering adding absorbtion material behind the slatted doors.

Cheers

 

[timm]

That's what I thought as well



That's what I thought as well



I've tried different angles on all planes, and various thickness of absorbing matterial behind them: The more I added, the better the image AND the transparency.
I've now got curtains to hide the 4" thick acoustic foam behind the speakers.
Aside from the loss of efficiency (the power amps work harder for the same volume) it is all positive acoustically.

Well this is an excellent thread... You know when all of a sudden you 'get it'. I've seen threads like this before here - but this was really helpful to me... thanks to all for the info....

Do we typically put something directly behind the speakers? A lot of the setups I have seen put things between them....

 

[JonFo]

Here's another idea for something one could put behind the speakers. A segmented acoustic panel system that can be arranged in a more flowing position than foling doors .

The SilentFold panels:

 

[JonFo]

Well this is an excellent thread... You know when all of a sudden you 'get it'. I've seen threads like this before here - but this was really helpful to me... thanks to all for the info....

Do we typically put something directly behind the speakers? A lot of the setups I have seen put things between them....

Timm, the worst thing for a MartinLogan is to have the rear wave bounce right back at the panel. Most people toe-in their speakers so that the rear wave tends to bounce away from the speaker, but since there is usually a side wall nearby, the rear wave arrives at around 15ms or less as the front wave, which muddies the sound (through comb filtering and through positional cues).

Many of us find that doing something to better manage the rear wave helps not just soundstage, but tonal balance as well.

As for stuff between, it will affect the sound if the room had reflective walls between the speakers before the new object is placed there (like adding a TV). But if the walls are treated, the variance will be less noticeable, as the majority of the sound will be perceived to be arriving from the front of the speakers, since less of the reverberant sound field from the rear wave is present.