Does a Stepped Front Wall Adulterate the Rear Wave From a Dipole Speaker?

[Ron Resnick]

I am seeking replies based on direct experience and educated theories regarding the reflection off the front wall of a rear wave launched from the rear of a dipole speaker.

Please reply if and only if you:

1) own or have owned or have had significant, long-term experience with dipole electrostatic speakers or dipole ribbon speakers; and

2) believe in not absorbing the back wave of the dipole speakers (I do not want a debate on the merits of absorbing the back wave).

How figurative or literal are we when we describe the back wave reflecting off the front wall? What is the nature of this wave?

If we could dye the backwave with a color would we see a clean and uniform wave bouncing off the front wall and retaining its waveform -- would it look like a coherent waveform? Or does the wave break up when it hits the front wall and is reflected off the front wall in "pieces"?


This is a photo of the front wall of my future listening room:






The left side of the front wall (about 42" wide) sticks out from the rest of the front wall by about four inches. You can see the side profile of the four inch step here:






Please assume the dipole ribbon speaker will be about 8 feet in front of the front wall.

Please assume the middle of the left speaker driver will be directly in front of the 4" step. (The back of the ribbon driver will pretty much be aiming right at that 4 inch corner sticking out.) I do not absorb the back wave of dipole speakers. (Again, I do not wish to debate that point here.)

Part of the rear wave of the speaker will hit the white wall (which is not stepped out by four inches), and part of the rear wave of the speaker will hit the section on the left (which protrudes from the rest of the front wall by four inches).

I am concerned about the back wave of the left speaker becoming adulterated by the different depths of the front wall right behind it.

What will be the effect of the four inch step on the rear wave when the rear wave hits it?

Will the step "break up" the rear wave?

Will the step adulterate in some way the rear wave and prevent a coherent waveform from being reflected off of the front wall and towards the listening position?

Thank you!

 

[thedudeabides]

Hi Ron,

Stat owner for 30 years plus with very passive "refractive" treatment used during this time. If your speakers are eight feet out from the wall, I find it hard to believe that the 4" offset would have any audible timing smear effect on the planar back wave. Much more likely to hear the impact of the wall "surface" than anything else.

Best and good luck.

 

[Brucemck2]

I had big Infinity panels for many years. Russ Herschelman designed a wall where it was easy to add and subtract 2’x2’ RPG absorbers and Skyline diffusers. I ended up with 4” thick absorbers along the floor, ceiling, and vertical wall junctions and Skylines covering the rest of the surface. In that room that sounded considerably better than a bare wall or a largely absorbent wall.

My working hypothesis was that the corner junctions propagated irregularly directed/focused waves, while the diffusers propagated “uniform” waves back into the room.

 

[the sound of Tao]

Ron,
You are not really going to know exactly how your room is going to respond to your speakers for sure until you are in there playing tunes with them.

That said, at the level you are playing at I’d suggest that your capacity to modify the sense of presence in your setup may involve playing with your treatment on that wall behind the Gryphons. I had a slight asymmetry that required just a slight shift so I couldn’t just mirror the speakers.

It may not be as obvious how critical this will be or how to respond until you get the dialling in of the big Gryphons exactly right.

If my experiences with setting up dipole radiation patterns in a near field setup I found that a slightly diffused treatment of the reflections from the front wall are really critical for the last umpteenth of a degree of speaker whispering. The timing of that delay between direct and reflected gives your hearing perceptions the vital second bite at an associated sound that makes tonality seem so much more immediately obvious and right.

Dialing in dipoles is quite a joyous learning curve and once you get a handle on it you feel so much more comfortable with dialling in any other speaker type. I was lucky and was mentored in this by the guy who is the Magnepan distributor for Australia and who has been dialling in various Maggie setups for a gazillion years (perhaps a slight exaggeration) but he is truly a master at it. Between burn in and then ultimately dialling in to the point where no more doubt lingered took me quite a while... maybe 6 to 8 months before I felt it was exact but it is so good when you get it there. We are talking millimetres difference between almost right and just right. Ribbons return on investment are great when you give them the exact exactness they need. You’ll love the feedback that your next system will give you. Enjoy the setup and refinement process that a system with the pedigree you are putting together will likely give you. Dialling in with a ribbon dipole in play is a rewarding thing. But what is behind the speaker is every bit as critical as what is in front of it IME if you are chasing the best from a dipole.

 

[JackD201]

Hi Ron

It shouldn't be a problem but if it bothers you visually then just go for symmetry and build up on the right side. Shouldn't cost much.

 

[VT Skier]

Why not ask Mr. Rasmussen of Gryphon directly? He may also be able to give you contact info for other Gryphon owners who could share their acoustical treatment experiences.

 

[Kingsrule]

Hi Ron

It shouldn't be a problem but if it bothers you visually then just go for symmetry and build up on the right side. Shouldn't cost much.

This....It will always be an issue even if it isn't one.........

 

[Ron Resnick]

Why not ask Mr. Rasmussen of Gryphon directly? He may also be able to give you contact info for other Gryphon owners who could share their acoustical treatment experiences.

Thank you, but this is an extremely narrow, idiosyncratic and esoteric question the answer to which -- whatever that may be -- applies to the back wave of all dipole speakers. The question is in no way unique to the Pendragons.

I can imagine that the only person who could knowledgeably answer this question accurately is some physicist in a university technology lab who happens to work on analysing the precise shape of audio frequency waves, and the nature of their shape and coherency upon reflection off of different types of wall designs.

 

[Ron Resnick]

Hi Ron

It shouldn't be a problem but if it bothers you visually then just go for symmetry and build up on the right side. Shouldn't cost much.

Dear Jack,

If I knew that the answer to my question were "the step does not distort the reflecting wave" then the step would not bother me at all visually.

If the step does distort the reflecting wave in some way, then adding the symmetrical step on the right side would simply distort the right channel back-waves as well, no?

 

[astrotoy]

Hi Ron, have you considered hiring an acoustician - who may also help with any other problems in the room. I know that Steve went that route when he moved back to SoCal. Also, Maier Shadi may have some advice about wall treatments. I think he did a great job for his main room at his shop in Santa Monica. Larry

 

[Kcin]

I've been dipole user for 30+ years. Many types of amps driving them: OTL, SET (believe it or not), SS etc.

Since you are not looking for a debate on the back wave and absorption, I think what Jack suggests in post #5 is reasonable- shim the remaining wall to the right to match the existing step and have one plane. Your issue real or not is solved--- this is not adding another step to the far right to match; this is shimming to carry on 1 plane straight across.

I like symmetry or straight lines.. and I have lost space in rooms to ensure this symmetry to account for construction features that could not be modified (structural beams, or chases such as yours).

Make it go away- probably cheaper that measuring it in and out with an acoustician. In the end, everything matters, how much it matters is the issue. If you "remove the situation" you won't be concerned with it any longer. YMMV and all that. Have fun.

 

[Ron Resnick]

Yes, I am in contact with two acousticians.

But, on this particular question, I am not looking for an educated guess. I am looking for a definitive scientific answer.

 

[sbo6]

Between the <5msec delay for 4" plus scattering multiple times / attenuated plus your advantage of having speakers out 8' out from the front wall there will be virtually no affect. With my dipoles I had diffusion on the front wall behind the dipole; you could put even diffusion panels covering and minimizing the 4" depth difference.

 

[Ron Resnick]

Thank you, gentlemen, but I am curious about the exact shape of the audio waveform, and how the shape of that wave behaves when it hits the 4" step-out.

 

[microstrip]

Yes, I am in contact with two acousticians.

But, on this particular question, I am not looking for an educated guess. I am looking for a definitive scientific answer.

Although it is only an educated guess, if one of my Soundlabs was 4" in front of the other it would compromise the sound in the whole room - not just in the listening position. It happened by mistake and affected the seamless of the big sound wall created by the speakers. But my 31 feet long room has RPG diffractal diffusers in the front wall, and Soundlab panels are very large full range dipoles.

I would consider equalizing this distance using diffusers. High reflective front wall creates high Q nulls by cancellation, that probably highlight this asymmetry. But surely an acoustician should know better than me ...

 

[JackD201]

Thank you, gentlemen, but I am curious about the exact shape of the audio waveform, and how the shape of that wave behaves when it hits the 4" step-out.

Hi Ron the difference in amplitude will be infinitesimal. The 4" will be centered around 3.5k. The level difference will be a less than 1/10th of a dB between the two planes while the transition will be diffracted. Tonal difference? I would say no. Enough to cause an image shift? Maybe. One that one probably wouldn't notice or can be adjusted for speaker placement alone? Yes. Will just looking at that 4" bother the hell out of me? You bet. Visually, I like symmetry. If the look of it bothers you deal with the look even if you wouldn't likely hear the difference of could acoustically deal with it. At the end of the day we invest in our systems and rooms to remove stress and not create it That would be my advice.

 

[microstrip]

Although it is only an educated guess, if one of my Soundlabs was 4" in front of the other it would compromise the sound in the whole room - not just in the listening position. It happened by mistake and affected the seamless of the big sound wall created by the speakers. But my 31 feet long room has RPG diffractal diffusers in the front wall, and Soundlab panels are very large full range dipoles.

I would consider equalizing this distance using diffusers. High reflective front wall creates high Q nulls by cancellation, that probably highlight this asymmetry. But surely an acoustician should know better than me ...

Just one point - I am not saying that such room will not sound good - I have listened to great sound in symmetrical rooms. But probably the fine tuning of speakers will be much harder to execute.

 

[Ron Resnick]

Although it is only an educated guess, if one of my Soundlabs was 4" in front of the other it would compromise the sound in the whole room - not just in the listening position. It happened by mistake and affected the seamless of the big sound wall created by the speakers. But my 31 feet long room has RPG diffractal diffusers in the front wall, and Soundlab panels are very large full range dipoles.

I would consider equalizing this distance using diffusers. High reflective front wall creates high Q nulls by cancellation, that probably highlight this asymmetry. But surely an acoustician should know better than me ...

Dear Francisco,

Thank you for your post. The speakers will be on the same line; one speaker will not be 4 inches closer to the listening position than the other speaker.

 

[microstrip]

Dear Francisco,

Thank you for your post. The speakers will be on the same line; one speaker will not be 4 inches closer to the listening position than the other speaker.

I know, it is why I clearly wrote "it would compromise the sound in the whole room - not just in the listening position" . In a dipole a significant part of the sound will be sound coming from the reflective front wall and will be added to the sound coming from the front of the dipole. As phases will be different due to 2 x4" different length path, we can expect significant differences in the patterns of interference of both speakers direct and reflected sound. A pity my SoundLabs are in storage - I would try measuring them with a 4" difference in position.

This is science - but perhaps other effects, such as global diffusion will mask this difference!

I remember I could slightly move the Aida's with little change in the measurement, but any small change in Soundlab's position changed significantly the dips and peaks in the midrange.

 

[Mark Seaton]

Thank you, gentlemen, but I am curious about the exact shape of the audio waveform, and how the shape of that wave behaves when it hits the 4" step-out.

Actual sound radiation or theoretical ideal the audiophile community clings to with line sources? Yes, that was meant to be sarcastic. Fortunately with the tall, dipole towers you have it has more in common with the conceptual ideal than not.

Two important points of understanding and consideration:

1. 4" is 1 wavelength at ~ 3400Hz. Things cancel when the path difference is 1/2 wavelength, or around 1700Hz. It is usually a reflection that is 1/4 wavelength behind/forward that creates a 1/2 wavelength path difference (there and back) which is around 850Hz. Below 400Hz the wall is acoustically flat and the 4" bump out has no effect.

2. Your dipole columns mostly produce a wide/round figure 8 of sound front and rear when viewed overhead, and that figure 8 radiates in mostly a partial cylinder front and rear, with very little sound in the plane of the speaker baffle (straight up or 90 deg from on axis horizontally.

The only real concern is that energy hitting the stepped forward area will come back with about an 8" head start on the rest of the wall, or about 0.6 ms sooner than the energy from the other side as the sound has to travel to the further surface and back. If it happened to be significant enough to be audible, it would pull the image toward the 4" step forward. With the step only 4" deep, the diffraction of the speaker's rear wave hitting the break in the wall will really only create a diffraction re-radiation at higher frequencies above that 1700-3400Hz range. Fortunately the dipole column of your speaker towers greatly reduces energy hitting the upper ceiling asymmetries so their impact will be modest until much lower frequencies, where the tall column of woofers will help some.

Like others have stated, I think the psychological/visual aspect might be more significant than the audible impact.