Statements positioning: Why cant they be placed closer to the rear wall

[tommarra]

I am a newbie to DIY Audio...and was planning to build the statements (based on the terrific reception by those who have built it and for the well..the visual statements).


Stalking the forum for a few months now, I have realized that these need to be placed approximately 18 inches from the rear wall.

Can someone please explain who do they need this much distance from the wall.

I understand that the mids are open back transmission line, but what does it mean in terms of sound and placement.

Thanks in advance!

[impala454]

I think there are plenty on here that could give a much more technical explanation, but as a Statements builder/owner, I'll give it a shot. the mids are open backed, which means the design is combining both the sound waves facing forwards, and those that go backwards, bounce off the wall, and come back at you. If you have the speaker too close to the wall (or blocked in some way), those backwards firing waves will not bounce back forwards and combine properly, and you'll be missing a large part of the overall sound.

[tommarra]

the mids are open backed, which means the design is combining both the sound waves facing forwards, and those that go backwards, bounce off the wall, and come back at you. If you have the speaker too close to the wall (or blocked in some way), those backwards firing waves will not bounce back forwards and combine properly, and you'll be missing a large part of the overall sound.

Thanks for your reply Check - so by placing the speaker close to the wall the cabinet itself would block the reflections from the back wall.

Now if I had to place it closer could one add some sort of reflector (e.g. a Convex Mirror) on the wall behind the mid transmission line - that would both diffuse and reflect the sound forward?

[kbzx35]

let me disclaimer first that I'm in no means a guru of diy audio but a newb as well. I think its safe to say that trying to compensate the design of the statements by adding anything to try to mimic its design intentions in an unsuitable environment will only be exponentially more difficult. A major portion of obtaining great sound is to find or design a speaker that would match the environment it will be in. For me, the statements would be out of the question as I have to place my speakers in a large wall cavity (media niche). placing those speakers in my niche will most likely create a lot of unwanted resonances and smeared imaging creating a nightmare. In fact, even rear firing ports may be a no no for me as well due to boundary loading of the cavity. I think you will be more pleased finding a different design that is more suitable for your situation with wall placement.

Thanks for your reply Check - so by placing the speaker close to the wall the cabinet itself would block the reflections from the back wall.

Now if I had to place it closer could one add some sort of reflector (e.g. a Convex Mirror) on the wall behind the mid transmission line - that would both diffuse and reflect the sound forward?

[impala454]

Thanks for your reply Check - so by placing the speaker close to the wall the cabinet itself would block the reflections from the back wall.

Now if I had to place it closer could one add some sort of reflector (e.g. a Convex Mirror) on the wall behind the mid transmission line - that would both diffuse and reflect the sound forward?

It's not a matter of the wall being able to reflect the sound so much as the distance required for the waves to converge.

Allow me to speak from experience. I built four Statements Monitors and a Statement Center, for my small apartment. I can tell a pretty major difference in sound quality when I pull them all out away from the wall. Granted, a lot of the time I do push them back within about 12", and they still sound pretty good, but if you want them to sound great then you have to place them correctly. If I had it to do over again, I'd probably build the sealed full sized statements. But since I'm probably not building anything else until I get a house, I'd probably re-evaluate the situation then. My advice, if you're on the fence about the Statements, is to analyze your room a bit before choosing a design to build. If you absolutely can't place the Statements correctly, it'd probably be worth choosing another design. Just my $0.02.

[tommarra]

Thanks Chuck! This helps a lot. I totally get the need to place the Statements correctly.

I also built the Statement Monitors and Statement Center , but feel that the Monitors are lacking impact in some way (they do sound very good but I keep feeling I compromised by not building the full size statements.)

Therefore I am planning to build the full size speakers - knowing full well that in a year or so I will move to a bigger apartment / home.

As a stop gap arrangement I was hoping that if I put some type of diffusor / absorbing material on the wall behind the mid tunnels, I can get the same effect. But I guess since the problem is that the back-waves need the distance to converge with front waves, the diffusor is not going to help.

[impala454]

An absorbing material would hurt the sound big time, as you actually want the back waves to bounce off the wall.

[dlneubec]

There are likely several reasons for why they shouldn't be placed closer to the front wall. One would be that the design has been voiced at an optimum, minimum distance from the front wall and side walls, for that matter. Moving them closer is likely to make the open back output more prominent and could destroy that voicing balance.

Also, while the reflections are important to the soundstage enhancement that an open back mid offers, you want those refections to be delayed enough so as to not be confused by the ear with the first arrival, on axis sound. As you move the speaker closer to the wall, the arrival time of reflected sound gets closer to the arrival time of the on axis sound. If they arrive at your ear too closely together, then you tend to get a smearing of the sound, voices get less intelligible, etc.

[tommarra]

Also, while the reflections are important to the soundstage enhancement that an open back mid offers, you want those refections to be delayed enough so as to not be confused by the ear with the first arrival, on axis sound. As you move the speaker closer to the wall, the arrival time of reflected sound gets closer to the arrival time of the on axis sound. If they arrive at your ear too closely together, then you tend to get a smearing of the sound, voices get less intelligible, etc.

Got it! Now I see why we don't want to place open back near the wall - the enhanced sound stage of the Statements series was what drew me into building the Statement Monitors.

Wonder if the crossover can be modified (like the monitors near wall configuration) to enable the statements to work a bit closer to the call.

[dlneubec]

I'm not sure what all Curt and Jim did with the near wall version of the monitors. I'd guess they probably either changed the voicing to adjust for the increased effect of back reflections, stuffed the rear chamber more, or a combination of both. Perhaps you could move the Statements closer to the front wall if you were willing to play with the rear tunnels by adding some stuffing as needed to reduce the output a bit, though you may have to do that by ear. That would necessarily alter the soundstage a bit.

[Jim Holtz]

Dan is giving good advice. If you want to place Statements Monitors closer to the wall, build the near/far wall version with a switch and you can position either close or far away from the wall with the proper voicing at the flip of a switch. The soundstage is more expansive with the far wall version but the near wall sounds darn good too.

Check Curt's website for all the crossovers.

Jim

[J-Dub]

I've built the mini statements (modified a bit) and they are everything they say they are. Incredible!

No matter what speaker you choose to build you should pay attention to the design. Two key terms for you to look up. "Full Space" and "Half Space." These terms describe how speakers are designed in relation to how they will be positioned at the wall. On/In the wall or off the wall.

I will also attempt to help you with how the statements must be 18" off the wall. Being open mid transmission line, these are completely different than any sealed design in relation to Full or Half space. Another key term to look up is "Phase." Sound travels in waves. the relation of one wave to an identical one in the same space is phase. Frequency is relative to distance. I don't have the book in front of me right now so use this next sentence solely as an example. A 20Hz wavelength will be approximately 56' long. A 10,000Hz wavelength will be around an inch. Now back to the statements, A transmission line is also known as a "quarter wave tube" Essentially it does what it says. The sound passes through a tube roughly 1/4 the length of the fundamental frequency only to catch up with the sound from the front of the speaker 90degrees or 1/4 out of phase. This supports the front waves doing many things but i'll let you look up "Kings work on tranmission lines and MLQWT's" for yourself. (tough read if you don't have a lot of experience)(and know your math) Once again back to the statements, The 18" off the wall has been calculated into the distance necessary to put the back wave 1/4 out of phase with the front. Sound waves exit the tube travel 18" inches to the wall, bounce off the wall, travel 18" back travel the distance of the rest of the cabinet and combine with the front waves 90 degrees out of pahse. Any derivation forward or backward will only hurt the sound.

I'll leave you with one more gem to find and that is "The Master Handbook of Acoustics" by F. Alton Averest. Although it isnt a book on speaker building is is one of the most fundamental you will ever read regarding acoustics. Knowing these things will help you to understand other designs and begin to design your own in a predictable yet creative manner. I hope this helps.

[tommarra]

@ Dan: THanks for the tip. Just to put your suggestion into practice, I played around with the stuffing in the Statement Monitors (near wall configuration) I have. I started by taking out all the foam in the mid tunnel, which brought the mids forward (aggressive..dont know what the exact word is :-) ). Taking all the foam out also made the mid a bit garbled. So I started sliding some foam in the tunnel, and finally at about 2.5 inch from the Tangband and about 0.5 inch from the rear baffle, i found the sweet spot for the monitors, which are placed at about 14 inches from the wall. Moving the speakers farther than 14 inches did not make a discernible impact to me on the sound.

So thanks for the tip, it was really very helpful.

@Jim: First off, i must thank you and Curt for the awesome designs that the Statement Series are. I already have the statement monitors, the center channel and now trying to complete my HT with the full-size statements. The Statement Monitors replaced B&W 805 monitors and I haven't once regretted the decision.



@J-Dub: This is great great information. Thank you for explaining to me the quarter wave transmission line. This makes the physics behind the 18" distance requirement very clear to me. I will definitely read up on the two sources you mentioned.


Thanks everyone for the information. This was great help.