Thought for the day. 'Rear-field' clutter. How important?

[Zoom 373]

I've owned a pair of DCM-TF350s since the early 90's.  Probably my second set of nice speakers I've ever owned.  I still have them.  Vifa drivers with rubber surrounds.  Maintenance free and nice, tight, and clear with transmission line ports for even bass extension.

 

So,about 2001, when offered a chance at a set of TF-600s, the bigger brother, I jumped.  They have a pair of front firing Vifas, and three tweeters,  2 time-aligned diagonally to the rear/side and one forward. I got them home, set them up in a shitty sloped roof room, with lots of interference, and they sounded like shit.  I couldn't figure them out.  I was severely disappointed. 

Until I moved.  They went first.  Into an empty room, fed by an old JVC personal disc player, earphone to rca input to a ... a PM-1200 I hadn't even used yet, but knew I needed a couple for the new to me concrete floor.

So, JVC personal disc player input to PM-1200 output to a pair of DCM TF -600s.  All alone in a room.

They ROCKED.  filled the whole damn room.  Sounded excellent.  Unbelieveable.  They are currently my best set of fronts for critical listening, fed by my M-500t.

 

So, my question is,

For the rear-firing equipped speaker, and for any dipole/flat panel, infinite baffle, how important is 'Rear-field' contamination, and how should it be set up?  Are there requirements with new dipoles for rear reflection surfaces?  Any proof?  

 

Along with my DCMs, and many others, the RS-IIs Bean is picking up have rear firing drivers.

This question came to me especially hard after I read the thread with the link from Phil about the designs of the Dahlquists.  And how the engineer talked about resonance from instruments going 360 degrees.  I thought, 'Wow, he's right'  But if you are trying to throw an audio signal rearward to simulate rear ward resonance, what happens to that rear signal if there is rig in the way?

 

Makes ya wanna go hmmmmmmmm

 

 

[Bean 384]

Ideally there shouldn't be anything between the speakers.... notice the quotes around ideally.  This is true for most stereo speakers such as Carver AL-III's, Magnepans, etc....

 

But ideally and reality dont always meet... 

 

So keeping as much stuff out of the way as you can is good.  Or pull the speakers out where they are NOT flush with the system if possible.  I honestly am not sure how I am going to set mine up just yet.

 

I have a "plan" but until I get them in the room I wont really know.  I am hoping I can minimize things keeping my gear a bit behind the speakers but again not 100% sure...  I think in my case as long as I can keep the gear at or below the height of the rear firing tweeter I will still get a large part of that wrap around effect...

 

Ideally I would break into the closet behind my setup and build an enclosure that comes into the room but is flush with the wall.

 

I may still do that eventually lol......  I also need to investigate the linen closet in the bathroom thats behind that wall to see where it falls.  If I can take some of that for a rack, I probably will but again I wont know w/o drilling a tiny little hole lol....

 

while my wife isnt around  

[dazed_and_confused 194]

Hmmmm...

 

Dipoles radiate in a figure of 8 pattern, so they don't interact as much with the side-walls as a monopole arrangement. So the two critical waves are the direct wave and the rear reflected or reverberant wave. (A slight over-simplification).

 

So at the listening seat what you hear is the direct wave and the rear reflected wave combined. If there's sufficient time delay between the two, your ear/brain is able to separate the arrival of the two waves and mostly ignores the rear reflected wave which is arriving later. If there's insufficient time delay in the arrival of the waves then the affect is that certain frequencies overlap and you get a smearing and/or cancellation effect.

This is why you need to get a dipole out into the room, to give sufficient time difference for the arrival of the two waves. This is more important where you have a hard reflective surface behind the speaker, which does little to break up that rear wave.

If you can't get the speakers out into the room then you can 'treat' the back wall either with absorption or diffraction/reflection or a combination. Clutter behind the speakers will act as diffraction and maybe absorption, depending on the material, so it can be helpful where the speaker cannot be pulled out into the room. 

 

Unfortunately, clutter is unpredictable and you don't really know what it's doing. It could be absorbing/diffracting more of the rear wave from the right speaker than the left, so it can seriously throw off the soundstage and positioning of instruments/performers within the stage.

 

So personally I would remove clutter and try to do a more controlled correction. Some dipole speaker manufacturers make rear wave diffractors specially for these situations. SoundLab have a product called a 'SALLIE' which sits behind the speaker and looks a bit like a louvered door only with bigger louvers....it directs the sound up/down and away from a direct reflection off the front wall (front wall is the wall behind the speaker in this case).

 

There are many DIY recipes for diffraction/absorption devices for treating that front wall. In olden days people used those egg carton trays, the 12x12  variety without the lid! You can plaster these over the wall and see what happens. Just think long and hard about what to tell the wife when she says 'what the fuck is that'.

 

What was the question again? 

 

Gotta run, there's a lot more detail to be had here, mine is just a quick pass... 

 

[dazed_and_confused 194]

ohhh.....here's a nice technical explanation along with math. http://www.linkwitzlab.com/rooms.htm

[AndrewJohn 10,467]

ohhh.....here's a nice technical explanation along with math. http://www.linkwitzlab.com/rooms.htm

 

Oh boy, good read.  I have some work to do! 

[Bean 384]

ohhh.....here's a nice technical explanation along with math. http://www.linkwitzlab.com/rooms.htm

 

too many words... bookmarked for later lol...... 

[PDR 1,194]

Before my big QRD on the front wall I used a poly diffusor.

It worked well with the dipoles......quick and easy to try.

 

Two ratchet straps and a $10.00 sheet of Masonite.

You can adjust the straps for more or less radius in the poly.

An afternoon while the wife is away will let you know if it

does what you want.

 

[Bean 384]

 

 

Before my big QRD on the front wall I used a poly diffusor.

It worked well with the dipoles......quick and easy to try.

 

Two ratchet straps and a $10.00 sheet of Masonite.

You can adjust the straps for more or less radius in the poly.

An afternoon while the wife is away will let you know if it

does what you want.

 

 

By any chance have any pics or links to what your talking about? 

[PDR 1,194]

This is the poly I had in my room.

It is on a wood frame, but you can make a temp one

with just a sheet of Masonite with two straps.

Like I say, just tighten or loosen the straps for a bigger or smaller radius.

That way you can customize to your room and taste.

Behind it you can stuff insulation to form a bass trap as well.

 

 

 

 

 

[dazed_and_confused 194]

Very chic and stylish, and you won't have to shell out too much...

 

 

In every room where I've had dipoles I've always found that leaving the front wall untreated and having the speakers well out into the room, sounds better than having any sort of random treatments on the wall. 

I went to the time and expense of making a bunch of treatment panels from acoustic foam, attaching 2x2 foam panels to 1/4" plyboard, and I always preferred the sound without the panels on the front wall.

You can see one of the panels in the picture below. I used them to deflect/absorb sound away from a custom crossover I made for my old Maggie 3.6's.......

 

 

You can see it more clearly here....with Bandit the cat (RIP)......note the windows behind the speakers and the distance from the wall...

 

 

 

 

 The image below, borrowed from here http://www.linkwitzlab.com/rooms.htm is telling - you can see how the energy coming from the front and rear is equal with a dipole or open baffle design and how the sideways energy is significantly less. (Pertinent to Carver Amazing owners too, where the cone woofers are open baff and act as dipoles mostly)

 

[Zoom 373]

 

 

This is the poly I had in my room.

It is on a wood frame, but you can make a temp one

with just a sheet of Masonite with two straps.

Like I say, just tighten or loosen the straps for a bigger or smaller radius.

That way you can customize to your room and taste.

Behind it you can stuff insulation to form a bass trap as well.

 

 

 

 

 

This -- is seriously -- kick ass.

Impossible, obviously. when integrating HT.

I wonder what my wife is gonna say when I tell her I need another room for my dipoles?  hehehehe.  Well, we already have cats and dogs, so negotiation in that respect is out.......

Those 3.6s are SWEET also, dac.  Wow.  Big Maggies are right near the top of my list.....

[Zoom 373]

OK.  lots of great information here....I'm picking on this post because I don't have the hang of multiple quotes, yet...

 

Hmmmm...

 

Dipoles radiate in a figure of 8 pattern, so they don't interact as much with the side-walls as a monopole arrangement. So the two critical waves are the direct wave and the rear reflected or reverberant wave. (A slight over-simplification).

 

So at the listening seat what you hear is the direct wave and the rear reflected wave combined. If there's sufficient time delay between the two, your ear/brain is able to separate the arrival of the two waves and mostly ignores the rear reflected wave which is arriving later. If there's insufficient time delay in the arrival of the waves then the affect is that certain frequencies overlap and you get a smearing and/or cancellation effect.

 

I buy into this 100% 

 

This is why you need to get a dipole out into the room, to give sufficient time difference for the arrival of the two waves. This is more important where you have a hard reflective surface behind the speaker, which does little to break up that rear wave.

If you can't get the speakers out into the room then you can 'treat' the back wall either with absorption or diffraction/reflection or a combination. Clutter behind the speakers will act as diffraction and maybe absorption, depending on the material, so it can be helpful where the speaker cannot be pulled out into the room. 

 

And this, they need some space for the rear reflection to delay enough that it sounds real, and doesn't produce cancellation waves to the forward wave.  I can see treating the rear wall, but only if you cannot produce enough space to allow a delay.  Because, isn't the very purpose of producing the rear signal to use it effectively, not diffract it.   ie. the only reason to try to lessen or diffract the rear sound wave, is if you don't have enough rear space.....Otherwise, why produce the rear wave in the first place?

 

Unfortunately, clutter is unpredictable and you don't really know what it's doing. It could be absorbing/diffracting more of the rear wave from the right speaker than the left, so it can seriously throw off the soundstage and positioning of instruments/performers within the stage.

____________________________________________________________________________________________

 My rear clutter, if I were to use a pair of flat panel speakers in my HT room, includes my rear stacks, my center TV/racks, and in the corners, bass cabs.  That's a lot of diffraction if I want a clean rear wall deflection from the rear driver/open baffle.

 

______________________________________________________________________________________________

 

So personally I would remove clutter and try to do a more controlled correction. Some dipole speaker manufacturers make rear wave diffractors specially for these situations. SoundLab have a product called a 'SALLIE' which sits behind the speaker and looks a bit like a louvered door only with bigger louvers....it directs the sound up/down and away from a direct reflection off the front wall (front wall is the wall behind the speaker in this case).

 

Gotta talk to the wife about freeing up another room.

 

[PDR 1,194]

The difference, for me in my room, with the QRD is something I'll not go without with the open baffles I own.

 

I was lucky in the fact that I started with a empty dedicated room....

 

 

It made my system sound like a piece of crap. I barely listened in there till the treatments went up.

Started with a few, just to see.....

 

 

Then added more, including the poly and the ceiling absorption along with the bass traps....

 

 

The QRD just brought depth and width to the soundstage I didnt have before.....

 

 

But heres the thing.....when I place my arrays in the room......its not quite right, something sounds off.

I'm guessing the QRD would do much better on the back wall, than the front with these.....

Although when I use my GR speaks for the low end...dipole subs...it gets better 

 

 

I like what the diffusion does with the open baffles.

I think if your curious the temp poly might give you an idea of what diffusion behind your fronts

will do to your sound with dipole/bipole type. 

 

 

 

 

[dennismiller55 4,809]

That is a great room Perry.  I have always been impressed with your system and room treatments.  I am sure I have seen each of the pictures in past post, but it is great to see them in one place from beginning to now.  

[trav0810 586]

I would LOVE to be able to hear Perry's system. I have heard a lot of nice equipment, but never in a room that was specifically set up to optimize the system. Nice job Perry!

[AndrewJohn 10,467]

That is a very nice series to show the progression, Perry.  

 

My head was thinking how could I build something like this into my system..., and wondered if it was "guess" or calculated. 

 

Like a lot of things, some of this was over my head - so I pulled Google into service, and stumbled on this for a brief explanation of Quadratic Residue Diffusers (QRDs).

 

This might be interesting from the QRDude on the Subwoofer Builder site HERE. 

 

And, the QRDude has a software package that seems to run on my WIN 7 system called the QRD Calculator, and shows a few design variations..., I haven't finished playing with it yet - need to do a thing or two today..., but wanted to share.

 

Quadratic Residue Diffuser Calculator link HERE. 

 

It may be rudimentary, but an interesting read -