Curtains muffling RF/C-7 bass?

[ottscay]

Ok, I just finished setting up my theater in my new house, and now I'm a bit concerned. There are two windows behind my TV I wanted to cover up, so I strung large dark-red movie theater like curtains across much of the back wall. It looks nice and really adds to the "theater" ambience. But it occurred to me that since my RF-7s and RC-7 are rear ported, the curtains may be affecting bass. I've tried A/Bing it, but it takes a while to change the curtains, so I'm not sure if it's a real difference or not. It does seem that the base was getting lost, especially on the center channel, which is entirel in front of the curtain.

Is this likely to be my imagination, or should I expect a curtain covered rear wall to interfere with the bass emanating from the rear ports? It's easy enough to movce the curtain ouf the corners to give the RF-7s more wall space (they're corner loaded as well), but I would basically have to completely rethink my decor if I have to move it from behind the RC-7. What do you guys think?

Thanks!

Scott

[Anakin26]

Hmm... I think anything that reduces wall reflection is pretty much welcome in a ht. I myself have the wall behind my rf-7s and rc-7 covered with heavy stage cloth. I would says YES it eats some of the lf, BUT it also makes for a more controlled bass reproduction. What I found really important is, that you give the RF-7s enough air to breathe between the ports and the wall. With the cloth it becomes even more important.

Nick

[ottscay]

I have about a foot and a half between them and the corners, although the diagonal measurement may be a tad closer. Should I give them more space?

[Anakin26]

Definitely. Bass will improve a lot if you do. I am not sure but I believe Klipsch recommends approx. 1 meter from the rear wall, which would be around 3 feet.

Nick

[DrWho]

Ok, so with the curtains you think it sounds more on the muddy side? If this is the case, then this is an indication that the mids and highs are being absorbed (which would make sense considering what you're doing). What you MIGHT percieve as muddy bass, is actually the absence of upper frequency reflections.

I would argue that your curtains aren't in any way affect the sound of your port (which will be emitting frequencies centered around 30Hz). These wavelengths are over 20 feet long and there is no way even a 1" curtain will have any significant impact. Heck, wavelengths this long go through walls with no problems.

So though you describe the differences with changes in the low end, the real root of your problem is prob in the upper frequencies (if in fact you think it is a problem). I personally would think the curtain would make the sound cleaner...much more so than having windows behind the speakers.

Btw, having a foot behind your speakers is more than enough. The absolute minimum distance would be twice the diameter of the port (which I think comes to about 9 inches?) I don't have the RF-7 so I can't measure for you.

[Anakin26]

I had my ht for quite a while before i installed the black stage cloth on the front wall. It was amazing how much this changed the room accoustics. I have my pc in the same room. Even typing on the keyboard sounds different with the cloth in the room.

I found it improved the sound a lot. I have a big window on one side of the room which I cover up with a heavy curtain when listening to music or watching a movie. This way I reduce unwanted reflections thus making the sound clearer and more defined.

I have to check on where I got the information from. I sure this big distance from the rear walls is mentioned somewhere. I like your theory better though... because space doesn´t permit my RF-7s to stand this far away from the rear wall. Twice the port diameter is more like it!

Thanks

Nick

[DrWho]

well think about it logically...

-too close to the wall and the cabinet starts to act like a sealed cabinet...so lots of loss in LF (kinda like a port tuned to 1Hz).

-Close to the wall (near the diameter of the port) and you're essentially making the port longer, which will lower the tuning of the cabinet...this will cause a dip in the response probably around 40-50Hz with a peak around 20Hz.

-Far enough away from the wall and the port tuning raises all the way to its 35Hz tuning.

So basically, from touching the wall to moving it out far enough, you are changing the tuning from 0Hz to 35Hz (or wherever the actual tuning is).

[Anakin26]

Hm... never thought about it that way. I thought that the distance to the rear wall has no influence on the porting other than, if too close, it will chancel out the waves before they are able to spread.

I tried a few different positions now, finding that there is one particular distance that kind of smoothens the LF response. In my case thats approx. 1 foot from the rear wall.

Thanks again!

Nick

[PrestonTom]

Dr Who is correct (as he frequently is).

Certainly anything below 100Hz will be not be effected by a drape or curtain. When the wavelegths are that large (over 10 ft), the wave will travel right through it. For a curtain to be effective it would have to be at least 1/2 wavelength and that trick only works for higher freqs (smaller wavelengths).

Room geometry (in conjucnction with the geometry of the speaker placement) will dictate the frequnecy response at these low frequencies.

Good luck,

-Tom

[colterphoto1]

If you are concerned about the lf energy being absorbed by the curtain, try this out as a test. Lean a small sheet of plywood against the curtain behind your RF7's (careful with edges next to fabric) . Does the LF energy change? You could sit in your sweet spot, adjust balance to one side, and have an assistant place and remove the board for an accurate A/B comparison.

This would tell you without a doubt if your fears are justified or not. But I suspect DrWho is correct. He usually is. That's why we're sending him to school!

Michael