Axiom Home Page Forums General Discussion Home Theater Perforated Screen & Equalization Problems

I just came across this statement by Alan in another post (Speakers for 30 foot x 40 foot screening room):
"Don't put the speakers behind the screen unless you are prepared to do equalization to compensate for the losses of the perforated screen."

I was planning on putting L/C/R speakers behind my screen, out of spatial necessity. In my months of planning, nobody mentioned anything to me about requiring EQ in that situation. I'm planning on getting the Denon 3806 with Audyssey MultEQ, so maybe I won't have to worry about any problems...
From my understanding, it is actually preferable to have the center channel behind the screen, so the voices come directly from the screen (like I was told movie theaters have).
Does anybody else have experience with perforated screens?

Geno, although perforated screens are sometimes referred to as "acoustically transparent", as Alan points out this often isn't the case and commercial theaters apply equalization to compensate for the losses involved. A good discussion of the problem for home theaters can be found here . There are some claims to have overcome the problem.

... but if they called them "acoustically translucent" nobody would buy them

It's worth searching for reviews and tests -- there seem to be a number of claims by different mfgs of only making a 1 or 2 dB difference across 20 Hz to 20 KHz but the test results showed more impact. One brand (ClearTex or something like that) seemed to come off the best.

I learned some really interesting things from the tests and articles :

1. The sound problems aren't just limited to rolled off high frequencies. Because the screen is in front of the speaker, you get reflections (typically 1 mS delay if screen is 6" away), often more than one (sound bounces off front of speaker and reflects AGAIN) so you end up with some respnse bumps in the midrange, say 1-3 KHz depending on speaker to screen distance. Those are harder to EQ.

2. The eq-ing issues isn't just limited to having to apply the equalization -- another problem is that if you have to push the treble up too far you can overdrive your tweeters at high volumes. M80s and VP150s have multiple tweeters which should help.

Anyways, there's more to it than I first thought...

JohnK,

Thanks for the link. Very interesting, and it does seem that one of the screens, the ClearPix, does best overall.

I'm inclined to agree with the reviewer, who generally advises people not to put good speakers behind perforated screens, but I do understand the demands of decor and the look of expensive custom installations.

It just bugs me that we go to great lengths to achieve linearity and smooth out glitches in frequency response to then have it undone by putting the speaker behind a not-so-acoustically transparent screen.

Of course, any customer can decide how much compromise in sound quality to accept by simply listening to the speaker behind the screen and then without the screen. After all, we've gotten speaker grilles to the point where there is no audible degradation at all with the grille in place.

Regards,

Maybe the best approach is to buy speakers with a bumpy midrange and ear-piercing treble... putting them behind a transparent screen wouldn't hurt the midrange and would definitely help the treble...

... actually I can think of a couple of brands which meet that description... both with a history of success in large theater-like rooms...

... Hmmmmm.

Bridgman,

Excellent! I can think of a couple of high-end critic's darlings that might just sound acceptable with a big, thick horse blanket thrown over them.

This may redefine "acoustic treatment''. . .

Regards,

"Don't put the speakers behind the screen unless you are prepared to do equalization to compensate for the losses of the perforated screen."

Well, I am that "30x40 foot guy" on a slow boat to China (when it comes to Audiology, Ph.D.). So... what is exactly to be afraid of equalization? Maybe I should start with an easier question: what is equalization and how does one do adjusts it in this instance?

What I am gathering here from the thread is that there is no "EQ" needed when the speakers are NOT placed behind the screen?

Most movie theaters/cinemas have all front speakers mounted behind perforated screens and still can get THX certified and still can blow you out of your seat with the volume. So, I just don't see the problem here, I suppose.

However, I am getting scared, since everyone seem to be so afraid of this big bad wolf of additonal equalization requirement. Sounds like something seriously difficult and painful.

Help, anyone? Thanx!

The Baba

Don't worry, it's not that bad. Equalization generally means "correcting the frequency response" through the use of an equalizer, basically a set of fancy tone controls.

Equalizers come in two major flavours -- "graphic" and "parametric". In the same way that normal tone controls have two "bands" -- bass and treble -- a graphic equalizer might have 8 to 16 bands, each covering a narrow frequency range from perhaps 30 Hz up to 15 KHz. They normally use sliders to set the levels, so the position of the knobs gives you a crude graph of what the resulting frequency response will be (assuming a flat input) -- hence the name graphic equalizer.

A parametric equalizer typically has fewer bands but you can adjust the frequency of each band (whereas the frequency range for each band on a graphic equalizer is fixed). If, for example, you need to do some pretty complicated adjustments in a narrow frequency range (say, for example, you had a screen in front of your speaker and needed to compensate for the reflections ) you could set all the channels close together and get the same result as having a 50-band equalizer but only using the bands in the narrow range.

The main problem with equalization is that it takes some time to get the settings right and never gives you results as good as fixing the real problem. For example, you will see threads here about using equalizers to correct subwoofer response when the real problem is room placement or lack of room treatements. EQ helps but in the case of a subwoofer you can only get the sound right in one place in the room...

The second problem with EQ-ing for screen effects was mentioned earlier -- speakers normally are designed to play at a decent volume with the normal distribution of energy at different frequencies. If you use an EQ to take the amount of treble way up and then turn the volume up, you can fry a tweeter at much lower volumes than before.

I don't feel like this is helping