Suggestions...John?? knightfly??

Chaz · Post by **Chaz** » Fri Jul 25, 2003 1:17 pm

History....

I designed and built a two-room facility back in 1994. Unitl recently, I was using JBL4312's, which are three-ways, which have a round port in front. These were repalced a couple of months ago with JBL LSR28P two-ways, which have an oblong port in back of the monitor.

Problem & Question.....

I am now getting reflections off the control room glass due to the rear port. Though the room has a very tight and mostly dead sound, I am not sure if these reflections are helping or hurting my mixes; especially in the low and low-mids freqs.

I have listened to reference CDs of which I am familiar, but I still am not sure what to do. Right now, I am a little frustrated because I just do not know what to do or whether I should do anything.

If anything, I would probably build an absorbant panel to fit inside the window frame to cut down on the reflections coming off the glass during mixdown. But then again, I am not sure right now as to what to do. Biz has been slow for the past couple of months and funds are limited. Either way, I still need to make an informed decision as what to or not to do at this time.

Anyways.....Any help and/or suggestions will be greatly appreciated. I do understand studio design and have done a few designs myself, but at this point in my life, I think I need help from those with more experience in this area than I.

For reference as to the room itself...go here... www.havenmp.com/stpics.html

Thanks.

Chaz

knightfly · Post by **knightfly** » Sat Jul 26, 2003 1:44 am

Chaz, I think you are like a lot of us, and you just need someone to give you permission to do what you already know is right - that's a lot of glass behind the board and the combination of that and rear ported speakers is a marriage made in hell...

If you intend to keep the LSR-28's I would consider maybe building a 2-piece, hinged, heavily absorbed gobo for that glass (unless it's your only way in/out) and hinge it horizontally in such a way that you can flip down the upper section while tracking, for visibility; then, when it's mixdown time, flip the upper section back up in place to completely cover the glass.

If the glass is the only way in/out, you'd need to build a rolling gobo or something similar, again heavily absorbed .

BTW, there is NO reflection with that short a path that will EVER do anything but hurt your sound - the phase cancellations should be driving you crazy by now.

I'm curious why you abandoned the 4312's - are you heading toward 5.1 and can't afford the space, or is there something wrong with the 4312's? Are you running a sub to make up for the smaller speakers, or what?

Anyway, without actually listening to your room those would be my opening thoughts on the matter... Steve

Chaz · Post by **Chaz** » Sat Jul 26, 2003 2:31 am

Hey Steve,

Thanks for responding so quickly to my post. I really DO know what I have to do, but I posted more out of frustration than anything else.

As for the 4312's...well...one has a cracked woofer cone and will cost $300 to re-cone. And, I also need to replace the cross-overs in both monitors. Plus, due to the size of my CR (13Wx8Dx7.2H), they are too much speaker.

I like the sound of the LSR's. They work with the room size better than the 4312's, but now the aforementioned rear port hole has created a beast.

Anyways....thanks again for the support and comments made in your post.

BTW....I will eventually repair the 4312's and use them in conjunction with the LSR's. I am looking into building another studio at another location with a larger CR, etc.

barefoot · Post by **barefoot** » Sat Jul 26, 2003 4:55 am

Chaz,

Just some words regarding the rear port. The rear port isn't your problem. Or, should I say, this isn't your primary problem.

The port is active only at the very lowest frequencies of the speaker's range - approximately one octave above and below the lower -3dB cutoff frequency. The lower f3 of the LSR28P is 46 Hz, so we're talking no significant port output above 100Hz. In this frequency range the wavelengths are many times larger than the cabinet. Therefore, the wavefronts radiate from the port omni directionally. It doesn't mater which way the port is facing.

The close proximity of the rear wall and window can enhance the acoustic loading on the port (increases coupling efficiency with the room) resulting in a rising bass output, but the distance is too short to cause any sort of spatial interference effect because the direct and reflected waves have effectively no relative phase difference. . And , again, this enhanced loading is present regardless of the port orientation.

Ports can, however, produce unintended higher frequency distortions. A noise effect called "chuffing" is caused by turbulence due to high air velocities in and around the port. Ports can also generate pipe organ type resonances. And finally upper midrange frequencies from the rear of the woofer can radiate directly out of the port. Some manufacturers put the port on the rear to try and aim these problems away from the listening position. Of course, if you have a reflective surface close to the rear of the monitor, the noises are going to simply bounce right back at you. So, some simple method of absorbing front wall reflections in the midrange could indeed help with these kinds of port issues. [On the other hand, fixing proximity induced bass lift in the lowest frequencies is a pretty major endeavor requiring large bass traps.] Still, remember that these noise effects, if present, are essentially due to monitor design flaws and not part of the "normal" functioning of the port.

Finally, we get to the area that is most likely causing the majority of your problems. This is the upper bass and lower midrange. In this frequency range the wavelengths are long enough such that some of the energy from the woofer cone can wrap around the sides (diffract) and radiate towards the rear of the cabinet. They will then bounce off the wall/window and radiate back towards the listening position. Unlike the deep bass, however, the wavelengths are short enough such that the path length differences between the direct and reflected waves can cause significant spatial interference effects - i.e. comb filtering. Probably the best way to fix this problem is to simply move the monitors away from the wall. This might not be a workable alternative, so you're left with either absorbing , diffusing, or reflecting that reward energy such that it doesn’t come back and interfere with the direct sound. (Of course, soffit mounting solves the whole issue).

Again, remember that this upper bass/lower midrange problem has nothing to do with the port. The port produces practically zero output in this frequency range. Most any speaker, rear ported, front ported, or sealed would suffer from this same problem.

Hope this helps!:)
Thomas

barefoot · Post by **barefoot** » Sat Jul 26, 2003 5:25 am

Oh, yeah.

So why wasn't this such a big issue with your 4312's?

Well, for one thing I think the 4312's have a bigger cabinet, right? The sound wave's ability to diffract around towards the rear depends on the wavelength relative to the size of the speaker cabinet. A smaller cabinet will allow shorter wavelengths to diffract to the rear.

Also the 4312's drivers are probably just more directional than the LSR28P's, so less of the midrange heads towards the rear of the speaker.

Thomas

Chaz · Post by **Chaz** » Sat Jul 26, 2003 7:22 am

Well Thomas,

That hepls me to understand a lot! The room was primarily designed with the 4312's in mind, but now that they are gone I will compensate for the LSR's.

The studio design, however, is pretty solid as I designed it with an acoustician and pro studio designer/builder. Sorry....can't think of his name now. The rear wall has a diaphragm system designed to handle low end freqs.

Thanks for the info and suggestions! It is greatly appreciated.