Frequency Gap (RM6600, PSW 350)

jesyjames · February 2003

Hello there. Hope someone can help me with this.

I recently moved my Home Theater to a different room in the house and was going through the calibration using the Avia Home Theater disk.

I have the system set-up per polks recommendation of running the front left and right through the sub. On the receiver sub is off, fronts to large, and all other speakers are small with crossover at 120hz(Denon AVR-1803). I have the filter on the PSW-350 in the "white" area of about 130-150hz.

I got everything set up and calibrated the way I want it. I then decided to run the Frequency Sweep using a radio shack decible meter. I noticed that when I run the sweep from 200hz on down(using the front left and the sub) that the db level is essentially steady up until I hit 100hz. At 100 hz it dips down from about 80db to 60db and lower and stays that low until at 80 hz things pick up again and it's steady down to 30hz where the sub seems to just give up.

What would be causing this "gap"? I would think it was a crossover issue except that since the sub is essentially playing everything from 130 or so down to 100hz it doesn't make sense that there would be a crossover problem at 100hz. Could room acoustics cause something like this? Is there a simple solution? Would I notice a 20hz frequency gap? etc. etc. etc.

Also, does anyone know if there is an internal filter in the RM6600 sats/center, and if so what frequency does it begin to roll off at?

Appreciate any help.

-Eric

jesyjames · February 2003

Not to bog you guys down with questions, but I have one more I'd like to ask if someone has an answer:

On my receiver there is a setting for the subwoofer that is +main and it states that "when the +main playback mode is selected, the low frequency signal range of channels set to LARGE are produced simultaneously from those channels and the subwoofer channel. In this playback mode, the low frequency range expand more uniformly through the room, but depending on the size and shape of the room, interference may result in a decrease of the actual volume of the low frequency range."

So... if I were to set all speakers to large, sub to yes, and run the sub through the sub out and fronts through the receiver... would I then be able to bypass the "double" filtering that Polk is trying to get me to avoid by running the fronts through the sub? And wouldn't this essentially be better anyway because I would be able to avoid double filtering *all* the channels? Am I just making stuff up now or does this make sense? What am I missing here?

Tour2ma · February 2003

Im not the authority here, but Ill ask a few questions first, then maybe a thought or two to tide you over until others respond

Originally posted by jesyjames
I recently moved my Home Theater to a different room in the house and was going through the calibration using the Avia Home Theater disk.

First, can we assume that the problem did not exist in the other room?

I have the system set-up per polks recommendation of running the front left and right through the sub. On the receiver sub is off, fronts to large, and all other speakers are small with crossover at 120hz(Denon AVR-1803). I have the filter on the PSW-350 in the "white" area of about 130-150hz.

This may be Polks way, but it is not the Club Polk way which is use LFE if available, cross over around 80 Hz, all speakers to small, etc. Many posts in this area detail this approach, if your AVR is capable. And it appears from your second post that it is.
The decreased LF volume Denon is warning you of is the cancellation risk from producing bass out of multiple sources.

I then decided to run the Frequency Sweep using a radio shack decible meter. I noticed that when I run the sweep from 200hz on down(using the front left and the sub) that the db level is essentially steady up until I hit 100hz. At 100 hz it dips down from about 80db to 60db and lower and stays that low until at 80 hz things pick up again and it's steady down to 30hz where the sub seems to just give up.

Are you applying the low frequency correction factors for the RS SPL meter? Also posted around here. The 100 Hz dip sounds like a room cancellation to me (see below)

What would be causing this "gap"? I would think it was a crossover issue except that since the sub is essentially playing everything from 130 or so down to 100hz it doesn't make sense that there would be a crossover problem at 100hz. Could room acoustics cause something like this? Is there a simple solution? Would I notice a 20hz frequency gap? etc. etc. etc.

Crossover is not instantaneous, but rather the mains rolling off as the sub rolls up. Does all this occur with the sub alone? With the sub/ right combo? If yes, then room response is very possible cause. Try moving your sub. Small moves can make a difference, both for the better and worse. Experiment.

Also, does anyone know if there is an internal filter in the RM6600 sats/center, and if so what frequency does it begin to roll off at?

Cant address this one

Dr. Spec · February 2003

jesyjames:

I saw no evidence of double filtering per your set up since your mains were set to large and the sub was set to no/off.

I suspect this problem may be phase related, or possibly room cancellation. Before you do anything, reverse the phase on the sub and run the sweep again and post the results here.

No one here much likes the speaker level conection to the sub - as tour2ma insinuated.

In the future, you may want to set all your speaks to small, set your filter to 120 Hz (because you have small sats), set your sub to yes/on, and run a sub cable from the sub-out jack to the LFE (unfiltered) input on the 350.

jesyjames · February 2003

First, can we assume that the problem did not exist in the other room?

I knew I should've been more explicit because it sounds like that is what I'm saying. The unglorious truth is that I simple never ran the test before w/ a sound meter.

Are you applying the low frequency correction factors for the RS SPL meter?

Definetly not as I've never heard of such correction factors. Thanks for the tip. I will take a look around and see what I come up with.

I suspect this problem may be phase related, or possibly room cancellation. Before you do anything, reverse the phase on the sub and run the sweep again and post the results here.

I did switch the phase switch on the subwoofer and repeated the test. I had gaps all over the place once I switched it... so I think it is safe to assume that I have it in the correct position.

In the future, you may want to set all your speaks to small, set your filter to 120 Hz (because you have small sats), set your sub to yes/on, and run a sub cable from the sub-out jack to the LFE (unfiltered) input on the 350.

I've had it set up like this before, but there was always the Polk owners manual warning for the sats going in my head that gave me a guilty feeling ; ) On a side note, when I ran the LFE sweep with the center/sub there was another gap from about 130hz-135hz. I assumed that this *was* an example of double filtering as the center probably has it's own filter and the receiver is applying one as well, and the subs knob is in that general area(sort of sounds like triple filtering).

Anyhow, thanks for the responses. I'm somewhat limited on where I can put the sub owing to the dynamics of the room and the ever important opinion of my wife, but I will see if I can make some small adjustments as well as rewire it so I have the speakers running from the receiver and a direct out to the sub. Will post back if I have any noticable findings.

Thanks again.

Dr. Spec · February 2003

jesy:

Your sats (center, mains, surrounds) do not have a filter inside of them.

Don't worry about what Polk says - they need to revise their owner's manual.

It is to their credit that they provide an LFE input on most of their subs, and it is a concession to the rest of the word because most of us like to filter via digital bass management.

Your speaker level and your line level L/R connections all go through the internal filter on the sub.

All the LFE input does is BYPASS the internal filter on the sub. Using this input means you must filter at the receiver via digital bass management.

Digital bass management is more flexible and reduces the load on the receiver amp, and is easier to connect to the sub than two sets of speaker wire. This is especially true in your case, because your AVR has user selectable high pass filter points.

I recommend setting all your speaks to small, the sub to yes/on, your receiver filter point to 120 Hz, and run the cable to the LFE (unfiltered) input.

Set your LFE volume on the receiver to a little less than the midpointof the range (e.g., -3 on a scale of -10 to +10). Make sure there are no other limiters running like "LFE Trim" or "Midnight Mode". If they are, disable them.

Set all your other internal speaker volumes to the midpoint (i.e., 0 on a scale of -10 to +10). Run your center channel test tone and increase the master volume until it reads 75 dB on the RS meter on C-Weighted Slow while placed at the couch facing forwards, on a tripod or box, at head level, tilted 40 degrees upwards.

Leave the master volume alone and write down its setting for future reference. Run the tones for all the other surround speaks. If they are not 75 dB, adjust the internal setting (not the master volume) for each speaker until they are all 75 dB.

Run the sub test tone and adjust the volume at the SUB PLATE AMP (not the master volume, and not the LFE receiver volume), until you also get 75 dB (it will fluctuate, so take an average).

Re run the sweep and tell me what your results are. If you are still getting a hole in the FR, it is room cancellation with the sub and experiment with placement.

The RS correction factors for low bass were just posted over in another thread below this one. Go there and check them out.

If you get 75 dB on average from the sub test tone, it will actually be 2 dB "hot", since the RS meter reads a bit low on the test tone. Most people like the sub a bit on the hot side for HT, and this will be just about right.

jesyjames · February 2003

Quick question and then I will try and get everything setup later this afternoon:

I have two options for adjusting the subwoofer level and it's unclear to me which one would be the best way to do it.

In the same setup menu where I can adjust the front r, l, c, surrounds, etc there is a subwoofer volume that ranges from -12 to 12. This is the menu where I would adjust the speaker volumes to compensate for any variations when calibrating to 75db. Would this be the proper place to set it?

Because there's also a "surround parameters" menu that allows me to set the "LFE" from -10 to 0db. From what I gather it seems to be a "peak" limiter for the LFE channel.

Now, I'm assuming we don't want to impose the artificial limiter, but merely lower the subwoofer output volume? On the off chance I'm wrong, I thought I'd better check.

Really appreciate the help. My goal is to get it to a point where I can just hit "play" and know I'm getting the most out of it.

Edit: Nevermind. Seems clear that the peak limiter is exactly one of the settings you said to make sure was off. I'll try and get this calibrated and see if that gap is still as bad.

Dr. Spec · February 2003

Originally posted by jesyjames
In the same setup menu where I can adjust the front r, l, c, surrounds, etc there is a subwoofer volume that ranges from -12 to 12. This is the menu where I would adjust the speaker volumes to compensate for any variations when calibrating to 75db. Would this be the proper place to set it?

Because there's also a "surround parameters" menu that allows me to set the "LFE" from -10 to 0db. From what I gather it seems to be a "peak" limiter for the LFE channel.

Now, I'm assuming we don't want to impose the artificial limiter, but merely lower the subwoofer output volume? On the off chance I'm wrong, I thought I'd better check.

Yes, the -12 to +12 subwoofer volume control is the one you want. Set it to -3 and leave it there.

DISABLE that effen "LFE peak limiter" - those damn things are responsible for more "hidden" bass management problems. That's why I pointed it out, above in my previous post. MAKE SURE you understand how that limiter works and how to disable it completely.

The LFE peak limiter (sometimes referred to as LFE "trim control") limits the dynamic range of the LFE channel and helps prevent the sub from bottoming out on heavy passages. It has no place in any "real" HT system with a decent subwoofer.

YOU are the ultimate "limiter" in your system, with your Master Volume control. Keep everything else out of the limiting loop. Even check your DVD player for limiters - they often have one in the audio circuit. They are usually set to "off" but electronics have a knack of spontaneously altering their settings and I check mine from time to time just to make sure.

TonyPTX · February 2003

Not to knock the sats, or throw a monkey wrench into this thread, but this "gap" that Jesse is referring to is one of the main reasons I "traded-up" my RM6700's for some RTi70 towers. The small enclosure of the sats just means that they can not faithfully generate the frequencies below say 150Hz. Per Polk, my 6700's had an internal cross-over at 150Hz. Customer Service recommended that I run the sats through the sub and crank up the sub low-pass x-over all the way to the max. Maybe this will help, but I still noticed the lack of low end mid-bass and couldn't take it anymore. If you're still stuck with the missing frequencies and the suggestions from the posts above don't help, and you're still stuck with the need for a sat/sub combo, then I suggest you look at maybe the RM7600 series. Those are some sats with a decent set of drivers.

jesyjames · March 2003

Just wanted to report my findings after some experimentation...

Simply moving the Subwoofer to the other side of the TV eliminated the frequency gap from 100hz to 120hz when doing a frequency sweep. It did, however, induce a a gap from 80hz-100hz. However, by marginally moving the sub I have it in position now where that gap has been minimized. Sure, there are some variations, but nothing major...

Until you get to 65hz... the bass has a major bloom(12db). I have no idea how to naturally tone that down and it certainly lends a very boomy/one-note quality to the bass. I've tried rotating the sub in a variety of positions but I can't seem to kill it.

Other than that, things are pretty good and thank you for the help.

Frequency Gap (RM6600, PSW 350)

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