10" & 15" subs in tandem w/crossover?

[EricH]

I'm running a Velo CHT-15 on my HT system and while I love the sub, it starts sounding "boomy" in my room with the crossover set much above 60 hz. Mains and surrounds are set at 100hz. I have a couple questions. Can I set them at 60hz, or this that basically like setting them on Large? Can I run a smaller sub off the same pre-out with a Y-cable, and set it to handle the 60 to 100 hz frequencies? Is there a loss in signal using a Y-cable to seperate subs? Any and all help appreciated.


HK AVR 630
RTi70 mains
CSi40 center
CSi30 side surrounds
Rti38 rear surrounds
Velo CHT-15 sub

["Polk" Paul DiComo]

There is NO reason to high-pass your RTi70s that high. Personally I prefer to run main speakers full range and low-pass the sub at a very low frequency. In my systems I have found that I get better main/sub blending and tighter bass that way. The only rational reason for high pass filtering the main speakers is for power handling and distortion issues associated with small speakers and/or stupidly high volume levels. Unless you are a volume animal you don't have to worry about that with your main speakers.

Give my method a try and see how it sounds. Experiment. It is good for the soul.

Other issues associated with boomy bass are placement and room problems. If you've been listening to the wrong people and stuck you subwoofer in the corner, you are doomed, doomed, DOOMED to suffer from crummy bass. Did I get a little carried away just now? Read this article about subwoofer placement:
http://www.polkaudio.com/home/faqad/...ticle=subsetup

Room issues associated with flabby walls and floors are a whole other topic covered to a small extent here: http://www.polkaudio.com/home/faqad/...e=optimizeroom

Let us know how this turns out for you.

[Dr. Spec]

If you've been listening to the wrong people and stuck you subwoofer in the corner, you are doomed, doomed, DOOMED to suffer from crummy bass.

Gosh, I wouldn't go that far, Paul. :)

Room modes are generally too complex to make a sweeping generalization like that.

Corner loading does excite all room modes. This is not necessarily a bad thing, because it minimizes the potential for nulls, which in turn improves the chances for decent bass response at the most listening locations in the room.

We all know moving just a few feet in the room can drastically alter the bass response. If one is looking for optimal bass response at one seating location, start with corner loading (because all modes are energized), and evaluate the FR at the listening location.

If the FR looks great at the seat, you got lucky. If not, I agree - start moving the sub to optimize the FR at the sweet spot. Room acoustics are so complex that there really isn't a hard fast rule for good subwoofer placement.

These are pretty comprehensive articles on subwoofer placement that are interesting reading.

http://www.harman.com/wp/pdf/multsubs.pdf

http://www.harman.com/wp/pdf/Loudspeakers&RoomsPt3.pdf

[EricH]

Thanks. Yes, the sub is in a corner. The room has been a 11' x 23'. It's in the process of being taken out to 19' x 23' and I'll have many more options on sub placement.

I used to have my RTi38's as mains and heard them clipping badly one night during The Two Towers. They were set on large at the time. I haven't yet recieved my new HK AVR 630 which has adjustable crossover points. I'd really like to run the RTi70's and surrounds at 60hz, as I don't blast my system. Theatre levels yes, The Who and Concorde no.

Thanks for the articles, I'll read 'em all.

[Dr. Spec]

Unless your AVR has the ability to independently set the xo for the LFE channel, I wouldn't go much less than 80 Hz for HT applications.

Doing so will lop off the top of the LFE channel since the typical AVR bass management circuit low passes the LFE channel along with the signals from the speaker channels.

http://www.hometheaterhifi.com/volum...rs-9-2002.html

["Polk" Paul DiComo]

OK, maybe I was being a bit harsh on the corner generalization but with the exception of sealed subwoofers, corner placement wil usually yield boomy, one note bass. Yes, room modes are too complex to predict what will work best for you.

Have you tried running the RT70s full range yet? They should not have the same volume limitations of the RTi38s. there is no harm in trying.

[Dr. Spec]

Truth be told, my sub is not corner loaded either - so I know where you're coming from, Paul. (thumbs up)

Mine is 1/3 wall width away from the corner and with some tweaking is +/- 3 dB from 35-100 Hz (with an 80 Hz xo).

No small feat considering the pre/pro imposes a 4th order low pass on the sub and a 2nd order high pass on the speakers. This disparity is a carry-over from the THX days, where you are supposed to use a small sealed speaker with an F3 of 80 Hz.

Sealed speakers roll-off 2nd order, so the high pass imposed by the pre/pro and the natural roll-off of the speaker resulted in a combined 4th order roll-off - not coincidentally the same rate being imposed on the subwoofer.

Unfortunately, most speakers these days are vented with an F3 typically in the 40-50 Hz region. Since the vented speaker will not be naturally rolling off 2nd order at 80 Hz, imposing only a 2nd order high pass typically results in a mid-bass hump in the 60-80 Hz bandwidth.

I solved that problem by engaging the 2nd order low pass filter in the subwoofer, essentially creating a 6th order roll-off for the subwoofer (yes, the dreaded double filtering). This compensated for the shallow 2nd order roll-off of the mains, and created the equivalent of a 4th order roll-off for both the speaks and the sub.

Of course, all of this could be solved if today's pre/pro's came with the option to select different filter rates for the high pass.

[EricH]

Have you tried running the RT70s full range yet? They should not have the same volume limitations of the RTi38s. there is no harm in trying.



No, I haven't. From scrounging around on this site, general consensus seemed to be running the RTi70's on small crossed at 80hz. I believe I may try running the mains and center full range + LFE, and just settle for whatever volume I can get without distortion. Also, I'll be going from an HK AVR 125 to a AVR 630 which should make a difference.

AVR 125 45 watts per/HCC rating: 25 amps
AVR 630 75 watts per/HCC rating: 50 amps

By the way, is it feasable to run 10" and 15" subs off the same pre-out with a y-cable? 10" for punchier mid bass, 15" for the bottom end?

[RuSsMaN]

I still say large, on just about anything with multiple 5.25" woofs or larger. Small shelf speakers and smaller center channels, try it both ways.

Originally, 'small' was intended for the sub/sat (micro) systems that were and are flooding the market, and large (normal) for everything else. By going small on your 70's, you are essentially negating the port design of the speaker. You might as well have bought the 38's, and gone for more of a 'point' source if you choose small.

In the end, of course what you hear and what setting YOU like is all that matters. I just find it amusing that people have been, and still do listen to 2ch music, with no bass management or filtering - yet different rules somehow seem to apply to home theater.

Cheers,
Russ (of the large camp Russ's)

[Dr. Spec]

I like the 70's on small, since they are a true 2-way speaker.

With an internal xo of 2.1 kHz, the dual drivers handle some fairly delicate midrange.

Excessive cone excursion (as what might be experienced if set to large in a HT application) will significantly modulate those midrange frequencies, reducing clarity. This is known as intermodulation distortion.

Also, small conserves amplifier power, something in very short supply with the typical AVR.

Sure, a Y splitter can be used to run both subwoofers. You might need to calibrate them to different levels, based on their individual capabilities. The 15" will probably play louder than the 10" and should be calibrated accordingly.

[RuSsMaN]

I understand where you are going and what you are saying doc. My civil retort as follows.

First - I don't want to hear the power excuse. If you need more power, get more power. Beit a bigger, better AVR, or go separates. Polk speakers are fairly efficient, and I would guess 9 times out of 10, it's not a continuous power issue, but rather a headroom / current issue.

Second - Intermod dist, very true - but thats why Polk designed the 'dynamic balance' drivers to combat JUST that. On another note, what delicate midrange frequencies are you trying to discern during a passage with enough lower-bass information to put the woofer cones at or near peak xmax? Much less discern them during a movie, with things flying around the room, and all the visual stimuli on the screen? (I'll give you center channel dialouge during an action sequence)

Not fighting, just talking. ;)

Cheers,
Russ

[Dr. Spec]

Second - Intermod dist, very true - but thats why Polk designed the 'dynamic balance' drivers to combat JUST that. On another note, what delicate midrange frequencies are you trying to discern during a passage with enough lower-bass information to put the woofer cones at or near peak xmax? Much less discern them during a movie, with things flying around the room, and all the visual stimuli on the screen? (I'll give you center channel dialouge during an action sequence)

Hey Russ:

Yeah, center channel dialogue pretty much put me over the edge, and that was with the 2.5 way CS400i no less.

The scene was in U-571 (dts) which has full range bass in all channels. Set the speakers to large and watch them practically bounce out of their baskets on certain scenes.

During a depth charge scene, Tank radios control and says "Control, aft torpedo tube ready in all respects" (or something to that effect). On large, with the CS400i cones bouncing around, that dialogue was hard to hear clearly. On small (80 Hz) it came through crystal clear, with the cones barely moving.

While I can't point to examples for the L/R fronts, the same issue applies conceptually to any 2 way. A female voice singing over an acoustic bass might be a good example?

If the RTi70 had a dedicated woofer with a 100 Hz xo, I would be more inclined to recommend large, like I have for the RTi150, RTi10, and RTi12.

Plus, I measured the bass response of the RT800i and it starts to roll-off just under 50 Hz and is several dB down by 40 Hz (and that's in-room). Even fed plenty of clean power, the 800's struggle to hit low E on a bass guitar with any genuine authority (and by authority I don't mean loud, I mean not rounding off the bottom of the note).

I guess my point is people won't be missing much extension by setting the Polk 2-way towers to small (say 80 Hz for theater and maybe 60 Hz for music), but they do pick up better mids when the cones aren't trying to play that deeper bass.

For the 150's, 10's and 12's, definitely run 'em on large (even if they can't do a legitimate 20 Hz or whatever).

No fighting here either; I can respect both POV's, but I'm firmly in the small camp for the 2-way towers.

[EricH]

Thanks Paul, Russ and Doc. All great info. I guess the best thing about the differing opinions is that I'm going to try all of your ideas, and decide for myself. Thanks again for the help.

[EricH]

Last night I set all of my speakers on Large as Paul suggested. I also kept the volume at a reasonable, but enjoyable level. Big improvement! Playing The Two Towers, Gandalf and the Balrog fighting while falling sequence, my system sounded fuller and "meatier". It also sounded more enveloping, with more effects coming from different directions This is due to the fact that the sounds are coming from the speaker that the sound effects people intended, instead of the sub.

I replayed the scene on Small settings to see if any clarity was gained. Maybe a small amount, but for the most part, everything simply sounded thinner. However, if monster volume levels came into play, all of this would change.

Since I also set my surrounds on Large, I checked them at a higher volume for any distortion. Not even close. It had a fuller, more complete sound also. Seems to me that most decent size surround speakers should be set on Large, given the smaller signal being sent.

I will also try the small setting on my speakers crossed at 60hz and 80 hz when the new AVR arrives. Thanks again.

[brody05]

Hey Eric, sounds great. Good for you for trying out both and making up your own mind on what you like.

I too (along with Russ) am a person that enjoys the benefits of a full range signal to all my speakers as well, I also have the LFE and mains sent to the subs for added bass, so the mains do the mains bass and the subs double up on that signal plus the LFEs.

I know what you mean about the rears ect having that bit more 'fuller' sound, todays DVDs have good signals going to all speakers and I notice a difference with the center center channel in regards to male voices such as Mel Gibson and Sean Connery. It was just a week or two ago I had a play around with different combos as I am in the throws of changing system components ect and really noticed it with Independance Day, The Patriot and Dragonheart.

Cool..... Brody


:p

[EricH]

Well I got the AVR 630 and set the crossover points at 60hz on all speakers. I did this to send the lower frenquencies to the sub. I don't think it sounds quite as nice as full range, but need to do more testing. With a 5 month old in the house, testing time is severely limited. This brings up a new question however, what exactly is in the LFE signal? I'm gonna start a new thread on this question.