bass content in surround speakers

Paul Spencer · 02-22-2004, 07:53 AM

I'm considering the idea of desiging all speakers in a surround system to have F3 down to at least 40 Hz in all speakers. I'd like some comments on this approach, especially from those who have tried it with experiences to share.

* does it give smoother in-room bass response?
* is it bass information from the centre, main and rear channels important when you have a subwoofer?
* should you set your speakers to "small" even though they have good bass response in order to get more dynamic range?

Wayne A. Pflughaupt · 02-22-2004, 02:32 PM

Hi Paul,

Quote:

* does it give smoother in-room bass response?

Generally, the more sources you have spread around the room generating low frequency signals, the worse bass response becomes. This is because low frequencies react to and have cancellations at certain frequencies. The cancellations are a function of the distance from the source to boundaries – walls, floors, ceilings. Plus, the halfway point between two low-frequency generators also behaves like a boundary.

Then you have phase issues as the different low frequency sources arrive at the ear at different times.

Quote:

* is it worth bass from the centre, main and rear channels?

Huh?

Quote:

* should you set your speakers to "small" even though they have good bass response in order to get more dynamic range?

This is certainly a viable option. Often movies (especially the action genre) have intense and elevated levels of bass that can actually negate the crossover slope and extend bass energy an octave or more below the crossover frequency. So having bass-capable speakers, even set to large, can help assure that the woofer will never be driven to its limits.

Regards,
Wayne A. Pflughaupt

Paul Spencer · 02-22-2004, 09:06 PM

Wayne,

Regarding phase issues, are you allowing for the fact that speaker distances are resolved by digital delay?

Regarding smoothing room response, I am getting a lot of conflicting views. This actually seems to be a very controversial point.

I edited my 2nd question to make more sense, which was written very late at night!

"So having bass-capable speakers, even set to large, can help assure that the woofer will never be driven to its limits."

I assume you mean "even set to SMALL"?
small speakers may in fact reach their excursion limits without adding any real bass output ...

another question:
how low should the frequency response of surround speakers extend? Is there any point going below 100 Hz. Or is it worthwhile getting them to extend to 60 Hz, then filtering bass below that point with an active filter to prevent extreme excursions.

Wayne A. Pflughaupt · 02-23-2004, 04:52 PM

Paul,

Quote:

Regarding phase issues, are you allowing for the fact that speaker distances are resolved by digital delay?

No. Naturally, if delay is available it would help. However, the boundary issues remain. Plus, even with delay available, you can only set it for one seating position.

Quote:

* is it bass information from the centre, main and rear channels important when you have a subwoofer?

I’m still not sure I get it.

Quote:

So having bass-capable speakers, even set to large, can help assure that the woofer will never be driven to its limits." I assume you mean "even set to SMALL"?

Yes. Sorry.

Quote:

another question:
how low should the frequency response of surround speakers extend? Is there any point going below 100 Hz. Or is it worthwhile getting them to extend to 60 Hz, then filtering bass below that point with an active filter to prevent extreme excursions.

Well, there are several factors at play here, like how power-capable the speakers are (not necessarily the same as how low will they play) and how hard you drive the system and crossover slopes. Fortunately most receivers these days use 24dB/octave slopes, which are much better at protecting small speakers from being overdriven (provided the low pass frequency is not set too low ).

For instance, my mains have good extension to 30Hz, but the woofer is small and not really heavy-duty. They work fine for music, but since we have a large room and have to crank it pretty hard, they could not handle a crossover frequency of 60Hz without severely bottoming out during demanding action flicks. I had to move the crossover frequency up to 90Hz, which is about and octave and a half above their extension rating.

Quote:

Regarding smoothing room response, I am getting a lot of conflicting views. This actually seems to be a very controversial point.

Yes it is. However, I’ve seen published tests from people like Tom Noussain, taken in a variety of rooms, complete with response charts, and all got the same results, that bass response deteriorated with multiple low frequency sources. By contrast, I’ve yet to see any multi-location proponent back up their claims with the kind of rigorous data that Noussain has. Indeed, I can’t say I’ve ever seen one post even a single response chart.

Not saying it can’t be done, but it’s difficult. It took me 10 times the effort to EQ two separate subs as it did when they were both in the same corner. I’ve tried and measured dual sub placement in a separate 16 x 14 room in the house and tried all the usual panaceas said to deliver superior performance to single-corner placement - opposing-cornered placement, two corners on the same wall, asymmetrical placement (i.e., one on a corner the other centered on a different wall). None delivered as good a response curve as the single corner placement. The opposing corner and symmetrical readings were so bad not even equalizing would have shaped them up.

Of course, all rooms are different, so your mileage may vary. It never hurts to have speakers with good extension, even if you don’t use all of it. I actually prefer mine that way.

Regards,
Wayne A. Pflughaupt

Jonathan M · 02-23-2004, 05:48 PM

Just some points to consider:

Most receivers actually use 12dB/octave Butterworth highpass filters. It's the lowpass filter to the sub that is usually 24dB/octave Linkwitz-Riley. This conforms with the THX spec. It's designed to work with a sealed cabinet with F3 of 80Hz (or whatever the receivers xover frequency is) for the 5 (or 6) "full-range" channels. (Effectively the cabinet is acting like a 2nd order Butterworth highpass, which somes with the electrical filter in the receiver to give the 24dB L-R filter to match the lowpass to the sub.

I am of the opinion that the best response for most rooms is to keep it simple. Have all the speakers the same - sealed units with the same F3 as your chosen crossover point. Then have the sub (with NO crossover - your receiver handles this) placed in the best place possible. You can always tame the subs response with EQ later.

For most situations, I feel that 80Hz is sufficiently low for this - this is the THX standard, which was actually designed by competent people, before it was abused and made a into a "sticker" that could even be applied to cables!

Note that even with the high-ish F3 point of 80Hz, make sure the speakers themselves are not too power or excursion limited down low. A small amount of excursion limit exceeding is not a problem in my experience (see my DIY HT page for info on my system, which uses 5 1/4" woofers with only 2-3mm or so excursion), but if you want to go to very loud levels on a regular basis, make sure the woofers are capable of it!

Hope this helps

Cheers,
Jonathan

**HTF Ads**

Joseph Sabato · 02-23-2004, 06:53 PM

Unless you have a lot of flexibility in placement and a lot of time and patience, having multiple bass sources will more likely give you worse response than a single source in an optimized location. I have had that experience and will be moving both drivers to the same location as soon as I get motivated. See my site for the results of two sources.

measurements

That said, I agree with having rear speakers with better bass capability to keep distortion levels down at high levels. I have JBL S38's in front and S36's in back and at very high levels have heard signs of distress from the rear on bass heavy effects (while the fronts are just fine). They are all set to small.



Skin Chooser:	Contact Us - Advertising Information - Home Theater Forum - Archive - Top