Miscellaneous Ramblings on Subwoofer Crossover Frequencies September, 2002 (updated May, 2004) Colin Miller and Brian Florian
Many die-hard conservative audiophiles at first eschewed the use of a subwoofer as a novelty trick of home theater, unsuitable for music reproduction. As the frequency extension benefits of a dedicated subwoofer became more and more undeniable, many shifted to the policy of running every loudspeaker “full-range” with a “pure” signal, and then setting the subwoofer to simply “fill in” where the other speakers left off. As knowledge moved on, the position has retreated to recommending lower crossover frequencies for larger, floor-standing loudspeakers, such as between 30-50Hz, as opposed to higher crossover frequencies in the 80-100 Hz range.
Higher crossover points
for subwoofer integration, regardless of the range of your main or
surround speakers, have some potential advantages. The higher the
• LFE Channel integrity. When it comes to the "subwoofer jack" on the back of the surround sound decoder, most everyone assumes that bass is taken from channels set to "Small", combined with the LFE channel and bang, you have a subwoofer output.
In the majority of surround sound processors and receivers, FULL RANGE copies of all channels set to "Small" are combined together with the LFE channel, and the sum is low-passed. Think about that. Strictly speaking, any* such processor with a sub/sat crossover frequency set lower than 120 Hz is "discarding" the upper end of the LFE channel. THX units are NOT exempt from this. With the standard THX 80 Hz 4th order crossover, the top of the LFE channel gets chucked.
Don't panic. This has been going on since day one, and virtually nobody has noticed . . . with good reason. I've said many times before, and I will say it again: THX did not pull their crossover out of thin air. It is the product of much development, and, when used in concert with THX speakers (or others which exhibit the correct roll-off), represents the best overall compromise of minimizing localization, extending dynamic range, and as it turns out, minimizing LFE truncation. When Dolby Digital was coming to the consumer marketplace, THX looked at an inordinate number of modern 5.1 soundtracks and guess what they found in the LFE channel: not much at all in the region of 80 Hz - 120 Hz, making their original choice of 80Hz rather fortuitous. Dolby Digital's LFE channel has a digital brick wall at 120 Hz, not a roll-off, so content creators almost always roll-off their stuff, usually somewhere around 80 Hz. Therefore, chucking the top band of the LFE is no big deal but the argument here is that a standard SSP crossover set much lower than 80Hz or so may actually be costing you bass content.
• More consistent frequency response between channels. If the bass from all channels is routed to the subwoofer, the frequency response over the range that the subwoofer produces will be identical between channels. The greater the range of the subwoofer, the greater this benefit. In contrast, any low frequency information produced by separate loudspeakers will not only have different responses in the room due to different physical locations, but there will be cancellations between channels due to distance between them. For instance, the bass response of the right front loudspeaker may be different from the bass response of the left front loudspeaker, which will be dramatically different than the bass response from the left and right front channels combined, and very much NOT an average of the two, but a haphazard mix, most likely a boost at extremely low frequencies, and a dip at frequencies just slightly higher, such as a rise 20 Hz and below and a dip at 50 Hz for a pair of speakers separated by 10 feet.
In such a case, the 100 Hz crossover point is
certainly worse than the 50 Hz crossover point, and very likely
Mixing high and low frequency crossovers in a multi-channel set up
Once you wrap your head around the fact that in most products you are setting a high-pass on each main channel and a single low-pass on the sub, the use of a different setting for each speaker (or pairs of speakers) no longer sounds like such a good idea (pun most definitely intended). Lets take an extreme scenario, just to illustrate the point.
We set the high-pass on the main left and right to 35 Hz because we think its in the best interest of our massive tower speakers. We set our center channel high-pass to 100 Hz because it isn't very big. What is the subwoofer low-pass in the processor going to be?
If set at 35 Hz to complement the main speakers, the center channel signal will have a huge hole from 35 Hz - 100 Hz. Whoa! Lots of bass on that channel we don't want to miss out on. So let's try setting the subwoofer low-pass to 100 Hz. Oops! Now we have IN-ROOM 6 dB too much from 35 Hz - 100 Hz on the main channels because BOTH the main speaker and the subwoofer are voicing it. You CANNOT correct for this. If you lower the subwoofer level, you lower it for everything, and now you don't have enough bass from the center channel.
By now some of you are thinking, "Why not low-pass a copy of each main channel at the various frequencies I want and sum that with the full LFE channel?". Possible, yes, and if fact there are some SSP models which do this, but at a price: doing so inherently results in frequency response aberrations due to phase issues. Bass is often common to the front three channels and even more often common between the LFE channel and the fronts. Summing different low-passed copies of the same material would by definition result in a messy frequency response. Take the ubiquitous 4th order low pass as an example: At the crossover frequency its phase has come around to 180deg, absolutely inverted (compared to material it is being summed with). Granted the relative amplitude of that low pass at the crossover frequency is down 6dB but is still enough to create the aberration.
The THX design manual references the Dolby Digital licensing manual which mandates that the subwoofer output be arrived at the way it does for these reasons. If there was a better way to do this, without adding a lot of cost and/or making the product overly complex, I think Dolby would tell us.
One alternative found in some decoders is to take a low-pass copy from the center (in our extreme example, at 100 Hz), add that to the front left/right and still high-pass those at 35 Hz, the balance going to the subwoofer (though you still waste 35 Hz - 120 Hz off the LFE channel). This can be both good and bad, depending on the rest of the design:
- Unless proactively addressed, you can still have the phase issues described above.
- When mixing channels digitally, S/N is lost (approximately 6 dB when two channels are added for example), because after the summing, the combined level has to be attenuated to the original level. Might not sound (pardon the pun) like much but its something a designer has to consider when weighing the pros and cons of doing something.
We acknowledge that a different crossover point for each speaker is a desirable thing from the point of view of real world acoustics and dynamics. The different positions of the speakers in the room virtually dictate it, and the various members of a mismatched speaker set will each have different points of intersection for increasing dynamic range and maximizing bass performance. But without also having a selection of slopes in the SSP and some VERY expensive measuring equipment, one is likely to end up further behind than ahead.
If you want consistent bass response from each channel of your 5.1 system, in our opinion, you're best to set all speakers to "Small", set them all to the same crossover point, and set that point no lower than what you are comfortable throwing away from the LFE channel. If your main left and right speakers are genuinely full range (be honest now!), then you are better off running them full range as opposed to high-passing them at a ridiculously low frequency. Short of that, high passing floor-standing speakers at 70 Hz is not "wasting" them in any way shape or form and in fact will more than likely extend their dynamic range thanks to the relief they'll be getting from the high-pass. Alternatively, setting center and surrounds as "Small", the mains as "Large", subwoofer as "None", and implementing an external two channel crossover to the subwoofer is a valid, and in some situations an advantageous way to go.
- Colin Miller & Brian Florian -