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 crossover frequency.
www.harman.com/wp/pdf/Loudspeakers&RoomsPt3.pdf (If you haven’t read this already, you really should.
As always in this ying and yang world, some downsides…
1. Their crossovers (or the low-pass slopes in the surround processor or receiver) aren’t fast (sharp) enough in their transition. It’s not that 100 Hz is really that easy to localize, but that frequencies a bit above it are. Keep in mind that the shape of the low-pass filter is an issue, and in reality a curve which varies from crossover to crossover in how quickly it transitions from no slope to it’s steepest slope, the rate specified, such as 12 dB/octave or 24 dB/octave. Still, consider a theoretical impossibility, for the sake of illustration- the immediate crossover that goes from completely flat to a straight angle down exactly at the stated crossover frequency.
Take 400 Hz and 800 Hz tones as content to be filtered out by our subwoofer crossover as an example of localizable content. A crossover at 50 Hz, 12 dB/octave will be €¦
-12 dB @ 100 Hz
-24 dB @ 200 Hz
-36 dB @ 400 Hz
-48 dB @ 800 Hz
Contrast that with the same 12 dB/octave slope kicking in at 100 Hz, and you get€¦
-0 dB @ 100 Hz
-12 dB @ 200 Hz
-24 dB @ 400 Hz
-36 dB @ 800 Hz
In such a case, the 100 Hz crossover point is certainly worse than the 50 Hz crossover point, and very likely problematic.
On the other hand, consider a 24dB/octave crossover slope implemented at 100 Hz, and the results show€¦
-0 dB @ 100 Hz
-24 dB @ 200 Hz
-48 dB @ 400 Hz
-72 dB @ 800 Hz
As you can easily see, a subwoofer with a 12 dB/octave crossover would not be suitable for a remotely higher crossover frequency. However, that would not preclude 100 Hz as a crossover frequency, as the sharper filter slope that started at 100 Hz would have less content above 200 Hz than the first, shallower filter that started at 50 Hz.
2. Some subwoofers aren’t suitable for higher crossover frequencies because of distortion. Harmonic distortion components are multiples of the original content, be it an original fundamental or harmonic itself. For example, harmonic distortion of a 20 Hz tone will generate energy @ 40 Hz, 60 Hz, 80 Hz, 100 Hz, etc., usually with the lower components higher in amplitude in the case of loudspeakers. In fact, many less experienced listeners, even some experienced musicians, will actually PREFER the distorted low frequency reproduction, as it provides MORE low bass, with the added spectrum in the more audible range. It sounds louder, and richer. When these subwoofers are allowed to run at higher frequencies via higher crossover settings, their harmonic distortion components reach a higher spectrum as well. It’s not that they necessarily produce more distortion than at lower crossover settings, but that the distortion is easier to hear, and easier to locate, and unlike the original content that fed the subwoofer, impervious the attenuation by the crossover, as the distortion is generated after the filter circuit by the power amplifier or the driver itself. In such a case, many may blame the crossover frequency for the increased localization problem, when in fact it’s just making the distortion problem more obvious. While I mention this in the context of subwoofers that have problems with higher crossover points, for those looking for any measure of fidelity, I would go so far as to offer this as an indication of a subwoofer unsuitable for use, period.
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.
* there are a few units which add the LFE channel to subwoofer output without low-pass. Electrical summing of low-passed and not low-passed material results in phase issues and so this method of handling the LFE channel is not advised.
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