Originally Posted by Rythmik
One of my customers, Dave, asked me to verify how AVR handles LFE signals. Under his recommendation, I bought Aria 2 calibration dvd. First, I set the crossover of my Denon 3805 to 150hz in order to preserve as much LFE signal as possible going into SUB pre-out and I used my data acquisition system to capture the waveform and do a fast FFT on it. And to my surprise, that track only has frequencies up to 80hz and the roll-off of 80hz is almost of brick wall type. Here is the waveform and frequency domain plot.
What is more interesting is the xover does not affect LFE going into SUB and at least from Aria 2 the bandwidth is limited to 80hz. In other words, when I set the xover to 40hz in my Denon, the sub preout outputs the same signal. What the crossover point affects is the front channels and other channels that are set to "small". As someone else already pointed out, it is very likely that most DVD will limit the LFE content to below 80hz, apparently to avoid localization of the sub. If the LFE does contain content up to 120hz, we need to use stereo subs to avoid the sub becoming localized. It is actually very easy to do that check. All we need to do is to have a HPF on sub preout to detect any signals above say 85hz.
To make the analysis complete, I also captured the waveform for front channel pre-out and the FFT result is:
BTW, Dave also mentioned a case that I think worth sharing with everyone. His friend used the bypass switch on a Hsu sub to demonstrate that when he disabled bypass and set the low pass filter knob to 80hz, he lost a lot of signals. Actually that makes prefect sense. This is because Hsu's LPF is 24db/oct and at xover point the phase shift is exactly 180 degrees, which is equivalent to reversing phase. So an apple-to-apple comparison is after he disabled bypass, he should set the sub's distance 6ft further so that he can compensate the 180 degrees phase shift at 80hz xover point (BTW, the wavelength at 80hz is 12ft. 180degrees is half wavelength, or 6 ft). So do not just flip that switch and expect everything else is same. As for our sub, we are only 12db/oct. So the phase shift is 90 degrees, and when one switch from our LFE position to AVR/12 position in the Low pass filter setting, he needs to add 3 ft to the sub's distance to get the same phase as in LFE position. Conversely, it one has optimized his system in AVR/12 position, when he switch from AVR/12 to LFE mode, he needs the do the opposite, which is to subtract 3 ft from sub's distance. All above discusssion assumes the xover point is set to 80hz.
I am Dave's friend with the HSU sub, and I have a Sony 5300ES receiver.
Did you mean Avia 2, not Aria 2? FYI, the first Avia DVD has an LFE channel sweep that goes from 100Hz to 20Hz. This is the disc I have and use for calibrating my audio/video components.
I believe different brands of receivers handle LFE differently. On one thread here, a certain receiver's manual stated that the speaker/subwoofer crossover point would be applied to the LFE channel as well, and filter out signal above that point from the LFE signal sent to the sub. I don't believe it stated or was clear if the speakers received the LFE signal above that point. My own receiver does not seem to change the LFE output based on the sub/speaker crossover point. So this would need to be tested and verified on a case-by-case basis.
There's a lot of talk about the Rythmik subs in your post, but let me clarify what I spoke with Dave about.
First of all, I told him that I lost a significant amount of bass from my sub during movies when I engaged the HSU's internal 90Hz crossover (that's the max frequency it can be set to). Here's a measurement from REW with the bass going to the mains and subwoofer with an xo point of I think 70-80Hz or so, but with the main speakers turned off to show how much more bass the second xo filters out of the signal that it should be playing. The red line is the normal roll-off slope determined by the receiver to blend with the mains, plus whatever room effects there are. The blue line is with the sub's own 90Hz xo engaged, adding a second filter to roll off the bass even quicker.
The phase setting here shouldn't matter, because it's only the sub playing, no speakers.
I told Dave that engaging this xo meant led to a significant decrease in the amount of bass during movies. The extra filter was reducing the output of the signal that was coming from the other speakers, plus filtering out any LFE signal there could have been up to 120Hz.
This came up because he was looking at I believe several different amp models of Rythmiks that had different frequency responses. I think one rolled off above 90Hz or so, and the other was much flatter to 150Hz or so. I was telling him that the version that rolled off above 90Hz~ could give him reduced levels of higher bass because of this, with the bass management for the mains, plus LFE potentially up to 120Hz.
You make good points about phase and changing the sub distance setting to match. I think I spoke with Dave about this a bit.
Here is a graph of the response from my HSU sub and my speakers, using a single phase setting on my sub, with sub distance setting distances ranging from 3'6" to 8'.
I was able to find the optimum sub distance setting for both 0 and 180 phase settings on the sub (which ended up being about 3'6" (could not go lower than 3'3"), and 6' for the opposite phase setting, with my crossover in the receiver at 70Hz), and after doing so, I optimized my BFD filters to make each variation flat. This graph compares the sub+speakers playing with the sub at 0 and 180 phase settings, with the sub distance changed in the receiver and the BFD eq filter set at the optimal levels.
Graphs are from the gallery in my signature.