Subwoofer X/O frequency experiments

I have really been going crazy trying to find the best X/O frequency for my elaborate 6-driver subwoofer set and my three Maggie MG1.6. I finally had the thought that the optimum frequency depends on the kind of music being played. So I bought an electronic crossover that I can locate in the listening room and make various adjustments (including X/O frequency) while music is playing. (My old X/O was located in the cellar with my power amps, and the frequency can only be changed by plugging in a different module…not very convenient for experimentation). Initial tests confirm what I thought.

First I played a CD of Wurlitzer theatre organ music. Heavy duty bass! No doubt that the best results were with the X/O set to 300-400Hz. Not all subwoofers can do 400 Hz, but mine can. Interestingly when I cranked it up to about 800Hz harmonic distortion became evident, but not unpleasant…it sounded exactly as if the organist had pulled out another stop. Organ pipes produce very pure tones, and little pipes are played simultaneously (via stops) so as to produce harmonics and make it sound nice. I did the same thing electronically. Sort of “tube amp sound”.

Next I played a Mozart Symphony DVDA (MDG 940 0967-5). Again this is a massive complex sound source, mostly loud, and 130 Hz or thereabouts seemed best.

Last I played a DVDA Emmylou Harris, Producer’s cut (Warner R9 78174). On this one anything over 50 Hz was bad. It has the usual pop music bass line, probably exaggerated to sound better on inferior systems.

I know that the audiophile party line is that the subwoofer should be crossed over where the main speakers roll off, generally below 50 Hz. This is a “safe” bet if you have to pick one frequency and stick to it. Been there, done that. However, I suggest that there is (or ought to be) a significant overlap of frequencies that both the main and the subwoofer can reproduce well, and that the optimum X/O varies depending on the music.
My experiments indicate the same.
In fact I have found that music from the same disc/recording also has differing optimum X/O frequencies!
This problem has driven me crazy as well.
It would be great if a box of electronics could recognize this and automatically adjust the X/O optimumly track by track.
It is so frustrating achieving the correct X/O for every track/recording that my subs often get turned off.
I think the main reasons for this problem is that all audio engineering environments (monitoring hardware etc) are not identical.Also engineers differ in their approach to the varying material they are working on.
I love my subs with music but find it hard to conrol them because of the constant adjustments required for optimum performance.
I think all subwoofer owners,regardless of the make they own,have encountered this problem when listening to music.
Anyone out there with advice/solutions to achieving perfect balance of low frequency that does not requre constant adjustment?
...advice/solutions to achieving PERFECT balance of low frequency that does not requre constant adjustment? (my emphasis)
Haven't found any:( As Eldartford, above, I found that 120-130Hz on classical (+0/-1 octave, LR 24 or Bessel) is the easiest compro to live with -- but I still end up adjusting the VOLUME on the sub amps...

I use a two 15" driver, open baffle, subwoof per side. I experimented in the same way as both of you using a friend's Behringer 2496... not only did the "better sounding" frequency oscillate from recording (or even track) to recording, but the "optimum" SLOPE did, too.

There are too many variables that complicate our lives here... maybe setting up a mike in the room & feeding input to a preset equaliser (s/thing like that B&O spkr thingy)? Rather ugly, complicated, and with too many opamps in the signal path...
>>"Anyone out there with advice/solutions to achieving perfect balance of low frequency that does not requre constant adjustment?"<<

Try setting your sub's crossover 1 octive above your speakers -3db point, and use the subs volume control for blending.

If your speakers are full range, and lets say their -3db point is 25hz, set the sub crossover at 50hz, if you are using a mini monitors whose -3db point is lets say 40hz, then set the crossover on the sub at 80hz.

Now all you have to deal with is the standing waves, and bloat from the rooms Primary Resonate Frequency points.
Here is an interesting link related to your question.
My experience is that playback volume strongly influences how well the main and subs mix. What works at low volume levels is way to prominent at normal and high volumes.
Onhwy61: absolutely. But that's related to our ears' sensitivity curve (the Fletcher/Munson "phon" curve). You would typically lower the volume on the sub at higher levels.
Davehrab...What you explain is the conventional wisdom, and what I described as a "safe bet". In many, but not all, cases significantly higher X/O sounds better.
... "safe bet".....

Eldartford, I'm surprised at you. A man who works with precision guidance controls using a "safe bet".

Would you accept a "safe bet" if you where targeting that "Russian school filled with 18 year old blond women".... I think not!!

I should have prefaced my comments by saying they applied mainly to box speakers, in fairly rectangular shaped rooms.

Now, with your Maggie's or any other panel or Stat's type speaker ... all bets are off.

Certainly the room can cause "anyone" it's share of the problems. I think the sub sometimes takes the rap for a bag room. It only takes a few bad room modes, in the mid and upper bass, to give that sub a reputation for being slow, bloated, and a-musical.

After you go through enough subs, with the same results, you come to realize it's your ceiling's 7 foot height that is causing a 70hz peak, and maybe the last 3 subs weren't really the bad guys.

Tough challenge seem the sub to the mains, and then that package to the room.

>>"my elaborate 6-driver subwoofer set"<<

Man thats AlottA warheads

Davehrab...Hey, those Russian babes really got to you! I am sorry to say that they are about 21 by now, but still quite desirable.

With regard to "safe" design decisions, military stuff is much more conservative than commercial. The consequence of error is worse (like that Chinese Embassy). Our chips, for example are larger and run slower than commercial functional equivalents (but they can fly through a nuclear fireball and still work).

My recent observations are based on planar speakers, and an unusual subwoofer array, but I think that similar results will be seen with box speakers. The important factor is a subwoofer that doesn't conk out at 100 Hz, but you would want that anyway even if you crossed over at 50 Hz.
The important factor is a subwoofer that doesn't conk out at 100 Hz...
True, especially to avoid sub drivers' distortion as the frequency goes up.
Generally speaking, I've noticed that most of us use subs as woofer complements, i.e. to offer extra energy that's lacking in the 90-40Hz region -- rather than to introduce energy in the 16-40 region.
The difficulty with most subs is, drivers cannot reproduce both (say) 16Hz & 300Hz (about 4octaves!) signals at the same time w/out some form of distortion and major difficulties in blending with the sats. They hit mechanical & electrical limitations. With the sub set high the resulting sound in the room may "sound" pleasing at first, but, the reproduction anomalies quickly show up in classical music (to cite one example).
Gregm...What you say suggests that the subwoofer should really be two-way, 20 to 50 and 50 to whatever, and I agree with this. Each of my three subwoofers consists of two drivers, 15" and 12" driven by separate power amps, and I have actually tried the two-way subwoofer thing. It doesn't seem worth the trouble, so I use both drivers through the full SW range so as to take advantage of maximum cone area to minimize cone excursion. Perhaps because the cone excursion is much less than most SW, my drivers (remember there are six of them)don't have the IM distortion problem which you describe...not to any noticable extent.
"Blending" with the main speakers is also not a problem even with the higher X/O frequency. My SW are located directly (2 ft) behind the MG1.6, and off the floor so that their sound goes through and around the Maggies, just as if the Maggies were producing it.

For the most accuracy you need your sub-woofer configured such that its sound sums flat with the main speakers at your seating location (this requires the appropriate magnitude and phase relationship)

For the most accuracy from any speaker you need it placed so that you're not getting cancellations from the reflected waves as the difference between direct and reflected sound approaches a 180 degree phase shift (282 feet to the point of reflection/ frequency) and so that the bass/midrange increase you get as it couples to the wall behind it is addressed with its cross-over baffle step compensation.

For the most accuracy from a system you need to avoid pushing drivers high enough that they break up or beam, or low enough that the distortion that goes with excursion is not too high.

The type of music, playback level, etc. may change which distortions are less objectionable but can't change physics.

Transfer functions are all that matters whether they result from acoustic or electrical effects.

An analog low-pass or high-pass filter or digital filter that acts the same (Signal processing is outside my area of engineering) has an inherent phase shift. The outputs from high and low pass filters of the same type and order put the outputs 90 degrees out of phase for each order.

The highest bass accuracy also results when you don't put energy into a room below (565 feet / longest dimension) Hz.

All this explains why big speakers don't work well in small rooms and what sub-woofer/main configurations produce the flattest (or most enjoyable with a given type of music) response.

If you have a sealed speaker its mechanics mean you have a 2nd order roll off. You get it and the woofer to sum flat with a 2nd order electrical low-pass on the sub that inverts its signal, the F3 points match between speaker and cross-over, and both share the same Q. This is why sealed speakers mate well with something like an REL that has a cross-over with adjustable frequency and Q.

The attenuation and phase shift an octave into a system's pass-band are negligible. You can high-pass a set of speakers an octave above their F3 point and low-pass your sub at the same frequency. They'll sum flat with a fourth order electrical cross-over. Or a second order electrical with the sub inverted. This is why fourth order cross-overs an octave above the speaker's F3 point work well.

In a sealed speaker excursion doubles for each octave lower (halving of frequency) you go. Once a ported speaker starts rolling off its excursion quadruples with each octave drop. Distortion comes from excursion, so distortion increases faster in ported speakers below their cross-over point. A 4th order cross-over maintains constant excursion on the speaker so excursion isn't increasing. This is why a higher order cross-over works better with ported speakers.

When you pair a sealed speaker with a second order high-pass and sub with fourth order low-pass you can get them to sum flat. THX home theater systems do this with sealed speakers having poles at 80H Q=.707, a second order butterworth electrical high-pass, and fourth order Linkwitz Riley low-pass. You have an LR4 response on both.

When you get a 1/2 wavelength length difference (say 3.5' from a wall for 80Hz) you get cancelations. With a speaker 4' out you don't have cancelation above 80Hz. With the sub-woofer in a corner you don't get a cancelation at any frequency. By the time frequency drops enough for a speaker to couple to the wall behind it it has little output. The sub-woofer is always operating in the same space so it is unaffected by this. This is why softit or in-wall speakers can work very well.

A higher order sub-woofer low pass keeps its cone out of breakup.