Subwoofer Crossover Importance

F

fredk

Audioholic General
You see posts of and on about the importance of crossovers and the need for good crossover design, but that conversation never seems to extend to subwoofer crossovers.

Crossover design is supposed to be specific to drivers, yet we are quite happy to use whatever crossover is provided in our reciever of choice. Is there something different about crossovers below a certain frequency or is it one of those compromises that we have come to accept without question?
 
TLS Guy

TLS Guy

Seriously, I have no life.
You see posts of and on about the importance of crossovers and the need for good crossover design, but that conversation never seems to extend to subwoofer crossovers.

Crossover design is supposed to be specific to drivers, yet we are quite happy to use whatever crossover is provided in our reciever of choice. Is there something different about crossovers below a certain frequency or is it one of those compromises that we have come to accept without question?
You have put your finger on the Achilles heel of sub sat systems. A crossover is a crossover, and has to be designed specifically like any other. So crossover equalizers can help.

However this is the reason the for my main system, I designed true full range speakers for the left and right mains and rears, and use no sub.
 
Swerd

Swerd

Audioholic Warlord
You have put your finger on the Achilles heel of sub sat systems. A crossover is a crossover, and has to be designed specifically like any other. So crossover equalizers can help.

However this is the reason the for my main system, I designed true full range speakers for the left and right mains and rears, and use no sub.
I agree with TLS, especially with a sub and satellite system where the crossover frequency to the sub might be high.

But if the crossover to the sub is low enough, then potential problems are less likely, and the active electronic crossovers in most HT receivers are OK.

Here's an example of a decent 12" sub woofer. If you look at the manufacturer's frequency response curve for this driver, you'll see its pretty flat up to about 400 Hz. The bad problems are above 1000 Hz, but no one should use this driver in that frequency range. Most HT users, set their subwoofer crossover at no higher than 100 Hz and often lower. For this driver, in a sealed cabinet of 1.8-2.0 ft³, they'll get decent bass down to about 25 Hz. So, even though TLS is right, with the crossovers in standard HT receivers used correctly, this should not be a problem.

And yes, I also agree with TLS that an electronic parametric equalizer can help smooth out the kind low bass response problems most of us have due to room reflections.
 
F

fredk

Audioholic General
But if the crossover to the sub is low enough, then potential problems are less likely,...
Why? What is different about lower frequencies?

So crossover equalizers can help.
Something like a BFD1124?

I am interested in Dr. Geddes aproach to smoothing/evening bass response, but wonder at what compromises there are in introducing yet another crossover point.
 
highfigh

highfigh

Seriously, I have no life.
Why? What is different about lower frequencies?



Something like a BFD1124?

I am interested in Dr. Geddes aproach to smoothing/evening bass response, but wonder at what compromises there are in introducing yet another crossover point.
The octave from 20Hz to 40Hz, while covering the same number of musical note as any other octave, is still only 20Hz wide. The common room dimensions are sometimes too small to have the same effect as a larger room and the ratios are such that, although they can still be problematic, they will often seem like it's only one note that causes issues with bass smoothness. For every EQ center frequency used, phase shift enters the signal and some equalizers that use more than 1/3 octave spacing will only start that above the bass region. In the low end, they may have 5 bands from 16Hz to 125Hz but using 1/3 octave spacing in that octave would place the bands at approximately 16Hz, 22Hz, 29Hz and 32Hz. The long wavelengths of bass notes makes it possible to be at a null point and in order to be at a location where its energy is at maximum, moving the listening position slightly may smooth the bass by minimizing destructive interference at that point but it mahy also alter the general image. In the midrange, where the human ear is most sensitive, the problems are often multiplied by the fact that the sound reflecting from one surface may be sufficient to reflect from a second surface (and possibly a third), which can cause multiple problems in the response. The crossover point of a subwoofer is commonly the cause of a peak or valley in the response and in most cases, trial and error must be used to cure this in the absence of test equipment. Also, a full range speaker system can be phase-aligned more easily in many cases because few sub crossovers have more than a 0°/180° phase switch is far from sufficient, IMO.
 
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highfigh

highfigh

Seriously, I have no life.
You have put your finger on the Achilles heel of sub sat systems. A crossover is a crossover, and has to be designed specifically like any other. So crossover equalizers can help.

However this is the reason the for my main system, I designed true full range speakers for the left and right mains and rears, and use no sub.
How did you treat your main room?
 
Swerd

Swerd

Audioholic Warlord
Why? What is different about lower frequencies?
All I meant to say was that as long as the subwoofer crossover point is kept much lower than the frequencies where breakup occurs, it will behave OK. In the example I showed, breakup starts around 1000 Hz, well above the usual subwoofer crossover point. If an 80 Hz crossover point is chosen, and a crossover slope of 12 dB/octave is used, noise from breakup for that sub, roughly four octaves higher (1280 Hz), will be quieted by 48 dB.

In midwoofer to tweeter crossovers, care must be taken to avoid or suppress the midwoofer's breakup noise. With most subwoofers, this is not such a problem. So for that reason I said custom designed crossovers may not be required for subwoofers. Most subwoofers are used over a smaller range than other drivers are.

Something like a BFD1124?
Yes, like the Behringer Feedback Destroyer, DSP1124P.
 
highfigh

highfigh

Seriously, I have no life.
All I meant to say was that as long as the subwoofer crossover point is kept much lower than the frequencies where breakup occurs, it will behave OK. In the example I showed, breakup starts around 1000 Hz, well above the usual subwoofer crossover point. If an 80 Hz crossover point is chosen, and a crossover slope of 12 dB/octave is used, noise from breakup for that sub, roughly four octaves higher (1280 Hz), will be quieted by 48 dB.

In midwoofer to tweeter crossovers, care must be taken to avoid or suppress the midwoofer's breakup noise. With most subwoofers, this is not such a problem. So for that reason I said custom designed crossovers may not be required for subwoofers. Most subwoofers are used over a smaller range than other drivers are.

Yes, like the Behringer Feedback Destroyer, DSP1124P.
Are you assuming that the speakers will be pushed hard, all the time? That's usually when they break up anyway, so if the system isn't run hard, breakup should never really be much of an issue, right?
 
no. 5

no. 5

Audioholic Field Marshall
You see posts of and on about the importance of crossovers and the need for good crossover design, but that conversation never seems to extend to subwoofer crossovers.

Crossover design is supposed to be specific to drivers, yet we are quite happy to use whatever crossover is provided in our reciever of choice. Is there something different about crossovers below a certain frequency or is it one of those compromises that we have come to accept without question?
I don't think so; it seems to me just to be something most people just don't think about. It's optimistic to think that an AVR's one size fits all crossover will perfectly integrate a subwoofer into a system, but for the most part it's what we're stuck with.
 
TLS Guy

TLS Guy

Seriously, I have no life.
How did you treat your main room?

I did not use any sound absorbing panels.

I set the room dimensions as optimally as the layout of the home permitted.

I used wool carpet.

The objects in the room were arranged to produce as much irregular reflection as possible. The equipment wall makes for a highly irregular surface.

The the windows on the opposite side also scatter the reflections. The rear wall houses my LP collection. There is a window, which adds a little liveliness there, as well as giving a good view of Benedict Lake.

However I think the major reason for the bass clarity, is the fact that it is an aperiodically damped TL system. These systems have a bass that is quite unlike reflex or closed box systems. It is very different in many ways. The bass reproduction is inherently non resonant but at the same time deep and powerful. So the Laplander drumming in the first movement of Aho's first symphony, is literally as "tight as a drum".
There is no ring or carry over. It sounds like bass drums all round the room.

Tymps sound like they really do, the bass strings sound just like they should as well as piano bass strings. The "being there" realism is quite uncanny. The lack of boom and muddle in the bass line, allows for clarity of texture through out the frequency spectrum.

Many visitors have heard this system now, and the clarity, balance and realism of timbre of all instruments and voices is always the focus of their comments.

We had close friends from Minneapolis visit this weekend. They brought along a couple, both high powered lawyers, visiting from California. They are opera buffs. They frequently listen at a close friends house who apparently has an extremely costly system. They watched opera here for a significant part of the visit. To say they were astonished was putting it mildly. I put on a first class production of Mozart's Marriage of Figaro, from Glyndebourne. A stellar cast under Haitink, the whole production beautifully filmed and recorded. They were totally taken in with the realism of the sound.

I think there is another reason why TLs are kind to rooms, and that is a phenomenon that organ builders refer to as the encircling nature of the sound from speaking pipes. By that they mean that pipes fill a room in a very uniform fashion, unlike a usual speaker system, guitar or piano. A pipe organ has a remarkably uniform spl. throughout the space, whereas the others loose spl, very quickly as you put distance between you and the source.

A TL is a very specialized pipe. Much more of the total output comes from the pipe mouth, and over a far greater frequency spectrum than from a reflex port.
This phenomenon of encircling has never received much attention outside the realm of organ builders. However after an experienced organ builder demonstrated the truth of the phenomenon, I have often wondered if that is a big reason as to why TLs seem to maintain clarity in a wide variety of listening rooms. In fact I believe the nature of the output from reflex ports is a root cause of lumpy bumpy bass reproduction.
 
TLS Guy

TLS Guy

Seriously, I have no life.
I don't think so; it seems to me just to be something most people just don't think about. It's optimistic to think that an AVR's one size fits all crossover will perfectly integrate a subwoofer into a system, but for the most part it's what we're stuck with.
I could not agree more.
 
highfigh

highfigh

Seriously, I have no life.
I did not use any sound absorbing panels.

I set the room dimensions as optimally as the layout of the home permitted.

I used wool carpet.

The objects in the room were arranged to produce as much irregular reflection as possible. The equipment wall makes for a highly irregular surface.

The the windows on the opposite side also scatter the reflections. The rear wall houses my LP collection. There is a window, which adds a little liveliness there, as well as giving a good view of Benedict Lake.

However I think the major reason for the bass clarity, is the fact that it is an aperiodically damped TL system. These systems have a bass that is quite unlike reflex or closed box systems. It is very different in many ways. The bass reproduction is inherently non resonant but at the same time deep and powerful. So the Laplander drumming in the first movement of Aho's first symphony, is literally as "tight as a drum".
There is no ring or carry over. It sounds like bass drums all round the room.

Tymps sound like they really do, the bass strings sound just like they should as well as piano bass strings. The "being there" realism is quite uncanny. The lack of boom and muddle in the bass line, allows for clarity of texture through out the frequency spectrum.

Many visitors have heard this system now, and the clarity, balance and realism of timbre of all instruments and voices is always the focus of their comments.

We had close friends from Minneapolis visit this weekend. They brought along a couple, both high powered lawyers, visiting from California. They are opera buffs. They frequently listen at a close friends house who apparently has an extremely costly system. They watched opera here for a significant part of the visit. To say they were astonished was putting it mildly. I put on a first class production of Mozart's Marriage of Figaro, from Glyndebourne. A stellar cast under Haitink, the whole production beautifully filmed and recorded. They were totally taken in with the realism of the sound.

I think there is another reason why TLs are kind to rooms, and that is a phenomenon that organ builders refer to as the encircling nature of the sound from speaking pipes. By that they mean that pipes fill a room in a very uniform fashion, unlike a usual speaker system, guitar or piano. A pipe organ has a remarkably uniform spl. throughout the space, whereas the others loose spl, very quickly as you put distance between you and the source.

A TL is a very specialized pipe. Much more of the total output comes from the pipe mouth, and over a far greater frequency spectrum than from a reflex port.
This phenomenon of encircling has never received much attention outside the realm of organ builders. However after an experienced organ builder demonstrated the truth of the phenomenon, I have often wondered if that is a big reason as to why TLs seem to maintain clarity in a wide variety of listening rooms. In fact I believe the nature of the output from reflex ports is a root cause of lumpy bumpy bass reproduction.
I definitely agree that pipes produce a more uniform sound than speakers in a ported box and I have never liked the sound from ported speakers, especially when the program material goes below the tuning frequency. I like the damping of sealed boxes, although it's usually so hard to have the extension into the lowest frequencies that it's often just a matter of deciding that "It's low enough and I can't afford to really get what I want/the room just won't work for the lowest F3 without a major redesign". My brother sang & recorded with John Weissrock in the '70s and I have a friend who made some TL subs in the late '70s/early '80s and I have heard the way they "put you where the music was played" more than other designs. Ports are too much of a compromise IMO, and as you say, the tuning is hard to match to the woofer/box in order to keep the response/impedance/phase from being like a roller coaster. The air moving across the mouth of the pipe being the actual sound generator and the port being there as a "helper", positioned so close to the actual driver, is one of the biggest reasons I think the response is as irregular as it is.
 
F

fredk

Audioholic General
Thanks for all the responses folks. You've given me lots to think through.

So it seems the consensus is that, with the right equipment, the additional crossover when using an 'MBM' as well as a sub would be manageable if you have the right equipment (like say a dcx2496).

I find the Geddes approach quite interesting in that he likes to use multiple high quality pro drivers (sealed) to handle sound production in the music frequency range.
 
F

fredk

Audioholic General
The argument of ported vs. sealed vs. TL vs.... is interesting, but I wonder if it becomes increasingly less important as more capable drivers arrive in the market. I personally suspect that there are a number of different solutions that would work equally well and what you choose depends on your personal situation.

I do not have space to put multiple 20cu.ft. ported subs tuned to 12Hz. in my livingroom to reproduce the low end. If I can place multiple smaller mbm-type subs in my room to take advangage of the acoustics (or more appropriately minimize the empact of small room acoustics) in the most important bass region - 40 to 150Hhz - that would work best for me. I then only have to acomodate one larger sub capable of handling the stuff below 40-50Hz.
 
F

fredk

Audioholic General
TLS guy. Do you have a link to the design principles behind TL subs? this is one area I have not looked in to at all.
 
highfigh

highfigh

Seriously, I have no life.
The argument of ported vs. sealed vs. TL vs.... is interesting, but I wonder if it becomes increasingly less important as more capable drivers arrive in the market. I personally suspect that there are a number of different solutions that would work equally well and what you choose depends on your personal situation.

I do not have space to put multiple 20cu.ft. ported subs tuned to 12Hz. in my livingroom to reproduce the low end. If I can place multiple smaller mbm-type subs in my room to take advangage of the acoustics (or more appropriately minimize the empact of small room acoustics) in the most important bass region - 40 to 150Hhz - that would work best for me. I then only have to acomodate one larger sub capable of handling the stuff below 40-50Hz.
The thing about different cabinet configurations is that they don't all give you the same Q factor. A high Q rings like an undamped drum head and a really low Q gives an extremely damped woofer, although it can only dampen the movement so much. A properly designed sealed box gives a Q of about .707 and it tends to be much more versatile than a box with a high Q, like a ported box that's too small. If you want smooth bass without large peaks, a TL is best, followed by a sealed box. Another thing that needs to be considered is that even though a woofer/box combo models a particular way in the computer, it won't necessarily give you that response because of the way the energy transfers to the room. A huge woofer in a small room will usually be thumpy and you will often hear the lowest notes when you leave the room. A small woofer in a large room will generally go unnoticed unless you put a couple of horsepower into it or it happens to be really sensitive.
 
F

fredk

Audioholic General
high Q, like a ported box that's too small.
You had me until that qualifier. How about a ported box that is the right size or properly designed, since that is the qualifier used for sealed?
 
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