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Pairing a subwoofer to a 2 (or 5 or 7) channel system: How?

post #1 of 8
Thread Starter 

I'm in the market for a powered sub to add to my powered monitors (and I started an advice thread in that other equipment subforum) and I've just started trying to work out what the important factors are when trying to update a 2.0 setup to a 2.1 setup.

 

The reason I started this thread to ask the Sound Science community what the important principals are that one should focus on when adding a subwoofer to a two channel system. My google-fu comes up empty on recent threads in this forum on the topic.

 

I guess what I'm interested in determining is firstly, what the issues are, and secondly, how should those issues be addressed. In my other thread, I started asking a bunch of questions which I can repeat the technical ones here:

  1. What size transducer should I look for? 
  2. What is appropriate low frequency range? 
  3. What is appropriate high frequency range? How does the high frequency range relate to where I would want to set the crossover frequency and/or my monitors frequency response?
  4. What kind of cross-over network should I look for? Passive or Active? minimum phase or maximum roll-off or other?
  5. At what frequency would I want to set the crossover network for optimally matching with the speaker's response? Namely, if the speakers have a corner frequency at 100Hz, would I set my crossover there?  lower?  higher?
  6. Do I want a 12 dB/octave crossover? 24 dB/octave? Does it matter and/or do I choose based on the low-end roll-off of the speakers?
  7. Front-firing vs down-firing?
  8. Ported vs not?
  9. Are there any questions I'm not asking, but that I should be considering?

 

 

 

So... any thoughts on what questions I should be asking and anyone have a good approach for finding answers?

 

 

Cheers!

post #2 of 8

 

 

Quote:
Originally Posted by ab initio View Post
 
  1. What size transducer should I look for? 

 

Generally, a larger transducer means deeper bass but probably less accurate in the higher range of its response. Some will claim you should choose based on room size, but a lot of other factors like enclosure design and amp power (and how much of that power the subwoofer driver can take) can overcome driver size.


Personally, I'd choose based on these factors:


i. Practicality : will it fit your room? will you be able to enjoy all the power and cone area without pissing off people, in your house or the neighbours?

ii. What exactly do you need it for? : If it's purely for action movies, then personally I'd get the biggest one with the lowest distortion I can afford. If it's purely for music, don't waste space and get an 8" that is known for accuracy; even thundering classical music or pounding hip hop wouldn't be obviously lacking in a dedicated room. Little bit of both? Get an 8", 10" or 12" known to be great for music that has a lot of power, then use the gain settings (on the HT receiver or the sub) to get more or less depending on what you're watching.

iii. Matching well with the other speakers : more on the frequency response below, but basically with the first point considered, get the largest, and more powerful you can afford that is practical.

 

Quote:
Originally Posted by ab initio View Post
 

2. What is appropriate low frequency range? 

 

 

Lower limit should be as low as possible, but if you're choosing between a $300 8" that has its -3db point at 25hz  and a $500 10" with its -3db point at 20hz, the only reason for spending the extra $200 is if despite being 8" its enclosure was designed to go that low at the expense of speed and accuracy in music.

 

Quote:
Originally Posted by ab initio View Post
 

3. What is appropriate high frequency range? How does the high frequency range relate to where I would want to set the crossover frequency and/or my monitors frequency response?

 

 

High frequency response should be chosen based on the main speakers' response, so that the total system response has them catching each other. Take note however that just manufacturer stated specs won't tell the whole story. You can have a monitor with 5" drivers going down to -3db at 60hz, but depending on the levels of the speaker amp/receiver and the sub amp, you might encounter more difficulties matching them properly. A crossover on each side doesn't just splice the sound and it's gone; see the ratings for crossover "slope"? It's kind of like the -3db point on speakers - it just introduces a roll off but even the sharpest settings (on subs that's usually -24db) the frequencies above that point don't disappear. Ergo, depending on the room (or the car) you might have to put a gap or overlap in the settings. My car's receiver-processor for example gives the sub a low pass -18db at 60hz, but the front midwoofers are crossed at high pass -12db at 80hz; put the same stuff in a different car and I don't expect the same settings.

 

Now, personally, any decent 5" mains in an acoustically decent room should do well for movies tuned by ear. It's really music that can complicate the match, in which case if it was my set-up, I'd get the best 6" speakers that go down to 50hz, that way if I use the sub for music (like for hi-def concerts), most of the music comes through the midwoofer. Alternately, some people would prefer less freqs on each driver and making them more specialized, but I'm more of the Fostex fullrange kind of philosophy than putting a half dozen drivers on each speaker just so it can produce a lot of bass, or putting too much of the bass on a subwoofer that sits separately from the mains (bass may be non-directional, but that doesn't mean it's immune from room modes).

 

Quote:
Originally Posted by ab initio View Post
 

4. What kind of cross-over network should I look for? Passive or Active? minimum phase or maximum roll-off or other?

 

 

Many subs just have an auxilliary passive crossover input for 2.0 use, using the speaker output of the amplifier to pass through it before going to the speakers. However there are other ways to wire up a system to use the active crossover. In a 2.0 system, if your integrated amp is one of those full-size ones that have a tape loop, you can hook up the sub there. You won't get a high pass on the speakers but you can just lower the frequency of the variable low pass filter on the sub (IF it has a variable setting) just to fill out where the speaker rolls off.

 

To ensure all those settings, it might be a better option to go with an HT receiver. Their active filters have various settings* for phase (this is just whether the driver pumps normally, or reverses when it pumps out with when it pumps inwards), slope (-12db, -18db, -24db) and AFAIK some of them use the sub even with 2.0 recording (except instead of a dedicated ".1" track they just run the 2.0 track through the filter). Take note not all affordable receivers have all these features, but that's still usually a lot more control than what's on most active  subs.


*For cars, this kind of receiver will be like the Pioneer P80RS and P90, Clarion HX-D2, etc.

 

Quote:
Originally Posted by ab initio View Post
 

5. At what frequency would I want to set the crossover network for optimally matching with the speaker's response? Namely, if the speakers have a corner frequency at 100Hz, would I set my crossover there?  lower?  higher?

 

There's also the room modes, as I've mentioned above, so you might need to put a gap or overlap. You'd have to use your own ears or get measuring tools (like a USB Microphone, or if it's a home theater receiver, trust Audyssey) and decide where it should be. At least it's not a car - back in the day I'd probably waste gas sitting in my car in a parking lot for 30mins just getting the settings right. Imagine how some car audio competitors were hit with a surprise that their cars would be evaluated with the engine switched off, when they did their tuning with the motor idling (and the system getting close to or a bit over14volts) and the climate control running. You'd still worry about neighbors though, something a guy parked at his house with the windows closed won't have to deal with.

 

Quote:
Originally Posted by ab initio View Post
 

6.Do I want a 12 dB/octave crossover? 24 dB/octave? Does it matter and/or do I choose based on the low-end roll-off of the speakers?

 

 

Again, room modes. It's best if you have variable settings on an HT receiver, or the sub's input has some control over these variables if you're not using an HT receiver. Remember also that manufacturer provided frequency response at best was tested at a different room; worse some could be problematic for other reasons, which means having the option to tweak is important when you're going to complicate the system beyond a 2.0 where you just accept the realities of small speaker response or what large speakers would require in space, amplification, etc.

 

Quote:
Originally Posted by ab initio View Post
 

7. Front-firing vs down-firing?

 

Again, room modes. However not being able to try out a variety of subs actually matters less than other stuff you can do to the sub. When I added a sub to my 2.0 home system, either one didn't matter much as the room as too small, ergo the bass is easily localized to be coming from the floor (besides, either way chances are you'd have the sub or hte port facing you or the floor and vice versa. Solution? I got an old subwoofer box I used in my car, loaded it with a couple of rusted dumbells and a sandbag, and put the sub on top, putting the height of the down-firing sub close to the height of the speakers.

That's just for music bass though. On movies, the rumbling bass wasn't easy to localize, unless it was moving (like a helicopter flying from front to back - yes this was just a 2.1 and it can simulate that).

 

Quote:
Originally Posted by ab initio View Post
 

8. Ported vs not?

 

While technically there's a difference, think about it this way:

1) How many active subs come sealed? (unless you make one)

 

2) Your mains speakers are very likely ported to start with. How problematic would an otherwise properly designed ported sub be?* Chances are if it's "inaccurate" it's probably in terms of gain settings and room modes, not necessarily the ported box making the bass flabby.


*Of course, in some cases, the box was designed to squeeze the most low frequency extension out of a small subwoofer, which means overall response can be affected

 

 

 

post #3 of 8
Thread Starter 
Warning: Spoiler! (Click to show)
Quote:
Originally Posted by ProtegeManiac View Post



Generally, a larger transducer means deeper bass but probably less accurate in the higher range of its response. Some will claim you should choose based on room size, but a lot of other factors like enclosure design and amp power (and how much of that power the subwoofer driver can take) can overcome driver size.


Personally, I'd choose based on these factors:


i. Practicality : will it fit your room? will you be able to enjoy all the power and cone area without pissing off people, in your house or the neighbours?
ii. What exactly do you need it for? : If it's purely for action movies, then personally I'd get the biggest one with the lowest distortion I can afford. If it's purely for music, don't waste space and get an 8" that is known for accuracy; even thundering classical music or pounding hip hop wouldn't be obviously lacking in a dedicated room. Little bit of both? Get an 8", 10" or 12" known to be great for music that has a lot of power, then use the gain settings (on the HT receiver or the sub) to get more or less depending on what you're watching.
iii. Matching well with the other speakers : more on the frequency response below, but basically with the first point considered, get the largest, and more powerful you can afford that is practical.


Lower limit should be as low as possible, but if you're choosing between a $300 8" that has its -3db point at 25hz  and a $500 10" with its -3db point at 20hz, the only reason for spending the extra $200 is if despite being 8" its enclosure was designed to go that low at the expense of speed and accuracy in music.


High frequency response should be chosen based on the main speakers' response, so that the total system response has them catching each other. Take note however that just manufacturer stated specs won't tell the whole story. You can have a monitor with 5" drivers going down to -3db at 60hz, but depending on the levels of the speaker amp/receiver and the sub amp, you might encounter more difficulties matching them properly. A crossover on each side doesn't just splice the sound and it's gone; see the ratings for crossover "slope"? It's kind of like the -3db point on speakers - it just introduces a roll off but even the sharpest settings (on subs that's usually -24db) the frequencies above that point don't disappear. Ergo, depending on the room (or the car) you might have to put a gap or overlap in the settings. My car's receiver-processor for example gives the sub a low pass -18db at 60hz, but the front midwoofers are crossed at high pass -12db at 80hz; put the same stuff in a different car and I don't expect the same settings.

Now, personally, any decent 5" mains in an acoustically decent room should do well for movies tuned by ear. It's really music that can complicate the match, in which case if it was my set-up, I'd get the best 6" speakers that go down to 50hz, that way if I use the sub for music (like for hi-def concerts), most of the music comes through the midwoofer. Alternately, some people would prefer less freqs on each driver and making them more specialized, but I'm more of the Fostex fullrange kind of philosophy than putting a half dozen drivers on each speaker just so it can produce a lot of bass, or putting too much of the bass on a subwoofer that sits separately from the mains (bass may be non-directional, but that doesn't mean it's immune from room modes).


Many subs just have an auxilliary passive crossover input for 2.0 use, using the speaker output of the amplifier to pass through it before going to the speakers. However there are other ways to wire up a system to use the active crossover. In a 2.0 system, if your integrated amp is one of those full-size ones that have a tape loop, you can hook up the sub there. You won't get a high pass on the speakers but you can just lower the frequency of the variable low pass filter on the sub (IF it has a variable setting) just to fill out where the speaker rolls off.

To ensure all those settings, it might be a better option to go with an HT receiver. Their active filters have various settings* for phase (this is just whether the driver pumps normally, or reverses when it pumps out with when it pumps inwards), slope (-12db, -18db, -24db) and AFAIK some of them use the sub even with 2.0 recording (except instead of a dedicated ".1" track they just run the 2.0 track through the filter). Take note not all affordable receivers have all these features, but that's still usually a lot more control than what's on most active  subs.



*For cars, this kind of receiver will be like the Pioneer P80RS and P90, Clarion HX-D2, etc.


There's also the room modes, as I've mentioned above, so you might need to put a gap or overlap. You'd have to use your own ears or get measuring tools (like a USB Microphone, or if it's a home theater receiver, trust Audyssey) and decide where it should be. At least it's not a car - back in the day I'd probably waste gas sitting in my car in a parking lot for 30mins just getting the settings right. Imagine how some car audio competitors were hit with a surprise that their cars would be evaluated with the engine switched off, when they did their tuning with the motor idling (and the system getting close to or a bit over14volts) and the climate control running. You'd still worry about neighbors though, something a guy parked at his house with the windows closed won't have to deal with.


Again, room modes. It's best if you have variable settings on an HT receiver, or the sub's input has some control over these variables if you're not using an HT receiver. Remember also that manufacturer provided frequency response at best was tested at a different room; worse some could be problematic for other reasons, which means having the option to tweak is important when you're going to complicate the system beyond a 2.0 where you just accept the realities of small speaker response or what large speakers would require in space, amplification, etc.


Again, room modes. However not being able to try out a variety of subs actually matters less than other stuff you can do to the sub. When I added a sub to my 2.0 home system, either one didn't matter much as the room as too small, ergo the bass is easily localized to be coming from the floor (besides, either way chances are you'd have the sub or hte port facing you or the floor and vice versa. Solution? I got an old subwoofer box I used in my car, loaded it with a couple of rusted dumbells and a sandbag, and put the sub on top, putting the height of the down-firing sub close to the height of the speakers.


That's just for music bass though. On movies, the rumbling bass wasn't easy to localize, unless it was moving (like a helicopter flying from front to back - yes this was just a 2.1 and it can simulate that).


While technically there's a difference, think about it this way:


1) How many active subs come sealed? (unless you make one)

2) Your mains speakers are very likely ported to start with. How problematic would an otherwise properly designed ported sub be?* Chances are if it's "inaccurate" it's probably in terms of gain settings and room modes, not necessarily the ported box making the bass flabby.



*Of course, in some cases, the box was designed to squeeze the most low frequency extension out of a small subwoofer, which means overall response can be affected

Wow! Thanks for the detailed response!

There's a lot in there to think about, but I know I will have more questions to ask. In my head, I was thinking about pairing a sub with a pair of powered monitors for high fidelity sound (vs for the big booms in action movies---although that's awesome in it's own right).

I guess I'm worried about issues like how the crossover might affect the phase of the sub and monitors and whether that leads to strange in-phase/out-of-phase effects across the common frequencies. I dont know if there is a formula or SOP for choosing a crossover frequency given the type of crossover and characteristics of the transducers. I guess I totally forgot about the importance of the room.

Cheers
post #4 of 8
Quote:
Originally Posted by ab initio View Post


Wow! Thanks for the detailed response!

There's a lot in there to think about, but I know I will have more questions to ask. In my head, I was thinking about pairing a sub with a pair of powered monitors for high fidelity sound (vs for the big booms in action movies---although that's awesome in it's own right).

I guess I'm worried about issues like how the crossover might affect the phase of the sub and monitors and whether that leads to strange in-phase/out-of-phase effects across the common frequencies. I dont know if there is a formula or SOP for choosing a crossover frequency given the type of crossover and characteristics of the transducers. I guess I totally forgot about the importance of the room.

Cheers

 

Quote:
Originally Posted by ab initio View Post


Wow! Thanks for the detailed response!

There's a lot in there to think about, but I know I will have more questions to ask...I guess I totally forgot about the importance of the room.

 

Actually, I couldn't provide a straight answer, but only highlight how the room is very important.

 

 

Quote:
Originally Posted by ab initio View Post

I guess I'm worried about issues like how the crossover might affect the phase of the sub and monitors and whether that leads to strange in-phase/out-of-phase effects across the common frequencies. I dont know if there is a formula or SOP for choosing a crossover frequency given the type of crossover and characteristics of the transducers.

 

There are formulas of sorts, but that also means that you should be able to measure the response in the room prior to and after applying the formula. In any case, there aren't usually infinite settings in those HT receivers, and not much more on the active subs, so you can always just use your own ears instead of calling in an acoustics professional (whose skills depend on the market of the local HT+audio stores, which would be expensive if you consult them but get your stuff online).

 

Or, alternately, if you're going to use an HT receiver, just trust Audyssey or YPAO :D

 

Quote:
Originally Posted by ab initio View Post

In my head, I was thinking about pairing a sub with a pair of powered monitors for high fidelity sound (vs for the big booms in action movies---although that's awesome in it's own right).

 

If it's purely for music, how is the layout in your room, and the size? If you can sit far enough from the speakers, larger towers that go down to 40hz should make enough difference in bass, you just need to pick a quality set (ie the entry-level ones that don't use the same 0.5in MDF panels on three sides of the cabinets) and an amp for them. That isn't too hard if you know what amps to look at - I've used my NAD304 to drive a few of those, with its 35w per channel well below the 50watts or so recommended for the speakers, and man can it drive them hard. I was demonstrating the difference between a dedicated 2ch amp cheaper than HT receivers for a few friends, so we went to an HT showroom and hooked up my NAD304 to the B&W, Mirage, and Polk towers they had. The Denon was too warm, the Yamaha was a bit thin, and both had the bass distorting at lower dB levels than the NAD rated at 35wpc. All these had multiple at least two 6" drivers, and the B&W that had multiple bass drivers (but without the subwoofer amp built into it) produced enough low frequency that I could feel house music bass pounding my chest.

post #5 of 8
Quote:
Originally Posted by ab initio View Post
I guess I'm worried about issues like how the crossover might affect the phase of the sub and monitors and whether that leads to strange in-phase/out-of-phase effects across the common frequencies. I dont know if there is a formula or SOP for choosing a crossover frequency given the type of crossover and characteristics of the transducers. I guess I totally forgot about the importance of the room.

 

Phase isn't really a problem. It has to be WAY out for any phase cancellation to occur. My sub has a phase dial, but I keep it at the detent for in phase and it works fine.

 

The crossover depends on the low frequency ability of your mains. If you have big speakers that go down low, cross over at 80Hz. If you are using bookshelf speakers, cross over at 120Hz.

 

The size of the room dictates the power of the sub. Size isn't a good determiner because some small subs produce more powerful sub bass than large ones. It all depends on the design.

 

In a good sized room, a sub can definitely add a lot to a music system, especially in classical music where there is more sub bass info than in classic rock.


Edited by bigshot - 11/24/13 at 4:49pm
post #6 of 8
Quote:
Originally Posted by bigshot View Post
 

 

 

 

The crossover depends on the low frequency ability of your mains. If you have big speakers that go down low, cross over at 80Hz. If you are using bookshelf speakers, cross over at 120Hz.

 

The size of the room dictates the power of the sub. Size isn't a good determiner because some small subs produce more powerful sub bass than large ones. It all depends on the design.

 

 

 

x-over should always be set according to your mains low end to start, then ear tuned from there according to the room.  ideally you want some overlap between the mains and the sub, and set the x-over to avoid nodes/reflections in the room as well as maintain the smoothest "transition" between the sub and the mains.  THX recommends 80Hz simply because it often works, but use it as a starting point.  personally i level match with a meter first then tune by ear afterwards.

 

displacement (Xmax) is never a substitution for size.  distortion always goes up with Xmax and often the suspensions / materials on large Xmax subs are stiffer/heavier which can degrade fine bass response.  there are some excellent small subs, no doubt (i LOVE my 8")...but in general a larger cone of the same quality will perform better.  but it's all about what you want sound wise in comparison to room size and layout. FWIW my opinion is that with the higher quality subs, the old rule of thumb that big guys are flabby but deep while the little guys are tight but less deep is a bit of a myth nowadays. 

 

agreed on phase, the real life use (IMO) for the phase switch is running multiple subs where the cancellations can be quite real and much harder to deal with in the room.  i run multiple subs in a smallish room and after a (very) long setup period, it sounds better than any single sub could ever hope to achieve.

 

i don't always agree with it, but this site has some excellent points and will make you think

 

these days i'm becoming more interested in using software x-over rather than at the sub or receiver, after i got a program that does it (JRiver) and have been experimenting, i think there are benefits to this and far more control can be exerted.  i'm sure there are some associated issues as well...but after years of building subs and x-overs with varying levels of almost nil results it looks like software may be the answer...


Edited by ferday - 11/25/13 at 2:32pm
post #7 of 8

I have a Sunfire 12 inch sub, which has a super small cabinet, but the damn thing could tear the walls off if I didn't have it dialed down. The easiest way to tell how loud and low a sub is going to be is to look at wattage. It takes a LOT of power to push out 20Hz at a good volume. I use the equalization curve of the mains to finesse the cross over at the bottom end. The nice thing about crossing over at 80Hz is that it only leaves two octaves to the sub, and even if it isn't stone flat through those two octaves, it isn't serious because it's so narrow and so low. I find that it's a lot easier to EQ the bass of my mains (12 inch studio monitors) than it is to get the sub perfect, because it goes below the range of hearing, so the feel of 16 or 17Hz can throw off my tuning by ear. The mains are perfectly capable of putting out 80Hz, so they aren't straining. The more the mains do, the less the sub has to do.

post #8 of 8

I have Hsu subwoofer. Their website has a wealth of information.

 

http://www.hsuresearch.com/faq.html

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