The benefits of two subs

[JohnA]

Just a quick note to stress how good two subs can be:bowdown:

My room is relatively small, and theoretically the SL3s shouldn't need much help.
In reality there are a couple of nasty room nodes ( let's say at 40 and 80Hz) that spoil the fun. With the bass settings at -3db things get a bit better, but frequency response tails off pretty sharpish.

I have experimented with all sorts of relocations of speakers and furniture, yet the nodes are always there.
I've just cracked it (Flatish response down to 30Hz) by using 2 subs at two different locations and quite out of phase with each other and the SL3s as well.:rocker:
Both subs are self-made. They are not even similar! The first one has two 12" drivers firing upwards and downwards, the second one is more conventional-looking but with a 18" long-throw driver and back-breaking 38 - 57mm MDF enclosure.

Short of bringing down structural walls, I didn't really expect the nodes to ever go away. We're talking 8-10db peaks here

Well it can be done, by dialling the phase of a single sub carefully, but this ends up in different (but smaller) peaks and troughs. That's where the second sub comes in: it can be phased in to totally cancel them out - and keep the response from tailing off at the same time. (every cancellation is energy lost, here we trade a lot of hard-earned acoustic energy for flat response)
It's actually funny when afterwards you reduce the gain of the first sub and boominess goes up

So if you can't absorb them, actively cancel them:musicnote:

I'm a very happy bunny (for now!)

 

[risabet]

A very creative response to a difficult problem. One would guess that two identical subs would allow a finer match and greater cancellation? Marvelous!

 

[JohnA]

Why identical?

They need to be in different locations and deal with different frequency ranges.
What's important is to have phase settings on each, because they heavily interact with each other, on top of the MLs
...and only change one thing at a time.

 

[JonFo]

...and only change one thing at a time.

That's the secret right there

Glad to hear you have that smooth bass you were after.

What mesuring rig do you use?

And what crossover/processor are you using to manage bass phase, crossover (and EQ?) ?

 

[JohnA]

That's the secret right there

Same as automotive tuning.
Otherwise you never know for sure what effect your changes had.

What mesuring rig do you use?

1. Stereophile's test CD #1 (for the test tones 1KHz to 20Hz)
2. Pen & Paper printouts to record the db readings/tone
3. Handheld digital decibelometer

Not fancy, but very flexible and very repeatable (well within 1db usually)

And what crossover/processor are you using to manage bass phase, crossover (and EQ?) ?

The plate amplifier (500w @ 4Ohms) of the first sub (2x12") has gain, adjustable low-pass frequency, 0-180 deg phase, which are enough I'd think.
The other one (18") is driven by a 1300w @ 4Ohms beast with adjustable low pass frequency and gain. Phase was adjusted by reversing the polarity (so no intermediate values available )
SL3s have their own bass set to -3db

If you have any ideas on a rig that will be substantially better, I'm all ears

 

[JohnA]

I've just come across this PC-based solution:

http://www.etfacoustic.com/demoroom.all.html

Anyone have experience with ETF?

 

[Bernard]

Just a quick note to stress how good two subs can be:bowdown: .......................

Have you experimented with room treatments (bass traps, etc) ? It just seems to be that you should be trying to remove the nodes passively rather than actively cancelling them - more complexity. Just my thoughts - I'm no acoustician.

 

[JohnA]

Have you experimented with room treatments (bass traps, etc) ? It just seems to be that you should be trying to remove the nodes passively rather than actively cancelling them - more complexity. Just my thoughts - I'm no acoustician.

Not massive bass traps, they wouldn't fit in the room.
I've got acoustic foam on the wall behind both panels, to absorb some of the rear wave (they are not as far from the front wall as I would have liked). That works well.
The 60Hz node I couldn't passively get rid of, no matter what. I guess it's a standing wave right where my listening position is. Moving a foot forwards it disappears:banghead:

 

[JonFo]

I've just come across this PC-based solution:

http://www.etfacoustic.com/demoroom.all.html

Anyone have experience with ETF?

John, that's exactly the software I use in my measurement setup.

Actually, I use the newer R+D version, which is pretty slick at helping set EQ's.

You can see how I used it if you read my SL3XC thread, linked in my sig.

A PC based setup does require a mic and mic interface to get the most out of it.

The ETF site recommends some stuff, and I've personally had great experience with the M-Audio line. Items like the AudioBuddy $99 mic preamp or my newest, the Firewire 410.

A good Mic (Calibrated preferably) a mic stand and some XLR cables and you're good to go.

I figure between software and hardware, a decent measurement rig sets one back anywhere from $300 to $500. First class mics (Earthworks M30) and top line interfaces could add up to around $1K.


This is one of the best investments in an audio system one can make. As we say in business: 'you can't manage what you don't measure'

 

[JonFo]

...
The 60Hz node I couldn't passively get rid of, no matter what. I guess it's a standing wave right where my listening position is. Moving a foot forwards it disappears:banghead:

John, this is why man invented EQ's

I'd recommend you investigate a bass EQ or processor of some kind to both tame the room modes and to maybe get even finer grained control of the bass.

A Behringer BFD1124 is good EQ, see endless discussion about it here as applied to Subwoofers.

Another good option, and one that includes crossover functions as well, is the Behringer DCX2496. Just do a search on AVSForum or HomeTheaterShack and be prepared to read for a while

The high-end is the DBX DriveRack series that I use on my system, these cost a good bit, but can do pretty much anything and are very clean and transparent.

 

[JohnA]

I'll look closer into this one then. More gizmos.....whooohoooooo

I've got access to a very good mike as well.:music:

We'll see if we can improve further this active equalisation thingie:rocker:

 

[tonepub]

I would seriously look at some bass traps before adding more electronics to the mix. Throwing some GIK bass traps in my room did more to improve the low end than anything I've ever done!

 

[JohnA]

I would seriously look at some bass traps before adding more electronics to the mix. ...

ah, but we're not throwing more electronics to the mix.
In fact I'm obsessed with anything that is in the signal path and doesn't absolutely have to be there

Subs are at the end of the chain anyway, so the low-level signal is not affected at all. The extra electronics that cancel the nodes are in parallel to the main ones.

The main sub fixes some issues and creates some new ones.
The second sub undoes the damage of the first one, filling the blanks.
It does sound like the long-way round, but it sure works!

 

[JonFo]

John, Jeff is right in recommending room treatments before you start tuning with EQ's.

While EQ can tame the worst nodes, it does so at a price. As the cut is applied no matter where you stand in the room.

An appropriate room treatment will fix the in-room sound in a more cohesive fashion.

If you then still have peaks (and there usually will still be some, albeit less intense), then the EQ is applied to smooth out the prime listening positions.

Also, as per the R+D recommendations for measurements, you want to measure and EQ the room, not just the listening position. This provides for a much more 'balanced' sound all-around.
I find it takes 32 measurements around my room to get a good picture of where I need to correct.
The correction this leads me to are quite different than those I might select if I just measure the listening area.