Cascaded/Multi Driver Subwoofers

[dallasjustice]

Assume a rectangular room with two dominant axial length modes. The modes of interest are the 1st order axial length mode (1,0,0) and the 2nd order axial length mode (2,0,0). In most medium to large listening rooms with a rectangular shape, these modes are very unlikely to be treatable with room treatments. Inverse EQ filters aren't a perfect solution either.

One way to eliminate these modes would be a two sub array I've discussed before here called the "source/sink" method. This works very well but can be a little complicated to setup. The benefit for the source/sink method is that it mostly eliminates the above mentioned problem modes. The downside is that low frequency time alignment is less than perfect. IMO, it's not audible and the benefits greatly outweigh the negatives.

The proposed cascaded method for eliminating these modes would require the subwoofer to be placed in the so called null position for each respective mode. For example, if a subwoofer were placed at the midway point between the front and back walls, and played low frequency logsweep, the 1st order axial mode would NOT be excited. However, the 2nd order axial mode would be excited by this midwall subwoofer, which is very undesirable.

A second subwoofer could then be placed at the null position for the 2nd order length axial (2,0,0). In this position, there would be no (2,0,0) excitation but the 1st order mode would be excited, which also isn't desirable.

Normally, subwoofers are low passed with a crossover point around 80hz. What if there were a cascaded series of subs each of which operate only within the frequency range which does not excite the unwanted mode? Using the example above, the midwall sub would be low passed slightly below the 2nd order axial (so it doesn't excite the 2,0,0) and crossed over to the second sub which is low passed up to the normal crossover frequency (say 80hz). IOW, there could be a series of subs each working only within the frequency most optimal to each sub's physical location in the listening room. I've thought about doing this before, but I always thought the crossovers might be a little too complex.

It looks like the newest version of audiolense may be designed to build more complex subwoofer crossovers to include a cascaded subwoofer array.

Here's an email I just received from Bernt concerning the new audiolense 5.0.

New in Audiolense 5.0:

ü The core algorithms have been completely reworked

ü TTD correction per speaker and, optionally per driver too.

ü Partial correction has been extended to include partial TTD correction

ü Clock drift correction in measurement is now practically perfect.

ü Sophisticated pre-ringing prevention TTD correction, including:

o Hands-on noise removal from the measurement

o A test run that enables the user to inspect removal of problematic reflections

o An option for selective removal of problematic reflections

o Improved TTD pre-filtering that is much more robust and effective than earlier with regards to pre-ringing

o Incrased flexibility in bass management, including multi-driver subwoofers and cascaded bass offloading.

o As a result, it is much easier to achieve a TTD correction without Audioble pre-ringing.

ü Target Designer enebles various combinations of minimum and linear phase targets.

ü Improved crossover handling with cleaner pulse, especially combined with TTD Correction.

ü TTD combined with minimum phase crossover is now included.

 

[DonH56]

That is the scheme I used and it has been around a long time. Does not invalidate the approach, natch; it's still the way most of us deal with room modes. Andy C has written a very nice multi-sub analysis program that can be very helpful (links over on AVS, have to dig it up later, and I think he has posted on here as well). I used it to help place my four subs optimally, or as optimally as possible given other physical constraints (doors and such).

 

[dallasjustice]

Yes, I know about AndyC's tool. But correct me if I'm wrong; his tool assumes each subwoofer is low passed the same like most other multi sub arrays.

That is the scheme I used and it has been around a long time. Does not invalidate the approach, natch; it's still the way most of us deal with room modes. Andy C has written a very nice multi-sub analysis program that can be very helpful (links over on AVS, have to dig it up later, and I think he has posted on here as well). I used it to help place my four subs optimally, or as optimally as possible given other physical constraints (doors and such).

 

[RayDunzl]

A nice interactive room mode calculator/display

http://amroc.andymel.eu/?l=18&w=14&h=9&ft=true&r60=0.6

 

[watchnerd]

A nice interactive room mode calculator/display

http://amroc.andymel.eu/?l=18&w=14&h=9&ft=true&r60=0.6

That's pretty cool.

But how do I calculate if my room is open on half of one side?

 

[RayDunzl]

But how do I calculate if my room is open on half of one side?

I just look at mine. It's pretty obvious. No wall.

 

[watchnerd]

I just look at mine. It's pretty obvious. No wall.

Is this a zen riddle?

Can you hear the sound of one hand clapping if one wall is missing?

 

[DonH56]

Yes, if your system is resolving enough and your ears are good enough...

 

[dallasjustice]

@mitchco inspired me to give AL5 a try.
https://www.computeraudiophile.com/...ker-and-room-correction-software-walkthrough/

I think I’m going to move my subs and give the above described cascaded sub array a try. I currently have subs setup on opposing walls in a “source/sink” setup. It’s worked well. The only downside to the source/sink method is that the “sink” sub can’t be perfectly time aligned with others speakers. The above proposed cascaded sub method promises to eliminate the two dominant axial modes while remaining perfectly time aligned.

I have not moved my subs in a couple of years so this will be like going through a breakup. But the new girl seems like she could be better than the old one. I hope I have time to do the work this weekend. I’ll report back.

 

[dallasjustice]

REW SPL plots for my first cascaded sub setup.

 

[svart-hvitt]

View attachment 9858 REW SPL plots for my first cascaded sub setup.

Nice!

And with less averaging?

 

[dallasjustice]

Nice!

And with less averaging?

1/12 per octave is pretty high resolution. You won’t be able to hear anything beyond that especially less than 200hz.

 

[DonH56]

Those graphs and similar (e.g. Ray's) are why I do not want to post mine. 1/12th octave smoothed or not.

 

[dallasjustice]

Here’s a little better measurement after tweaking sub levels. It’s labelled “both subs” but it’s actually two cascaded subs and a full range speaker. Next post shows the individual crossovers for each. All SPL 1/12 per octave.

 

[dallasjustice]

This shows full range speaker crossover and each cascaded sub’s crossover.

 

[dallasjustice]

This is the full range system measurement.

 

[dallasjustice]

The question is:
Why setup subs in this way?

In almost all cases, a single or even multiple subs setup in front of listening position WILL result in rear wall Allison effect. IOW, the bass travels to the back wall, reflects off of the back wall and creates a “null” at a specific frequency which varies depending on distance from listening position to front sub and rear wall. You can move things around but the reflection will be there and result in a null at some frequency. Now, some people have enormous rooms or rooms that aren’t rectangular. This may not be as much of a problem in those situations. But for me, the front sub clearly loses energy 25-45hz; see green trace. This is an unacceptable result and would mean anemic bass no matter the EQ settings; EQ can’t fix non-minimum phase phenomena.

The rear wall sub can fix this problem; see purple trace. BUT, audiophiles are not allowed to have a subwoofer behind the seated position! That breaks all the rules because everyone knows that you can “hear” the bass behind you!!!

The ears start to localize bass at 80-90hz according to Toole and many others. This makes perfect sense when you consider wave length and size of head (even audiophile size heads). This has been born out by my experience. There is no loss of stereo image using mono subs with reasonable crossovers.

In the post above you can see that the rear wall sub gets a one octave (very steep) low pass filter crossed at 45hz with front sub. Why did I pick 45hz? Look at where the front and rear subs intersect as one loses energy and the other flattens out; 45hz! I also like 45hz because there’s absolutely no way I can perceive any sound coming from behind seated position at these frequencies. I’ve tried all types of music. It’s like the rear sub doesn’t exist.

Anyway, all the superlatives apply, as usual. Merry Christmas.

Michael.

 

[dallasjustice]

 

[DonH56]

In this context what does "cascaded" mean?

 

[dallasjustice]

In this context what does "cascaded" mean?

More than one subwoofer handling all R/L speaker low pass frequencies wherein each subwoofer only handles a fraction of total low pass frequencies. In the case of two subwoofers, one subwoofer will have a low pass filter and the other will have a high pass and a low pass filter applied.