Multiple Small Subs - Geddes Approach

I never had much luck with anything more then one sub, using the JBL 2245H 18 in sub. After a few years I dragged an other one in the listening area, and when using two subs it would create dead spots or boom bass in certain areas of the listening room.
If it is going to work, they all will need to be placed in the same area firing out in the same direction.
 
I never had much luck with anything more then one sub, using the JBL 2245H 18 in sub. After a few years I dragged an other one in the listening area, and when using two subs it would create dead spots or boom bass in certain areas of the listening room.
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Reminds me of what Churchill said about democracy: not great but better than the other systems.

I put a phase-reversal switch (DPDT) on my second mixed-bass sub and have run many REW curves. Both phases create some (very) unwanted boosts and cuts at my single-seat listening location, like you said. But either is better over-all than my single Klipschorn alone.

I suppose other rooms, other available sub locations, and other less-heterogeneous speakers (I'm not sure why you'd want a second speaker the same as your first one) than mine might have other results.

But there's a literature (covered in Toole's great book) arguing the more the better (up to maybe 4, says Toole, if I recall). The data from the research using real rooms is unruly and the researchers are struggling to find the right kind of parameters for characterizing goodness (variation in tones, average differences among different seat locations, etc.). Complicated but so was the development of harmonic and inter-modulation distortion parameters.

Ben
 
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Input2, you'll find this, with a room being what it is. When you find a position that has an effect on certain frequencies that are of concern, you might play with some low pass filtering. There are two aims...you'll want to eliminate the higher frequencies and those above where the sub is being helpful, and to adjust the phase of those below so that they are constructive toward the end result. It doesn't matter whether you sit down with a box of inductors and electrolytic capacitors or use something more elaborate.

Bentoronto, in doing this (adding subs one by one from a single sub starting point) there is no logical reason why they should (necessarily) be the same. Is this what you're saying because I'd agree but I wouldn't approach it at random if I were trying to get the best result from as few subs as possible, and I've had some good results with only two subs (plus mains).
 
I never had much luck with anything more then one sub, using the JBL 2245H 18 in sub. After a few years I dragged an other one in the listening area, and when using two subs it would create dead spots or boom bass in certain areas of the listening room.
If it is going to work, they all will need to be placed in the same area firing out in the same direction.

Seems a little extreme to write-off the entire multi-sub approach based on a single trial. Set up properly - did you EQ each sub separately for best summation of the bass to the mains? - the technique in this thread works very well.
 
not quite correct

I never had much luck with anything more then one sub, using the JBL 2245H 18 in sub. After a few years I dragged an other one in the listening area, and when using two subs it would create dead spots or boom bass in certain areas of the listening room.
If it is going to work, they all will need to be placed in the same area firing out in the same direction.

Sounds like an ill informed generalization... and contrary to my experience of over 18 years with multiple sub placement.

I have 4 x 12", one mid short wall right, one on second floor balcony crosswalk, two behind main listening area either side of 7 ' couch

Works far better than any single sub ever did

John L
 
Since I've been tooting the horn for heterogeneity - spatial as well as using distinctly different speakers - I'd like to hear some pro's and con's about avoiding using multiples of the same sub.

For sure, we are getting into questions that are hard to pin down research-wise. Just fooling with locations (and using the identical sub) leads to mammoth research and mammoth articles.

I have a Klipschorn bass (need I add, in a corner) and 20-Hz 15-inch OB mounted on a big irregular shaped board mid-wall. Cross-over somewhere just north of 100 Hz.

Ben
 
I put a phase-reversal switch (DPDT) on my second mixed-bass sub and have run many REW curves. Both phases create some (very) unwanted boosts and cuts at my single-seat listening location, like you said. But either is better over-all than my single Klipschorn alone.

I suppose other rooms, other available sub locations, and other less-heterogeneous speakers (I'm not sure why you'd want a second speaker the same as your first one) than mine might have other results.
Ben,

Reversing the polarity of one of two subs which have quite different phase response will, as you mention, create some (very) unwanted boosts and cuts around the room and at your listening location.

Since a front loaded and horn loaded speaker have quite different phase (and time arrival) response, switching polarity on one simply changes the cancellation frequencies.

Using multiple subs each having the same phase response would result in a smoother frequency response.
Without testing the individual phase response, the easiest way to insure the speakers have similar phase response is by using multiples of the same type.

The "mix and match" approach can work by luck, or with careful placement and time/phase alignment but in general we have better "luck" using multiple subs of the same type rather than different.

That said, I have four different types of subs in my shop, and the results are "better" than any one alone, yet I have a single sub in my home theater, and it sounds better than the four subs in the shop, though won't get as loud.

Art
 
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Ben,

Reversing the polarity of one of two subs which have quite different phase response will, as you mention, create some (very) unwanted boosts and cuts around the room and at your listening location.

Using multiple subs each having the same phase response would result in a smoother frequency response.
Without testing the individual phase response, the easiest way to insure the speakers have similar phase response is by using multiples of the same type.

The "mix and match" approach can work by luck, or with careful placement and time/phase alignment but in general we have better "luck" using multiple subs of the same type rather than different.

That said, I have four different types of subs in my shop, and the results are "better" than any one alone, yet I have a single sub in my home theater, and it sounds better than the four subs in the shop, though won't get as loud.

Art
Having a single sub involves a lot of luck* and having two identical subs also involves a lot of luck... so we need a "luck hierarchy." Perhaps I've been studying the stock market too long (and statistics theory) but I'll take heterogeneity luck over monolithic luck.

But this kind of polarity guess-work also applies to the cross-over north of the sub. I'd be glad to hear a rational procedure for getting the polarity right between a corner horn and two meter-square bipolar electrostatics standing away from the wall. It is ALL luck.

Perhaps I should have emphasized more that polarity of the second sub either way is better than the "luck" with a single woofer in my room, even one with the superior room-wrangling power of a respected folded corner horn. But I hasten to add, the main eye-popping advantage of the second sub with either polarity has to do with extending the response south**, with the influences on other points a trade-off between polarities.

Ben
*I think we are both using the word "luck" not as a spiritualistic concept but as a replacement for "kind of too hard to anticipate easily by calculation, let alone accurately and changes all the time when I move my reading light and footstool."
**Yes, the OB is the sub sub. You read it here first.
 
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But this kind of polarity guess-work also applies to the cross-over north of the sub. I'd be glad to hear a rational procedure for getting the polarity right between a corner horn and two meter-square bipolar electrostatics standing away from the wall. It is ALL luck.
Ben,

Getting the polarity right between a corner horn and two meter-square bipolar electrostatics standing away from the wall does not require luck or guess work, just measurement- the correct polarity will result in more output in the crossover region.

To get the various speaker's phase to align in the crossover region would require time alignment (in addition to proper polarity), the tops would need to be delayed by approximately the horn path length plus the distance between the corner exit and the top speaker's position.
Unless your electrostatics are equidistant from the corner woofer, only one (left or right) can be time/phase aligned at the listening position.
And since the phase response and time of flight to your listening position of your two subs is also presumably different, at the listening position, the front loaded sub would also require a delay to integrate phase in the crossover region.

Art
 
I have been reading this thread with interest.....

My system is 2 channel with OB main speakers and two subwoofers. The room has one stud/plasterboard wall, quite a long way behind the listening position, all other walls are plastered brick and the floor is solid concrete, not suspended. I had managed reasonable bass integration with the main speakers and the main sub but there were always issues around the crossover point and interaction with certain room modes

After trying numerous types of passive acoustic treatment, none of which were ideal, I decided to jump into digital correction, adding a DEQX processor in 2012. This required a large learning curve but the system is now optimized to a level which is way beyond my original expectations

I am by no means an expert on digital room correction but I do have quite a lot of hands on experience in this area. There is much more to setting up a sub - more than just room placement or phase, as I have learned (please forgive the length of this post and I welcome comment from those more technically knowledgeable than me)…

The main sub is a M&K MX-200 which has served me well for a long time. When first purchased, positioning involved placing the subwoofer at the seating position and moving myself around the room listening for the best sounding spot for the sub (and then moving it there and me to the chair !). It has been in that position ever since. I am lucky that I have a dedicated music room with just one chair so placement was not an issue. As it happens, the MX-200 is slightly behind the LH main speaker, facing the LH wall, angled slightly forward but not particularly close to either the side or rear walls, or the corner

Things got a lot more challenging when I added a second sub (B&W PV1D because I liked the fast transients) and that's when I decided to go digital because with 2 subs the room was sometimes getting unmanageable. The B&W is slightly behind the listening position, similarly angled to the M&K & closer to a wall but has drivers diametrically opposed rather than front-down (M&K). Subs with different properties definitely complement one another when you can individually correct and align them


Using subwoofers in a room, the situation becomes very complex because of the interaction between several interrelated factors…..

1) Room / position
2) Crossover point / slope to main speakers
3) Phase
4) Time alignment

All these areas affect each other and in particular good time alignment becomes really critical . Most ‘solutions’ address phase first and then ‘basic’ time alignment but few I could find also allowed steep crossover slopes between sub-mains and a method to blend these seamlessly. DEQX does that.

The biggest revelation has been discovering that all frequencies have differing wavelengths and there is no ‘perfect’ time alignment, just a compromise spot where it actually ‘sounds’ perfect in that particular room

Let me explain….

I can give an example of this last comment from experience – my speakers are anecholically optimised for an almost flat measured in room response 16hz upwards. Rather meaningless I know because it is how a system sounds playing music and not how frequency response looks on a computer screen. It’s a good start however

So, the speakers are digitally calibrated ‘flat’, the subs are in the optimum position based on room listening, the DEQX has ensured a phase aligned crossover to the subs and I have set the time alignment between both subs and then to the main speakers by using a mic at the listening position. So far so good but it wasn’t ‘quite’ perfect…still a few occasional bass issues. However, I have found that getting the optimum time alignment is actually much more complex because of the wavelength issue

The universally accepted method is based on impulse response, either aligning with the first peaks on all speakers or on the initial impulse rise (slowing those closer to the listener back to the timing of the furthest, in my case the M&K sub). I have found that this in itself is fraught with potential errors

Time alignment with what?

The impulse response plot of a main speaker and a sub is the sum of all frequencies from that speaker and as they all travel at different wavelengths, what are the peaks and rises actually showing? They give a good start point but further tweaking is needed to finally hit the sweet spot where that particular system in that particular room sounds as near as dammit perfect. It’s like focusing a camera lens and when you get there it’s very clear - I now have no frequency issues whatever I throw at it. Bass is fast, dynamic and clean and the room has completely disappeared. Believe me, I play a lot of music and this comment is based on experience for hours per day over many months. Although DEQX is expensive and fiendishly difficult to perfect, the end result has to be heard to believe

Now, this is interesting…

Just by changing time alignment on this ‘perfect’ setup, suddenly peaks and dips in bass response reappear, I guess because now some of the crossover frequencies or those interacting with the room have become out of sync .

Here are a couple of ‘real’ examples…

Changing timing i.e on the main speakers by an additional 2.7ms to the subs creates a noticeable boomy peak around 85hz. No change to equalisation to cause this either. It is because the timing at that frequency is now out of step with the rest of the range and it just doesn’t sound right. Equalisation doesn’t cure that because all it does is increase or reduce the volume around 85hz to make the hump more or less noticeable and the music no longer sounds as real

If I move alignment to, say 5.3ms then the audible change appears elsewhere in the low frequency response. In this case bass appears to decrease, probably because the room or crossover interaction is now cancelling a particular frequency which is out of step time wise. Return to the correct time alignment and it jumps back into real performers sounding just as if they are in front of you with ‘space’ around each one. Amazing really that such minute differences can be so noticeable. Therefore, with subwoofers, I believe room equalisation or passive treatment alone are insufficient, as is just setting impulse response in simplistic terms

The end result can be stunningly realistic. This is a way to stop a poor room ruining the enjoyment & I like to listen to music, not ‘hi-fi’ or noticing abberations in the sound. It still surprises me how much difference I obtained the very first time I (properly time aligned) corrected my room from its original incarnation, using the same sources, amps, cabling and speakers. ‘Night and Day’ is an overused term but in this case pretty accurate

Anyone reading this is quite entitled to be sceptical and I do not claim to understand everything about the subject but subwoofer integration once it’s implemented correctly is stunning. It just takes a lot of effort and a reasonable financial outlay to get there

All of this may not be particularly scientific and I am not saying that DEQX is the only solution, however it works for me and I now feel I am getting the best out of it

My wife regards me as obsessive with my music, almost to the point of OCD over the past 30 years & I used to be one of those people who always felt my system should sound better. That is no longer the case
 
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I use absorptin material like 1 in foam in all my subwoofers, I find that using foam gives me much more of a natual sound. Foam is great for small subwoofers like the 12in KW-120-THX klipsch enclosure.
My 18in JBL 2245-H with the recommended subwoofer enclosure greatly improved the sound quality.
Foam is available at a low cost from a few suppliers that will ship it to your door in any size shape and quantity.
 
The universally accepted method is based on impulse response, either aligning with the first peaks on all speakers or on the initial impulse rise (slowing those closer to the listener back to the timing of the furthest, in my case the M&K sub). I have found that this in itself is fraught with potential errors

That method is fine for aligning satellites but it becomes moot in the subwoofer crossover region. Here we have to find the best compromise of crossover slope/frequency and delay. This can be done live with an RTA. No DEQX required.
 
Markus

Have you found delay to be useful in setting up multiple subs? I ask because I have not found this to be the case. I have never used delay in any of my sub setups and tend to get very good results. When experimenting with delay I found that huge changes result across the band and it was very difficult to get a particular problem in a narrow region resolved without making other regions worse. This is not so with say a narrow band boost or cut filter.
 
^
I was talking about getting the splice between sub and satellites right.

Todd Welti's SFM uses delay besides level and EQ to find the best summed response. So, yes I believe delay is a useful parameter but I don't know how to use it effectively when setting up multiple subs manually.
It could and should be done in software though.