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Old 3rd September 2013, 10:12 PM   #1611
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Default 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

Last edited by Drewan77; 3rd September 2013 at 10:36 PM.
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Old 9th October 2013, 01:48 PM   #1612
gtb is offline gtb  France
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Quote:
Originally Posted by gedlee View Post
AT very LF the stuffing does little to absorb the sound waves, but does add an apparent volume increase that is a very good thing in a closed box.
When you use closed box subs with multisub approach, you can extend bass with equalisation. So do it still exist a reason to fill the sub or can we go without it ?
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Old 9th October 2013, 02:55 PM   #1613
gedlee is offline gedlee  United States
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Yes, you need to use EQ when using small closed box subs, but this is kind of standard these days. I cannot see a downside to using absorption in a closed box, only positives, so I would say use it. Never overstuff the box, but mostly filled is a good approach.
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Old 9th October 2013, 11:01 PM   #1614
Input2 is offline Input2  Canada
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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.
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Old 10th October 2013, 12:38 AM   #1615
gedlee is offline gedlee  United States
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I find that using foam gives me much more of a natual sound.
I use foam as well, but I must have missed the part about it sounding more natural.
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Old 10th October 2013, 07:08 AM   #1616
Omholt is offline Omholt  Norway
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If the goal is to reduce resonances, high density rock wool would work better then foam. It absorps lower and with a higher coefficient.

Those wavelengths are long though, so I can't see the box creating much problem unless on crosses over very high.
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Old 10th October 2013, 08:06 AM   #1617
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Quote:
Originally Posted by Drewan77 View Post
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.
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Old 10th October 2013, 02:09 PM   #1618
gedlee is offline gedlee  United States
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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.
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Old 10th October 2013, 02:35 PM   #1619
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^
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.
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Old 10th October 2013, 03:05 PM   #1620
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How can you locate multiple subs smartly in your room and also time align them? Not to mention having a two or more seats for listeners in the room, different frequencies, wall reflections, ports....

Ben
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