My multi location subwoofer thread inspired by Welti,Devantier,Geddes !!! - Page 7

I should have been more specific in my question about ceiling bounce. I'm contemplating a corner horn which sits on top of a direct radiator base bin or a base horn, such as the pi7 corner horn, and has no flanking subs. I don't see enough spatial separation for the direct radiator to fill in the null but, when I penned that question, I hadn't considered the path length of a corner horn, which add to the effective spatial separation. Ultimately though, there ought to be something I can do with a miniDSP equalizer on the woofer to introduce a peak at the null frequency to smooth out the null, at least for my own seat. The ability to intrroduce phase shift just at the null frequency would be nice but that isn't quite as simple as a PEQ term. (any suggestions?)

The calculated null in my geometry is 235 hz with the speaker centered at 3' elevation. Should I just lower the speaker 6" and raising the null.to 325 hz and then hope I get enough attenuation from the carpet and pad to make the null go away?

Jack

What does spatial averaging measuring technique have to do with seat to seat FR consistency?
Edited by zheka - 1/12/13 at 7:35pm

The seat-to-seat consistency is really described by the deviation of a series of measurements. But one of the proposed approximations is to take an average of the measurements. I personally think the averaging procedure is less important than the mean deviation, whether done mechanically by physically sweeping the microphone or by taking a series of masurements and mathematically averaging them together. Averaging tends to tell power response and deviation tells seat to seat consistency.


You're on the right track here, in my opinion. The way I handle this is to use the proximity to the apex of the corner remove any possibility of anomalies from the adjacent walls. The distance from midhorn to woofer and their overlapping band helps smooth the vertical anomalies, much like a truncated line array.

In fact, line arrays are what made me come up with the whole flanking sub / helper woofer approach in the first place. We were having those early multisub discussions and I also recalled how line arrays suffered no notch from ground reflection like a point-source speaker elevated off the ground does. So I tried a pair of drivers elevated off the ground and found this did a great job of mitigating floor bounce too. That's when I knew that the approach was effective.

So my suggestion is to put the woofer near the ground and the mid a couple feet up, overlapping them in the low midrange. That will mitigate the notch when it falls in the region of the band where the midhorn isn't directional enough to limit off-axis output. That's what my design does anyway.


That's possible too. Geddes seems to shift around and change position sometimes. But the multisub configuration is (or should be) intended to reduce seat-to-seat variation and improve smoothness of response, simultaneously. Otherwise, I see little point in going to the trouble to use multiple subs.

The conversations we had in 2005, and the internet discussions that followed it, were about using multiple subs for the simultaneous goals described above. You can see that in the threads. Geddes was pitting his approach against Welti's, and even bet me that his approach would do the same things and do it better. It did not prove to be true, which is perhaps why he decided to morph his stated goal.

If one decides to tackle just one problem by itself, they've reduced the problem to a much simpler one. Like I said earlier, if all you want to do is make the bass response really smooth indoors, get a nice sweep on a response chart, then just close mic the subwoofer. It will be as smooth as an anechoic chart. No need for multisubs at all.

I think in the post sdurani quoted Geddes expressed skepticism about utility of spatial averaging, not importance of seat-to-seat consistency. hence my comment.

I guess since spatial averaging really just means smoothing, that comment could probably interpreted two different ways. One interpretation would be spatial averaging of the sound produced and the other would be spatial averaging of the measured datasets.

As for me, I would want spatial averaging of the sound field, and to know how well I achieved that, I would not want spatial averaging of the measured data.

It's probably impossible to avoid getting both attributes simultaneously. Even if subs were being placed purely to minimize spatial variance, the frequency response will likely get smoother. Likewise, using subwoofer placement to improve frequency response will probably yield improvements (even if they are slight) in more than one seating location.

When the local home theatre group I belong to got a tour of Harman a few years back, I got a chance to ask Alan Devantier about the multi-sub investigations he worked on with Welti. They seem to be starting from the premise that EQ (manual or automated) would be used at some point during set up. Indeed with automatic room correction being ubiquitous on consumer gear, their premise was not incorrect.

However, one of the biggest concerns with EQ is that fixing a problem at one seat can cause problems elsewhere. For example, if you measure at 6 seating locations with Audyssey, it's not going to pull down a peak it sees at a couple of locations only to create dips at the other four seats. With that in mind, Devantier said that their goal was to get as close to the same response in every seat; doesn't matter if the response is crappy, just as long as it is as similar as possible across the seating area.

Once that is accomplished, you can apply global EQ without concern for ruining other seats. So, Welti & Devantier do have a point in using subwoofer placement exclusively for improving seat to seat consistency, with zero concern for improving the frequency response itself (that part comes later). Of course, for people that are not interested in more than one listening position and/or prefer not to use electronic equalization, this approach isn't for them.

In my room, the mid opposing walls position per Welti did not help with the modes and seat-to-seat consistency. The only pace where I could get a relatively smooth response was in the center of the room, though the over all SPL was lower there for some reason.

What is "spatial averaging of the sound field" and how does one measure it?

"Spatial averaging of the sound field" is just another way of saying smoothing over a wide area to achieve good response in every location in the listening area.


I agree, and in fact, that's the point of the multisub configuration - to get both simultaneously. It's not terribly difficult to get smooth response in one spot, but more difficult to get it over a wider range of listening positions. But like you said, adding more subs will probably make most spots smoother than having just one sub.


You know the two-opposing walls configuration is not the best Welti configuration. If it gave you smoother response in the center of the room, I guess it was working to some degree though. Still, I think I would have done the same thing you did - which is to make my placement choices based on models and/or measurements. With just two distributed subs, you really need to place them carefully.

Actually, as you know, if I only had a pair of subs, I'd run them in a flanking configuration. I'd only run distributed subs if I had three or four to work with. I always place the flanking subs as first priority, and then run one or two more distributed subs when possible for smoothing the lower frequency modes.