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Old 19th February 2012, 05:07 PM   #1471
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Quote:
Originally Posted by bentoronto View Post
Good points from weltersys.

I have quite a big collection of organ music and many other goodies and I have spent a lot of time watching this music on a spectrum analyzer. Pretty rare for anything that low. Betcha if you hit that bottom key on the piano, there's not much fundamental present. Likewise for organ pedals which mostly have proxie tones, not the advertized frequencies.
Just because the fundamentals might be lower in amplitude than their harmonics doesn't mean we should then just throw them away entirely. Any further losses of the fundamental due to the speakers roll off will be even more noticeable thanks to the fletcher munsen curve.
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Anyway, your brain reconstructs those fundamentals just fine although I'd love to see one of those debunking ABX blind experiments done with and without output below 35 Hz.
Your brain reconstructs the fundamental in that you interpret the note as being of the frequency of the suppressed fundamental, however that does not mean that it sounds the SAME as it would if the fundamental was present. The weight of the note is missing.
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BTW, no such thing much as "room gain" unless you lived inside a sealed propane cylinder.
Before we reopen the whole pressurisation or not debate, when I say room gain I mean any gain that is over and above the free field response of the speaker at the same distance. In other words how much would the low bass drop if we kept the speaker and mic in the same place and somehow removed all the walls and ceiling.

Below the lowest room mode all room reflections will arrive constructively near-in phase so room "gain" is maximum. In the modal region some reflections will arrive constructively in phase and some destructively out of phase so on average room gain is reduced in the modal region, aside from the peaks of the modes themselves.

Even with a dipole which can't pressurise a room there will still be some downwards tilt in the bass with increasing frequency in the transition from below modal to the modal region of the room.

Pressurisation of the room isn't required for gain below the modal region of the room, (even with large room openings it still occurs) although a sealed box will introduce a small amount of additional pressurisation gain at quite low frequencies.
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Anyway, a mic picks up "room gain" as well as your ears do... so it isn't some extra mystery benefit not visible on acoustic curves... even if it existed.
So if a room has room gain in the bass the microphone will record it, and if we then play it back in the same room we will now have 2x the room gain. Why is this good ?

The microphone should capture the room gain of the original instruments, but we don't want to add a second lot of room gain during reproduction.
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Last edited by DBMandrake; 19th February 2012 at 05:11 PM.
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Old 20th February 2012, 01:51 AM   #1472
gedlee is offline gedlee  United States
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Quote:
Originally Posted by DBMandrake View Post
Before we reopen the whole pressurisation or not debate, when I say room gain I mean any gain that is over and above the free field response of the speaker at the same distance. In other words how much would the low bass drop if we kept the speaker and mic in the same place and somehow removed all the walls and ceiling.
By your deffinition of room gain, all modes are "room gain". That is not the usual deffinition - its more the [pressurization mode that is considered room gain, and it is pretty much a falicy. But if the modes are the "room gain" then you are just talking about LF modal response in a small room. I don;t think that I agree with your characterization of it however.

Quote:
Below the lowest room mode all room reflections will arrive constructively near-in phase so room "gain" is maximum.

Pressurisation of the room isn't required for gain below the modal region of the room, (even with large room openings it still occurs) although a sealed box will introduce a small amount of additional pressurisation gain at quite low frequencies.
- this is simply a description of the "zeroith mode" in a closed room - this is the pressurization mode. In any real room however, there are substantial leaks and this mode is diminished in level and moved away from zero by these leaks. In any space bigger than a car it is not really a factor.
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Old 20th February 2012, 02:42 AM   #1473
badman is offline badman  United States
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Since you're on the line doc, wondering what you think of "dead" subs- that is, lossy suspensions needing some minimum displacement before they come "alive". Seems to me that if you have suspension issues in subs, you need to cross over lower and the increasing wavelengths will make any suspension loss insignificant. What's your take?
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Old 20th February 2012, 01:14 PM   #1474
gedlee is offline gedlee  United States
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A sub needs a driver with a lot of BL for good efficiency. This makes it naturally well damped as the electromotive damping is dominate and very high. Any additional damping is IMO just wasted energy and insignificant anyways. There is no point to it (nor benefit that I can see).
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Old 20th February 2012, 01:42 PM   #1475
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Originally Posted by gedlee View Post
A sub needs a driver with a lot of BL for good efficiency. This makes it naturally well damped as the electromotive damping is dominate and very high. Any additional damping is IMO just wasted energy and insignificant anyways. There is no point to it (nor benefit that I can see).
Do you believe that a big woofer with motional feedback in a corner can actively "eat" room resonances? Would the effect be more than trivial?

Ben
I wonder if it is the same people who believe in special speaker wires who also believe in "room gain"?
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Last edited by bentoronto; 20th February 2012 at 01:48 PM.
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Old 20th February 2012, 02:50 PM   #1476
gedlee is offline gedlee  United States
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Originally Posted by bentoronto View Post
Do you believe that a big woofer with motional feedback in a corner can actively "eat" room resonances? Would the effect be more than trivial?
"Can"? - yes, but it is not that trivial, especially if "motional" feedback is used. Putting a mic at the source location and forcing the pressure at this mic to go to zero will signififcantly "change" the modal pattern, basically it changes the boundary conditions of the whole room problem. But what it will not do is elliminate the modes. In all likelihood there will be the same number after as before, just all different. So is this a "cure-all" - no, it barely even changes the problem.
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Old 20th February 2012, 04:01 PM   #1477
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In as much as there are two kinds of loudspeaker feedback, there are two kinds of room adjustment.

You've conceptualized acoustic feedback with a mic and discussed the consequences of that kind of active manipulation of the acoustics of the room. Very interesting. Thank you.

Actually, I had in mind the other kind which controls the motion of the cone without separate concern for the actual output of the cone. In that case, I now believe, the object of motional feedback when the driver has no signal, is to keep the cone stationary, like a brick wall. Indeed, that's what it tends to feel like when you press against a motionally fedback cone... within the frequency range of the amp, of course. I suppose that would have no more effect on room modes than any other piece of wall.

Ben
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Old 20th February 2012, 04:08 PM   #1478
badman is offline badman  United States
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Quote:
Originally Posted by gedlee View Post
A sub needs a driver with a lot of BL for good efficiency. This makes it naturally well damped as the electromotive damping is dominate and very high. Any additional damping is IMO just wasted energy and insignificant anyways. There is no point to it (nor benefit that I can see).
"Insignificant anyways", fair 'nuff. That was my read too but thanks for your input.
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Old 20th February 2012, 04:14 PM   #1479
MaVo is offline MaVo  Germany
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Hi Ben and Earl,

do you refer to a device like the "shadow", as spoken about in Active Absorbers ?

regards,

Mathias
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Old 20th February 2012, 04:30 PM   #1480
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Amazing. Seems Olson invented it!

May or may not work in regular rooms or special spaces, dunno. But since you are dealing with very long waves (which are usually a real pain to work with elsewhere), just might be helpful technology.

One weird claim in the patent application is that you can put such a gizmo behind a bipolar electrostatic speaker and it eats the rear waves before they can sneak around to the front.

Ben
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