Open Baffle Efficiency

[badman]

I've seen some casual mention of efficiency gains from open baffle loading- specifically 3dB per driver (presumably from the backwave). Now, I would think that our hearing mechanism (not to mention distance-loss) would eliminate much of this sensitivity, as it would be delayed relative to the frontwave (only reflections, really, as the side-pattern would largely be cancelled via... you guessed it, dipole action!). A short gated measurement might eliminate such gains.

Would anyone care to hypothesize further? Is there another mechanism which is 'unleashing the beast' here? Is the gain more measured than perceived? What do y'all think?

Flamesuit on!

[nunayafb]

From various sources one being the Linkwitz site, the total acoustic output from a dipole is 4.8dB lower than a monopole. Now for open baffle, The frequency of where dipole behavior begins is much lower than dipole so I doubt as much is lost - just speculating though.
at certain frequencies I could imagine the front and rear waves being in phase although with alot of delay, such that a continuous tone could add to a +3dB gain, but then at other frequencies they would be out of phase and cause dips.

[planet10]

Originally posted by badman
casual mention of efficiency gains from open baffle loading- specifically 3dB per driver

I've always just dismissed that as not worth commenting... you may get overall flatter power response (just as you do in a bipole), but efficiency as measured in the midband where we have a 2-pi radiator will not change fromt he same driver in any other box.

dave

[Rudolf]

This is a simulation of a 15" driver in a 50x50 cm OB with a listening position 3 m away on axis.

There is a lot of gain from the backwave - and up to the first dip in frequency response there isn´t much delay to speak of (phase-wise). At higher frequencies you see, why attenuation of the backwave could be a good thing.

I still can´t make up my mind why the gain from freefield response (red) to backwave response (blue) could be more than 6 dB. I should ask MJK for that.

Rudolf

[MJK]

I am not sure the responses below say 100 Hz should be looked at as a gain in efficiency. I think the differences between the roll off lines at any given frequency are more a function of the first peak in the response location on the frequency axis. As the rear wall gets closer a second image of a dipile is adding to the response and the first peak drops in frequency from about 320 Hz (100 m or no rear wall) to 120 Hz (for 1.7 m distance) and finally to 70 Hz (for 1 m distance). The response rolls off below these frequency values leading to three parallel lines at three different vertical positions. The average SPL of all three curves above 300 Hz looks to be about the same if you smooth out the rounded peaks and sharp nulls.

[badman]

Most specifically, I'm wondering about the response behavior on the linkwitz site, with respect to midrange, rather than bass: He's saying that the 2 parallel midranges are measuring a voltage sensitivity of 103dB/2.83V in the phoenix. Certainly well beyond what one would expect in a sealed or vented setup.

I suppose some testing is in order. Not a lot of fun from where I'm sitting.... I hate testing)

[MJK]

You could run a one and then a two driver OB simulation using my MathCad worksheets and see what you get for an on axis SPL response. Maybe that would be enough to answer your question.

[PHilgeman]

In an OB Dipole, in some of the frequencies above where the dipole cancellation occurs, there is an increase in 'sensativity' or output level for a given input at those particular frequencies.

In order to create a natural sounding speaker, this needs to be partially notched out, usually while also compensating for open baffle losses using a "Bridged-T" circuit.

This usually brings the speaker right back to where it was (in a closed box) with regard to sensativity in the 400-1000Hz reigon.

-Paul Hilgeman

[MJK]

In order to create a natural sounding speaker, this needs to be partially notched out, usually while also compensating for open baffle losses using a "Bridged-T" circuit.

I have no idea what a "Bridged-T" circuit is so I must not be using one on my Lowther OB design. It still sounds very natural to me.

[Rudolf]

Originally posted by MJK
I am not sure the responses below say 100 Hz should be looked at as a gain in efficiency. ... As the rear wall gets closer a second image of a dipole is adding to the response ...

I would like to look at it from a power response point of view. As I see it the red line represents the energy radiated to the listening position from the front half of the dipole 8. If the back half of that 8 is completely reflected from the backwall, it should maximally double the energy - giving 6(?) dB plus. But the simulation tells >10 dB for low frequencies. What am I missing?

An interesting aspect is the kind of mirror image a dipole generates when being close to a wall. Is it really a complete second image of a dipole? Making it effectively a linear quadrupole source as discussed by D.Russell: http://www.kettering.edu/~drussell/Demos/rad2/mdq.html

Or is it a three-point-source with only the backward radiation been mirrored?
Some experiments of a friend with a very small OB positioned very near to the wall seem to back the former theory, but my common sense likes to support the latter.
http://img508.imageshack.us/my.php?i...41000229pk.jpg

Any opinion or proven physics about that?

Rudolf