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In Reply to: baffle step loss compensator posted by Brad on November 13, 2002 at 01:09:54:
n/t
Follow Ups:
You've got two things happening. With a 6" wide baffle there's a 6dB diffraction step happening around 2KHz; an 18" ring the way you have it would lower that step to 800 Hz or so, giving a lot more mid presence. The second thing is Allison effect, the out of phase reflections coming back off the wall behind the box, which are also being pushed lower in frequency by that ring. If you want to maximize performance cut a hole in the wall and flush-mount the face of the cabinet with the wall. Now you have no diffraction step or Allison effect- but maybe one very pissed off landlord.
Bill,Must disagree. f3=380/W(B)=760Hz Where W(B)=baffle width in feet
f3=115/W(B)=115/.1524=754.59Hz W(B)in meters
Take your pick. Have I misunderstood?But going from 2pi to 4 pi is prevented if stuffed in wall, so no step loss.
This link may me more informative.
Tim
The diffraction step occurs at the point where the radiating surface is one wavelength across, and you go from a free-space to a half-space configuration; with a six inch cab front the diffraction frequency is 2256Hz to be exact- at sea level, standard atmospheric pressure, not raining,etc,etc. The Allison effect takes place when the diffraction step has not been reached and the distance from the radiating plane to the rear wall and back again is 1/2 wavelength. A cabinet front a foot from the rear wall will see that 1/2 wavelength at two feet, causing a dip at 564 Hz. The THX specification has the entire speaker array walled on the radiating plane to eliminate both the diffraction step and Allison effect.
Bill,
As I said, if you stuff the speaker in the wall, there is no loss or step down. No 2 Pi to 4 Pi transition. I disagree with your diffraction loss frequency being equal to one wavelength of baffle width. Olson figured the loss on a 24" round baffle at or near 190Hz, not 560.Apparently Murphy, and Olson before him, use ~1/3WL. With Olson's calculations, and furthered by Murphy, the formula he uses is based on f3=380/W(B)in feet.
Murphy adjusted his calculations to allow for the fact that in a home setting the -6dB loss will not be realized due to some reflection back to the listener. Also, the loss is gradual over ~300Hz, so pick the mid point of the slope. Compensation is then 3dB.
On the Allison effect, or boundry response interference, Dickason states that this is evident in lower frequency ranges where the distance of the driver to wall or floor is less than 3/4 WL. In other words, woofers mounted .4-.6 meters(16"-24")away from the wall will show a dip between 120-160Hz at 3-10dB in power response, depending on the room. Allison then suggested that the woofer be kept .75 WL away from the boundry at 300 Hz xo frequency, or 45", and fire away fom the corner. This, while keeping the mid at 45" off floor and out from corner. Both Acoustic Research and Allison Acoustics used this arrangement.
Tim
I haven't bothered to go to the trouble of creating diffraction plates or measuring for Allison effect myself, nor worrying about them either, as my wife would not tolerate my tearing into the walls of my house to mount speakers, and in live sound you have to manage with the club you're in- and it's far easier to use RTA & EQ to adjust to the environment than to rebuild every stage. But when Tom Holman created THX he did go to the trouble of trying every permutation, and he found the 6dB diffraction step when the baffle was less than a wavelength, and Allison effect losses of some 15dB with the distance from the baffle to the wall at 1/4 wavelength. In his case using A4s that happened at about 100Hz, which led to the spec that THX arrays are flush mounted. That's one reason why KHorns work so well in the bass- they have no reflections off the rear wall as the rear wall is part of the radiating plane- and thus no Allison effect.
Make that Allison effect for a foot away from the wall 282Hz- oops.
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