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Old 11th July 2011, 01:39 PM   #31
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Quote:
Originally Posted by eric beek View Post
Baffle-step response can be flatted out by glueing sound-absorbing foam to the so narrow made baffle as is possible ,the transient response improves as well ,so does sweet-spot and polar patterns,see interference and diffraction under my name for picture
That is in an interesting idea, does anyone have any more information about it?
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Old 11th July 2011, 02:12 PM   #32
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Quote:
Originally Posted by eric beek View Post
Baffle-step response can be flatted out by glueing sound-absorbing foam to the so narrow made baffle as is possible ,the transient response improves as well ,so does sweet-spot and polar patterns,see interference and diffraction under my name for picture
While attaching foam near the edge of the baffle can reduce the ripples in the response (particularly on-axis) caused by baffle edge diffraction, the basic baffle step shift in response of 6dB will always remain regardless of the type of damping material used. All you're doing is getting a little bit closer to the theoretical smooth 6dB shelf response of a sphere, with less overshoot than you might otherwise get with a rectangular baffle.

I'm sure you know that, but some people reading could possibly mistake your comments to suggest that somehow the basic 6dB shift was being reduced, reducing the need for baffle step correction, which isn't the case.

With typical edge radii of front baffles I'd also suggest that foam is only effective above about 2Khz or so, and only at frequencies where the active driver has wide enough (180 degree) dispersion to illuminate the baffle in the first place.

For a midrange driver you would probably get more reduction of baffle diffraction ripples (especially below 2Khz) by simply offsetting the driver a bit, and for a tweeter (which is where the foam would be most effective) you can more or less eliminate baffle diffraction by using a tweeter incorporating a waveguide - on many such designs little or no signal is radiated at right angles along the plane of the baffle so there is nothing to diffract from the cabinet edges at those frequencies.

Whether there is any worthwhile effect depends entirely on the directivity of the drivers - small drivers with very wide dispersion may see a significant improvement, larger and/or directivity controlled drivers won't see any useful improvement in my experience.

Certainly worth trying though.
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Last edited by DBMandrake; 11th July 2011 at 02:16 PM.
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Old 13th July 2011, 09:41 PM   #33
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Thanks Allen! Perfect timing for me.

John
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Old 23rd July 2011, 06:52 AM   #34
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Great work Allen.
I think a simple tutorial like that has been long overdue here, you have filled that void nicely, kudos to you!

Mick.
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Old 24th July 2011, 10:09 PM   #35
Fedess is offline Fedess  Argentina
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Very nice tutorial, congratulations Allen!

And thank you so much!

Fede
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Old 24th September 2011, 03:26 PM   #36
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Many thanks, AllenB. Your information on the Tweeter helped a great deal with my homemade crossover, now it's more ballanced.
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Old 15th November 2011, 09:53 PM   #37
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Originally Posted by AllenB View Post
To attenuate the tweeter by 1dB, take the tweeter impedance value we've been using here and multiply it by 0.12 to get the resistor value. For 2dB multiply instead by 0.25, 3dB by 0.4, 4dB by 0.6, 5dB by 0.8, and finally for 6dB of attenuation use a resistor which is equal to the impedance value.
May I ask how you worked out those numbers?
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Old 15th November 2011, 11:12 PM   #38
AllenB is offline AllenB  Australia
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If we consider the resistor that is used for attenuating the tweeter (Rs) and the tweeter itself (if we call it Rtweeter for the moment), the total signal voltage (Vtotal) will be found across the series combination of Rs and Rtweeter. The voltage across Rtweeter (Vtweeter) can be found by
Code:
                     Rtweeter
Vtweeter = Vtotal x -------------
                   Rs + Rtweeter
The attenuation will be

Code:
            Rtweeter
20 x log( ------------ )
          Rs + Rtweeter

Last edited by AllenB; 15th November 2011 at 11:15 PM.
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Old 15th November 2011, 11:34 PM   #39
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Thanks for answering, but why are you taking the ratio of tweeter and total and expressed it as dB? I thought by reducing 1 dB in the context you are referring to the spl of the tweeter? or is it the ratio of voltage between the tweeter and total?
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Old 16th November 2011, 01:22 AM   #40
AllenB is offline AllenB  Australia
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Reducing the voltage (relative to the total) will bring the tweeter down and it can be expressed in dB. A halving of voltage will reduce the level by 6dB, which amounts to one quarter of the power. This is because the halving of voltage will also halve the current that flows from it.
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