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Old 23rd July 2006, 04:30 PM   #1
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Default System Q and IB v.s. OB

I am trying to get a handle on a method of predicting total system LF response in OBs and I think that Q may be where the solution lies but I need some help.

It is very simple to model response of IB using existing programs but I have not yet found a modeling program that handles OB. I see two factors affecting LF of OB v.s. IB. One is the cancellation of LFs due to rear wave interference and the other is the decrease of system Q with baffles of less than infinite size.

I am not sure if these are independent effects or not. In other words is the effective change in system Q just a reflection of the results of cancellation or is it a seperate effect?

Can we calculate the change in effective Q due to smaller than infinite baffle size? If so could we put the effective Q into a program as Qts and model IB to get the OB response? That is can we develope a transform to determine an effective Q that would substitute for Qts allowing us to determine OB response using and IB model?

mike
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Old 23rd July 2006, 05:34 PM   #2
infinia is offline infinia  United States
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Hi
I think in both cases OB and IB the system Q aproaches Qts, maybe just slightly higher for non IEC tests. The system Q is only really valid for predicting response near system resonance. In the IB case the LF response could be modeled fairly accurately using the usual box modeling ie (no midbass cancellations). For OB stuff I think re-reading Linkwitz can answer many of your questions here. Excel can most likely be an asset in modeling for design puposes. I don't know if I'm just repeating the obvious here or maybe your questions in my mind is not that concise.
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Old 23rd July 2006, 06:11 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

Decrease of system Q will smaller baffle size ? lost me there.

Q remains constant = free air Qts. Baffle loss changes with size.

The combined response - 3rd order bass roll-off, is a combined
fixed 2nd order + variable 1st order, you can have high Q 2nd
order + 1st order to give what appears to be a more damped
3rd order response, but the term Q applies to 2nd order.

Try "the edge" to sim this. http://www.tolvan.com/edge/

/sreten.
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Old 23rd July 2006, 06:37 PM   #4
infinia is offline infinia  United States
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Sreten,
Free air Qts does change (increase) with an IEC baffle for low Q drivers Q<0.4. Not too much though.
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Old 23rd July 2006, 09:16 PM   #5
Rudolf is offline Rudolf  Germany
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Default Re: System Q and IB v.s. OB

Quote:
Originally posted by mashaffer
I am trying to get a handle on a method of predicting total system LF response in OBs and I think that Q may be where the solution lies but I need some help.
mike,

it is really futile trying to correlate LF response with system Q alone in an OB since the room is such a dominant factor IMHO.

Quote:
I see two factors affecting LF of OB v.s. IB. One is the cancellation of LFs due to rear wave interference and the other is the decrease of system Q with baffles of less than infinite size. I am not sure if these are independent effects or not. [/B]
These are independent effects for sure. Changing the size and proportions of the baffle would result in something comparable to a changed system Q. As stated above, EDGE can give you same insight into what happens there.

But the cancellations and amplifications due to the reflected rear wave are independent of baffle size. They are a result of OB-to-wall distances only.

AFAIK the best (and only dependable) tool to simulate both effects in interaction are MJKs Mathcad worksheets for OBs: www.quarter-wave.com/Models/MathCad_Models.html

Rudolf
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Old 23rd July 2006, 10:11 PM   #6
Svante is offline Svante  Sweden
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...also the sharp version of Basta! can model OBs including the response of the driver. If I may say so.
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