[FlorianO]

Sorry if this is OT here.

I was wondering if there is any way to derive the f3 cut-off frequency for a driver on an infinite baffle (as a formula depending on fs or Qt, I would guess).  Pointers to relevant resources highly appreciated.

TIA,

Florian

[JohninCR]

Just use one of the box modelling programs and input a very large sealed box volume (say Vas times 10 or more).  That should give you a pretty good idea of IB response.  Then, if you input some different Q's and other driver parameters, it will help you develop a feel for how they affect bass response.  Then all you need to do is subtract dipole bass cancellation from the IB response and you've got an estimate of dipole response.

[FlorianO]

Well, thanks for that, I was hoping for a more exact formula.

The closest thing I could find about that is that the offshoot in bass response for a driver with a Qt=1.0 is 1.25 dB high and occurs at fs*sqrt(2).

[scorpion]

Johns method is good enough. Brian Steele has a subwoofer page also dealing with dipoles. There is an Excel calculator on the page which
among other things also calculates the element's IB response. Link: http://www.diysubwoofers.org/dipole/

/Erling

[johnk...]

F(-3dB)/Fs = Sqrt(X)

where X = [ 1/Q^2 - 2 + sqrt{ (1/Q^2 -2)^2 + 4 } ]/2

[johnk...]

Johns method is good enough. Brian Steele has a subwoofer page also dealing with dipoles. There is an Excel calculator on the page which
among other things also calculates the element's IB response. Link: http://www.diysubwoofers.org/dipole/

/Erling

That's reaaly not the way do do dipole bass.

[scorpion]

johnK,

In fact I agree with you with regard to how dipole bass speakers should be designed. Question was however the IB response of an element.
Your formula will be amply illustrated by the Excel-sheets figure for frequency response. The curious reader could 'prove' your formula (and get an explanation for the magical Q-value of 0.7) by noting that a Q-value equal to SQR(0.5) = 0.7071 will give F3 = Fs in IB for that element, a Q value less than 0.707 will raise F3 above Fs and a greater value will lower F3 below Fs. High Q-value speakers will have a bump round (and a bit above) Fs in their IB (dipole response) which explains the argument for using high Q drivers
in dipole speakers.

/Erling

[johnk...]

johnK,

[snip]  High Q-value speakers will have a bump round (and a bit above) Fs in their IB (dipole response) which explains the argument for using high Q drivers in dipole speakers.

/Erling

But I don't believe that is a good argument. 

[FlorianO]

F(-3dB)/Fs = Sqrt(X)

where X = [ 1/Q^2 - 2 + sqrt{ (1/Q^2 -2)^2 + 4 } ]/2

Perfect !!!  Would it be too much to ask for a reference ?

Thanks again John

[johnk...]

Reference? Me! I worked it out on the back of a envelope. The old fashion way!