How does linkwitz count baffle width for h-frames and u-frames

[thadman]

simply put, I remember seeing the information somewhere but after a long and arduous search...nothing

I'd like to input the values into spl_max1.xls(sp?) and compare them to standard flat values and determine if the extra efficiency will best the resonance caused by the design.

Also, how much lower in frequency is the H/U-frame resonance measured objectively than calculated? 1 Octave? 2 Octaves? 1/2 Octave?

 

[john k...]

There is no consideration of baffle size in the max SPL spread sheet.

As for what the actualy resonant frequency of an H or U frame is, that is a difficult question to answer. The resonant frequency will always be lower than the 1/4 wave frequency given by the length of the duct. It is dependent on both the length, L, and cross sectional effective radius, R. The 1/4 wave frequency based on length is only recovered as the ratio of R/L goes to zero. For example, for a 1.5' open ended duct the 1/4 wave frequency is 188 Hz. But if the duct has cross section with radius of 1/2' the resonance should drop to about 153 Hz according to theory.

 

[thadman]

There is no consideration of baffle size in the max SPL spread sheet.

As for what the actualy resonant frequency of an H or U frame is, that is a difficult question to answer. The resonant frequency will always be lower than the 1/4 wave frequency given by the length of the duct. It is dependent on both the length, L, and cross sectional effective radius, R. The 1/4 wave frequency based on length is only recovered as the ratio of R/L goes to zero. For example, for a 1.5' open ended duct the 1/4 wave frequency is 188 Hz. But if the duct has cross section with radius of 1/2' the resonance should drop to about 153 Hz according to theory.

You have to enter a baffle width for the SPL spreadsheet to calculate open baffle SPL

 

[Pano]

I'd like to know a bit more about U-frame baffles, too. I haven't seen a calculator that does them. Must be one somewhere, right? Did I miss it?

With my current baffles, The Edge shows a peak at 300Hz. That sounds about like what I am hearing. But that simulation does not include the side wings. I would be nice to be able to include them. They do change the FR, for sure.

 

[john k...]

SL's spread sheet only accepts the separation between sources as input. The width of a rectangular baffle would not be correct. It can be shown that a rectangular baffle is equivalent to a circular baffle with a specific radius http://www.musicanddesign.com/Equivalent_Baffles.html

Using this effective radius will give an indication of the flat baffle perfromance. For an H frame enteing the front to back length will give an estimation of the max SPL capability. For a U frame enter twice the front to back length. However SL's spread sheet will not calculate the resonance of the H or U frame duct. For a U-frame this resonace must be damped to achieve the potential performance gains. See

http://www.musicanddesign.com/u_frame.html

and

http://www.musicanddesign.com/NaO-II-U-frame.html
for a discussion on U-frame design.

Calculation of the H and U frame resonance peak requires calculation the driving point acoustic impedance for the duct which will depend on the duct geometry and damping. For an undamped duct you can get some insite here http://www.kettering.edu/~drussell/GMI-Acoustics/Filters.html