MJK - A Method for Designing a Compact Back Loaded Horn Loudspeaker System

[DrBoar]

I think I stand corrected.
http://www.mh-audio.nl/Acoustics/HHReso.html
Gives a resonanse frequency of 260 Hz with a circular opening of 9 cm
https://www.omnicalculator.com/physics/helmholtz-resonator
gave 650 Hz with a 17 x 3.5 cm opening

 

[David McBean]

If the chamber volume is 3.5 litres and the depth is 14 cm, and the port tube cross-sectional area is 62 cm^2 and the length is 1.2 cm, then the Helmholtz resonant frequency is close to 240 Hz, as shown in the attachment.

The calculator that comes up with the 650 Hz figure is not right because it doesn't take the port tube end corrections into account.

 

[DrBoar]

I dug out some nine year old measurement and I see a fairly flat response from 40 to 70 Hz then a 10 dB peak at or slightly below 100 Hz and then some lesser peak 180 Hz. This got to be made again more systematic and with better notes.Next time I do it like for publishing a paper.

 

[DrBoar]

This is with the pipe empty (black) and with the first section filled. Keeps the output below 100 Hz and reduce the peaks above by 5 dB or so

Filled section 1 and 2 of the pipe. Furher reduction to about 8 dB but also a 5 dB loss at 40 Hz

Finaly filling in all 3 sections, peaks down 10-15 dB with marginal losses in the sub 60 Hz range

The filling of the first section seem lika a no brainer. With regard to the other two sections I think I will experiment with selective damping like having 1 cm thick felt on the walls or having dense rockwool wedges at the bends. Something to reduce the peaks above 100 Hz and still try to keep the 35-60 output of the pipe