Edited this post as I discovered the data I had was incorrect. I still have a question about calculating box losses based on impedance data. This is for a subwoofer build. Following the fomulas given in Testing Loudspeakers, I have taken data for 3 versions of dampening material lining the box walls. No lining, walls lined with .75" material, walls lined with 1.25" material. This time I captured impedance with ports open then plugged.
Tuning was calculated with Fb = (Fl^2 + Fh^2 - Fc^2)^.5
No lining:
Fl 15.058
Fh 43.489
Fc 36.784
Fm 25.590
Rm 3.855
Calculated results:
Fb 27.65
alpha 1.41
Qtsb .55
h 1.16
Qb 17.67
Walls lined with .75" material
Fl 14.644
Fh 41.128
Fc 35.275
Fm 23.535
Rm 4.183
Calculated results:
Fb 25.72
alpha 1.27
Qtsb .556
h 1.098
Qb 8.143
Walls lined with 1.25" material
Fl 14.644
Fh 41.128
Fc 35.771
Fm 24.034
Rm 4.086
Calculated results:
Fb 25.028
alpha 1.20
Qtsb .54
h 1.04
Qb 9.89
Im not sure why the box losses for the 1.25" lining are lower then with the .75" lining. I would expect adding more lining material would increase the loss due to absorption. What am I missing?
Tuning was calculated with Fb = (Fl^2 + Fh^2 - Fc^2)^.5
No lining:
Fl 15.058
Fh 43.489
Fc 36.784
Fm 25.590
Rm 3.855
Calculated results:
Fb 27.65
alpha 1.41
Qtsb .55
h 1.16
Qb 17.67
Walls lined with .75" material
Fl 14.644
Fh 41.128
Fc 35.275
Fm 23.535
Rm 4.183
Calculated results:
Fb 25.72
alpha 1.27
Qtsb .556
h 1.098
Qb 8.143
Walls lined with 1.25" material
Fl 14.644
Fh 41.128
Fc 35.771
Fm 24.034
Rm 4.086
Calculated results:
Fb 25.028
alpha 1.20
Qtsb .54
h 1.04
Qb 9.89
Im not sure why the box losses for the 1.25" lining are lower then with the .75" lining. I would expect adding more lining material would increase the loss due to absorption. What am I missing?
Last edited:
Looking at the data a little more, I thinking there is either too much material lining the walls with the 1.5" thick stuff or its too tightly compressed.
The drivers FS in box or Fsb rises with the thicker material when it should be decreasing. Also the drivers minimum impedance between the peaks or Rm starts to decrease.
No lining:
Fsb = 23.67hz
Rm = 3.85ohm
.75" material
Fsb = 23.41hz
Rm = 4.18ohm
1.25" material
Fsb = 24.06hz
Rm = 4.08ohm
The drivers FS in box or Fsb rises with the thicker material when it should be decreasing. Also the drivers minimum impedance between the peaks or Rm starts to decrease.
No lining:
Fsb = 23.67hz
Rm = 3.85ohm
.75" material
Fsb = 23.41hz
Rm = 4.18ohm
1.25" material
Fsb = 24.06hz
Rm = 4.08ohm
