Shielding A Seas CA18RLY Woofer, Measurements

[Winter]

Just in case anyone is interested, listed below is some test information on shielding a Seas CA18RLY. I used Woofer Tester 2 to measure the Fs and Qts parameters. A 27" CRT television with a blue screen (no input signal) was used to judge magnetic interference. When a bucking magnet is placed in proximity to the woofer magnet rear plate with similar poles, initially they repel. As they get closer than 1/2", they actually start mildly attracting. This force can be managed by hand, and the bucking magnet can be centered relative to the pole piece, and glued easily if desired. The magnet and shielding cup is a different matter. The magnet with cup can be placed to contact the woofer magnet rear plate. However it cannot be centered by hand as the lateral magnetic forces generated by the cup are too great. I had to superglue six 1/4 inch wide, 1/5 inch thick, wood spacers around the sides of the woofer magnet to guide the shielding cup to center. The bucking magnet size guideline (no shielding cup) of 1/2 to 3/4 the size of the woofer magnet appears to be good as shown by the TV picture interference distances below.

Woofer: Seas CA18RLY
Bucking Magnet, 1/2 Woofer Magnet Size: 2 x Vifa #707517
Bucking Magnet, Full Woofer Magnet Size: Seas H1120
Bucking Magnet with Cup, Full Size Magnet: Seas H9915

Woofer Configuration...............Standard....Bucking Mag 1/2....Bucking Mag Full....Bucking Mag/Cup
Fs...........................................42... ..............42.....................42........... ..............42
Qts.........................................0.45.. ............0.41..................0.39............ ...........0.37
TV Front Interference Distance.....14".................6"............... ......16"..........................8"
TV Top Interference 'Distance........8".................3"............ ..........3"...........................0"
Calculated Sealed Box F3 Hz..........66................72.................. ..76...........................81