Cassette Magnetic Shield

tapemuzik.de

I had to transfer some cassette spools last year into new shells. In the process I noticed that new and old magnetic shields are very different sizes. Most are flat and roughly 25 mm x 12 mm. I've seen a few that had rounded angles on the outside edges but that seemed to just hold it in place inside the shell.

My thesis is that these have maintained roughly the same dimensions, but have decreased in thickness over time. I don't own a gaussmeter (dang) so I can't measure the effectiveness. I'm using a cheap Fowler caliper that uses a 1⁄1000  scale so we are doing this in thousandths like a luthier. I'm also using a mix of blank stock and retail cassettes to see if there's any difference there. This does require in some cases that I destroy the shell to access the magnetic shield. For their sacrifices we give our thanks.
 

artist/title	MFR	Year	size
The James Gang / Passin' thru	AMPEX	1972	30 thou
J. Geils Band	N/A	1973	28 thou
Rolling stones - Beggars Banquet	N/A	1976	25 thou
Certron HD90	Certron	1977	24 thou
SONY CHF 90	SONY	1978	30 thou
Maxell UD II 100	Maxell	1981	20 thou
Jorma Kaukonen (bootleg)	N/A	1982	28 thou
Rich Little - Comedy Edition	Radioshack	1983	9 thou
Maxell UDS II 90	Maxell	1985	21 thou
JVC dynarec GI-90	JVC	1988	21 thou
FUJI DR-II 90	FUJI	1990	22 thou
Rhino Humpers - Intense	N/A	1994	12 thou
P.U.S.A.	N/A	1996	12 thou
Reach the Sky - s/t	N/A	1997	10 thou
WKDU Live Vol 1	N/A	2013	11 thou
Allston Pudding - localz only	N/A	2014	15 thou
Tankini - dousuk	N/A	2017	12 thou
Heavy Discipline - demo	N/A	2019	10 thou
Strange Mono- bail fund benefit	N/A	2023	21 thou
Magnetic Responsibility / Naan Cul	N/A	2023	12 thou

I kept commercial tapes to a minimum in the sample because their date of manufacture is harder to narrow down. The few I kept are slot or coffin cases which more definitively date to the 1970s. Hypothetically some may date to the late 60s. Despite the haphazard sample, the trend here is pretty clear. I think the overage thickness has decreased by about 30-40%.  The thinnest shields are either very new or from old novelty recordings. (I'm looking at you Rich Little.)  

 While reading up on this I found a Billboard article from 1986. The article quoted Glenn Maenza, the manager of electronic engineering for CBS records and IEEE member. [SOURCE]

"Cassette quality is not affected by elimination of the shields since the shield typically reduced the 60/120 Hz level from -23 to -26 dB, relative to audible threshold levels..."

Maezenza did go on to say that he wasn't advocating removal of the shield. But ever cheaper manufacturing delivered the change anyway. Today new blank tapes (if you can find them) often have no shield at all!  Starting around 2015, I started noticing new blanks from TDK, Sony and Maxell completely lack the shield. Usually you can't tell from the audio alone. But depending on your deck and how close the motor is to your read head, you might pick up more 60 Hz noise with these newer, cheaper tapes. But if you're using cassettes... you're probably not a hard-core audiophile anyway.

**I may expend this article if I find more reliably dated recent blank tape stock.