Shielded, low distortion driver?

[weinstro]

Hi,

What are my options for a high quality, shielded subwoofer?

Could be 10" or 12", sealed application preferred.

Alternatively, is there a way to shield something like a Peerless 830843?

I've checked Madisound, PartsExpress, and Zalytron. Not seeing much that jumps out.

[GM]

Use an AlNiCo motor driver or build a bipole with the motors physically coupled together. Of course this will require using non rear vented drivers. Another option is to line the cab with inexpensive 10 ga. cold rolled sheet steel (CRS).

GM

[weinstro]

Originally posted by GM
Use an AlNiCo motor driver or build a bipole with the motors physically coupled together. Of course this will require using non rear vented drivers. Another option is to line the cab with inexpensive 10 ga. cold rolled sheet steel (CRS).

GM

Regarding the sheet metal lining idea, it's intriguing, but I have a couple of questions:

Can anyone tell me a success story where they've implemented this approach?

How much field attenuation can one get? How would I measure this?

How much attentuation do I need before a CRT HDTV is affected? The drivers would be 18" from the base of the TV.

How do you keep the sheet metal from resonating? I'm worried that this will end up sounding like a pimped out car stereo with a vibrating trunk lid.

Does the sheet metal lining gradually become magnetized over time? Should I care if this happens?

Many thanks!

[montreal]

Originally posted by weinstro


Can anyone tell me a success story where they've implemented this approach?

How much field attenuation can one get? How would I measure this?

How much attentuation do I need before a CRT HDTV is affected? The drivers would be 18" from the base of the TV.

How do you keep the sheet metal from resonating? I'm worried that this will end up sounding like a pimped out car stereo with a vibrating trunk lid.

Does the sheet metal lining gradually become magnetized over time? Should I care if this happens?

The inside of my factory built Tannoy Saturn bookshelves are lined with a steel sheet (except the inside back and inside front).

There is also a smooth metal cap which surrounds the magnet structure. The metal sheets appear to be laminated with some special rigid glue onto the inside speaker walls which are made of high density particle board.

These speakers work fine on top of my Sony 34" CRT HDTV cabinet.

Then I moved these Saturns away to allow a large 15" Tannoy custom built monitor to go above the same HDTV. This driver sits in a box I build of similar dimension to the whole HDTV, so it looks a lot like one TV sitting on top of the other with 6" of gap between the top of the TV and the bottom of the speaker box.

I immediately had rainbow distortion all over the TV screen. So I ordered an expensive sheet of that special metal shielding material "MU metal" (probably paid close to $100) and placed this sheet under the box.

This did nothing to help the rainbows, so I called the supplier of the sheet (some inventor guy) and he told me to go into a heating and ventilation metal parts supply store and buy in different sizes what are called duct terminator caps, metal caps shaped like tin cans designed to seal off an existing round heating duct that is no longer connected to anything.

These caps come in diameter of 4" ,5", 6" and so on, and each final can is made by cutting a metal duct of corresponding diameter to a strategic length and attaching the cap to it.

Using some high density extruded polystyrene board, I was able to make up a number of rib shaped spacers (sticks) to sit inside each can so that 3 or 4 cans could be concentrically placed over the 15" speaker's magnet structure, like putting one top hat over a slightly smaller one.

By building up 3 or 4 layers of these cans, I completely blocked the magnetic radiation from entering into the HDTV.

I even put on a final top hat made of the special anti-magnetic sheet material, but this was overkill as it did not improve what was already perfect.

So I now have that original 30" by 20" metal shielding sheet under the speaker box and 3 concentric top hats of cheap HVAC parts and one final top hat made from the expensive material.

If I remember correctly, the final diameter of all these cans is about 12" and the final depth is also about 12". The smallest can diameter is 6", followed by 8" followed by 10" and finally the 12" outer can. The length of each can is also chosen so the back of each can is 1" away from the back of the next larger sized can. The styrofoam spacers are strips of 1" by 1" cut to proper length.

One of the downsides of having a 12" diameter can sitting directly on top of the magnet structure is that there is some restriction in how easily air gets pumped by the speaker cone.

The Tannoy 15" speaker frame has four large holes cut in the assembly to allow air to flow. But my large top hat does create a partial barrier to how easily the rear travelling wave can travel backwards. So I might be getting some sonic interference which impairs the sound slightly coming from this large center channel full range speaker.

For my same approach in a sub-woofer application, this interference would be less of an issue I imagine.

This has been working for 5 years.

[GM]

Well, it's not hearsay on my part and it has worked for others that have followed my advice, but haven't kept a record, so you'll have to do advanced searches through a number of forums to find them.

You get enough that one of these can be laid against a TV/monitor without visibly affecting its picture: http://www.lansingheritage.org/html/...nts/421-8h.htm

I assume a gauss meter.

Don't have a clue. Take the driver in question and slowly move it towards where it will be positioned to find out if you even need shielding.

You either install the plates internally with non-hardening glue so that they 'float' or you can glue them solidly in and treat them the same as you would any other reflective surface.

Don't know for sure, I never checked, but drivers shielded with a cover don't, so I assume not. All I know for sure is that it works long term. I had the aforementioned 15" setting on top of a 25" console RCA TV for >20 yrs with just a ~2 ft square IIRC scrap 10 ga. piece stuck under the speaker and hanging out over the CRT cover and no one ever noticed any color/whatever shift. Indeed, folks were always rather impressed with its ~Technicolor-like saturation, especially as it got older.

Regardless, all this is easy/cheap enough to prove it to yourself, just make sure it's CRS scrap from the local sheet metal/'tin knocker' shop, not the more common HRS, which won't help enough to matter.

GM

[just a guy]

Personally I would either move the speakers (or the tv) and not worry about this issue at all.

OTOH it's probably time for a new (non crt) tv (or projector) anyway.

I would NEVER let the tv limit my driver choices, I'd get rid of the tv altogether if it came down to it, rather than attempting to force them to play nice together.

[Jack Hidley]

*****Shameless plug warning******

Shielded, low distortion, cast frame 10", long throw and cheap:
http://home.comcast.net/~jhidley/index.html

Scroll down to NPT-11-083-2.

[sreten]

Originally posted by Jack Hidley
*****Shameless plug warning******

Shielded, low distortion, cast frame 10", long throw and cheap:
http://home.comcast.net/~jhidley/index.html

Scroll down to NPT-11-083-2.

Hmmm......

Unusual driver. High Re means pairs would be best. Low Fs means
sealed. High Vas (low Fs) and medium Qts indicate to me isobaric.
The assymetry indicates a pair in push pull isobaric will be better.

Quick modelling indicates 35L to 50L well stuffed sealed isobaric.
(A parallel pair = 140L to 200L = very big boxes)

Massive moving mass of 400g total yields very low efficiency but
quite oustanding sealed box bass extension, below 20Hz, which
seems somewhat unnecessary for a 10" driver.

Unless I've misunderstood the parameters possibly this driver is
intended to be used sealed with around 1cuft per driver (2cuft
for a pair) with active EQ to flatten the peaking bass alignment.

Hard to recommend - unless I've missed something ....

/sreten.

[Jack Hidley]

I think you must have misread the TSPs from the spreadsheet. The Mms for the driver is 98g, not 200g. This driver is listed in row 17. Other than the high Re (it is designed to be used in pairs) and the extremely high ratio of Vd/$, its not that unusual.

[sreten]

http://home.comcast.net/~jhidley/Kli...I_Detailed.htm

Mms 202.658g
mechanical mass of driver diaphragm assembly including voice-coil and air load

Hi,

Hard to recommend - unless I've missed something ...

The parameters in the spreadsheet referred to are more sensible.

They yeild an isobaric 40L sealed arrangement with f3 @ 30Hz and f8 @20Hz. (q=0.7)

Which is very good, but of course not very efficient. Probably
not best suited to HT duty but very good for quality stereo.

A parallel pair (i.e. not isobaric) could be arranged push pull, they
need a well stuffed box, size is hard to say depending on the Q
tolerable for the application, 0.7 = effective 160L / 6cuft, whilst
0.9 = effective 80L / 3cuft.

So you are right its not that unusual, but in parallel pairs some
EQ control (LT etc) would be handy for more compact hifi use.

/sreten.