Silly's 1st Sub -- 8" MCM 55-2421 - diyAudio
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Old 24th May 2006, 07:43 AM   #1
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Default Silly's 1st Sub -- 8" MCM 55-2421

sreten has been nice enough to push me in reasonable directions for my first small sub in this thread.

However, it's badly titled as "cheapy passive sub", causing poor slobs to actually read it ;-) So, I created this new thread that more accurately reflects the state of things.

As it stands, I have this 8" subwoofer and this 150w plate amp being shipped.

sreten is now about to tell me about the required enclosure ;-)

Okay, seriously, modelled in WinISD, the smallest port opening I can do is like 3.5" (90mm) to keep airspeed down to 32 f/s (10 m/s), which would require a 70" (175cm) port, not exactly something that fits in a 0.35 cu ft enclosure (10 liter).

So, that leaves us with a passive radiator.

sreten already told me to get this 10" passive radiator from MCM, but like... I ignored him, cuz I was in a hurry to order before my trip to Vegas

Before I go ahead and finally order that... is there any other options I should consider? For example, this 10" PR from Creative Sound? Or some wacky non-PR solution? Or do I have a fundamental lack of understanding and there actually *is* a reasonable ported solution? A slot?
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Old 24th May 2006, 08:20 AM   #2
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<sigh>

Trying to read about passive radiators like a good boy... http://www.diysubwoofers.org/prd/

Okay, so I want to figure out the required mass of my PR for a 10L box at 32hz.

According the diysubs-- we do it like a port, with
Radius = (Sd/PI)^0.5

and MCM says the Sd for that PR is... Sd: 346 cm^2
Radius = (346/3.14159)^0.5 = 10.5 cm

We then throw that in WinISD, and it tells us we need a port length of 246.10 cm.

Volume = (PI*R^2)*h
= (3.14159 * 10.5^2) * 246.10cm
= 85,239 cm^2
= 0.085239 m^3

So then...
Mass = Volume * Density of Air
= 0.085239 m^3 * 1.2 kg/m^3
= 102g

Okay, is that good or bad? Cuz according to mcm specs...
Mms: 400g

I am not sure what Mms is, but if that is related to the 102g I just calculated I am in big trouble, since it will be hard to remove 300g from the PR

Am I confused?
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Old 24th May 2006, 09:39 AM   #3
sreten is offline sreten  United Kingdom
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Hi,

I don't like the look of the bass exciter - as it only has the surround
suspension - the MCM has the surround and central spider also.

Some PR calculations assume the The PR has no stiffness.

However I did my calculations in a rather ad-hoc method.

Plugging the PR specs into winISDPro driver editor gives a Vas of 40L.

This is very reasonable. The typical mass of a 10" sub cone is ~ 100g,
so the supension with a 100g cone would give a driver with an Fs
of 32Hz, as the PR cone is 400g, Fs = 16Hz.

Given the Vas = 40L I them manually calculated Fbox as though it is
a sealed box giving Fb = 9L/37.5hz, 10L/36Hz, 11L/34.5Hz, 12L/33Hz.

I then modelled it as a vented box using averaged specification
and measured values for the 8" MCM main driver.

As far as I can tell even if the Fs of the PR is way out, as long
as the PR mass and Sd is correct it will make little difference.

And yes to the question "are you forced to go with a PR".
Qts of the 8" driver is too low to be used effectively sealed.
So it must be vented, as its small the port is umananagable.
So PR it must be.

/sreten.

P.S. my estimate is you'd need a cone mass of ~ 500g for
32Hz in a 10L box (including the PR suspension stiffness).

I suspect your calculations have mixed up a radius
and a diameter giving you 100g instead of 400g.
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Old 24th May 2006, 08:42 PM   #4
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Quote:
Originally posted by sreten

I suspect your calculations have mixed up a radius
and a diameter giving you 100g instead of 400g.
You are a damn genius that is exactly the problem.

Using the correct port diameter (10.5*2) gives 999cm port length, which ends up giving 415g.

I am guessing my 415g versus your 500g has to do w/ "PR Stiffness", can you explain that a bit more? How exactly does the stiffness of the PR affect things?

It would seem to me that since a stiff PR is harder to move... it should look "heavier". So... I would have thought ignoring stiffness would cause my weight to be higher, not lower.

Guess I should start calculating the volume adjustments to my enclosure for my driver/pr/amp. I am a bit freaked by how tiny 10L is...
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Old 25th May 2006, 12:17 AM   #5
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I'm not an expert on the subject, but I think the stiffness of the suspension has the opposite role than what your thinking. Usually speakers with stiff suspensions have a higher Fs because it cant resonate easily at lower frequencies. I don't think you can calculate the stifness as part of the PR's weight. In my experience with PR's, a stiff suspension is a bad thing, it takes more energy to move the PR so you end up with slightly lower output.
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Old 25th May 2006, 01:18 PM   #6
sreten is offline sreten  United Kingdom
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Hi,

No suspension means Vas = ~ and Fs = 0 (yeah right).

The PR stiffness has the same effect as driver Vas.

/sreten.
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Old 25th May 2006, 06:42 PM   #7
sreten is offline sreten  United Kingdom
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http://www.partsexpress.com/pe/showd...number=295-494

the Vas of this is 110L (not 40L as above). In a 10L box it will make little difference.

So for this PR you'd be nearer 400g than 500g.
But still slightly heavier than assuming no suspension.

Note to go with that Vas (110L) if you up mms to 400g Fs becomes 10Hz.

But you still end up with near 32Hz in box.

In a sense it doesn't really matter how big Vas / low Fs is.
But if Vas is low enough to interact with Vbox you can't ignore it.

Personally I think a reasonable Vas (not too high) for a PR is a good thing.

/sreten.
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Old 27th May 2006, 03:51 AM   #8
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Well, the good news is I now have plenty of time to wait for my passive radiator to show up...

The sub arrived today, completely mangled. MCM clearly has no idea how to pack a box...
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Old 29th May 2006, 11:12 PM   #9
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While I wait for a replace sub and PR, figure I could probably get started building the enclosure.

Question: How do people calculate the extra volume required for the amp/pr/speakers?

Do I really wrap the speaker in saran wrap and figure out its volume via water displacement? Do I just use some rule of thumb (some fixed percent of extra volume) ? Do I use use measurements from the pieces and throw down some mad math skillz to calculate?
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Old 30th May 2006, 10:36 AM   #10
sreten is offline sreten  United Kingdom
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Hi,

I'd use some sensible estimations.

Wrapping drivers in clingfilm will not be accurate due the airspace within the frame.

Also assuming they are all externally mounted don't forget the volume of the cutouts.

Also any foam lining will add a few percent to the effective internal volume.

The secret is to have perfectly acceptable behaviour +/- 10% of your target.

How do you intend to mount the drivers ? And on what faces.
I suggest you do not use removable baffles, build a solid box.

With the amp on the back i'd put the driver on the bottom, the
PR on the front and add a single brace for the top and sides.

/sreten.
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