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?
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?
<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?
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?
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.
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.
Attachments
sreten said:
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...
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.
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=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.
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.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?
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?
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.
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.Attachments
Man this stuff can be frustrating for n00bs. So, you are saying that the volume of the airspace within the frame should be counted as part of the volume of the enclosure. That seems to contradict http://www.diysubwoofers.org/faq.htm where the pictures shows that portion excluded from the volume of the box.
Your 2nd point is pretty interesting, I would not have even considered the volume of the cutouts
So, the yellow sections in my enclosure picture you consider to be part of the enclosure volume. That leaves the black (magnets) of the speakers, plus a bit of framing to subtract from the volume, and the smallish red area from the cutouts to add to the volume. So, roughly the area of the magnets can be subtracted from the volume, the other stuff roughly cancelling each other out.
For the amp, the cutout will add a decent amount of volume, where as the huge *** transform will subtract quite a bit. Again, I suspect a near cancellation here, but I need to do some measurements there.
So, here is the thing. We are targetting 10L enclosure. Do you have any idea how freakin small that is?
20cm*20cm*25cm, or roughly 8" * 8" * 10" for us folks on the other side of the pond.Plate Amp is 10-5/8" x 8-1/4" (27cm x 21cm). It doesn't exactly fit.
So, on one hand, I am thinking... with 3/4" mdf, and the longest length being maybe 11 inches, do I really need bracing? On the other hand, to fit everything in the enclosure it sorta needs to be bigger, so eating up some extra volume with bracing might be a good idea anyway
lol -- I am such a n00b. Thanks for all your help sreten.
Attachments
Hi,
the volume of the Driver is the volume of the frame
+ the volume of the magnet + volume of the cone.
The volume of the cone can be a lot less the the
airspace of the frame. So the FAQ is wrong I'd say.
apparently needs a 9.5 inch x 7 inch cutout,
what do you mean it doesn't fit ?
Using 3/4 inch MDF gives a box ~ 9.5 x 9.5 x 11.5 inches.
/sreten.
the volume of the Driver is the volume of the frame
+ the volume of the magnet + volume of the cone.
The volume of the cone can be a lot less the the
airspace of the frame. So the FAQ is wrong I'd say.
An externally hosted image should be here but it was not working when we last tested it.
apparently needs a 9.5 inch x 7 inch cutout,
what do you mean it doesn't fit ?
Using 3/4 inch MDF gives a box ~ 9.5 x 9.5 x 11.5 inches.
/sreten.Whoops, I looked at the wrong figures for the plate amp.
Overall dimensions 10-5/8" (H) x 8-1/4" (W) x 4" (D)
Required opening: 9 1/2" (H) x 7" (W)
Ignore that part
So -- bracing. Is that... required, highly recommended, or mostly optional?
I haven't sent the bent subwoofer back yet (3day weekend here in the states) so I will take some measurements/estimates on its volume before I send it back so I can start guessing at enclosure volume
Oh, and as for placement of amp/pr/woofer... I haven't thought really hard about it. It sounded like you were recommending down-firing? Any particular reason? Can all subs handle that? Is there a minimum distance the sub needs to be off the ground?
By default, I probably would have done 3 sides of non-top/bottom. Is that to be avoided?
Overall dimensions 10-5/8" (H) x 8-1/4" (W) x 4" (D)
Required opening: 9 1/2" (H) x 7" (W)
Ignore that part
So -- bracing. Is that... required, highly recommended, or mostly optional?
I haven't sent the bent subwoofer back yet (3day weekend here in the states) so I will take some measurements/estimates on its volume before I send it back so I can start guessing at enclosure volume
Oh, and as for placement of amp/pr/woofer... I haven't thought really hard about it. It sounded like you were recommending down-firing? Any particular reason? Can all subs handle that? Is there a minimum distance the sub needs to be off the ground?
By default, I probably would have done 3 sides of non-top/bottom. Is that to be avoided?
Hi,
The placement was just a thought. 2 sides and the back will be fine.
You do need to take careful measurements before committing saw to wood.
The box will have to be circa 11" high and 11" deep for the PR.
Ideally the PR would go on the left (from the front) the driver the
right. You need to make sure they will clear the amplifier parts.
Hopefully the width will work out 8 to 9 inches.
To this you add two side grilles. These will need to clear the driver
and PR at Xmax so I presume these would be 3/4" or 1" thick, so
when you add these you will end up with an ~ 11" cube.
Bracing - I'd add a single brace to the 3 free sides as shown before.
/sreten.
The placement was just a thought. 2 sides and the back will be fine.
You do need to take careful measurements before committing saw to wood.
The box will have to be circa 11" high and 11" deep for the PR.
Ideally the PR would go on the left (from the front) the driver the
right. You need to make sure they will clear the amplifier parts.
Hopefully the width will work out 8 to 9 inches.
To this you add two side grilles. These will need to clear the driver
and PR at Xmax so I presume these would be 3/4" or 1" thick, so
when you add these you will end up with an ~ 11" cube.
Bracing - I'd add a single brace to the 3 free sides as shown before.
/sreten.Well, disaster struck... I should have figured that since my first diy speakers really had zero problems that it should all go to hell this time
After making all the calculations to substract out volumes of internal components and such... I got everything cut, glued, painted. Got the amp in, the driver in... and then, I went to add the PR.
Ooopies, guess I didn't measure 3 times and cut twice or whatever that saying is ;-) The PR hits the driver about 1/4" before settlings nicely into its home. <sigh>. Luckily I have plenty of MDF, time to start over.
I had to at least hear *something* tonight, so I simply flipped the PR over, and screwed it in place. while strikingly ugly, and really throwing off the volume to be too big... it was still quite nice to have a sub! I am finding it even harder to evaluate a sub than my main speakers.. but I will say this... Holy Hell, It Moves Serious Air! I must have the plate amp at 20% power to keep the plaster on my walls. good times.
After making all the calculations to substract out volumes of internal components and such... I got everything cut, glued, painted. Got the amp in, the driver in... and then, I went to add the PR.
Ooopies, guess I didn't measure 3 times and cut twice or whatever that saying is ;-) The PR hits the driver about 1/4" before settlings nicely into its home. <sigh>. Luckily I have plenty of MDF, time to start over.
I had to at least hear *something* tonight, so I simply flipped the PR over, and screwed it in place. while strikingly ugly, and really throwing off the volume to be too big... it was still quite nice to have a sub!
I am finding it even harder to evaluate a sub than my main speakers.. but I will say this... Holy Hell, It Moves Serious Air! I must have the plate amp at 20% power to keep the plaster on my walls. good times.Hi,
Perhaps you can simply add about 3mm to the baffle thickness
(e.g. hardboard) for both drivers moving them the required
distance apart, depends on the way you've built the boxes.
Unlike the average AV "subwoofer" you will find what you have
built not only goes lower than the above, it can also shift a lot
more of the volume displacement required at low frequencies
to give clean undistorted low bass.
That is whilst it will not play that loud (relatively) it will play low,
and (for its size) produce levels of low bass far above the norm.
/sreten.
Perhaps you can simply add about 3mm to the baffle thickness
(e.g. hardboard) for both drivers moving them the required
distance apart, depends on the way you've built the boxes.
Unlike the average AV "subwoofer" you will find what you have
built not only goes lower than the above, it can also shift a lot
more of the volume displacement required at low frequencies
to give clean undistorted low bass.
That is whilst it will not play that loud (relatively) it will play low,
and (for its size) produce levels of low bass far above the norm.
/sreten.lancer said:
Hi,
Two (I think) happy punters for the price of one can't be bad.
Re: integration and compound croosover slopes.
The issue here is group delay and phase tracking, i.e. transient response.
The sealed sattelites by default have a 2nd order high pass roll-off.
The sub amplifier should be set with its bass roll-off at maximum
setting giving it a presumably a 2nd order low pass roll-off.
On the AV amplifier the c/o point is set, this should be ideally
the square root of low pass frequency x high pass frequency.
This will give the AV amplifiers c/o a chance to work well. If its
2nd order LR or 4th order LR the result will be modified 4th or
6th order overdamped acoustic functions - best compromise.
/sreten.So let me get this straight... The enclosure for this is only 10L?! Wow, I think I need to build this!
What passive radiator is being used and how low does it play? If I build this it'll be for extreme nearfield PC listening with some vifa tc9fd full range drivers or modded minimus 7's so I don't need much SPL at all.
What passive radiator is being used and how low does it play? If I build this it'll be for extreme nearfield PC listening with some vifa tc9fd full range drivers or modded minimus 7's so I don't need much SPL at all.
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