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Old 28th October 2014, 05:43 PM   #1
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Default Building passive radiators

Hello

So lately I need more punch out of a small box I designed and vented is a bad choice.
But, finding suitable PR is really tough.

So was thinking of designing and building own sets of PR. Ofcourse, a PR is a speaker without magnets. But finding an "active speaker" matching closely to the requirement becomes difficult. Moreover, I really like rectangular ones.

So suppose if I buy a 2x3 inch active driver and remove its magnet, is there someway I can adjust its Fs and X-max and Sd and other relevant parameters.

By the way, as a sidenote, how is the Fs of a speaker "set" when m/f are building it. There are many youtube videos explaining the steps to build a real professional driver. But how are the rest of the parameters "set", in the sense there are always drivers for almost every setup, vented or sealed.

Thanks.
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Old 28th October 2014, 10:16 PM   #2
GM is offline GM  United States
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'Sounds' like you need to learn the basics, so start by figuring out how large and/or how many PRs you need, then it's usually best overall to make your own discs using MDF/whatever based on the mass required for a given total area [Sd] that you can buy rectangular replacement surrounds for that will get you close enough to add mass if need be to get the desired tuning: mh-audio.nl - Home

Then it's just a matter of a 'spider' attached to a cardboard tube or wood dowel that's attached to the center of the disc to keep it moving straight and true at a very high compliance, so a string or similar webbing is as good as any for the smaller/lighter PRs you apparently need.

WRT to designing them, one uses the same math to determine a driver's Fs, Vas, i.e. Sd, Mms, Cms.

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Old 28th October 2014, 11:19 PM   #3
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It may be worth studying passive radiator theory, but I think it's still a bit of a crap shoot. I once built a bookshelf speaker system with two 8 inch "full range" drivers and a tweeter. crossed at around 2kHZ. One woofer was hooked to the amp through the crossover coil, and the other was used as a passive radiator. I put either a short or an 8 ohm resistor across the coil of the 8 inch that was acting as a passive radiator (can't remember which - it's been almost 40 years), for damping, and I must have had beginners luck, it sounded VERY good. A voice coil is always also a generator. When it's shorted out, it dampens itself. By varying the ohms of the short, you are tuning the damping to some degree.
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Old 29th October 2014, 06:50 AM   #4
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Originally Posted by GM View Post
WRT to designing them, one uses the same math to determine a driver's Fs, Vas, i.e. Sd, Mms, Cms.
Since all those parameters depend on the Q, I guess those math involves you in figuring out and properly working with a driver. Do correct me if I am wrong.

But what about, say I want to design a driver, active or passive, with Fs of 40Hz. Which component of the driver controls that?

Last edited by arjunm009; 29th October 2014 at 06:54 AM.
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Old 29th October 2014, 06:53 AM   #5
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Originally Posted by Bob Richards View Post
It may be worth studying passive radiator theory, but I think it's still a bit of a crap shoot. I once built a bookshelf speaker system with two 8 inch "full range" drivers and a tweeter. crossed at around 2kHZ. One woofer was hooked to the amp through the crossover coil, and the other was used as a passive radiator. I put either a short or an 8 ohm resistor across the coil of the 8 inch that was acting as a passive radiator (can't remember which - it's been almost 40 years), for damping, and I must have had beginners luck, it sounded VERY good. A voice coil is always also a generator. When it's shorted out, it dampens itself. By varying the ohms of the short, you are tuning the damping to some degree.
That was very interesting. Just by shorting the voice coil, you tuned the PR.
Please can you explain the theory behind it?
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Old 29th October 2014, 09:20 AM   #6
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As I understand it in a vented system the port is effectively a helmholz resonantor. The 'mass' of the air in the port bounces on the 'spring' of the air volume in the speaker box to create a resonant amplification centered on the ports resonant frequency which is set to just below a standard drivers unit input capability.
In a passive radiator the same resonating mass / spring principle applies only the mass is that of the diaphragm, and the spring is that of the diaphragms support. With a single degree of freedom mass / spring system the resonant frequency is determined only by this mass and its supporting systems stiffness, or in TS paremeters Fs = 1/2Pi * Sqrt ( Cms / Mms).
So if you have a speaker with known Fs, Cms and Mms parameters then the stiffness may be too hard to change but you should be able to work out how much mass is shift is needed to achieve a target Fs. Modeling a vent in programs like WinISD shoud eb adequate for setting the passive radiators Fs target.
This is about adding energy to fill in an area where a constant Voltage / frequency input might otherwise be lacking. An equaliser such as the linkwitz transporm that increase the drivers input voltage in the low frequency range can do the same job within the drivers displacement limits.
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Old 29th October 2014, 11:44 AM   #7
xrk971 is online now xrk971  United States
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Originally Posted by arjunm009 View Post
That was very interesting. Just by shorting the voice coil, you tuned the PR.
Please can you explain the theory behind it?
A moving coil in a magnetic field acts as an EMF source or generator/microphone - the opposite of a speaker driver. If you apply a resistive load across the voice coil, the movement of the cone experiences a force loading due to the power being dissipated across the resistor. Varying the resistive load changes the damping force on the cone motion. Try it sometime with a bare driver sitting face up on a table. Leave it open and drop a small rubber ball on the cone and watch the motion of the cone. Then short it with a resistor and drop the ball again - it won't bounce as much. Or try connecting two bare drivers in parallel and push the cone on one, the other will move in reaction to the motion on the first.
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Old 29th October 2014, 06:51 PM   #8
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In a box as small as the one you are planning there is going to be a clearance issue between the magnets inside the box, which you are probably aware of. In theory the magnet structure can be removed, in practice that might be difficult to do without causing damage to the frame or suspension. I haven't tried it myself, so I don't know how difficult it really is. I've seen some videos, but when reclaimers salvage the magnets they aren't worried about destroying the rest of the speaker...

WinISD Pro allows you to add mass to the passive radiator and will calculate the new tuning frequency and response. Assuming the initial parameters are correct you will know how much mass to add for a given tuning.

Sd is determined by the cone size and you cannot change that. Qms is another factor in passive radiator design, a higher Qms is desirable. Low Qms represents mechanical losses, and a low Qms passive radiator will not produce as much output as a high Qms PR. Changing Qms would logically involve changing suspension compliance and fs too. But I would think a "loose" suspension is a good thing for a PR. It would probably be a good idea to "exercise" a speaker to break it in for use as a PR.

For a one-off design you wouldn't want to mess around with getting a custom passive radiator design, but if you are contemplating a production run it might be worth it to investigate having a manufacturer make a batch of PRs for you. After all, a PR is just a speaker without a magnet or voice coil. There are companies that make the components (cones, frames, etc) which other manufacturers use for their speakers, google around and you can find them.

I have also heard of early speaker hobbyists making their own PRs from a surround and cone, glued directly to the hole in the box. I'm pretty sure they were dealing with larger size "pro" speakers which are expensive, much abused, and have re-coning kits available. There are kits available for smaller speakers, but it isn't a cost-effective solution.
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Old 30th October 2014, 01:45 AM   #9
GM is offline GM  United States
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Quote:
Originally Posted by arjunm009 View Post
Since all those parameters depend on the Q......

But what about, say I want to design a driver, active or passive, with Fs of 40Hz. Which component of the driver controls that?
Hmm, normally one designs a PR for the desired Fs, Vas and the Q is whatever it turns out to be, which should be very high [Qms] to ensure a high acoustic efficiency.

Thiele/Small - Wikipedia, the free encyclopedia

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Old 30th October 2014, 01:51 AM   #10
GM is offline GM  United States
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Originally Posted by Bob Richards View Post
....for damping, and I must have had beginners luck, it sounded VERY good.

By varying the ohms of the short, you are tuning the damping to some degree.
An expensive way to get a damped vent, but a good way for sure.

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