Questions re. passive radiators

[Dave Jones]

I've been reading this web page:

http://www.diysubwoofers.org/prd/

It says,

ALWAYS use a passive radiator that is larger in diameter than the active driver, as the displacement of the passive radiator usually has to be 1.5 to 2 times that of the driver. If it's not possible to use one large passive radiator, then you can use two or more smaller ones, and tune them by working out the effective diameter from the combined surface area of the radiators.

Q1 - How do you calculate the mass when you use two PR's? Assuming the two radiators are the same size, should the radiators each have half the mass that you calculate for the one radiator with the same surface area as the two you are using?

Q2 - The Lance Dickason book takes various PR parameters into consideration that the procedure on the web page doesn't. In fact, the only parameter that the web page uses is the effective diameter of the radiator. Is the procedure on the web page "good enough"? Is it good enough if the radiators are motorless versions of the driver, like in the Peerless XLS line?

Q3 - Does anyone know of a good EBS design using the 10" XLS and two 10" XLS passive radiators? -- or a 10" driver and one 12" PR?

 

[Svante]

The "helmholtz" frequency using a PR is

fh=c*S*sqrt(rho0/(M*V))

if the compliance of the PR suspension is ignored. c=345 m/s S=PR surface, rho0=1.2kg/m3, M=PR mass, V=box volume

So if S is doubled M has to be quadrupled (doubled in each PR) to maintain fh.

The statement that the PR needs a larger area is not nessecarily true. A PR usually has a larger linear excursion range than a driver, since it the main limiting factor is the suspension (it does not have a voice coil, obviously). A doubled excursion is equivalent to a doubled area with regard to max volume displacement.

 

[Dave Jones]

The statement that the PR needs a larger area is not nessecarily true. A PR usually has a larger linear excursion range than a driver, since it the main limiting factor is the suspension (it does not have a voice coil, obviously). A doubled excursion is equivalent to a doubled area with regard to max volume displacement.

Thanks much! So I'll just use a 10" XLS driver/PR pair, or maybe even the 12". Now, the questions are,

0) Is the 10" good enough? I'll be using it in a small room.

1) How big should the box be? I want an EBS alignment.

2) Should I get a plate amp with, say, a 4db boost at 25Hz or something? I have a Richter Scale Series III I can use for EQ.

 

[Dave Jones]

I've poked around a bit more on the net. I think I have finally decided (at long last) what my first subwoofer project will be. For the office, I'm going with the 10" XLS woofer and 10" XLS PR/400 gram in a box that's nominally 35 liters. If you've got a better suggestion, please let's hear it.

And please indulge me just a few more questions.

1) How do I figure how much the driver and PR displace (to add to the 35L number)?

2) Is damping material necessary, and if so, how does it affect the effective box size?

3) Should I get a plate amp with LF bass boost?

4) Does anyone know where I can buy a ready-made box a bit larger than 35L?

 

[Vikash]

I personally think 35l is on the large side. Use Unibox to model the XLS10 and passive combo.

When considering room gain, LF boost may not be neccesary.

I would rather have two 10" passives than a single 12". I think it would provide potential for cancelling vibration if tightly coupled.

You can use the volume of a fustrum to calculate approximate driver displacement.

You can also model the affect of damping material in Unibox.

 

[ozynigma]

I've poked around a bit more on the net. I think I have finally decided (at long last) what my first subwoofer project will be. For the office, I'm going with the 10" XLS woofer and 10" XLS PR/400 gram in a box that's nominally 35 liters. If you've got a better suggestion, please let's hear it.

And please indulge me just a few more questions.

1) How do I figure how much the driver and PR displace (to add to the 35L number)?

2) Is damping material necessary, and if so, how does it affect the effective box size?

3) Should I get a plate amp with LF bass boost?

4) Does anyone know where I can buy a ready-made box a bit larger than 35L?

Dave,

You might get some good design ideas from here for your PR,

http://www.adireaudio.com/Files/PR15andDPL12ComboKit.pdf

Also I would not recommend building a sub without first reading

http://www.woodartistry.com/linkwitzlab/thor-intro.htm

After reading this I decided to build a sealed box.

I just built a DPL12 in a 60l sealed box, no PR and it sounds good.

1) seems very hard to find this out, I would allow about 3 litres for driver volume unless someone or the manufacturer can provide more accurate information. Also don't forget to allow the volume of the amp module and any bracing.

2) "Practical effective volume increases of 15% to 25% are quite attainable" The Loudspeaker Design Cookbook, Dickason.

So if you are forced to build an enclosure smaller than ideal design you could use some stuffing to compensate for that. Also some stuffing should be used to dampen resonance. (I still have to do that with my sub).

3) This depends on too many things to say. Your sub will interact with your room and this will be dependant on placement in the room. If you do get an amp with boost it would be best if you were able to change the boost setting. My amp has resistors you can change to do this.

Howard

 

[Dave Jones]

I personally think 35l is on the large side.

You are right. I messed up the calculation, (and the graph also).

I think these numbers are more realistic:

XLS 10" with 10" passive radiator

290 gram -- Vb = 35 liters
400 gram -- Vb = 26 liters

But then, I might have messed up again. :-(

 

[Vikash]

Looks ok. I must have said this in a few posts now, but I think I'm going with 25l to 30l (probably the lower end) if I decide to stick with one 400g passive.

Have you looked at these threads/pages:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=32957&highlight=
http://www.diyaudio.com/forums/showthread.php?s=&threadid=19180&highlight=
http://www.diyaudio.com/forums/showthread.php?s=&threadid=30314&highlight=
http://www.geocities.com/adrian_mack/homepage.html

 

[Dave Jones]

Looks ok. I must have said this in a few posts now, but I think I'm going with 25l to 30l (probably the lower end) if I decide to stick with one 400g passive.

Have you looked at these threads/pages:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=32957&highlight=
http://www.diyaudio.com/forums/showthread.php?s=&threadid=19180&highlight=
http://www.diyaudio.com/forums/showthread.php?s=&threadid=30314&highlight=
http://www.geocities.com/adrian_mack/homepage.html

The first one uses the 265 gram PR in a 2 cubic foot box. Hmmm. http://www.lautsprechershop.de/hifi/index.htm?[url]http://www.lautsprechershop.de/hifi/bau_xa10.htm[/url]