Passive Radiator Placement

mp006ltk

Member
2004-10-02 5:54 pm
I will be building a sub using the new Peerless XXLS and the XLS Passive radiator. How should I design the box to casue the least amount of cancelation between the Sub and the Slave. All the pictures in programs seem to show them on the same face but I would think this would casue the most cancelation due to alternating directions of tavel. I was thinking of having them on apposing sides or on adjacent sides. Any merits to one or the other. Also the 33 liter box sugested in the engineering notes seems a little small could I go up to 44 liters or would I casue some unforseen problem? Lastly are the Peerless Slaves still able to have mass added?

Thanks!
 

Ron E

Member
2002-06-27 10:41 pm
USA, MN
mp006ltk said:
I would think this would casue the most cancelation due to alternating directions of tavel.

IT doesn't matter how you put them on the baffle, there will be no difference in response - no cancellation effects unless they are widely separated (as in 5-10 feet).. BTW there is an inherent cancellation at the PR resonance that you can do nothing about, but not to worry, this is usually well below the frequencies where the sub is making any sound anyway... ;)

The only thing you have to worry about with PR's is vibration when the PR's are very heavy. When you have a kilogram swinging back and forth a centimeter or so 30 times per second you can actually have the cabinet walk across the floor. Putting the sub on wheels is therefore not usually a good idea unless the wheels have locks.
 

sek

Member
2003-05-21 2:51 pm
Berlin
Hi,

the Peerless application note calls for the 400g PR model in order to achieve the advertised response.

My PR has a mounting hole to add a bolt (in order to bolt on weights), but it's not required. 33l seems okay, too. I've built mine (using the XLS10 driver) with even a little lower volume.

But increasing it too much could lead to the PR bottoming out (due to too low resonant damping), and you don't want that (>400g hitting the suspension 30 times per second). Also, it might sound boomier than at 33l, and you don't want that, too (as it already isn't the most precise configuration). ;)

Sebastian.
 
The passive radiator does not simply move out of phase with the output of the active driver, as you think. Instead, it behaves like the column of air in a reflex port. What is not important is what direction the drivers are facing, since the cones are too small for the driver to be a velocity source. What is important is the net pressure differential, since at frequencies where the wavelength is greater than the driver diameter, the driver functions as what is called a "pressure source". That is why net displacement is so important for bass drivers and not so important for high frequency drivers. The bass you hear is coming from the driver pressurizing the air around you. The passive radiator tunes the enclosure. It does not simply track the motion of the active driver but in reverse phase, but at certain frequencies the passive radiator is designed to contribute to the output, so it must be moving in phase with the active driver at this frequency, and "bouncing" on the air mass inside the box, which behaves like a spring. This is because of that "net pressure differential" thing.