[RLDWV]

I'm trying to wrap my head around speaker building theory (and it hurts, believe me

), and one topic came up that really puzzles me - passive radiators. How does a rear-mounted PR NOT create an out-of-phase sound wave in relation to the front-mounted driver? I imagine that they must move in lock-step, but maybe that's where I'm wrong. Is it an inertia effect that prevents it from happening (i.e.: delay in getting the mass of the PR moving)? Otherwise, you'd think that the PR would just bounce the back-wave of the driver off the wall behind the physical box (facing the PR), creating an out-of-phase waveform.


I know that they do work - I still have a really old pair of Klipsch KG-4's that have a large PR in the back, it's just "why" they work that's hanging me up.


Go easy on me - I'm new to this...

Bob

 

[Bill Fitzmaurice]



Quote:
Originally Posted by RLDWV /forum/post/18237716


I imagine that they must move in lock-step, but maybe that's where I'm wrong.

Just as with a bass reflex the air volume within the cabinet acts as a capacitor, storing energy long enough so that at and above Fb the PR output remains in phase.

 

[RLDWV]

So if you were to actually view them in super-slow-mo at Fb they would be travelling in opposite directions, is that correct? A lot of what I've read talks about PR's being easier to plan for than ports, but I guess the internal volume calculation is still critical here. Does a PR contribute to overall output for a wider frequency range than a port does? I've kind of thought of ports as providing the same effect as blowing air over the top of a soda bottle - one frequency based on the internal volume, but I'm probably over simplifying that, too.

Bob

 

[noah katz]



Quote:
Originally Posted by RLDWV /forum/post/18237787


So if you were to actually view them in super-slow-mo at Fb they would be travelling in opposite directions, is that correct?

Only at some freq.


You can read up on vibration at wikipedia or googling.

 

[Bill Fitzmaurice]



Quote:
Originally Posted by RLDWV /forum/post/18237787


So if you were to actually view them in super-slow-mo at Fb they would be travelling in opposite directions, is that correct? A lot of what I've read talks about PR's being easier to plan for than ports, but I guess the internal volume calculation is still critical here. Does a PR contribute to overall output for a wider frequency range than a port does? I've kind of thought of ports as providing the same effect as blowing air over the top of a soda bottle - one frequency based on the internal volume, but I'm probably over simplifying that, too.

Bob

Ports and PRs function almost identically, the main difference being that a port is a vibrating mass of air, a PR is a vibrating drone cone. The advantage to a PR is that it will work in a box too small for a port with the requisite air mass to fit inside, the disadvantage is they're harder to tune and are usually more expensive to construct.

 

[John_E_Janowitz]

A passive is tuned much the same as a port of the same diameter. Example, take a 15"PR with 530sqcm Sd. You can take a port with that same 530sqcm Sd and figure the length for that port to get your required tuning. Then you calculate the mass of the air inside that port. The PR diaphragm will need the same mass as that volume of air to get the equal tuning.


For general stuff on PRs you can check out the Passive Radiator FAQ we have up too.
http://www.aespeakers.com/PRFAQ.php


John

 

[RLDWV]



Quote:
Originally Posted by John_E_Janowitz /forum/post/18239068


For general stuff on PRs you can check out the Passive Radiator FAQ we have up too.
http://www.aespeakers.com/PRFAQ.php


John

I'm reading it now - very good stuff (Just added to my browser favorites) Thanks for the info! The lights are starting to come on for me now (dimly, but on nonetheless).


Bob