Passive radiators are used when the reflex tuning of an enclosure would result in a port which would be too long for the diameter required to prevent air turbulence within the port.
With no space grabbing large port, the cabinet volume can be made smaller when using a passive radiator.
Soundwise, the use of a passive radiator means there will be no chance of hearing the chuffing noises which can be produced by the air turbulence in a port.
With no space grabbing large port, the cabinet volume can be made smaller when using a passive radiator.
Soundwise, the use of a passive radiator means there will be no chance of hearing the chuffing noises which can be produced by the air turbulence in a port.
Some passive radiator loudspeakers can have minimum usable volume. It is the volume under which the passive radiators do not effectively move. If the Pr suspension stifness is a bit too much for woofer to move, it will sound bassless until enough volume is applied. That will probably never be the case when listening in large rooms but in smaller ones it can be a problem.
The larger cone also moves with a smaller amplitude, resulting in the same behaviour as compared to an ideal ported system. The main deviatons from ideal are that ported systems suffer from port losses (only at high air velocities) and port resonances, while passive radiators have a suspension stiffness and loss (at all cone velocities) and are limited in peak displacement as well.
That would imply that the passive radiator behaves non-linearly at low amplitudes, which I think is not true. Springs, like the suspension of a passive radiator, usually are linear at low amplitudes and become non-linear at high amplitudes.
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The main problem with PRs is that they are heavy compared to the air column in a port with all that entails.
For example if the woofer is a 12" or 15" job the air column in the appropriate port is around 3g while the appropriate PR tuned to the same frequency can easily reach or exceed 300g.
More difficult to excite via air coupling as Zvu said but also harder to stop.
That said for small desktop speakers a port tube can be larger than the cab and then a PR is the only choice.
For example if the woofer is a 12" or 15" job the air column in the appropriate port is around 3g while the appropriate PR tuned to the same frequency can easily reach or exceed 300g.
More difficult to excite via air coupling as Zvu said but also harder to stop.
That said for small desktop speakers a port tube can be larger than the cab and then a PR is the only choice.
Yes, it should be 1.5-2x larger than the driver otherwise run the risk of bottoming out the PR.
It's a tiny bit more efficient when comparing same size effective piston area [vents are simmed as a piston], but it's just a little bump at Fb, so not audible, though obviously if the vent is smaller, then there could be an audible difference in sheer output.
Sound wise there's a massive difference since the equivalent large, long vent will have TL pipe harmonics low enough in frequency and high enough in amplitude to comb filter with the driver's output.
That said, PRs have a nasty notch ~an octave? below tuning, so combined with some suspension noise, personally don't like to use them much above a 20 Hz tuning and preferably below it.
GM
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