Passive Radiator vs Port Tube
Passive radiators have become increasingly popular in small portable speakers. So lets examine why that might be the case and what are the pros and cons of a passive radiator compared to a port tube.
Both a port tube and a passive radiator serve the same purpose. That is to extent the low frequency response of a speaker. They work by adding a resonanting component to the speaker system at a frequency range a little below the bass drivers roll-off point. For a given driver in a chamber there is a fundamental resonant frequency below which the audio output starts dropping off. If we add a resonating component in this frequency range it can ‘collect’ the energy from the driver and radiate it as sound.
A port tube is an embodiment of the helmholtz resonator principal. What happens is the block or air in the port tube acts as a mass and moves in and out against the air volume in the speaker box. The frequency can be tuned by adjusting the diameter and length of the port tube. As the diameter of the tube increases the chunk of air in the tube tries to compress a larger amount of air thereby increasing the stiffness component and increasing the tuning frequency. As the tube gets longer a bigger chunk of air is created increasing the effective mass and lowering the tuning frequency.
Generally we want the port tube inside the speaker enclosure for aesthetic purposes so there is a limit as to how long we can make it. Sure the tube can bend around a little but for a small speaker there is just not that much space available. So in order to get a sufficiently low tuning frequency we would need to make the port diameter smaller. Making the port tube smaller give rise to two other issues. The first is that the surface area of air mass radiating to the outside world becomes smaller and less effective at pumping low frequency into the room. The second is the velocity of air movement inside the port increases and creates port noise.
Passive radiators to the rescue. Passive radiators do the same job as a port tube but instead of having a block of air as the mass we have a solid piece of material. That means we no longer have to consider a tube length. We do however have to be worried about the surface area of the passive radiator as this will equate of an effective stiffness. Again if we increase the stiffness we increase the tuning frequency. With the passive radiator we also need to consider the range of movement. It is going to have some kind of a suspension as a support and this will have a limit of excursion. The port tube air however does not really have a limit like this. If we make the passive radiator heavy, in order to tune lower, we will need a stiffer support to prevent it from sagging so we cant completely escape the relationship between mass and stiffness.
Smart people have come up with accurate calculations for tuning frequency of a port tube based on the enclosure volume, and port length and diameter. For passive radiators it gets slightly more complicated because the support for the radiator adds to the system stiffness. Calculating this stiffness is not as easy, at least it tends not to be accurate, so we inevitably end up with more trial and error getting the tuning right. That makes the development of a passive radiator system more cumbersome than the development of a port tube system.
The passive radiator has the advantage, primarily that it does not occupy enclosure volume and so for small speakers it has become the preferred option. In small speakers the demand for very low frequency output is much less and so the limitations of a passive radiator, being excursion limits and stiffness are not a big a issue. In larger speakers the port tube remains the preferred choice. In larger speakers the loss of enclosure volume is not as significant while the demand for low frequency output increases. The port tube is also less costly and easier to assemble. Either way careful design and clean implementation will beat a sloppy job every time.