Ok, so I have a question about passive radiator systems. I talked to Jim last night about these type of sub/woofer sections and he mentioned 2 key requirements. One is that the total area of the passive radiators is greater than the area of the driver, and the other is that the xmax is also greater, so that full excursion of the driver won't bottom out the passives.
For example, the HT4 woofer uses a 12" driver with dual 12" passives I believe. What are the tradeoffs of using different size passive radiators? I'm looking to use a 15" Rythmik driver, and Jim mentioned that dual 12" passive radiators would probly work fine, or dual 15" passives as well. I've seen people build subs with 12" drivers and 18" radiators and so forth. What changes as play with the size of those passive radiators? Is there a key to being "optimum" for a given driver and cabinet size?
I wouldn't want to go out and have a 15" driver and dual 18's if it would perform the same as a 15" and dual 12's or any other combo. I'd simply like to buy the best setup.
As you go lower in frequency the displacement requirement increase significantly. For a vent this means it must be larger and larger in diameter in order to avoid excessive air velocity from the port. The larger diameter requires the port be larger in order to tune to the same frequency. Before long the size of the port is too big to be practical. These demands are proportional to the amount of displacement that the active driver possesses. This is the reason why passive radiators are used so often with high excursion woofers, like the ones Jim uses, if a vented alignment is being pursued. It is the only way to tune to a low frequency and still allow enough air displacement to be adequate to avoid high port noise and compression of low bass output.
At Fb (the cabinet tuning frequency) the excursion demands fall almost entirely on the passive radiator (or port in a vented speaker). The driver's displacement is reduced about 20dB at this frequency and it's travel is very low in the narrow frequency range. Because of this, as a general rule, passive radiators need to move at least twice as much air by volume as the woofer is capable of moving. This can come from a combination of increased surface area and increased excursion, but the total volume needs to be about twice that of the woofer -as a minimum. The the best low bass reproduction without distortion or compression it is best to go with as much passive radiator displacement as is practical. The bigger you can go, the better, but I understand that this needs to stop somewhere.
With a standard 15" woofer with a typical higher excursion passive radiator using one 15" radiator would probably be acceptable. However, when using a woofer like the Rythmic 15" with its high excursion cababiliies I would not recommend less than two 15's to keep up with it, and even this may push the passive radiators if a lot of very low frequency demands are placed on the system (ie; a lot of signal below 20Hz).
The other question is - Is the Rythmic 15" an appropriate driver to use in a vented / passive radiator system.I have not modeled it to see. Perhaps Jim already has, but I don't personally know at this time. I also know there has been some talk of using the Rythmic servo system with passive radiators. I am not sure how Rythmic is doing this. If the woofer's cone motion is braked at Fb and reduced by 20dB then the sensing coil on the woofer signaling the servo would see this, and not the output from the passive radiator and this would cause the servo to make an incorrect correction. Maybe Brian Ding at Rythmic has worked out some circuitry to deal with this, but I am not sure how he is doing it accurately as this notch will change for different systems.
If it is not servo then it's just a matter of matching the woofer, box, and tuning.
Jeff B.