knowledgebase
Passive RadiatorFrequently Asked Questions
Passive radiators are devices that can function in the place of ports for many speaker designs. They are not direct replacements however as they do have some unique qualities. They have both benefits and drawbacks, but if implemented properly they can make for a superior subwoofer or speaker system.
Q13 What happens when I double the amount of passive radiators for a given cabinet and tuning frequency?
Use a passive radiator when you would like to have the extra output of a vented system, but are not able to fit an adequate port into the enclosure. The tuning of a PR is determined by the amount of mass on the diaphram, where the tuning of a port is determined by length. Often if you want to get low tuning in a small enclosure, an adequate port will just not fit. You either need to sacrifice output by going with a smaller diameter port, or sacrifice low end extension by going with a higher tuned port. For example, we use a single 15" woofer in an enclosure tuned to 20Hz with a pair of 18" PR's. If you wanted to get the same tuning with an adequate port, it would need to be 6" diameter and nearly 60" long and you would STILL have problems with vent noise.
You typically only want to use a passive radiator along with a woofer that is designed for a vented enclosure. A passive radiator enclosure is NOT a sealed box enclosure. You will typically want to use a lower Q woofer with a PR system. This usually means a woofer with Qts of .4 or lower.
You typically only want to use a passive radiator along with a woofer that is designed for a vented enclosure. A passive radiator enclosure is NOT a sealed box enclosure. You will typically want to use a lower Q woofer with a PR system. This usually means a woofer with Qts of .4 or lower.
A passive radiator does not have the pipe resonance, air turbulence, and escaping internal box noises that the simple port has. They are also easier to implement than a port when the tuning frequency is very low, and most importantly will not compress the output until they reach the limits of their suspension travel.
The cabinet will need more attention to the placement as they are effectively another driver cutout. The mass of the passive is capable of shaking the cabinet if care is not taken in the design. Do not attempt to use heavy weight passives with lightweight cabinets on tall stands!
Passive radiators are very sensitive to the surface area of their moving assembly. The larger the better usually until either the passive is too large for the cabinet, or the moving mass of the passive is too large to be used reliably. Larger passives will need more mass to tune to the same frequency as smaller passives. Larger PR area is beneficial as it lowers the effect of the PR Vas on the cabinet, increases the Qms of the passive closer to that of a port, lowers the PR "notch" in frequency, and can allow more output before the PR reaches its suspension limits. All of these combine to give more LF output.
The Vas of the passive radiator forms a notch filter when combined with the the cabinet Vb and will null the output near the free air resonant frequency of the PR. Using larger area PR will force this notch lower in frequency while keeping the tuning frequency of the cabinet the same. Hence there can be slightly more LF output below Fb with the larger PR area. Most of the time this is not a large issue. The resonant frequency of the larger PR's is down as low as 3.4Hz, so the notch does not affect any of the useable frequency range.
The PR should have a minimum of 2 times the volume displacement (Vd) of the active driver. If your woofer displaces 2L at Xmax, then you will want to have a passive, or passives that displace 4-8L to make sure you do not run out of excursion at the low frequencies. Typically most people use a passive one standard size larger than the active driver (e.g. 12" passive with a 10" driver). Some choose to add multiple passives to increase the passive area to the limit. You can use multiple PR in the same enclosure, but then you will need to ensure that the Fs of each are as close as possible. We prefer to use dual opposing PR as this allows the PR to mass cancel their moving mass, very important for keeping the cabinet from shaking.
If you use two passive radiators consider putting them on opposing sides of the enclosure so that the inertial forces cancel out.
If you use two passive radiators consider putting them on opposing sides of the enclosure so that the inertial forces cancel out.
The PR can be considered equivalent to a port with a surface area equal to the Sd of the passive radiator. The PR Mms is then equivalent to the mass of air volume that would be in the length of the port. They are so similar that you can tune them the same as a port with these simple calculations found here.
Yes and No. The best a passive radiator can do is to be equivalent to a port with roughly the same diameter. An 8" port is better than a 8" passive radiator in theory since the port does not have a significant Vas or Qms. However passive radiators create specifications that are far beyond what any realistic port can achieve. A port equivalent to most large passives cannot be used typically unless the cabinet was larger than most living rooms. The most extreme example is the dual 15" 1400g passive radiator cabinets. A port equivalent to these would be over 18" in diameter and 46 FEET in length! This is where the passive radiator works best, in situations that ports cannot be used under any circumstances. The example 18" port cited would have incredible output compared to a standard 4" port if it could be made to work, however the port size is not feasible while the passive radiator is.
Passive radiators have a Fs, Cms, Mms, Qms, and Vas just like a typical driver. Sometimes these are stated as Fp, Cmp, Mmp, Qmp, and Vap respectively for the PR. The most important thing to realize with these specifications is that the Cms, Qms, and Vas should be as large as possible. This is when the passive radiator acts closest to a port of the same diameter. The other important specification is the Mms of the passive radiator as that is what will determine the tuning frequency of the cabinet.
Not really. The passive radiator once mounted in a cabinet will have a new Fs that will be determined mostly by the cabinet volume and the Mms of the passive radiator. One should just look at what the tuning frequency of the passive in the cabinet will be. Simple hand calculator equations for this can be found here. We have found that free air Fs will shift around with temperature and humidity and its affects on surrounds especially. A 10degree F change in temperature is enough to shift the free air Fs of the PR significantly. The best way to ensure the design is right is to do an impedance graph and look for the tuning frequency dip. This is the minimum point between the two impedance peaks. When done well the dip will be very close to the Re of the driver (done poorly the dip is much higher than the Re of the driver. This shows you have a leaky enclosure).
The PR should never be down-firing, the high Mms will cause the diaphragm to be displaced downward immediately. The spiders will eventually be stretched permanently by this treatment, and the unit will need to be rebuilt.
If you double the surface area of the passive radiator, and keep everything else the same, you must increase the total PR Mms 4 times in order to keep the same tuning frequency. In other words, both passive radiators now need to each have double the Mms of the original single unit. This is similar to a larger diameter port needing more length to tune to the same frequency as a smaller diameter.
This is a very interesting concept that we have done some experimenting with. It works very well to get a large amount of output from a small cabinet. Using a single light PR for the high frequency chamber and a a pair of PR's for the low frequency chamber of a 6th order bandpass enclosure works extremely well. The trick is actually to get a light enough PR to use in the HF section.
This is entirely subjective, but if they are done correctly they create bass that is as tight as a sealed box cabinet with the incredible output levels and sensitivity of a maximum output ported cabinet. When a PR system is tuned very low, the rise in group delay evident at tuning is pushed lower in frequency, no longer affecting the sound. For an example, look at a 3cf enclosure with pair of 15" 1400gram PR's. The tuning is right around 20Hz, and above 35Hz, the group delay of the system is equal to what it would be with the same woofer in a sealed enclosure. Below this frequency, the group delay does not cause any audible effects. This again means that the PR system can sound as "tight" as the sealed enclosure, but with the extra low frequency output. Something important to consider is that most people using passive radiators comment that they do not like regular ported cabinets, but do like the sound of the passive radiators. Passive radiator cabinets really do have a sound of their own.
Also, many people have said that they have not heard a passive radiator system that they like. Keep in mind, most commercial passive radiator systems on the market are nowhere near ideal, and are made with extreme compromises. It is very difficult for a large commercial company to produce an ideal passive radiator system because of budget constraints. These commercial systems are not a good indicator of what can be achieved with a properly designed PR system.
Also, many people have said that they have not heard a passive radiator system that they like. Keep in mind, most commercial passive radiator systems on the market are nowhere near ideal, and are made with extreme compromises. It is very difficult for a large commercial company to produce an ideal passive radiator system because of budget constraints. These commercial systems are not a good indicator of what can be achieved with a properly designed PR system.
Passive radiators should not be used in systems with high Q woofers. A woofer with a high Q is designed only for a sealed box enclosure. Using a passive radiator with a high Q woofer will result in a very boomy sound, and not the tight bass that most people typically desire.