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Passive Radiator Speaker Design

Even though loudspeakers using a passive radiator are quite common in today’s market, there are many misconceptions about how a passive radiator works to make bass. Most of these are either incomplete or flat out wrong.

In this article, we’ll walk you through everything you need to know about passive radiators including how they work, their cost, their pros and cons, as well as the differences between them and standard ports.

What is a Passive Radiator?

There’s no denying that passive radiators and ports work on the same principles. Therefore, the response of a passive radiator system is relatively similar to that of a ported system using the same driver.

Unlike vented systems using standard ports which are often made out of cheap piece of plastic tubing, speaker enclosures using a passive radiator are fitted with an “active loudspeaker” (or main driver), and a passive radiator (also known as a “drone cone”) to extend the system’s low frequency response. This adds a whole lot more to the cost of a system than a standard port.

The active loudspeaker is a normal driver, and the passive radiator is just a flat diaphragm mounted via flexible suspension onto a frame.

The passive radiator, although it may at first glance look like a normal driver, it does not have any engine parts. That is to say it does not include a coil, magnet and power supply. In the same way, it will not be electrically connected to an electrical circuit or power amplifier.

The passive radiator must usually be at least two times larger than the main driver in the enclosure.

Passive radiators are used in a wide variety of speaker systems including but not limited to portable Bluetooth speakers, home stereo speakers, subwoofer cabinets and car audio speaker systems, particularly in cases where using a vent is difficult or impossible.

Furthermore, passive radiators are an efficient way to get a whole lot of extra oomph out of a
sound system without a ton of power while remaining quite small.

This is the reason why they are so common in portable speakers and computer-driven speaker systems.

P.S: It should be noted that when a passive radiator is used instead of a regular port, you still get quite the same response (more or less) like a vented enclosure. The acoustical behavior of both a passive radiator and a port is quite similar, although there are a few differences in practice.

 

How Does a Passive Radiator Work?

A passive radiator coupled with an active driver work together according to the Helmholtz resonator principle.

To put it simply, a passive radiator makes use of the internal air pressure generated by the active driver. This subtle and clever method allows the loudspeaker to deliver surprisingly great sound and solid bass performance from a relatively small footprint enclosure.

 

The way this work is by sealing the entire enclosure so that all (or a big chunk of) of the vibrations generated by the active driver are trapped in the enclosure and are forced to move through the diaphragm of the passive driver. Because the air pressure inside the enclosure cannot escape in any direction other than through the passive driver, the effect of a big, powerful woofer is effectively simulated.

How to Tune a Passive Radiator?

A passive radiator, like a port, must be tuned properly to work with a specific driver and enclosure. You can’t blindly put a passive radiator in an enclosure and expect it to work. It’s highly unlikely that you will get good results.

Without properly matching a passive radiator to your enclosure and to the active driver, it’ll result in either boomy or flat bass, muddy sounding bass, or bass that seems to drone at a single frequency, regardless of what note is being played.

The resonant frequency of the passive radiator is determined by its mass and the springiness (compliance) of the air in the enclosure.

Passive radiators are tuned to achieve the desired tuning frequency by applying mass to the diaphram (e.g. by adding weight in the form of washers/disks to a bolt behind the cone). In this way, you can easily tune your box to hit 20ish Hz without the need for very large enclosure and longer ports.

You can think of the weight of a passive radiator as the equivalent of the length of a port in a ported enclosure.

When it comes to tuning a ported box, the longer the port, the lower the tuning. Likewise, when it comes to passive radiators, the more weight you add to the cone of the passive radiator, the lower the tuning frequency and vice versa.

To tune a passive radiator, you’ll need to plug in the volume of the box, and the Thiele/Small parameters of the passive radiator into a modeling software such as WinISD or BassBox Pro. The software will help you adjust both the box size and added mass that will get you the response you are attempting to get. Of course, this is assuming your net box size is actually the size you entered in the program.

P.S: To get your net box size right, you’ll need to make sure to subtract out space that’s taken up by anything other than air (i.e. the main driver, the passive radiator, any box bracing or supports, polyfill stuffing, etc.).

 

Another easy way to tune a passive radiator is by keeping an eye on the cone excursion of your main driver while playing sine wave test tones. At the tuning frequency, the cone excursion of your main driver will barely move and will look like it came nearly to a stop. From there you can keep gradually adding weight until you get your tuning frequency where you want it.

When Should I Use a Passive Radiator?

There are many different reasons to use a passive radiator instead of a port and vice versa. But for the most part, you’ll opt for a passive radiator when you would like to have the extra output of a ported system, but are not able to fit an adequate port into the enclosure.

As mentioned above, tuning a passive radiator is done by varying the amount of mass on its diaphram, whereas tuning a port is done by changing its length. Often, if you want to lower the tuning frequency of a small enclosure, an adequate port will just not fit. This means, you’ll either need to sacrifice SPL, and run into chuffing and port noise by going with a smaller diameter port, or sacrifice SQ (sound quality), and low end boost by going with a higher tuned port.

Say for example you have a 15 inch subwoofer in an enclosure that’s tuned to 20hz with a pair of 18 inch passive radiators. If you were to get the same tuning with a port, its diameter would need to be at least 6″, and its length would need to be nearly 60″ long. And the worst thing is that you would still have a ton of problems with port noise.

When Passive Radiators Are Better Than Ports

There’s no denying that passive radiators add a whole lot more to the cost of a system than a standard port. So, is it worth it? Let’s find out…

There are a few important things you need to take into account when you’re designing and building a ported speaker system.

First of all, you need to make sure the vent air velocity is low enough to avoid chuffing. The generally accepted rule of thumb for preventing air turbulence is to design for a max velocity of 5% of the speed of sound, which comes to about 17 meters per second (~56 feet/sec).

Second, a ported system is tuned by a combination of enclosure volume and the volume of air (or the port’s cross-sectional area by the length) displaced by the port.

Make the port large, and you need to make it longer for the same tuning. Make the enclosure smaller and the port length has to get longer. This has to be taken into account when you are looking at reducing the air velocity of the port. Larger port means less port noise and turbulence, but that also means a longer port for the same size box. It’s all about tradeoffs. So choose your compromise.

To demonstrate how this work, we’ll use a Jl Audio 12w7ae 12 inch subwoofer. But, keep in mind that this problem is not unique to this particular sub. In fact, this is pretty common with all high displacement drivers.

The Jl Audio 12w7ae really points this out really well because it has a whopping 1.15 in (29mm) of Xmax and can hit ultra low bass notes in a rather small box.

This subwoofer looks great in a net 49 liter (1.75 cu ft) box tuned to 22 Hz.

A 3 inch diameter port in this enclosure requires a length of 20.20″ which seems fairly manageable. The dilemma, however, is that air velocity far exceeds 5% limit (17.3 m/s) at only 40 watts. Meanwhile, we are only using 12mm of excursion from the sub, so we’re far away from maxing out its potential.

If you make the port 4 inches wide, its length will need to be about 36.89″ to maintain the same tuning. This is quite big, but still doable. Nevertheless, 4” port still doesn’t keep air velocity low enough. I mean, the maximum vent air velocity (17.3 m/s) is easily exceeded at 120 watts while the driver still only hitting about 22mm of excursion.

So, what port size do you need to utilize the Jl Audio 12w7ae subwoofer to its full potential?

Well, If you were to push this subwoofer to its full potential of 1,000 watts RMS, a 7″ inch port would be required to stay under the 17.3 m/s rule. The downside, however, is that not only this is incredibly impractical to fit in such a small enclosure, but it would introduce resonances all the way down into the operating range of the subwoofer.

Furthermore, The port alone would take up to 80 liters of volume so your small box would now have to be over 125 liters before the port, which means that the small box isn’t so small anymore.

To sum it up, using a port to tune a small box low enough while preventing port noise (in the form of chuffing and turbulence) can be quite challenging. This is where passive radiators come into play.

What are The Benefits of Passive Radiators?

Passive radiators, by their nature, have many things going in their favor for small box use.

First, and most importantly the lack of port noise is one of the biggest advantages to passive radiators.

Admittedly, passive radiators do have an excursion limit on their suspension which should be taken into account, however, they won’t compress the output until their suspension travel limit is reached, and won’t ever chuff like a port.

Second, passive radiators take up very little space in your box. And by applying proper moving mass, the tuning frequency can easily be set to dig down into the depth of 20ish Hz without the need for very large cabinets and long ports. This will allow you to maintain that small footprint that most people typically desire.

Third, passive radiators are much easier to implement than a port when the tuning frequency is very low.

Fourth, passive radiators don’t have pipe resonances like a port, so you won’t ever get audible resonances in your operating band.

Are Passive Radiators Better Than Ports?

Well, passive radiators and ports are both popular options in the industry. We are not going to try to convince you that one is better than the other, as each one of them has its own pros and cons, which means that they suit different needs. So, it’s important to weigh the pros and cons to decide which is right for your enclosure.

Whether you opt for a passive radiator or a port, keep in mind that if you try to go with a smaller box, the port length will be too long to fit as a straight piece inside of the enclosure, or you end up with a really high-tuned subwoofer system.

Same thing goes for a passive radiator. I mean, if you use a really small enclosure, you’ll end up needing too much weight to avoid being tuned really high. And by using too much weight on the passive radiator, you’re basically turning your box into a sealed one.

Does the Size of the Passive Radiator Matter?

Passive radiators are very sensitive to the surface area of their moving assembly. Therefore you need the cone area of the passive radiator to be larger than that of the active driver.

Generally speaking, the larger the passive radiator, the better. However, it shouldn’t be too large to the point where it’s not convenient for the box, or that its moving mass is too large to be used reliably.

As mentioned above, the larger the passive radiator, the more mass you’ll need to tune it to the same frequency as a smaller passive radiator.

A passive radiator with a large cone area has several benefits including preventing over-excursion of the passive radiator, decreasing the effect of the passive radiator Vas on the enclosure, increasing the Qms of the passive radiator closer to that of a port, lowering that “notch” in the passive radiator’s frequency response, and best of all is that it allows for more output before the PR reaches its suspension limits.

For a good working setup, it is recommended for a passive radiator to have at least twice the volume displacement (Vd) of the active driver.

P.S: The volume displacement (Vd) is basically the effective surface area of the driver (Sd) multiplied by the maximum linear travel Xmax. Vd = Sd x Xmax.

 

For example, if your main driver displaces 2L at Xmax, you’ll need to use a passive radiator or a pair of passive radiators that can displace 4-8L to avoid running out of excursion at low frequencies.

Can I Use Multiple Passive Radiators in a Box?

Absolutely. You can use multiple passive radiators in the same enclosure to increase the passive area to the limit. However, you will need to ensure that the Fs of each are as close as possible.

Furthermore, if you use two passive radiators, you’ll need to mount them on opposing sides of the enclosure so that the inertial forces cancel out and prevent the box from shaking.

It must be noted that if you double the surface area of the passive radiator while everything else is kept the same, you’ll need to increase the total passive radiators Mms (the total mass of the subwoofers cone, coil, and other moving parts.) 4 times in order to keep the same tuning frequency.

That is to say, both passive radiators now need to have double the Mms of the original single unit. This is essentially the equivalent of a larger diameter port needing more length to maintain the same frequency as a smaller diameter port.

Do Passive Radiators Give More Output Than a Port?

Well, it depends! Theoretically speaking, an 8″ port is better than an 8″ passive radiator since the port does not have a significant Vas or Qms. However, passive radiators offer some exceptional benefits that no port can achieve.

I mean, think about it: a port that’s equivalent to the largest passive radiator can not be used typically unless the enclosure was bigger than a shipping container.

For example, a port equivalent to a dual 15″ inch passive radiator box would be over 18″ in diameter and a whopping 46 feet long. This is where a passive radiator comes into play. In situations where using a port is difficult or impossible.

If it could be made to work somehow, the example of 18″ port would have an outrageous output compared to a dual 15″ inch passive radiator box. However that port size is just not feasible while the passive radiator is.

What Is the Sound of a Passive Radiator Box?

Generally speaking, a sealed box is more accurate and better sounding while a ported box is louder but not as accurate.

Although this is entirely subjective, we believe that a passive radiator design strikes a perfect balance between the two subwoofer box designs and offers the best of both worlds.

When done correctly, a passive radiator box can deliver bass that is as tight as a sealed box with the incredible output levels of a ported one.

As you may already know, the group delay of passive radiators is lower above their tuning frequency. Yet, at the tuning frequency, group delay is through the roof.

So, when a passive radiator system is tuned very low, the rise in group delay evident at tuning is pushed lower in frequency. Thus it no longer affects the sound.

For example, if you have an enclosure that’s tuned to 20Hz, its group delay above 35Hz or so would be equal to that of the same woofer being mounted in a sealed enclosure. Below this frequency, the group delay does not cause any serious audible effects. This again means that the passive radiator box can sound as “tight” as a sealed one, but with the extra low frequency output.

Passive Radiator Vs Port — Which is Best?

The technical details can be confusing, and the fact is, all you want to know is “ What to opt for: a passive radiator or a standard port?“ Well, first of all, you really need to sift through the differences between them to determine which meets your needs.

If you are a seasoned audiophile who doesn’t mind shilling some extra dough to enjoy deep and rich sounds, then a passive radiator is what you need. Not only will it deliver great bass that is as tight as a sealed box with the incredible output levels of a ported one, but it’ll save you from the hassle of dealing with port noise and air turbulence, all while allowing you to maintain a small enclosure footprint that most people typically desire.

On the other hand, if you’re on tight-budget and don’t mind going the extra mile to get rid of port noise, and having a relatively bigger box, the a port is for you.

Advantages of a passive radiator design

  • Low tuning can be achieved in a small box
  • No pipe resonance
  • No port noise or air turbulence
  • No volume is wasted on the port
  • Easier to implement than a port
  • Passive radiators work great in portable systems

Disadvantages of a passive radiator design

  • Upfront cost – a passive radiator is more expensive than a port
  • Transient response below tuning point is worse than a ported box
  • Requires a well-built enclosure to prevent shaking
  • Faster rolloff may require a high-pass filter that an equivalent ported system might just scrape by without.

 

Alex Brown

Hey There, my name is Alex Brown, I'm an LA-based sound engineer with over 10 years experience installing, troubleshooting, and repairing commercial, automotive, and household sound equipment. I've installed highly competitive car audio systems, and everything from navigation systems to full car stereo systems, remote starters, alarms and beyond. I enjoy creating solutions and simplifying everyday needs. I also love helping people get great sounding gear, thereby, saving the world from bad sound one customer at a time.

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