Passive Radiators and You
What are passive radiators?
Passive radiators are a vent substitute, and are an unpowered woofer without a voice coil or magnet, and have some way of attached weight to them, typically in the place where a voice coil would be. Most car audio PRs use a bolt where you can add weight. They are used to take the place of a port in a bass reflex (vented, ported, call it what you will) enclosure. They act as a spring, in the same way the air in a port acts as a spring. Here is what mine look like:
What do they sound like?
Many people say a PR
enclosure sounds like something in between a sealed box and a regular ported box, and that comes fairly close to describing it. It can sound as “tight” and controlled as a sealed box, yet have the extra output and sensitivity that comes with a ported enclosure. Group delay is very minimal, on par with a sealed enclosure at frequencies above. Below tuning, there is steep rolloff, typically to the tune of 32dB/octave. In addition to this, there is a negative notch in output. Here is a visual of that, and a comparison between PR
, closed, and vented systems:
I may add some more technical jargon later, but I will leave it at this for now: they are extremely pleasing to the ear.
Why would I use a passive radiator?
PRs are very useful when adjustability, space, or very low tuning is a concern. You can tune very low with a PR
with no additional space subtractions, simply the addition of weight to the PR
enclosures will also never have port colorations such as resonant sounds, wind noises, or any internal woofer sounds escaping. Downsides to a PR
enclosure are cost, the need for additional baffle space, steeper and slightly higher cutoff frequency, and greater overall losses.
How do I design this contraption?
It’s easier than you might think, but there are a couple important things you need to pay attention to.
The first and most important thing we are concerned with here is volume displaced, or Vd. Finding Vd is simple, it is Xmax times surface area (Sd), and is represented in liters (generally). I have found that you want at least twice the amount of Vd in your PRs as you have in your powered drivers. You can have more, and should if you can to a certain point. That point depends on the force behind your powered driver(s), but is most likely more than four times as much. That is an area I don’t have a whole lot of experience with, but I wouldn’t think anyone would want that much anyway. However, LESS IS BAD! If you don’t believe me, ask me about my system with only 1.45 times the Vd.
Let’s say you’re using a pair of 12” powered drivers, with 30mm of Xmax each.
(Sd) 113cm2 x (Xmax) 30mm = 339mL
339mL x 2 = 678mL total
So, if you are shooting for twice the Vd, you will want 1.356L of Vd in your PRs. How you get this is up to you based on the PR
’s abilities and what size(s) you use.
Next thing you need to pay attention to is PR
placement. It’s best to mount the PRs on opposite sides of the box (if using more than one). This way, their opposing movement cancels the weight of each other’s. If you have them on the same plane, you better make sure the box is secured, because when those PRs get moving, you’re going to know it. This gets worse the lower you tune (because you have more weight on there). You also need to ensure that you have enough space on the backside of the PR
, some PRs use a bolt for weight addition, and this needs to be able to move.
The way they face generally should be the same as the way you would face a regular port in the same vehicle.
How do I tune a PR?
PRs are tuned by adding and subtracting weight, same as with a regular port, only now we are adding physical weights instead of increasing the mass of the air in the port. If that doesn’t make sense to you, think about this: when you tune a regular ported box, you are simply adjusting the weight of the air in the port. Let’s say you want to tune to a certain frequency, 30Hz in this example. Let’s say you figured out your required length of the port to be 25”. In order to tune lower, keeping everything else the same, you are going to have to increase the length of that port. By increasing length, you are adding more port volume, and as a result, more weight of air inside the port. Same goes for tuning higher; less port length, less air mass.
Understanding this, I will explain two ways to tune a PR
. One is extremely easy, the other is a bit longer and technical. The latter first:
To do the math, first you will need to figure out your required standard port length for the given surface area of your “port”. To get this, it’s probably best to use a program/calculator (I like WinISD) and plug in your box size, “port” size and amount, and desired tuning. You will get a length. Then you will need to find port volume, which will be port surface area times port lenth, or (Pi x R2) x L (if you already have surface area, it’s just Sd x L). After you find port volume, you will need port mass, which is Port volume times the density of air (which is 1.225kg/m3).
Example (I made up these two numbers for the example, this is inaccurate):
Sd = 830cm2 = 128.65in2
Required port length = 55in
Port Volume = Sd x L
= 128.65in2 x 55in
= 7075.75in3 = 4.1ft3 = .116m3
Mass = Port Volume x Density of Air
= .116m3 x 1.225kg/m3
= .1421kg = 142.1g
So, you would need to add 142.1g of weight to the PR
to find the given tuning.