Passive Radiators and You

[gckless 5,000+ posts]

Passive Radiators and You

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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. 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.

Example:

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.

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[gckless 5,000+ posts]

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):

15” PR 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.

Here’s another way:

They require absolutely NO guess and check. They are actually easier IME. First step is to download winisd alpha.
link:LinearTeam

Next put your driver into winisd alpha, t/s you can find on your own, or contact the company. Click new project, then click new, for new driver. Then click the parameter tab.. Here is the correct order to enter your parameters so it doesn't give you an error, make sure auto calculate is on

1.Enter Mms and Cms which results in Fs being calculated. If Mmm and Cms are not known enter Fs. If Fs and Mmm or Cms are known enter Fs and the other known value.

2.Enter Sd, Bl and Re. This will result in more auto calculations being made, but Qms and Qts will be blank. If Re is not available, go ahead and enter Sd and Bl.

3.Enter either Qms or Rms, whichever is available, generally Qms is available.

4.If Mms and/or Cms were not provided for input, now input Qes. This will cause some other fields to auto calculate.

5.Enter Hc, Hg and Pe if available. These are not required, however Pe (RMS Power Rating) will give a good idea of how much power signal input to model with.

6.Enter the number of voice coils. NOTE: Drivers using dual voice coils may cause the Bl and Re parameters to change when changing the connection wiring from parallel to series. Be sure to monitor these when changing the connection option, but WinISD should automatically change these values properly.

7.Correct Znom (nominal impedance) if needed. Many times this will change to 6 and should be 2 or 4... or the ultimate resistance (in ohms) you intend to run based on the voice coil configuration and your connection setting (parallel or series).

8.Enter Xmax and any other missing parameters if known and continue. DO NOT change any of the blue auto calculations. If they are only slightly different, this is okay. If they are off significantly, contact the manufacturer or vendor of the driver.

Then start a new PR project, it's the last enclosure choice. put the specs for the passive radiator in question into the program. You can usually find those specs as well on the companies website. You'll need their radiating area, FS, CMS, xmax and VAS.

First step once you've started the project, adjust your box volume FIRST, in the box tab.

Then you'll go and click on the PR tab. First change the number of PR's your using to match the number you have. Then check the instructions to see how many grams each weight weighs in your passive kit, and add weight groups of that much. So if it's 45grams, see what 45,90,135,etc each tunes you to. After that, each washer in your kit will weigh so much so you'll know what to add. If 90 grams gave you the tuning you want, then you'd add 2 washers to each passive. Just add whatever weight you think will give you the best response, based on the graphs you want.

Other thing to look at is your passive excursion graph. On the signal tab change how much power your giving the driver to match your amp, then go to the passive excursion graph in the file thing up top. It's one of your graph view options, just above your graph, click where you can change from Tranfer function magnitude and choose cone excursion PR. Make sure the passives stay below their xmax. If they don't, make sure you engage a SSF. I always run a SSF on passives, even if it's just as 20hz to make sure nothing gets into trouble for sure..

In the real world, sometimes tuning is a hair off, but always worry about how it sounds, not exactly what tuning you have. I have heard passives being next to a wall will actually increase your tuning, not lower it like with a slot port. I would think this is because of the wall making the suspension seems stiffer than it is due to the pressure on the cone. However, I've never ran a impedance sweep to test. IME, they seem to tune more or less where winisd says they will, more reliably than ports anyways. If you feel it isn't play low enough or is too boomy, add a weight, if it's play too loud down low or not quite as loud as you want, subtract one. Nice thing about passives is your not stuck with any particular tuning and can adjust easily once it's in the car.

Now for the simpler method:

Put the things in, do a resonance test with tones, and adjust from there. Your PR’s, just like the air in a port, are going to moving the most at tuning. Also at tuning, and also just like a regular ported enclosure, your powered drivers are going to be moving the least. This is do to resonances affecting internal pressure.

So anyway, once you do that, if you want to tune lower, add weight. If you want to tune higher, remove weight.

Here is a quick, basically non-instructional (lol) video I made a little while ago:



This is all I have for now. If you would like something added, or see something that needs correction, please let me know.