Pro car audio competition pysics of getting really loud?


2010-10-24 1:05 am
Hi, since I've been helped before with a question about the pysics of subwoofer resonance, I thought I would ask another question I need for my audio knowledge bank.

I am into car audio, I like competitions where people burp single frequencies to acheive staggering spl levels like 145-180 db! But for me, what I would like to work on is doing music as loud as I can with one 15", and 2 18"s later.

I have read about two types of subwoofers: mass controlled, and compliance controlled.

The way I take it is that for a compliance controlled subwoofer, xmax and cone area, are the two key factors for a subwoofer to get loud, and they tend to be inefficient, and not perform well in the upper mass controlled frequencies.

And how does using more force from higher bl make a subwoofer louder in the upper freqs, I know at like 40 and up, the motor has to interup the cones momentum so that it can move faster, thus a strong motor would controll better, but is there a point where there would be an end to needed motor force one you know that the subwoofer is contolled enough, and not gain any extra spl? How does this equate to higher spl? In conjunction with a vented box, and say a qts of .32-.34. So also, if you could use more force, then how would the subwoofer be atthe Sam frequency if the acceleration should change?

Question, for subsonic compliance controlled subwoofers, is xmax the main thing given same cone area, to get loud?

And I've read that mass controlled subwoofers get loudest with high sensitivity, so given the same cone area, xmax, suspension compliance, a bigger motor force will make it louder?

I am trying to understand mass controlled subwoofers and: power compression, ported box pysics in somewhat simple terms, why xmax dosnt matter supposedly with mass contolled subwoofers when it could move more air
with more xmax, is the amount of air moved not equate the spl? I thought It was everything, and maybe it is for compliance controlled transducers, but I guess a ported box and a low qts somehow change this all, no? If ported boxes can also be explained would be great, I've heard something about the phase of the vent air being opposite at time but don't understand how and why.
Last edited:
A mass controlled sub is another way of saying "its motion is resisted because it's so heavy." These kinds of subs are usually large. I know clarion made a 30" sub, and it was "mass controlled."

Compliance controlled are those that are dominated by the resistance of air pressure and the speaker's surround and spider. These are smaller subs.

Whatever you have, if your goal is to get gigantic SPL, what you're dealing with are primarily 3 factors (none of which have to do with the above two) -

1) Power output. You need a lot of watts to start with.

2) Resonance. Your vehicle and the box your subs are in will create a cavity that will resonate at certain frequencies.

3) Containment. Leaks of the pressure inside the car to the outside world, and physical motion of parts inside the care will bleed off energy and diminish SPL. The biggest "leak" is the body of the car, expanding and contracting like blowing up and deflating a big balloon. It's hard to avoid unless you build your car out of welded, reinforced bulletproof armor.

Once you have the power available, and the subs to turn that power into pressure, after that it's a matter of experimentation and trial and error to create an enclosure that matches the volume of the air mass in your car for resonant frequency. Get it right and you can get as much as 15 dB higher than you would if you had it all wrong.

In the SPL competition circuit, it is assumed you already understand what kinds of speakers you will need, and how to build a big, sturdy box for them. After that, it's watts, resonance, containment.


2005-12-01 12:10 am
Start by forgetting the mass/compliance controlled things you have read. I'm assuming you saw the Top Subwoofer Myth list posted somewhere.

They don't really mean anything, and are pretty pointless. Mass controlled drivers are not the huge drivers, they are your pro-type drivers, low xmax and high sensitivity. They are mass controlled because they are designed to operate at frequencies where the moving mass of the driver affects the acceleration more so than the compliance of the driver/enclosure combo, which is a greater factor at lower frequencies.

Look at it this way, a mass controlled driver is designed to run from 40Hz up several hundered hetz into woofer territory, and a compliance controlled driver is one that likes to stay below 100Hz, and dig to 20Hz and below.

This really doesn't tell you anything. If you want to burp a 20Hz, you are not going to use a driver that dies at 40Hz anyway, and if you are going for 80Hz you aren't going to waste your time with a low sensitivity driver with a huge xmax that will torch the VC long before half the xmax is used up. If you want something in the middle you use a driver that is a mix of both, no big deal. ;)

In the end it doesn't really mean anything, as you just pick the driver that does what you need it too. If you want 30Hz up then you need to look at pro audio designs. If you want infrasonics then you need drivers with low Fs, and very high xmax.

Xmax always matters though, you just don't need as much as you go up in frequency. ;)
Last edited:


2006-01-07 12:40 pm
I am no expert, but i think the maximum spl depends on power rating, xmax and enclosure size. Power rating is obvious since spl is directly related to how much watts go into the speaker, xmax is a necessary thing, since the more watts go into the speaker, the higher the excursion will be. Enclosure size isnt so obvious, but compare a horn and a closed box, driven by the same speaker. The horn will be much louder than the closed box. Within limits, the bigger the box, the louder it will be.

So, if you search for an optimal driver, first choose a bandwidth, then set a size limit, then figure out what drivers with which watt/xmax ratings are available for the possible enclosures and you should arrive at a pretty good spl value.


2010-10-24 1:05 am
Thanks everyone, and soho, I understood all of what you said, i had understood it from the myths article from audiopulse...I just didnt realize that mass controlled drivers were controlled mostly by driver-mass and not box/other, i actually figured it opposite, that, mass controlled subs were VERY dependant on enclosure mostly.

So what about xmax? I undertand alot about caraudio like tsps, and etc. But, it has been controversial that people claim that xmax isnt needed for pro audio subwoofers, and i do understand that because the driver cant move very far at higher freqs. But i figure, what if you can get a subwoofer with good xmax that is a good amount efficient, fictional or not, and have so much motor force that the driver can reach 2.5" PP excursion at like 50hz? Sounds impossible but, would this create much more spl?
I know people who will say: Amount of air moved (vd) dosnt make higher numbers, its about the acceleration. But isnt acceleration the frequency? Or is acceleration how quickly the driver can start the next wave? Because i thought rate of acceleration, and acceleration were the same? Im a little confused.

Also, things will change rom sealed i know, when going ported.

Btw, i have a subwoofer already, i dnt want to burp or compete per se. I just want a loud hard beating daily system for my truck. And i have 1 15" subwoofer that has a motor that looks like an atomic apxs or a little bigger and comming is a ss dtr 1700w, or 2200w amplifer, for the single sub.
Last edited:
For SPL competitions, it's been recommended to use a series tuned 6th order bandpass, where one chamber vents into the other. If you get the tuning frequencies the same, you get extremely hard hittting bass, but only at one frequency. You'd have to operate the cabinet solely at that frequency, but it will be very very loud.

Subwoofer Enclosures, Sixth and Eigth Order/Bass Reflex and Bandpass

For every day use, a sealed box would likely be better.


  • 1-SPL-Drooling.JPG
    421.5 KB · Views: 189
  • 2-Chicken-defeathering.JPG
    70.6 KB · Views: 181
  • 3-Why-not 194-dB-SPL.JPG
    3-Why-not 194-dB-SPL.JPG
    665.7 KB · Views: 185


2010-10-24 1:05 am
1. WHat is the pysics of a ported box, ive heard many things about them but i would like a deep explaination of it and how changing it changes things.

2. Just got into physics of standing waves, are they mandatory in car audio? And is cabin gain caused by standing waves, im having trouble understand it if you can help me.

3. People will say it takes alot of power, and efficiency to get really loud like 150, but i realize also that xmax may not matter at burping since the subwoofer hardly moves at all, but i dnt know much about how this works.

4. Most important: Some people claim that more force moves more air suposedly, or somehow creates more spl. I know higher frequencies need more force to overcome moving mass inertia alot quicker then lower freqs or natural frequency of vibration, but if you have 2 identical subs but with a stronger motor on the other, and the same box alignment as per the qts of the subwoofer and what have you, how does the more sensitive subwoofer possibly create more spl then the other if they are at the same frequency? I figure if the subwoofer could move with more force it would move faster, wouldnt this change the frequency??? So hoe does more energy create more spl with ported boxes? And is this voided with a sealed box? Making spl dependant more on xmax/radiating surface.

5. Any notes you can give me for both sq and spl subwoofers with high, low, and medium qts, and different compliances, and more importantly why and how to assume what will work with what for what, and why? Roll off frquencies? I believe lower qts subs rolloff faster then higher Qts subwoofers, thus a need for a port, but the WHOLE POINT of the thread right here is: How does the port pysically alow an efficent low Q sub get lower with its resonance, and how does it all work.

6. So what about the physics of the port in laymens terms? but i want deeep understanding so i can know how to build boxes better. I have build a bit of boxes before, and know things about sealed design like Qtc, and other tsps. Not much about compliance. I know about a .707 blah with sealed boxes has the lowest f3, and longest flat response.
...much less for ported boxes.
How port area changes things? etc.

A rundown on ported design...I already know how to clculate port area, and length etc, and general boxes sizer per sub size, and can model subs in programs, but want to know how it really works to create specific respnses?

Btw: My goal is loudest daily system i can have for music, and if loud enough would take it too the competitions.

But for now, it would be cool doing like 145 on music with my single 15" subwoofer, but i doubt it, but i have 2 18s i will have later on.

Also want to know for the knowledge, and because its my hobby.

Last edited:
I actually know a bit. And i dnt want to use a comp setup for listening, no way. I'm not trying to tune mid to highish and try enjoying it, nope. I just want to go ported, loud, and okay sound. I have done it many times before, i just wanted to learn more about the pysics more, so i can be even better at it. I know all the fundamentals and tips already.


2010-10-24 1:05 am
Update: Iv'e been reading this at an HT site, great read: Essay

Could this help me understand the pysics alot better, might want to scroll to the bottom to get too it, there is animations. I slightly understand the end of it with a ported box, but would be nice if someone could resummerize it.


2010-10-24 1:05 am
Update: Iv'e been reading this at an HT site, great read: Essay

Could this help me understand the pysics alot better, might want to scroll to the bottom to get too it, there is animations. I slightly understand the end of it with a ported box, but would be nice if someone could resummerize it.

Well check it out, i just now did an experiment with a shoe string and bolt, diff wieghts etc. I think i got the idea, but now, i ask: The string i used is obviously not rigid, is air in a box/port? And does this detmine damping? How slow/fast the energy is transfered from subwoofer to port? Or how rigid the air is or something? explain pls.

Also what mistake would cause the port to tend to takeover the active subwoofers motion? Or should i say make the active driver a little out of controll? Damping of the active driver? How is that changed with the physics of the reflex setup?
Last edited:


2009-06-02 4:47 pm
While to be the best at either competing or at musical reproduction the system should be purpose built to the desired need, you can sacrifice a bit and have both with a swappable port plug. I've seen a few done locally in a vehicles and it works out okay. One I've seen in a truck with 4 15's has a large port tuned high for competitions and a "plug" you can put in the put to cut it's cross sectional area while maintaining the same depth (read - lowering the tuning) for daily listening. That may be an idea you can apply if you're looking to have one box/ install that is easy to switch between competing and daily listening.
Last edited:
6. So what about the physics of the port in laymens terms? but i want deeep understanding so i can know how to build boxes better. I have build a bit of boxes before, and know things about sealed design like Qtc, and other tsps. Not much about compliance. I know about a .707 blah with sealed boxes has the lowest f3, and longest flat response.

But for now, it would be cool doing like 145 on music with my single 15" subwoofer, but i doubt it, but i have 2 18s i will have later on.

Okay here goers, i'm not officially qualified by any means but this is my take on it from being a SPL competitor (151db at 40hz was my best).

One tip to remember there's no such thing as free energy.

In a standard sealed box positive sound pressure radiates outwards from the driver.

In a ported box both the positive and negative energy of the driver is used, and sound radiates from both the driver and the port. The negative energy in the box is normally out of phase with the positive energy coming from the driver. Hence below the tuning point ported boxes have a steep roll off. At the tuning point the sound energy within the box is delayed another 180 degrees of phase and will radiate out the port in time with the driver creating a peak. If the delay caused by the port is 1/2 wavelength then you will get a 3db peak.

Now to get bigger than 3db peak you have to create a bigger delay so multiple cycles of negative energy are held within the box and then released through the port. You will see this in you box modelling software as group delay sky rockets.

As you get more and more adventurous with your port peak your enclosure will cause an impedance peak in the driver, as the impedance goes up the driver moves less as it is receiving less power, so the goal here is to get more power into it if you want to go louder. In your situation this is not much concern as you want wide frequency response and the impedance peak is only around the port tuning peak. In a one note wonder car you can use this peak to your advantage buy running your amps below minimum impedance.

I.e. my sub is a quad voice coil, each vc is 1ohm. With two amps strapped together the minimum recommended load is 2ohm. But I can safely get away with wiring my sub to 0.25 ohm as the impedance peak of my enclosure at tuning brings it back up to a safe level.
However if I play any frequency greater than +/- 5hz of tuning I risk putting the amps into protect or worse blowing them.

The next issue is ensuring the air slug (mass of air in port) in the port is closely optimized to the output you wish to achieve. A port that is to narrow will cause cuffing. A port that is two shallow will have the air slug blown out before maxiumum resonance has occurred. A port slug that is to big will act as part of the box and not a slug at all. I haven't done much research here, but I have some simple rules. Port length at 70z should be approx 20cm, Multiply length by 4 for every octave below this. And use the diameter as recommended by box software.

So to summarize porting boxes is a way of making use of the negative energy of the driver through introducing delays.

And if you can afford it, get a good spl meter.
Last edited: