Answers to a Few SPL Myths

on 30 November 2010
Created: 30 November 2010

With the increase in popularity or the comeback of SPL competitions in the past few years there has been an increase of theories as to what works and what won’t work to increase your SPL.  As an instructor I am asked a lot of questions regarding SPL applications and sound quality questions everything from, what type of product to use to what do I think is the best type of box to build, or what are the best speakers to use, and sometimes even what are the best type of cars should be used to compete with.  Being a novice competitor which most of SPL competitors are, it can be very confusing, hard to understand even frustrating. And taking the advice of experts that you have spoke to, you just went out and bought ten of the biggest gnarly speakers that they suggest.  And according to these experts you’ve spoke to they say, “there’s no way you can loose with these speakers”.  And after getting spanked at your first competition, and talking to other competitors they’re asking you why are you using those subs, why did you buy so many of them, and why in the heck did you build a box like that. Even worse, come to find out competitors in the in the two speaker class, are blowing your SPL scores out the door. 

So after reading this we hope you will have a better understanding of what the heck you’re doing when we go out buying product, building your car, and going to competitions.  We would also like to put to rest some of these SPL myths so you can just go out and have some fun cranking out some big numbers rather than subtracting big numbers from your pocket book. 

Are a ton of speakers better than just two speakers?

That is the million-dollar question.  With the way most of the sanctioning bodies are set up now you can be very competitive with just a two speakers system (not mention shattering world records) rather than going to the closest retailer and buying all of a small stores inventory of sub-woofers.  Don’t get me wrong, you should most definitely have at least one extra sub on stand by while you are at a competition.  So if you have a two-speaker system you should have a minimum of one sub rearing to go in case you launched one sub into your back seat of your car.  Keep in mind that the whole idea behind SPL competition is to drive those subs to there limit.  As far as why you shouldn’t always go with a ton of subwoofers is because it’s not always a guarantee that you will hit a higher SPL with more subs. And I can guarantee it especially if you don’t go with more power.

The approach towards box design and speaker engineering make it possible to do extreme SPL numbers with a two-speaker system.  Many competitors today are using a two-speaker system and doing very well due to the box design and a good understanding of acoustics. The performance results of using a two-speaker system is due to the style of enclosure the speakers are in.  The efficiency of different style speaker enclosures makes it very feasible to use a two-speaker system.  Another advantage of using a two-speaker system is you have the ability to apply the correct amount of power to the speakers.  For instance most competitors think if you have a lot of speakers you will automatically hit higher SPL numbers but obviously that’s not the case.  If you have a two-speaker system and supply a thousand watts to it in the right size box it’s going to crank.  Add another two speakers without another amplifier, you’re not going to gain anything except frustration.  It costs a lot of money to supply power for a four-subwoofer system rather than supping power for just two.  Obviously you have the cost of the subs in addition to the cost of the amplifiers needed to properly power all of your subs to be competitive. Depending on how many speakers you use will also determine the speaker enclosure you will be using. If you go with a two-speaker system you pretty much have no option as far as the type of box to go with to be competitive. So as to how many speakers should you use the answer is not only how much room do you have to mount them in the right size enclosure or style enclosure. But how much power you can afford to drive your subwoofers to their maximum limit, you need both in order to have the winning SPL numbers. Which leads us to our next clarification.

Sealed or ported?

According to experts that you might have spoke to may suggest Sealed would be the better box to go with because you’re going to have more control of your speakers and possibly be able to play lower frequencies.  That is not true, sealed won’t always be the better box to go with for SPL, again keep in mind the goal here is not to have the best sounding car it is to get the highest SPL numbers and it almost definitely not to be able to play the lowest frequencies.  This is probably the hardest decisions for most competitors if you don’t understand the difference from a ported and a sealed box.  Some people will go with a wall of four or more sub woofers in a sealed box and others will go with a wall of just two subwoofers with an enormous box and an enormous vent (port) opening. And depending on who you ask you will get several answers as to what is going to work the best.  If you were going to be doing strictly SPL competing with a two-speaker system you should always go with a ported enclosure.  Reason being you should always be able to get a higher gain from a ported box compared to a sealed box with all things being equal.  And depending if the box is done right you should be able to get at least a 3dB gain out of it.  The general characteristic of a ported box is they are more efficient than a sealed, therefore more output.  And for the first timers that have never built a ported box.


If you are not familiar with ported boxes or this type of speaker enclosure, if it’s not built correctly, I can just about guarantee that you will blow your speakers.  You need to ask yourself if you are capable of properly designing and building a ported box. If you don’t have a full understanding of how ported enclosures work, or know how to calculate box volumes, I would suggest calling the manufacturer for some advice as to proper box size, port diameter and port length.  Any good manufacturer will be more than willing to help you design a speaker enclosure. For a novice installer I would suggest sealed, for a more advanced installer I would absolutely go with ported.  Just be aware going with a ported box is going to require an extremely large enclosure with a pretty large vent opening to achieve winning SPL scores.  The vent opening has to be large to create another large piston area.  That large piston or vent opening is what makes the ported box have more output or an increase in SPL.  Whereas using a sealed box would be the exact opposite, to get higher dB readings use smaller enclosures to achieve winning SPL scores.  Hence more speakers can fit into a car in a sealed box.  That’s why you will generally see sealed speaker enclosures with cars that have a lot of subwoofers.  Where as ported, if done correctly, you would probably be able to only fit one or two speakers in a car with a ported enclosure, Due to the volume of space a two speaker ported box will take up. Yes we are talking that big. 

Resonant Frequency

Also one of the most popular things for competitors to do is to find the resonant frequency of the car or the standing wave in the vehicle.  Which is key to hitting high SPL numbers.  Although the biggest mistake done here is that it is usually done before the box or the wall is put in the car.  Which will dramatically change the characteristics of the vehicle.  So if you are going to take the time to figure the standing wave or measure the transfer function, make sure you do it correctly and put in a make shift wall in the car before you actually measure the acoustics of the car.  This isn’t so much a myth but one of the biggest mistakes done by installers or competitors.  Yet one of the biggest myths is to try and tune your box to the exact number or frequency of the standing wave of the car.  You should try to tune the box to about five to ten hertz below the standing wave of the car.  Tuning the box to the exact standing wave frequency can be difficult and it won’t allow the speaker box to get the gain it needs to naturally amplify that frequency.  So you should tune the box to about five to ten hertz or about half an octave below your standing wave of the car.  By doing this you will get a natural increase in volume due to the acoustics of the car.


And how about those speakers that look like they have an oversized bagel attached to the cone of your speaker, you know what speakers I’m talking about.  The speakers that have a massive surround, the ones that are a twelve inch speaker that have a eight inch cone, the one that probably has the same excursion as your old ten inch speaker.  Yah, those ones.  Well those might look pretty cool but most of them wont have the same overall output as a well engineered speaker.  Just by looking at the surround on a speaker doesn’t tell you how much excursion (one way linear motion) it has.  And you can’t tell by grabbing the cone of the speaker and moving it up and down, no that won’t do it either.  The only way to tell a speakers excursion is to measure the length of a speakers voice coil.  And the only way to do that is to take apart the speaker, and you can’t do that either.  So, what you have to do is ask the sales guy what’s the Xmax of this speaker is.  Xmax is the length of the voice coil divided in half and that’s the only way to find out how much excursion speakers have.  And honestly I’ve even seen this number fudged by the manufacturer before.  Not only that but look at the actual cone size on most of these speakers, they’re pretty small.  You can compare that cone to a cone of a normal surround speaker and determine which one has more cone area (Sd), the numbers won’t lie. Just about every time the speaker with the oversized surround will have a significantly smaller cone area (Sd).  One other measurement parameter you can look at is called Vd. No, not what you gave you girl friend but it s a measurement of or the amount of air the speaker can move.  And this is derived by taking the Sd, which is the cone area, and multiplying it by the Xmax that is the amount the speaker moves in or out.  This measurement parameter Vd gives you the best of both worlds, it takes into consideration the cone size and the amount of excursion.  Again SPL is determined by how much air you can move or push so a lot of these types of speakers can be deceiving. 

Well I hope I helped put to rest some of the things you might have heard from numerous installers, friends and competitors.  Most of all just go out there and have fun.  All the things you will hear are not always going to be true but don’t disregard any of them, the best thing to do is put them into practice and prove yourself if they work or not.  Sometimes unfortunately it can be expensive and time confusing but most of the time it’s a lot of fun experimenting.  As time goes by you will become more knowledgeable of the different theories and techniques and you will be one of those people that we look to for answers.  Good luck and have fun. 

Certifications & Accreditations

Accet logo
BBB Business Review logo
SBCA Certification logo
MESA Certification logo
MECP Certification logo
Florida Association of Veterans Education Specialists logo
Federal Student Aid logo