Last updated on August 9th, 2020 at 09:10 pm
Adding Bass Without Wasting Space
There has been a lot of discussion lately in the automotive audio circles about the advantages and disadvantages of free air subwoofers. Some people like them, some don’t, but what we have noticed at CarAudioNow.com is the massive amount of confusion about what a free air subwoofer is, how it functions, and what the differences are from infinite baffle setups or traditional boxed subwoofers.
In an effort to clear up the misconceptions and get you the best information and advice about free air subwoofers, we got in touch with professional installers. They gave wisdom and dispelled myths about these space-saving setups so we can arm you with the knowledge you need to make the right decision before investing tons of cash in a setup you are unsure will meet your desires.
What is a Free Air Subwoofer?
Let’s get this out there right away, as simple as possible: a free air subwoofer is a specially designed speaker that does not require installation in an enclosure. Typical mounting is through your car's rear package shelf or on a mounted panel in the trunk. Because a free air sub does not need a massive box, these setups will keep the space in your trunk open for all that junk.
Can Any Sub Be Mounted “Free Air”?
Back in the stone age of auto audio, when we were trying to pump up the bass from a tape deck in a 1984 Chevy Caprice, free air mounting through the deck was common. But, those speakers -typically between 5” and 6”x9”s- didn’t move air the way modern setups do. Today’s cars take advantage of decades of experience in sound isolation, insulation, and proper speaker design, unlike that rusty old Caprice. Those same speakers would sound like garbage in a modern BMW or Acura.
The reason is quite simple: the more power you put into a speaker, the more distortion you are going to experience. When mounting a subwoofer on the rear deck, you are trying to get performance the speaker wasn’t meant to provide. A subwoofer makes bass by moving air in front and behind the speaker. These waves are equal, but 180 degrees out of sync. What tends to happen when free air mounting a standard sub is the sound is better behind the speaker than in front of it. Great for tailgating parties, not so good for driving around. This is the primary reason most audio shops will refuse to free air install a non-free air sub in your package shelf. They know you will be back, complaining about the poor sound quality and the amount of rattling your car is making.
Why Subwoofers are Installed in Boxes
As speaker technology advanced, sub boxes became standard upgrades for stock stereo systems. A sub box contains the air movement from the rear of the speakers. The air inside the box acts like a spring to push the air forward. This is where that deep, feel-it-in-your-gut bass response comes from. Of course, the downside is the amount of room that a subwoofer box will take up in your trunk. Great bass is awesome, but when you have to ask your girlfriend to hold bags of groceries on her lap because your trunk is full of speakers, it’s not cool.
What about Infinite Baffle Setups?
Here is another term that is causing utter confusion these days. Too many people are under the impression that an infinite baffle system does not use a box, or that free air subs and infinite baffle subs are actually the same thing. To clear this up, free air subwoofers work sans-box, while infinite baffle systems use a large, sealed space -such as the trunk of your car. Both speakers are mounted similarly, often under the package shelf or against the back seat. I know it sounds like they are almost the same, and that’s where all the confusion comes from.
An infinite baffle system will require extensive modification to make sure that no portion of the rear waves interfere with the front waves. Installers do this with a baffle board, and it works great when the actual air space is more than 10 times the volume of air the speaker needs. For car audio enthusiasts, this means installing subwoofers that operate within the confines of the air space in the trunk. Too big of a speaker or too small of a trunk will not produce bass the way you want.
The idea is basically to design a system in which the listener is effectively inside the subwoofer box. That means lots of modifications, careful construction and design of sealing and mounting, and ultimately in car audio, some compromises. A free air subwoofer basically eliminates the complexity of an infinite baffle setup to create great sounding bass without the added effort of sealing the enclosure.
What's Better – Infinite Baffle, Free Air or Traditional Boxed?
You will need to determine what system will work best for the way you use your vehicle or boat. The size of subwoofer speakers often means that you will lose significant interior space, particularly if you want subwoofers that are 10” or larger. Subwoofer boxes will take up lots of room in your trunk, but even an infinite baffle system will require you to make compromises on space and functionality.
Many drivers today get excellent bass response from high-quality free air subwoofers paired up with a good quality amplifier. The ease of installation and limited number of modifications are a big plus for free air subwoofers. If your car has limited package shelf space, you may be faced with a compromise between speaker size and output. Most installations are going to require you to use baffles and to seal your trunk, even with a free air sub.
A sealed or ported subwoofer enclosure will always give you the clearest, deepest bass frequencies, provided of course you are using the right size box for your speakers. Massive speakers in a tiny box isn’t going to get you the result you seek. Figure that you will need a box that is 2” to 3” larger than the dimension of your speaker. For a 10” subwoofer, this means a box roughly 13” x 13” x 15”. Add in your amplifier and you can quickly see how much space that one speaker will take up.
Compromises to Consider When Choosing Subwoofers
Picking the right setup for your vehicle depends on the type of vehicle and the way you plan to use your car. If having a usable trunk is important to you, an infinite baffle system is the wrong choice. A free air setup works best in space limited applications, like smaller cars or vehicles that don’t have a lot of space.
If you decide to run an infinite baffle system, there are some things you must take into consideration. An infinite baffle system can hit lower frequencies than a boxed sub, but you also must be careful about the amount of power you push into the speaker.
Infinite baffle subwoofers can easily exceed xmax, which is the amount of movement the voice coil can move in one direction without leaving the magnetic field of the coil. Too much power and you will get bad distortion that will eventually destroy your subwoofer. The limited movement of the voice coil prevents the subwoofer from maximizing its bass potential.
Tips for Installation of a Free Air Subwoofer
Free air subwoofers don’t require the level of sound wave isolation that infinite baffle systems do. The subwoofers are also less susceptible to distorted sound response when you have things in your trunk that interrupt the sound waves.
With that said, the better job that you do of isolating the sound waves from interfering, the better sounding sub you will have. A typical mounting where the free air subwoofer is installed through the rear deck should incorporate sound absorbing and vibration absorbing materials, like Dynamat. You might even want to reinforce the rear deck to reduce rattles and vibrations. A more rigid surface will improve the speaker response.
If you are mounting a free air speaker to the back of your rear seats, your setup will benefit from using a baffle board. A sealed surround helps the speaker to sound better. Free air subwoofers often lack punchy bass. Your efforts to insulate will reward you with a better sounding system.
You need to consider the alterations that either a free air or infinite baffle system will require. If your subs are mounting through your package tray, you will need to make large holes that can’t easily be repaired. Insulation and sealing of the trunk adds some weight, and an infinite baffle system will suffer if you have items stored in the trunk.
Owners of cars with small trunk spaces may be better off considering using a small ported sub box in their trunk, rather than a free air sub. The bass response will be significantly better from a ported box than from a small trunk.
There are lots of good subwoofers on the market today that are designed for free air mounting. If you buy good-quality speakers, power them with a sensible amplifier, and insulate your car properly, either a free air or infinite baffle design will produce a nice bass response in your ride. In our next article, we will take a look at the best free air speakers you can buy.