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Selecting and Installing a Free Air Sub

Easy bass

by Julian Edgar

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At a glance...

  • Step by step install of free-air sub
  • Low cost
  • Very little loss of boot space
  • Straightforward design
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This article was first published in 2006.

People into mega sound systems seem to hate them but nearly all manufacturers of good in-car sound systems use them – including Lexus, who arguably make the best quality OE car systems of all. What are we talking about? Free air subwoofers, where the enclosure comprises only the boot volume, rather than a separate tuned box. Installing a free-air sub is much easier than building and fitting in a boxed subwoofer – there’s less woodworking (a lot less!) and the design process is simpler. The result also takes up much less boot space and is lighter.

And the downsides? You must select the correct driver for the application, the bass probably won’t be as ‘tight’, the approach best suits a booted sedan, and of course the quality of bass will vary with how much you’ve got in the boot. (But if you’re like most people, the boost will be chockas maybe 1 per cent of the time.)

So let’s put the approach into action, using as the guinea pig a Falcon EF sedan.

Selecting the Sub

Subwoofers generally use the air trapped behind the cone as an intrinsic part of the system design.

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In ported subs, the box volume and the diameter and length of the port are tuned in conjunction with the selected driver so that the system will work below the driver’s resonant frequency. In this approach, the vibrations of the air trapped within the box and the port serve to reinforce the sound generated by the front of the cone.

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In sealed subs, the energy trapped inside the box stays right there, being absorbed in the fluffy stuff that’s stuck all over the inside walls (or simply piled into the box). This means that the efficiency of the system (how loud it is for a given input power) of a sealed system is usually lower than a ported system, but the bass is usually perceived as being tighter - although it doesn’t go as deep. The latter’s especially the case with a small sealed enclosure – the air trapped behind the cone increases the speaker’s resonant frequency.

In both designs it’s important that the air pressure waves being generated behind the cone don’t just flow around the edge of the speaker frame to the front (and vice versa). If this is allowed to occur, the positive pressure waves being generated at the rear cancel out the negative pressure waves being generated in front of the cone, resulting in no deep notes being produced. (This is why even a big subwoofer driver run bare on the bench sounds awful.) In the ported design, the phase of the rear sound waves is reversed so that their effect adds to the front sound waves, rather than detracting from it. In a sealed design, the rear sound waves have no way of escaping.

And then there’s a third design approach. It’s called infinite baffle or free-air and is based on the idea that the front and rear of the speaker are separated by a panel that’s infinitely large. There’s no build-up of pressure behind the speaker (cos that baffle is infinitely large, so must the volume) and there’s no way sound from the back can cancel sound from the front (cos of the infinitely large baffle). This is the design approach that best describes a speaker installed so that the front of the cone is connected to the cabin and the rear of the cone connects to the boot space. Of course neither volume is infinitely large – although the baffle kind-of is – but in speaker terms, a boot volume of hundreds of litres represents a massive enclosure of the sort not normally found in subwoofers.

Which brings us to our next point – selecting the driver.

Driver Selection

A speaker designed for a ported enclosure has certain characteristics, and a speaker designed for a sealed enclosure has other, different characteristics. The same applies for a free-air application. That’s not to say that there isn’t some blurring together of the categories – some drivers can be used in both ported and free-air applications, but for best results, you want to use a driver that is designed to work in your specific application.

There are two ways of obtaining such a speaker. The first is to buy new – look for a manufacturer that lists the speaker as appropriate for a free-air or infinite baffle application. (See Installing a Free-Air Sub for a story on installing a new free air sub.)

The other way is to buy secondhand. You can source a driver from a car that ran a factory free-air sub such as a Lexus LS400 (8 inch), Soarer (10 inch) or Ford Fairlane (8 inch). Be sure when selecting these speakers that you have an amp to suit – some of these speakers are only 2 ohm and not all amps will be happy running a speaker with such low impedance. (See Cheap, Do-It-Yourself Bass) for a story on installing an ex-Soarer free air sub.)

Or, you can buy an aftermarket sub marked appropriately (these days, many manufacturers write the preferred mounting use on the speaker’s magnet).

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Finally, you can come from left field and use a speaker that was never designed for car use but which has the right mix of attributes. For example, home theatre 4 or 6 ohm speakers designed to mount in the ceiling or walls are perfect for free-air car use. In fact, that’s the approach being taken here where a fire-damaged Sonance 3-way 8-inch speaker was purchased for near nothing and the high performance woofer removed from the assembly for car use. (The only damage to the woofer was that the outer rubber suspension was peeling away from the cone in a few tiny spots.)

In all cases, the free-air speaker should not have a super floppy suspension. In other words, there should be some stiffness when you carefully push the cone back and forth with your spread fingers. You still want a long travel but in a free-air application, a super floppy suspension will easily allow the cone to overshoot and bottom-out. (This type of speaker is best mounted in a relatively small sealed box, where the trapped air adds to the springiness of the cone suspension.)

Installation

As with a number of cars produced over the last 15 years, the Falcon already has a cut-out in the rear deck designed to take a subwoofer. In this case, the hole is designed for an 8-inch driver.

This means that installing the new sub was as easy as....

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Removing the two way rear speakers...

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...and lots of trim pieces...

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and more trim pieces...

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...before the rear deck cover could be pulled forward...

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...to reveal the factory subwoofer speaker cut-out.

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The new speaker just dropped straight into the hole, although the mounting holes in the plastic speaker frame needed to be elongated a little with a file to match the holes in the metalwork. There’s something else important to note in this shot. Unlike cars of old, the centre seat position has a lap/sash seatbelt, with the inertia reel mounted on the rear deck. This means that the structural integrity of the rear deck must be maintained – that is, be very wary of cutting big holes in it to mount a speaker!

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A foam rubber gasket was cut out and sandwiched between the driver and the metalwork. This prevents airflow occurring around the edge of the frame.

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The amplifier used was one that we’d previously modified with a fan and better internal heatsinking (see (Amplifier Cooling Upgrade ).

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When selecting a sub woofer amp looks for these characteristics:

  • The amplifier should be able to be bridged, a process where the outputs of both stereo channels are summed to allow the amp to develop lots more power.

  • The amp should feature an in-built crossover. Preferably, the filter frequency should be able to be altered to tune the point at which the sub does its stuff.

  • Variable gain is important as it allows you to tune the loudness of the subwoofer within the system.

  • If the amp has the ability to take both left and right signal inputs and then sum them (ie L+R), you’ll be able to easily develop a dual-channel bass output. If the two channels of the amp can be bridged, this summing will occur automatically.

  • Another control that’s useful is an amplifier bass boost switch.

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The amp was mounted on a square of black painted particle board which was then bolted to the underside of the rear deck between the new sub and one of the original rear speakers. Note that the amp was spaced away from the board so that the fan can still operate effectively.

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Heavy power supply cabling was then run from the amp all the way forward to the battery. Most cars need to have a hole made in the firewall through which the power supply cable can pass – here it was drilled in an inset panel that also contains the steering column. Taking this approach avoids having to drill through a firewall that may be double-skinned.

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The cable must be protected where it runs through the firewall – either by a grommet (as shown here) or by having a short length of thick-walled hose slipped over it.

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At the battery the negative cable connected straight to the negative terminal. But the positive cable uses an interposed fuse - you must always install a fuse close to the battery when running power cabling to an amplifier.

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The audio feed for the sub amp was via twin RCA line level cables. These were run down the other side of the car to keep them away from the power feed (a possible source of electrical noise). A 5-metre cable is the right length in most cars.

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This head unit – as with most aftermarket head-units of the last 5 years – came with line-level outs. If there aren’t any RCA sockets on the back you’ll need to use a speaker lever to line level converter and pick up the audio signal from the rear speakers.

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The rear deck cover in the Falcon is moulded in shape so making a replacement wasn’t an easy option. Instead, two pieces of foam rubber were placed either side of the sub so that the cover would easily clear it. (Note also the circled foam strip placed under the brake light to stop it rattling.)

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Then the cover was re-installed with a later model factory Falcon speaker grille placed over a hole cut in the cover. The cover has enough flexibility to conform to the new required shape and looks good.

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And with everything tucked up out of the way, the loss of boot space is very small.

Conclusion

One of the hardest aspects of this sort of install is stopping bits of the car rattling when you crank up the level. As shown above, a foam strip was inserted under the brake light but even so, when the system’s loud, the cover can rattle a little, as can the centre inertia reel seatbelt! However, with that qualification, the sound is fine and the bass improvement massive.

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