Subwoofer Design - Part 1

This article was first published in 2003.

If there's one thing which everyone wants in a car sound system, it's deep bass. The amount of the bass is the major criterion which many people use to judge the quality of sound - can it really shake you, or do you just hear the notes? Do the sounds of the bass drum cause the images in the mirrors to blur into nothingness and send signals to those walking the sidewalk that you're around - or is it more like just a wet fart? You might know what results you want, but going into the nearest car sound supermarket without any idea of how you can get it can sometimes mean that you shell out thousands of dollars - to get a system which may or may not work.

Here we'll show you how to go about the design of a sub which will be effective and won't cost you an arm and a leg. But take note that we're writing this series for 'semi-normal' people - if we can put it that way. If you want to listen to hundreds of dB from an ear-bleeding distance of 10cm, you're bound to be disappointed with what follows. But on the other hand, if you like listening to music with a good range of kick, then this will be right for you.

How They Work

Before we get into subwoofer enclosure design, you need to know something about how a speaker works. If you don't, then the salesperson can (and probably will) tell you anything to sell you expensive bits.

A speaker operates by moving its cone back and forth. When the cone pushes forward it creates a high pressure wave which travels through the air, and when it moves back it causes a low pressure wave - sometimes called a 'rarefaction'. The ear picks up these air pressure waves and turns them into sounds. The speed with which the cone moves will determine the pitch of the sound heard by the ear. If the cone is vibrating slowly - like back and forth say 50 times a second - then a deep note will be heard. Higher pitched sounds are made by the cone vibrating more quickly - 1000 times a second (expressed as 1000 Hertz) is a squeal, 8,000Hz is a screech, and 15,000Hz is higher than many people can hear at all.

Since we're interested in bass, we'll stick to discussing the frequencies below 150Hz. A woofer - which is a large speaker designed to work at low frequencies - will sound lousy if it is not mounted correctly. Foremost is the need to separate the front and rear of the speaker. To see why this is needed, imagine a bare woofer sitting on a bench. The speaker is being driven by a powerful amp, and is running music with a lot of bass. The cone will visibly move forward and backwards with the music, but the bass will be lousy. The reason for this is that when the cone moves forward, a 'proper' pressure wave in the air isn't created. Instead, the air simply moves around the edges of the speaker frame, filling up the low pressure area created behind the cone. When the cone moves back again, the air flows the other way. Instead of pumping bass into the room, all the cone is doing is pumping air around its edges.

Separating the front and rear of the speaker doesn't mean that the driver must be in a sealed box - although that is one approach that works well. The separation of the pressure waves needs to occur acoustically, and in fact the enclosure design may well allow air to flow from the back of the speaker out the front through a port. But - and this is a key point - the enclosure needs to be designed so that the rear pressure waves add to the sound, not cancel it out. If you figure that this sort of design exercise isn't achievable by buying a no-name woofer, mounting it a box which your cousin made to store his tools in, and then cutting a vent hole in one end - well, you're right!

But if "a speaker operates by moving its cone back and forth", why can't all speakers reproduce sound down to, say, 20Hz? After all, it's easy enough to drive a cone at this frequency. The answer is that the cone must actually couple with the air: it must impart energy to the air so that the waves travel through it. And achieving this requires very tricky design - and the trickiest bit is the subwoofer enclosure.

Different Designs

Four major types of speaker boxes are used. (Note that in each case it doesn't matter how many drivers are put into the box, whether they're facing in or out, etc.).

The designs are:

• Sealed

In this type of box the sound waves coming from the back of the cone are effectively wasted. Instead of contributing to the sound that moves you, they're dissipating in the acrylic fluffy lining inside the sealed box. However, while this type of box produces less SPL (ie they are less efficient on a watts per dB basis) they are easy to make, have only a gradual bass drop-off as the notes get lower (and that drop-off can be counter-balanced by the rise in bass response that occurs within the closed confines of a car), and can potentially handle more power than a ported enclosure.

The effects of mismatching the speaker and its enclosure is also less severe when a sealed design is used, so if you have a driver of unknown specs, a sealed box is generally the only way to go. You can recognise a sealed box design easily - there are no openings in the box except for the driver(s).

• Ported

A briefly mentioned above, a ported enclosure additionally makes use of the energy coming from the back of the cone. It does this by using a connecting port (or vent) that joins the inside of the box with the outside. The port diameter and length are carefully sized so that the plug of air contained within the vent is excited into back and forth motion, but its movements are delayed just enough that when the cone of the driver is moving forward, so is the plug of air inside the port. In this way, the two air movements complement each other.

The advantages are twofold: firstly the efficiency of the system is greater (ie more SPL per watt of amp power), and secondly the bottom-end bass response of the system can be improved over the use of a sealed box. The downsides are that if the port isn't just right for the driver and box, boomy one-note bass can be the result, and even with well designed ported boxes, at ultra-low frequencies the cone of the woofer becomes unloaded - which can cause it to be destroyed if you're not careful in how you set up the system. And while the bass response holds on to lower notes, once it does start to fall away, it does so more quickly than with a sealed enclosure.

Ported designs can be easily recognised by the presence of one or more openings (usually round) that connect the other side of the driver to the atmosphere.

• Passive Radiator

A passive radiator design is a relative unknown in car systems - but it doesn't have to be. This type of design uses a passive radiator (a driver without a magnet) to act as a port - the cone and its suspension moves back and forth like the plug of air within a port, but at no times can the main driver become completely unloaded as is the case in a ported enclosure. The disadvantage is that passive radiators are rare - and finding the detailed specs on them which are needed to do good designs even rarer!

A passive radiator design looks to have two drivers in a sealed box (sometimes by just examining the system you won't be able to tell that one of the 'drivers' is in fact a passive radiator) although the radiator is usually bigger than the woofer.

• Bandpass

A bandpass design is a very tricky thing. Rather than producing frequencies from as high as the woofer can go - and then trailing them off at the other end as is determined by the enclosure design - a bandpass lets just a narrow spread of frequencies be emitted from the box. Because it is producing only frequencies from (say) 30 - 90Hz, it can be more efficient that the other box designs - it's concentrating all of its energies just in this narrow field of frequencies. All the sound comes from the ports, with the driver itself buried inside.

There is a variety of bandpass designs - some mount the driver on the division between two boxes and vent just one volume, while others vent both boxes. Still others connect one box to the other by means of a vent and then have a further vent in the first box. Bandpass boxes need very tricky design indeed, and if you're not careful either the frequency spread (the range of bass notes produced by the sub) or the efficiency (how loud it is for a given input power) can drop right down. In addition, the bandpass boxes are much more complex to make than other designs - there tends to be lots of pieces inside that need to be airtight, and fitting the ports inside the box can be a drama. However, get it right and you can have a loud and strong sub which is small and effective.

In a bandpass design the driver won't be visible (unless there's a plastic window fitted) - the speaker's cone does not connect directly with the air, except through a port.

Different Designs

If you're thinking to yourself, "Yeah, that's all fine, but where exactly does it leave me?" you're not alone. It's a bit like comparing the advantages of a V8 against a turbo four: both are good in some areas but specify enough criteria and you'll find that the selection starts to narrow. For a subwoofer these key criteria include:

• How loud do you want it? (Competition, sidewalk impressiveness or just good listening?)
• How much room do you have for it? (The whole boot? Squeezed to one side of the hatch?)
• What is the performance of the rest of the system? (Are there other drivers that will take the mid-bass load or does the sub have to do that as well?)
• Do you have a preferred driver (It's on special at a bargain price this week...)

The design of subwoofers has been revolutionised by PC design software. Using it can help shape answers to a lot of the above questions - it's easy enough to find out, for example, that your chosen driver will need an enclosure that is too big for your load-carrying aspirations, or that if you use a bandpass design you might need to beef-up the rest of the speakers to carry the response down to 100Hz.

But building your own subwoofer isn't very hard, and designing your own not a whole lot harder...

In the next article we'll look at some sub designs using PC software packages like BassBox.

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