I.C.E. ADVICE - Car Stereo Speakers & Subwoofers

Your choice of front and rear full-range speakers for your car will probably have the biggest effect on the sound of your system, apart from the low bass... which we'll talk about in a minute.

Choosing an 'Easy Fit' or 'Custom Fit' model should mean that the chosen speaker will be a drop-in replacement for the factory speaker pre-fitted to your vehicle, requiring minimal installation work. This not only reduces the fitting cost but ensures that the factory look is fully retained. But you need to make your choice carefully, especially if ordering online. The speaker sizes and mounting positions may well vary on different versions of the same base model of vehicle.

Even with so-called drop-in speakers it can be a real trial to fit them without breaking something. Many will simply drop in (well, you will need to release and refit a few screws from around the speaker chassis!) from the front nice and easy, once you've released the factory grille. But not all - you may have to release a trim panel to get in behind it. On older cars you can usually clearly see where the screws are to release the door trim, but on most current cars it can seem a real mystery. Even the pros will have replacement plastic door trim clips handy, because even though they know exactly where to ease them off, one or two are sure to break.

'Custom Fit' speakers designed as easy replacements for your car's factory units should line-up with existing mounting points and require no modifications to the trim or exisiting grille. However, going to the trouble of fitting higher quality speakers that require a little bit more effort to get them in can be worthwhile, and be done while retaining the factory look of the vehicle's interior. The sales staff at one of the Mobile Media Specialist Association's InCar Expert member shops will be very happy to discuss this or any other special cosmetic requirements, such as colour matching to existing trim.

Choosing Your Speakers
In any audio system, whether in the home or in the car, it is the speakers that have the most effect on the final sound. It's their job to take the electrical signal and turn it into something we can hear, and they do that simply by moving the air - pretty amazing really. It's a simple job and on the face of it you might think similar looking speakers will sound almost identical, but the audible difference may well amaze you. For this reason, you really do need to take time to listen to different models if you can.

It's difficult to rely on magazine or web site reviews, because with speakers there is nearly always a degree of compromise. We can assume without much doubt that what we are hearing is not exactly the same as the producer of the music track heard when they recorded it on studio monitors. But that doesn't matter, as long as we like what we're hearing. But does the reviewer like the same kind of sound that you like? This will vary with the music you listen to and, of course, your hearing. So choice of speakers is a very personal thing.

Power handling is the number often splashed across the packaging, making it seem all-important. Of course you do need to match the speaker power handling to the output power of your amplifier, and the rule of thumb is to double your amplifier's RMS power. So if your amplifier claims an output of 30W RMS per channel, look for speakers with a power handling of at least 60W.
But power handling is no indication of quality.
So how can you tell the quality of a speaker just by looking at it?

Frankly you can't, but you can get some pointers from having a good look and a feel. If you're allowed to hold one, weigh it in your hand. Does it seem relatively heavy compared to others of the same size? Look at the back, does it have a large magnet? These are positive signs but not conclusive, because the best quality speakers will often use higher efficiency magnets. These may well be smaller and lighter but will perform as well or even better.

Here's another trick. Have a look at the basket - that's the frame that the cone sits in. Is it a simple stamped metal or something more unusual? Flick it with a finger nail. Does it ping or do you get a dull sound? The best speakers will use a metal basket that has been treated (covered in a special paint), or perhaps a special plastic rather than metal for the frame itself. This helps to kill the 'ringing' that comes from thin metal frames, and while it may not necessarily be a big advantage in practice, it does show that the designer has tried to build a better product.

Main types of car speakers
The most popular car speakers tend to be the multi-axial type, usually co-axial (that is, two speakers mounted within the same chassis, with the tweeter in front of the bass driver) or tri-axial (same idea, but with a separate midrange and tweeter mounted ahead of the bass driver).

You can also find 4-way multi-axial speakers (though they?re not generally called quad-axial, maybe because that just sounds silly!). These usually have a super-tweeter added, but it's often of limited benefit. Looks cool but doesn't give a great improvement unless you like very tinsely highs.

The simplest speaker is the single cone unit. This is the cheapest but invariably the least impressive. The single cone is unable to cover all of the audio frequency band well enough to give true hi-fi sound. The higher (treble) frequencies are usually restricted. It wouldn't make sense to upgrade to a single cone speaker unless you were looking to sell the car on and just needed the radio to make a noise to prove it works!

Component speaker systems tend to be the highest quality but can work out quite expensive. These are physically separate speakers, usually 2-way (midrange plus a separate tweeter) or 3-way (a bass speaker, midrange and tweeter). As each speaker is separate, the best mounting positions can be selected for each part and it's possible to dedicate amplifier channels to drive each pair (with dedicated equalisation, crossovers and other effects).

It is possible to find hybrid co-axial systems that use a shared chassis but provide separate inputs to the drive units, allowing multi-channel amplifiers and electronic active crossovers to be used, or better quality passive crossovers. This can give the benefits of a component speaker system but in a compact package. Also, the drive units are usually very closely physically aligned (providing what's known as a single point source, as close as it's practical to get). This can give benefits in terms of creating more realistic imaging and staging (providing an accurate sonic stage where the performers are well defined).

CHOOSING YOUR SUBWOOFER SYSTEM
SOMETHING ABOUT SUB BOXES...
The differences between the main types of bass box is quite a detailed subject, but below are some pointers on which one to choose. To calculate a Bass Box using something a lot better than guess work, try the FREE online software on our sister site: Subwoofer Box Design Calculator.

FREE AIR : also known as 'Infinite Baffle' ('free air' is a misleading term but somehow got adopted as a description), this does not use a bass box in the regular sense, but instead uses the boot (or trunk, for our US friends) as a cabinet. The speaker may be mounted on to a panel (the 'baffle board') behind the rear seat, or suspended from the rear parcel shelf, so it fires sound through the ski hatch or simply through the back of the rear seat.

The rear and front sound waves from the speaker must be completely isolated one from the other (hence the term 'infinite baffle' or never-ending baffle - the 'baffle' being the name of the board or panel that the speaker is fitted to), otherwise one will cancel out the other (which causes what's known as phase cancellation).

Only use a subwoofer designed for 'free air' mounting, as these will have a stiffer suspension.

It's usually a cheaper method of fitting a subwoofer to your car, but the air space in the boot is generally too large and not sealed well enough to provide any extra help or damping to the speaker. The speaker can therefore easily over stretch itself (called over excursion), causing severe mechanical damage if driven too hard. Some speakers are made with a very stiff mechanical suspension to help cope with this - don't use a speaker designed to be mounted in a small box - and with good installation methods (sealing the boot area and using multiple speakers) this can still give good results.

CLOSED BOX : as the name suggests, this is a sealed box. It will generally give a tight, well controlled sound, with a natural roll-off. Good for rock, jazz, classical music and pretty much everything else. The trapped air helps to provide good mechanical protection to the speaker at high levels (this limits over-excursion). It's the easiest box to get right. But it tends to be less efficient (produces less output overall) than a ported system.

When designing any kind of enclosure, always avoid building square boxes - these can lead to standing waves building up inside the box, which can cause a very unpleasant boominess. Where possible, have at least one non-parallel panel. For example, having one panel angled (either snug up against the back of the rear seat, or with the sub mounted on it) helps to reduce standing waves. However, in most cases anything other than a square box will be okay, as long as it is sturdy and rigid - don't use thin cheap chipboard. 15mm thick MDF is the minimum, 18mm is better but don't forget the effect it will have on petrol consumption if it's a big heavy enclosure.

If using Plywood you can get away with using thinner panels - 12mm thick Plywood is very good (half that thickness is also fine for small and medium size boxes if you internally cross-brace the box, as Plywood is good stuff, naturally very rigid and less porous than MDF). 15mm Plywood is excellent but expensive.

PORTED BOX : also known as a vented box, has one or more vents which are tuned to the size of the box and the requirements of the speaker (not any old tube will do unless you're damn lucky!). This will generally give considerably more output (higher efficiency) than a closed box in the region between 30-80Hz. This gives it a big sound, well suited to dance music styles, but when combined with the natural boost caused by the passenger compartment, it can result in an unnaturally big and undercontrolled bass sound.

Only use a subwoofer designed for vented enclosures. Subs designed to be fitted to closed (sealed) boxes are likely to have a less stiff suspension and could be damaged at the point where the box 'unloads' below the tuned frequency... I see some blank looks. Okay, it means as follows...

Below the port tuned frequency the cabinet unloads (the air inside no longer acts as a brake) and this can cause a sudden over-excursion of the speaker driver. At high levels this can be terminal! For this reason, ported boxes designed for high sound pressure levels (SPL), especially competition, should have additional protection. For example, a sharp electronic subsonic filter on the amplifier (24dB per octave or better) with a very accurately calculated turnover frequency. Even then it's risky... but then big bass and high SPL is always risky, it's what makes us do it!

BANDPASS BOX : a hybrid of the ported enclosure, the bandpass box houses one or more speakers fully inside the cabinet, so that the speaker does not directly radiate sound in to the car. The sound exits only via the port, often a rectangular vent. As the speaker has a controlled mass of air on both sides of the cone, it has a high degree of mechanical damping and control. The result is a high efficiency, high output system. And see-through acrylic bandpass cabinets look very cool!

The down-side? Well, bandpass cabinets output a relatively limited frequency range because of the vent design - this isn't necessarily a problem but needs to be taken into account when designing the overall speaker system. Also, speakers can become toast before you know it, because having them fully enclosed hides the telltale distortion that would tell you to back off the volume control. Add to that the fact that bandpass designs are generally more complex to get right, so they tend to cost more to build, and you may wonder if it's worth it. But a well designed bandpass box can provide huge, very focussed bass output and very good dynamic impact. [KOB]

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