How to buy ... loudspeakers
- — 04 September, 2006 22:00
Whether you’re into hi-fi or home theatre no single component in your system will have a greater effect on overall sound quality than the speakers. They are, after all, the very last link in the chain before the sound hits your ears. They’re responsible for reproducing every single note you hear from crashing cymbals to spoken words to floor-shaking bass. They come in all shapes and sizes, cost as little or as much as you like and there’s a staggering array of brands and models to choose from. It’s no surprise then that no matter what your level of audio expertise, speakers are probably one of the most confusing components to shop for.
The Big Picture
With all the demands placed on it, a speaker needs to be versatile, by which we mean it must be able to reproduce most — if not all — of the sounds human ears are capable of hearing. But a speaker must also know its limitations; if it tries to reproduce a sound when it really isn’t capable of doing so (say, if you have the bass turned up too high) it’ll distort the sound and instead of listening to a faithfully reproduced passage of music, you’ll be listening to noises created by crumpling driver cones and resonating cabinets. Individual speaker cones are called drivers and so as not to overextend any one driver a typical speaker will usually contain more than one variety — usually a tweeter, a midrange and/or a woofer — each dedicated to producing only a certain part of the frequency spectrum. By combining two or sometimes all three of these types of driver in one box, a speaker manufacturer can produce a single unit that covers a large chunk of the frequency spectrum. These types of speaker are called full-range, two-way or even three-way speakers.
But there are no hard and fast rules — some manufacturers opt to create speakers with a frequency range of, say, 50Hz to 20kHz by using a single midrange driver and a tweeter and advise you to add a dedicated subwoofer to cover the frequency range below this point.
Drivers: Tweeter: A tweeter is primarily responsible for the high end of the frequency spectrum (sounds between roughly 2kHz and 20kHz and above). They come in a few different varieties such as dome, ribbon and horn-loaded and can be made from a number of different materials including cloth (usually coated with another substance) or metal. The material has a significant effect on the overall sound a tweeter produces — a metal tweeter, for instance, will provide sharper detail but can sometimes sound harsh, while fabric domes will make for a smoother sound at the expense of some detail.
Midrange: Designed to cover frequencies between about 200Hz and 4kHz. A typical midrange driver consists of a rigid cone or diaphragm with a voice coil and magnet attached to the apex, all of which is suspended in a metal frame called a basket. The entire cone segment can move in and out thanks to a flexible rubber (or similar material) surround.
Woofer: A woofer is constructed identically to a midrange driver except it’s usually bigger (typically 6 to 15 inches in diameter) to allow it to create deeper, louder bass. Like midrange drivers the material the cone is made from has a significant effect on the sound it can produce and some types of material offer different characteristics. Cheaper speakers will likely use some kind of paper or cardboard-based material for the cone simply because they can be manufactured cheaply. Others will use more sophisticated material such as Kevlar, polypropylene or even aluminium, all with the aim of producing a certain style of sound. Finding a speaker with a sound you like often comes down to what kind of material was used to construct the cone. Why all these fancy composites? The cone must be rigid enough to not flex under load (and thus create distortion), but it must also be light enough to move quickly to keep pace with the music. If a driver is too slow you’ll find music muddied and drums indistinct leaving your action movies a big blur of mindless bass without definition. The speed at which a driver is able to react is called transient response and is a term often used by reviewers to describe how well a speaker handles fast-paced music and movies.
Subwoofers: If you’re buying a home theatre system, chances are you’ll have a subwoofer anyway, but we’d recommend one even if you’re intending to purchase a stereo system for music only. Using a subwoofer will ease the load on the two front speakers since they no longer have to worry about creating deep bass, which will improve overall dynamics and detail and substantially improve bass response.
Crossovers: When shopping for speakers you’ll often hear the term ‘crossover’ mentioned. A crossover is a tiny circuit fitted to each speaker that at its most basic level filters the high frequencies and sends them to the tweeter while sending the low frequencies to the midrange and woofer. Like all parts of a speaker, crossover quality varies depending on the construction and materials. They’ll be set to a specific frequency, usually around 2–3kHz for a large floorstanding speaker.
Enclosures: The box that holds the drivers plays a more important role than you may think and the construction can have a dramatic effect on sound quality. Wood or MDF (medium-density fibreboard) tend to be the most popular materials due to their rigidity and tonal qualities, but in reality enclosures can be made out of just about anything, from extruded aluminium to lowly plastic (although we’d avoid a plastic-encased speaker as we’ve yet to hear a good one).
The trick when it comes to building the enclosure for a loudspeaker is to construct it in such a way that the cabinet itself doesn’t affect overall sound quality. Part of the problem is that sound emanates not only from the front of a driver but also the back. If soundwaves from the back of the speaker are bouncing around off the rear wall of the cabinet they can actually cancel out the front sound, resulting in a loss of bass output. Speaker manufacturers employ a number of techniques to avoid this problem, including internal baffling or a (typically more expensive) rounded cabinet design.
Resonance of the cabinet itself is also a factor in speaker design — if the cabinet starts resonating it can introduce sound that isn’t supposed to be there, a phenomenon you’ll often hear reviewers refer to as colouration. Complex internal bracing and further damping can alleviate this problem. If you’ve ever wondered why one set of speakers costs so much more than another, you’ll probably find the more expensive one has had considerably more work go into the internal sound dampening and other design features.
One final thing to pay attention to with enclosures is the type of cable connection. Cheaper speakers use a spring clip connector to connect to bare wire but a better option is the binding post. A sturdy multifaceted connector, the binding post can take banana plugs, bare wire and spades all with the intention of providing a cleaner connection to your amplifier, resulting in better sound.
Types of enclosure: During use a speaker moves a lot of air, and naturally this air needs somewhere to go — just how and where can have a major effect on sound quality. A number of methods are employed to shift air around an enclosure.
Reflexed: Perhaps the most common type of enclosure is a bass reflex enclosure which uses a carefully tuned (by its length and diameter) port to allow air to move and helps maximise bass response. Another variation on a reflexed enclosure is called a passive radiator. This is where a second ‘passive’ driver (a driver without a voice coil or magnet) is driven only by the air movement created by the electrically driven woofer. Like a bass reflex port it aids bass response but since there’s no lengthy port to incorporate, it can be used in a smaller enclosure. Both provide efficient ways of creating bass.
Sealed enclosure: As the name suggests, a sealed enclosure does away with any port or passive radiator and relies on internal pressures to help drive the speaker. Typically these enclosures provide tight or fast bass, but are not very efficient and require some serious amplification to run.
Make sense of the specifications
When it comes to buying speakers, knowing exactly what the figures in the little specification box mean can really help, not only in matching them to your amplifier or receiver but in giving you a little hint as to the kind of performance you might expect to hear from them.
Power: An easy one — match the power rating of the speaker to your amplifier. It’s better and safer to use an overpowered amp than an underpowered one.
Impedance: Measured in ohms and indicating resistance, the impedance rating tells you how easy or hard a speaker is for your amp to drive. A 4-ohm speaker offers less resistance than an 8-ohm speaker. Strange as it may seem, a speaker offering less resistance is actually harder for an amp to drive because it means the amp must push more current through the speaker to run it.
Frequency response: An important figure that tells you the dynamic range of the speaker. Usually it’ll look like this: 40Hz–20kHz. However, unless it’s accompanied by a further specification, such as +/-3dB, it’s all but useless. A speaker may well create 40Hz bass notes but they’re no good if they’re at minus 20dB and no one can hear them.
What to listen for
Now that you know something about speaker varieties, construction and what the specifications mean, here’s a quick primer on some things to listen for when demo-ing speakers.
Bass response: Listen for how a speaker responds to fast-paced music and movies. Is the bass well defined or does it blend into one big mass? How deep does it go? Is there bass where there shouldn’t be any — for example, do voices sound overly chesty? If the bass is boomy, chances are the speaker needs repositioning. Don’t be afraid to ask a salesperson to make alterations to the physical setup for you to see what differences positioning can have on sound.
High-frequency response: If the highs make you wince or hurt your ears the speakers can be described as bright or harsh. Listen for undue sibilance (when the ‘s’ sounds make too much of an ‘ssss’ noise). The high end should be detailed but certainly not cringe inducing.
Imaging/soundstage: When it comes to stereo speakers the ability to create a believable image is paramount. Ideally when listening to a well-recorded track the various instruments should be clearly defined and the sound should extend in front of and behind the speaker as well as to each side. The ability of a speaker to project the sound so it seems detached from the cabinet is a worthy trait.
Collapsed soundstage: When played loud is the sound still comfortable to listen to or has the soundfield collapsed into an unruly and indistinct mass of noise?
Ensure the speakers you buy fit both your listening environment and your tastes. If you live in a tiny apartment block in the middle of the city you may be cramped for space and of necessity will need something unobtrusive that can be hidden away or even attached to a wall. Too large a speaker in a small space will probably sound overly bassy and won’t be able to image properly. If you have a large lounge room by all means go for a larger freestanding style of speaker — you’ll need it to fill the room evenly with sound.
Related to these practical considerations are the constraints placed on speaker choice by significant others. You may spend a fortune on your dream set of speakers, but if your partner can’t stand the sight of them and makes you place them where they don’t sound their best, it will not be money well spent.
Everything we’ve covered here regarding speakers and their design can be applied equally to stereo and surround-sound setups, but always remember to take in account your taste in music. Take your own CDs and DVDs to listen in store and don’t buy speakers that sound fantastic with jazz if all you listen to is Eminem.