GOODSOUND!GoodSound! "How To" Archives

Published May 15, 2001

 

Loudspeaker Basics -- Part 1

In our last article, we learned how loudspeakers are in many ways the most important components in an audio system. I recommended that a large portion of your budget be allocated to speakers. The next-most-important consideration involves which amplifier or receiver to match to a particular loudspeaker. (And by the way, while I’m trying hard to explain the most common audio terms as we go along, if you come across a term that you don’t understand, you can probably find it in our Glossary.)

Matching the right speaker and amplifier to one another isn't complicated. In fact, most speakers will work with most amplifiers; but if you know what to look for, you can coax even better performance out of the combination. Before we go there, let’s first look at how a loudspeaker works.

To keep things simple, we’ll discuss only the most common type of loudspeaker here and leave the more exotic types for a future column. The first thing you notice when you look under the grille-cloth is that not all speakers use the same number of drive units. However (with very few exceptions), they all use a minimum of two. Why is that?

Sound is measured in Hertz (abbreviated as Hz), which is a unit of frequency measurement. If you take a string and stretch it between two points and then pluck it, it will make a sound because the string is vibrating. Each of those vibrations takes the string above and below its resting point, and one complete trip up and down is considered a cycle. One cycle per second is measured as 1Hz. High-frequency sounds are measured in the thousands of Hertz (also known as kilohertz or kHz), while deep sounds have far fewer cycles -- a low organ tone might measure 30Hz.


The Polk Audio RT35i loudspeaker is a two-way design.



Although Paradigm's floorstanding Studio/100 loudspeaker has four drivers, it is a three-way design. The bottom two woofers work together to produce the bass frequencies.

The range of human hearing extends from approximately 20Hz to approximately 20kHz. Actually, 20Hz is so low that we don’t so much hear it as feel it, like a rumble. Very few instruments can play this low (organs, synthesizers, certain very big drums). On the other end of the spectrum, a 20kHz tone/squeal/whistle is so high that many of us can’t quite hear it anymore. This depends on age and other factors like hearing abuse (high-level rock concerts or work-related noise pollution). "20 to 20" is the frequency spectrum a loudspeaker is supposed to reproduce. Note the word supposed. Very few speakers actually manage to reproduce this entire range. While that is a subject for future investigation, we can already mention that no single loudspeaker driver can reproduce this whole range except for headphones -- because they need to produce sound only inside the very tiny cavity of your ear.

Since a single loudspeaker drive unit can’t reproduce the whole frequency spectrum of sound, the full spectrum is usually divided into at least two parts: high and low. Specialized high-frequency and low-frequency drivers are each assigned specified portions of the speaker's response range. High-frequency drivers are called tweeters (in reference to the noises birds make), and low-frequency drivers are woofers (for the low barks and growls of a dog). The device that separates the frequencies is called a crossover.

So we now have a loudspeaker box with two round drivers attached to its front: one small tweeter and one bigger woofer. This basic type of loudspeaker is called a two-way design because the whole frequency spectrum has been divided into two parts. That’s the meaning of "-way" -- how many ways has "20 to 20" been divided. It’s also true that the speaker only has two drivers. However, it could have more drivers and still be a two-way. A designer might decide to use multiple drivers for each portion of the frequency spectrum. That’s hardly ever done with the tweeter, but it is done quite frequently with woofers. A designer might double up on the woofers, using two (or even three) of them. In a two-way speaker with multiple woofers, these woofers are said to operate in parallel. This simply means they’re doing exactly the same thing, like dual tires on the back of semi trucks.

Some speakers that use three drivers divide the frequency spectrum into three sections -- then they are called three-way speakers. And, not surprisingly, the names for the three drivers, corresponding to their low/mid/high responsibilities for the different frequency portions, are woofer, midrange (or mid) and tweeter. A speaker with more than three drivers could either be a three-way with multiple woofers, or a four-way or five-way. But most speakers are two-way or three-way designs.

We’ve talked about drivers quite a bit. In a conventional box speaker, the moving parts that produce sound are called drivers or transducers. They literally push the air back and forth like miniature pistons, to create the sound waves we hear as music. Remember what we learned about frequencies -- higher-frequency sound waves have more individual ripples, or up and down cycles, per second than lower ones. A 20kHz note has 20,000 peaks and valleys per second. To reproduce it, a driver has to push out and back 20,000 times per second. That’s much faster than a human eye can perceive. Watch a two-way speaker’s tweeter while it’s playing really loud -- it won't seem to move at all. But watch the woofer. Play the music loudly enough, especially with bass notes, and you will easily see the woofer move.

So far, we've dealt with the different drivers as though they were the same, but of course they aren't. Tweeters are usually small and dome-shaped, whether they are made from treated silk or ultra-thin metal, while woofers are much larger and cone-shaped or flat. Since tweeters deal with the high frequencies, they need to move back and forth very rapidly. A small, lightweight dome is perfect for this. But the lower frequencies of the midrange and bottom-end of the spectrum require both greater in-and-out motion (also called excursion) and greater surface area on the part of the driver. While the cones (or sometimes flat diaphragms) used for woofers look simple, they are very carefully designed from their shape to the type and thickness of rubber surrounds used to anchor them to their frames. Woofers can also be made from a wide range of materials -- from paper to metal to high-technology ceramic.

Next time, we will take a look at the internal parts of the driver and begin to talk about the basic loudspeaker specifications we will need to know to consider that all-important speaker/amplifier interface.


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