SPEAKER

  How many times every day do you hear recorded music on theradios on TV, in stores, in elevators—even in the street? You'd never hear music at all if it weren't for loudspeakers: electric sound-making machines. Most of the music we hear around us is played back with big loudspeakers attached to stereos or tiny earbudhead phones. Radios, televisions, computers, cellular phones, intercoms, and talking toys are just some of the electric gadgets that make sounds with loudspeakers. But what exactly are loudspeakers and how do they work?

Photo: These neat little Sony bookshelf speakers have a built-in amplifier and a long jack lead, so you can plug them into a CD player, MP3 player, computer, or anything else with a standard headphone socket. They pack quite a punch for something so tiny because they have a reasonably high sensitivity, which is explained below.

How loudspeakers turn electricity into sound

When things shake about, or vibrate, they make the sounds we can hear in the world around us. Sound is invisible most of the time, but sometimes you can actually see it! If you thump a kettle-drum with a stick, you can see the tight drum skin moving up and down very quickly for some time afterward—pumping sound waves into the air. Loudspeakers work in a similar way.

At the front of a loudspeaker, there is a fabric, plastic, paper, or lightweight metal cone (sometimes called a diaphragm) not unlike a drum skin (colored gray in our picture). The outer part of the cone is fastened to the outer part of the loudspeaker's circular metal rim. The inner part is fixed to an ironcoil (sometimes called the voice coil, colored orange in the diagram) that sits just in front of a permanent magnet (sometimes called the field magnet, and colored yellow). When you hook up the loudspeaker to a stereo, electrical signals feed through the speaker cables (red) into the coil. This turns the coil into a temporary magnet or electromagnet. As the electricity flows back and forth in the cables, the electromagnet either attracts or repels the permanent magnet. This moves the coil back and forward, pulling and pushing the loudspeaker cone. Like a drum skin vibrating back and forth, the moving cone pumps sounds out into the air.

Animation: Right: How a loudspeaker works. When a fluctuating electric current flows through the coil (orange), it becomes a temporary electromagnet, attracted and repelled by the permanent magnet (blue/red). As the coil moves, it moves the cone (gray) back and forth, pumping sound waves into the air (light blue).

Photos: Speaker cones are generally made from paper, though plastic and even light metals such as aluminum and titanium are also used. Left: Paper cone: The Sony loudspeaker in our top photo with its protective plastic cover removed. Now you can see the white speaker cone in closeup. The small black dome in the middle is called the dust cap. The black outer rim between the white cone and the frame that supports it is called the suspension. The frame itself is called the basket.

Photo: Right: Metal cones: The tiny titanium speakers in this laptop computer are less than 1cm in diameter.

How speakers make sounds of different volume and frequency

Loudspeakers will play loud when the cone vibrates a large amount, or soft when it moves a small amount. Why? Think about drums. Banging a drum skin really hard makes the skin vibrate a greater distance and produce a louder sound. In the same way, sending a bigger pulse of electricity into a loudspeaker makes the cone move further and generates a louder noise. Quieter sounds are made by smaller pulses of electricity.

We can reach the same conclusion by thinking about energy. The laws of physics tell us that we can't make energy out of thin air. So if we want to make a loud sound (one that carries lots of energy), we need to create a vibration with lots of energy in the first place (in other words, hit something really hard).

Some drums have pedals on them that make the skin tighter or looser. If the skin is tight, it vibrates more quickly when you bang the drum and produces a higher-pitched sound; if the skin is loose, the opposite happens and you get a much lower note. A similar thing happens in a loudspeaker. Bigger speakers with large cones (known as woofers) move more slowly than smaller speakers with smaller cones (known as tweeters)—so they are better for producing lower frequencies. Any speaker can produce a wide range of different sound frequencies by moving back and forth quickly (for higher notes) or slowly (for lower notes).

Photo: Left: A typical compact loudspeaker in a portable radio, photographed from behind. You can see the permanent (field) magnet and the cone quite clearly and the two cables sending an electric current into the speaker. You can't actually see the (voice) coil, though I've indicated roughly where it is (in front of and underneath the magnet, attached to the cone in the center).

How to make your speakers sound better

It's not just the moving cone that determines how a speaker sounds. Have you noticed how most speakers are built into wooden or plastic cases? That's not just to make them look nice: it drastically changes the sound. You probably know that a guitar's wooden body amplifies the sound the strings make by a process called sympathetic resonance. As the strings vibrate, they make the air around them vibrate too. That starts the air vibrating inside the guitar body in sympathy—and this is what makes the sound loud enough to hear. A loudspeaker case works in exactly the same way. Without the resonance of the case, you'd hardly hear a guitar or a loudspeaker at all.

Except for earphones, loudspeakers are usually some distance from our ears. The sound waves produced by the speaker cones have to travel through the air in a room before we can hear them. But sound waves travel out from speakers in all directions. They travel backward from the speaker as well as forward; they travel down to the floor and up to the ceiling as well. In practice, one single push or pull of a speaker cone sends sound waves traveling in all directions. These reflected waves bounce off the walls, floors, and furniture in your room and interact in many different ways, sometimes adding together and sometimes canceling out. With the same set of speakers, an empty room will sound very different to a room full of furniture; a living room with rugs and soft furnishings will sound very different to a kitchen or bathroom with lots of hard surfaces.

You can dramatically alter the quality of the sound your speakers make by putting them in different places. Always arrange them symmetrically (so if one is six inches from a wall, the other needs to be six inches from a wall too). Never fasten speakers directly onto a wall or stand them on the floor. Instead, try to mount them roughly at ear level. Put each speaker nearer to the center of the room so there are unequal distances from the speaker to the walls, ceiling, and floor. That will help to stop reflected sounds from interfering with the main speaker sounds. Speaker stands are a great investment: they usually make speakers sound twice as good!

Photo: A pair of old Mission loudspeakers. There are two speakers here, one on top of the other. Each cabinet contains a large woofer at the top to produce low bass notes and a smaller tweeter at the bottom for the higher treble notes. You would never normally stack speakers like this in a room.

Listening with both ears: stereo, quad, and binaural

When sound comes from a single loudspeaker, we say it's mono or monaural. Mono is like the sound of one person talking: the sound source is fixed in one place.

Stereo (stereophonic sound) is very different. The first time you hear stereo, it sounds like a miracle. Where are the sounds coming from? How do they move around your head like that? Stereo is a simple trick: two loudspeakers each play slightly different sounds and our ears and brains reassemble the noises into a two-dimensional soundscape. If you listen to music with headphones, you'll be used to the way stereo sound jumps back and forth between your ears. You might hear a drum beating in one ear and a guitar playing in the other, for example.

Photo: Most old audio systems had only one speaker, like this, and reproduced sound in mono. This is a wonderfully preserved Amplion speaker made in 1928. It's an exhibit at Think Tank, the science museum in Birmingham, England.

Although stereo is a big improvement on mono, it's still only two-dimensional sound. It is possible to make loudspeakers sound three-dimensional, but you need more speakers to do it. Quad (quadrophonic) sound is like double stereo: you have two speakers in front of you and two behind. Now the sound can move behind you or in front as well as from side to side. Surround sound used in movie theaters (cinemas) works in a similar way.

Binaural is a way of making a sound recording seem three-dimensional with only two speakers. Our ears are more than just holes through which sounds enter. The hills and valleys in our outer ears help us to work out where sounds are coming from and give the sounds we hear in the world their three-dimensional quality. Normal stereo recordings don't pick up this directional information, because ordinary microphones don't have the hills and valleys that our ears have. Binaural recordings are made a different way using a dummy's head with plastic ears shaped like a human's. Two microphones are placed inside the ear holes so they pick up noises like human ears would. When sound is recorded binaurally, in this way, and then played back with conventional headphones, it sounds strikingly different to stereo—and almost lifelike. A binaural recording of a jet plane taking off sounds just like it's moving through your head!

Why bigger and more powerful isn't always louder

Generally speaking, the more noise you want to make with your speakers, the bigger they'll need to be. Why? Because the amount of noise something makes is related to the amount of sound energy it sends out into the air. Bigger speakers sound louder because they have bigger cones that can pump out more energy per second, which means they're more powerful.

In the everyday world, words like "energy" and "power" are often used in quite a vague way, but they have very strict scientific meanings. In science, we measure energy in units called joules (named for English physicist James Prescott Joule) and the amount of energy something produces in one second is called its power, measured in units of watts (named for Scottish engineer, James Watt). A power of one watt means something is making or using one joule of energy every second. An ordinary 60-wattincandescent lamp takes in 60 joules of electrical energy every second, though probably puts out only about 3–6 watts of light and wastes the other 54–57 watts as heat. You might think a loudspeaker rated 100 watts would produce 100 watts of sound—but it's not quite that simple. Speakers are also fairly inefficient: much of the electrical energy you feed into the back of a speaker doesn't emerge from the front as sound energy but is wasted in the coil as useless heat energy.

Power

If you want to compare the output of two loudspeakers, you might do it by putting them side by side, switching them (in turn) up to maximum, and walking off into the distance to see how far you could go before the sound disappeared. In theory, an easier way to achieve the same end is to find out their power rating in watts and compare. This is what most people do when they buy speakers or home stereo systems.

You might expect that the higher the number of watts, the louder and more powerful your speakers would be. So, for example, those little Sony bookshelf speakers in the top photo are rated a puny 7 watts, whereas big, old-fashioned, home stereo speakers are more like to be rated at something like 50 watts. However, straight power measurements can be very misleading and are often deliberately used as a marketing trick by speaker and amplifier manufacturers: two similarly rated speakers may produce sound in very different ways and in practice, for various reasons that I'm not going to go into here, speakers that claim to have a higher power rating might sound quieter than ones with a lower power rating. Not only that, but simple power ratings tell you little or nothing about how you'll be able to use your speakers in the real world: I used to own some huge speakers that were so powerful I could never play them at anything other than minimum volume for fear of upsetting my neighbors. I had to play them so quietly that they simply never worked properly. It was like owning a Ferrari and driving at walking speed.

  Photo:  Now that's what I call loud! The gigantic 1400-watt "loud hailer" loudspeaker on the side of this Sea Knight HH-46D helicopter was designed for use during search and rescue missions at sea, but just how loud was 1400 watts in practice? And was it loud enough? Large outer photo by JO1Snaza ;  pullout closeup  photo by PH3 Sue Cain.Both photos courtesy of Defense Imagery.

Sensitivity

Instead of power, it's often better to look at a measurement called the sensitivity of a speaker, which is how much noise the speaker produces in decibels at a distance of 1 meter for an input power of one watt. (In effect, sensitivity is a rough guide to the efficiency of a speaker—or how much output it produces for each unit of input.) The higher the sensitivity, the more efficiently the speaker is converting energy from your amplifier into sound—and, often, the better it is. Most home speakers have sensitivities of about 80–90dB; the little Sony ones in our top photo come in at 82dB, which means if you put 1 watt of power into them and stand 1m away they'll produce a perfectly respectable 82dB of sound.  Now the decibel scale is logarithmic,so small increases in decibels translate into very much louder sound. Small bookshelf loudspeakers often pack a surprisingly powerful punch because, although they have low power ratings, they have high sensitivity.

How do electrostatic speakers work?

Unless you're a real audiophile, you'll probably never come across electrostatic speakers (sometimes known as condenser or capacitor speakers). Unlike the speakers we've considered so far, which generally have rounded cones mounted in square boxes pumped back and forth by electro magnetism, electrostatic speakers often look more like closets or radiators and work like capacitors. A capacitor is a device for storing electricity using two parallel metal plates separated by some sort of an insulator (usually air or plastic). Suppose you take a huge capacitor and rapidly change the electric charge on the plates. Since unlike charges attract and like charges repel, if the plates can move, the changing charge is going to make them vibrate, sending sound into the air. In practice, electrostatic speakers have two fixed plates on either side and a moving plate that vibrates in between them, which is analogous to the moving cone in a traditional magnetic loudspeaker. Although they can produce excellent sound, they have a number of drawbacks: they generally don't work well at low frequencies, they have to be fairly large and heavy, they can get very dusty (because of the static electricity they use), and they need to use high voltages to work effectively, which means they sometimes need transformers and consume a lot of power. The best known maker of electrostatic speakers is English company Quad Electro acoustics.