In your home audio system, speakers are as important as they are simple.

Audio speakers are opposite devices to the microphone. A microphone is used to capture an audio wave-sequence and transform it into an electric impulse, and therefore can be recorded - on a magnetic tape for example.

What speakers do is very simple to understand, but how they do it is a bit more complex. Their role is to transform the electric impulses back into audio waves.

Continuing the example of a magnetic tape, we have a magnetic deck that can read it. But all the magnetic deck manages to do is translate the information on the tape into electric signals that we can't hear.

That's because our ears are only sensitive to air vibrations, and not to electric signals. And here speakers get in the picture. These are devices that can translate electric impulses into mechanical vibrations that our ears can hear.

A short analogy

A simple example of a vibrating wave is what happens when we throw a rock into a still water. What we see is the rock producing radial waves that spread on the water surface, weaker and weaker, until they fade. When these waves meet an obstacle in their way, the shore for example, they are reflected back. If the obstacle were a fine membrane, it would oppose a weaker resistance to the wave pressure and the wave would make it vibrate.

Objects vibrating in the air create sound waves in a similar manner. The fast vibrations of the object create pressure variations in the air surrounding it. The particles in the air tend to move back and forth in order to restore the balance. The pressure variations are transmitted this way at lower and lower intensities, until they fade.

But if you're close enough to the vibrating object, the wave will also hit your eardrum, causing it to vibrate. This mechanical signal is further transformed into an electric signal and "read" by the brain as a sound.

How dynamic speakers work

An audio speaker works on similar principles, only in the reverse way. A magnetic or digital deck reproduces the audio information from the recorded support through electric signals. The speakers translate these signals into audio waves. If everything in the audio system works well, what we hear is a sound that is identical with the original, before it was recorded.

How speakers are made

• the voice coil - the electromagnet, usually made of a coil of wire that is wrapped around a piece of metal of high conductivity.
• the magnet - is used to produce a steady, non-changing magnetic field; the electro-magnet nearby, due to its alternating charge, will be either attracted to it, or repelled;
• the cone or diaphragm - this is the part that vibrates when the voice coil moves and produces the sound waves;
• the spider (lower suspension) - cloth disc that only allows the voice coil and bottom of the cone to move back and forth;
• the surround (upper suspension) - a ring that holds the top of the cone from moving to sides, and allowing it to move only back and forth. Together with the spider it forms the suspension system for the cone and voice coil;
• the dust cap - a cover glued to the cone;
• the frame or basket - a carcase that holds all the parts together.

Note: Also see the attachment called "Inside The Speaker".

Basically, most of the work is done by the combination of magnet and voice coil (the voice coil functions as an electro-magnet when the speaker is on).

When the electrical current flowing through the voice coil changes direction, the coil's polar orientation reverses. The proximity with the magnet causes the voice coil to move up and down, according to the polar orientation of the two magnetic fields. When they charge identically, they reject each other. When you reverse the flow of electricity, they attract.

The moving voice coil is connected to the cone (or diaphragm) and causes it to vibrate. The alternating current that passes through the voice coil is characterized by frequency and amplitude.

Frequency and amplitude are, theoretically, identically restored by the sound wave.

When referring to sound, frequency gives the pitch of the sound wave, and amplitude the volume (how loud the sound is).

Simply put, this is how the electric impulses are transformed into sound waves.

But the frequency-spectrum is quite large, and it can't be reproduced by only one speaker. Further we'll see how speakers divide the frequency ranges among them.

Separating frequencies

There are speakers that reproduce better high-frequency waves, and there are speakers that reproduce low-frequency waves or mid-range frequencies. That's the principle that classifies speakers into three categories:

• Woofers - are the biggest drivers, that can produce low frequency sounds;
• Tweeters - the smallest units designed to produce the highest frequencies;
• Midrange - reproduces frequencies in the middle of the spectrum.

Before the sound waves get to the drivers they are sorted by a particular device called crossover network (see image). A crossover network is made of, among other things, inductors and capacitors, that separate the sound waves into low- and high- frequency waves. Usually, all the drivers and the crossover network are contained in the same enclosure.

The crossover network is usually placed after the amplifier in the audio system, because this is the handiest and least expensive location. This is also called a passive crossover because it does not require an external power supply. There is also the alternative of using an active crossover that would be placed before the amplifier.

Speaker enclosures

Can you imagine what a pain it would be if you had to connect each one of these drivers to your amplifier? That's probably the basic reason why speaker enclosures are designed to hold all the drivers together. Enclosures are built of wood, metal or other solid material meant to absorb the vibrations that the drivers produce. But what makes the enclosures differ is the way the sound waves travel in and out of the box. On this criterion, here's how they classify:

• Acoustic Suspension Enclosure or sealed enclosure - the boxes are completely sealed and no air can escape. The front wave travels out of the box and the back wave travels only inside the box. These enclosures are not as efficient as other designs.
• Bass Reflex Enclosure - by making a vent or a port into the speaker, the backward wave is redirected outward and its pressure is used to supplement the power of the front wave.
• Dipole Passive Radiator Enclosure - here, the backward wave does not escape the port anymore, but is used to move an additional, passive driver. Some dipole enclosures have the active driver facing one way and the passive driver facing the opposite way, and the sound is diffused in all directions. This design is sometimes used for rear channels in a home theater system.

In a subsequent chapter we'll see how the enclosures classify due to their size and function in: floorstanding, bookshelf, sub-woofer, in-wall etc.

Amplifier: why we need it

Most devices that use a speaker to produce sound use amplifiers. They are needed because the electrical signal transmitted through wires to the driver is very thin, and therefore can't produce large vibrations to the driver's diaphragm. In order to enhance the vibrations so we can hear the sound, we need to input more power into the system. An amplifier only supplies power. Sometimes, amplifiers that move sub-woofers with ten pound magnets (or larger) need to generate 300, 400, or even 1,000 watts of power.

Alternative speakers: electrostatic, planar-magnetic, horns

Dynamic speakers are the most commonly used, but this doesn't mean there are no alternatives to those. Electrostatic, planar-magnetic and horn speakers were developed for this purpose.

Electrostatic speakers

These speakers are made of a large, thin diaphragm placed in an electrostatic field generated between two metal panels. Electrostatic speakers have a few advantages over a dynamic design (lower mass diaphragm, no need for a crossover); but they also have their inconveniences - e.g. they can rather reproduce mid-range and high-level frequencies. For true bass, they should be used in conjunction with a dynamic sub-woofer or should have a woofer built in.

Planar-magnetic speakers

The planar-magnetic speakers are a lot like the electrostatic speakers, but they replace the thin & wide diaphragm with a narrow metal ribbon. The ribbon is suspended between two powerful magnets instead of charged metal panels.

Because the functioning principle is so similar to electrostatic speakers, so are their advantages and shortcomings. That's why planar-magnetic drivers are also used in tweeters.

Horns

Horn speakers are a modified version of dynamic speakers. The traditional, dynamic drivers are, in fact, placed at the small end of a cone-shaped structure, that will guide and enhance the amplitude of the wave.

This is the same principle we use when we're putting hands to the mouth to shout louder or when we play a trombone.

Speakers that use traditional dynamic drivers have typical efficiency levels of 88 to 92 dB, while horn drivers reach sensitivity ratings of 96 to 98 dB.