In any sound system, ultimate quality depends on the
speakers. The best recording, encoded on the most advanced
storage device and played by a top-of-the-line deck and
amplifier, will sound awful if the system is hooked up to poor
speakers. A system's speaker is the component that takes the
electronic signal stored on things like CDs, tapes and
turns it back into actual sound that we can hear.
A small speaker set for computer
In this edition of HowStuffWorks,
we'll find out exactly how speakers do this. We'll also look
at how speaker designs differ, and see how these differences
affect sound quality. Speakers are amazing pieces of
technology that have had a profound impact on our culture. But
at their heart, they are remarkably simple devices.
Sound Basics To understand how speakers
work, you first need to understand how sound works.
Inside your ear is a very thin piece of skin called the
eardrum. When your eardrum vibrates, your brain
interprets the vibrations as sound -- that's how you hear.
Rapid changes in air pressure are the most common thing to
vibrate your eardrum.
An object produces sound when it vibrates in air (sound can
also travel through liquids and solids, but air is the
transmission medium when we listen to speakers). When
something vibrates, it moves the air particles around it.
Those air particles in turn move the air particles around
them, carrying the pulse of the vibration through the air as a
To see how this works, let's look at a simple vibrating
object -- a bell. When you ring a bell, the metal vibrates --
flexes in and out -- rapidly. When it flexes out on one side,
it pushes out on the surrounding air particles on that side.
These air particles then collide with the particles in front
of them, which collide with the particles in front of them and
so on. When the bell flexes away, it pulls in on these
surrounding air particles, creating a drop in pressure that
pulls in on more surrounding air particles, which creates
another drop in pressure that pulls in particles that are even
farther out and so on. This decreasing of pressure is called
In this way, a vibrating object sends a wave of
pressure fluctuation through the atmosphere. When the
fluctuation wave reaches your ear, it vibrates the eardrum
back and forth. Our brain interprets this motion as sound. We
hear different sounds from different vibrating objects because
of variations in:
Sound-wave frequency - A higher wave frequency
simply means that the air pressure fluctuates faster. We
hear this as a higher pitch. When there are fewer
fluctuations in a period of time, the pitch is lower.
Air-pressure level - This is the wave's
amplitude, which determines how loud the sound is. Sound
waves with greater amplitudes move our ear drums more, and
we register this sensation as a higher volume.
works something like our ears. It has a diaphragm that
is vibrated by sound waves in an area. The signal from a
microphone gets encoded on a tape or
CD as an
electrical signal. When you play this signal back on your
stereo, the amplifier sends it to the speaker, which
re-interprets it into physical vibrations. Good speakers are
optimized to produce extremely accurate fluctuations in air
pressure, just like the ones originally picked up by the
microphone. In the next section, we'll see how the speaker
Making Sound In the last section, we saw
that sound travels in waves of air pressure fluctuation, and
that we hear sounds differently depending on the frequency and
amplitude of these waves. We also learned that microphones
translate sound waves into electrical signals, which can be
encoded onto CDs, tapes, LPs, etc. Players convert this stored
information back into an electric current for use in the
A speaker is essentially the final translation machine --
the reverse of the microphone.
It takes the electrical signal and translates it back into
physical vibrations to create sound waves. When everything is
working as it should, the speaker produces nearly the same
vibrations that the microphone originally recorded and encoded
on a tape, CD, LP, etc.
Traditional speakers do this with one or more
drivers. A driver produces sound waves by rapidly
vibrating a flexible cone, or diaphragm.
The cone, usually made of paper, plastic or
metal, is attached on the wide end to the suspension.
The suspension, or surround, is a rim of
flexible material that allows the cone to move, and is
attached to the driver's metal frame, called the
The narrow end of the cone is connected to the voice
The coil is attached to the basket by the spider,
a ring of flexible material. The spider holds the coil in
position, but allows it to move freely back and forth.
Some drivers have a dome instead of a cone. A
dome is just a diaphragm that extends out instead of tapering
A typical speaker driver, with a metal
basket, heavy permanent magnet and paper
The voice coil is a basic electromagnet.
If you've read How
Electromagnets Work, then you know that an electromagnet
is a coil of wire, usually wrapped around a piece of magnetic
metal, such as iron. Running
electrical current through the wire creates a magnetic field
around the coil, magnetizing the metal it is wrapped around.
The field acts just like the magnetic field around a permanent
magnet: It has a polar orientation -- a "north" end and and a
"south" end -- and it is attracted to iron objects. But unlike
a permanent magnet, in an electromagnet you can alter the
orientation of the poles. If you reverse the flow of the
current, the north and south ends of the electromagnet switch.
This is exactly what a stereo signal does -- it constantly
reverses the flow of electricity. If you've ever hooked
up a stereo system, then you know that there are two output
wires for each speaker -- typically a black one and a red one.
The wire that runs through the speaker system
connects to two hook-up jacks on the
Essentially, the amplifier
is constantly switching the electrical signal, fluctuating
between a positive charge and a negative charge on the red
wire. Since electrons always flow in the same direction
between positively charged particles and negatively charged
particles, the current going through the speaker moves one way
and then reverses and flows the other way. This alternating
current causes the polar orientation of the electromagnet
to reverse itself many times a second.
So how does this fluctuation make the speaker coil move
back and forth? The electromagnet is positioned in a constant
magnetic field created by a permanent magnet. These two
magnets -- the electromagnet and the permanent magnet --
interact with each other as any two magnets do. The positive
end of the electromagnet is attracted to the negative pole of
the permanent magnetic field, and the negative pole of the
electromagnet is repelled by the permanent magnet's negative
pole. When the electromagnet's polar orientation switches, so
does the direction of repulsion and attraction. In this way,
the alternating current constantly reverses the magnetic
forces between the voice coil and the permanent magnet. This
pushes the coil back and forth rapidly, like a piston.
When the electrical current
flowing through the voice coil changes direction, the coil's
polar orientation reverses. This changes the magnetic forces
between the voice coil and the permanent magnet, moving the
coil and attached diaphragm back and
When the coil moves, it pushes and pulls on the speaker
cone. This vibrates the air in front of the speaker, creating
sound waves. The electrical audio signal can also be
interpreted as a wave. The frequency and amplitude of
this wave, which represents the original sound wave, dictates
the rate and distance that the voice coil moves. This, in
turn, determines the frequency and amplitude of the sound
waves produced by the diaphragm.
Different driver sizes are better suited for certain
frequency ranges. For this reason, loudspeaker units typically
divide a wide frequency range among multiple drivers.
In the next section, we'll find out how speakers divide up the
frequency range, and we'll look at the main driver types used
Chunks of the Frequency Range In the last
section, we saw that traditional speakers produce sound by
pushing and pulling an electromagnet attached to a flexible
cone. Although drivers are all based on the same concept,
there is a wide range in driver size and power. The basic
driver types are:
Woofers are the biggest drivers, and are designed to
produce low frequency sounds. Tweeters are much smaller
units, designed to produce the highest frequencies.
Midrange speakers produce a range of frequencies in the
middle of the sound spectrum.
And if you think about it, this makes perfect sense. To
create higher frequency waves -- waves in which the points of
high pressure and low pressure are closer together -- the
driver diaphragm must vibrate more quickly. This is harder to
do with a large cone because of the mass of the cone.
Conversely, it's harder to get a small driver to vibrate
slowly enough to produce very low frequency sounds. It's more
suited to rapid movement.
To produce quality sound over a wide frequency range more
effectively, you can break the entire range into smaller
chunks that are handled by specialized drivers. Quality
loudspeakers will typically have a woofer, a tweeter and
sometimes a midrange driver, all included in one
Of course, to dedicate each driver to a particular
frequency range, the speaker system first needs to break the
audio signal into different pieces -- low frequency, high
frequency and sometimes mid-range frequencies. This is the job
of the speaker crossover.
The most common type of crossover is passive,
meaning it doesn't need an external power source because it is
activated by the audio signal passing through it. This sort of
crossover uses inductors,
and sometimes other circuitry components. Capacitors and
inductors only become good conductors under certain
conditions. A crossover capacitor will conduct the current
very well when the frequency exceeds a certain level, but will
conduct poorly when the frequency is below that level. A
crossover inductor acts in the reverse manner -- it is only a
good conductor when the frequency is below a certain level.
The typical crossover unit from a
loudspeaker: The frequency is divided up by inductors
and capacitors and then sent on to the woofer, tweeter
When the electrical audio signal travels through the
speaker wire to the speaker, it passes through the crossover
units for each driver. To flow to the tweeter, the current
will have to pass through a capacitor. So for the most part,
the high frequency part of the signal will flow on to the
tweeter voice coil. To flow to the woofer, the current passes
through an inductor, so the driver will mainly respond to low
frequencies. A crossover for the mid-range driver will conduct
the current through a capacitor and an inductor, to set an
upper and lower cutoff point.
There are also active crossovers. Active crossovers
are electronic devices that pick out the different frequency
ranges in an audio signal before it goes on to the amplifier
(you use an amplifier circuit for each driver). They have
several advantages over passive crossovers, the main one being
that you can easily adjust the frequency ranges. Passive
crossover ranges are determined by the individual circuitry
components -- to change them, you need to install new
capacitors and inductors. Active crossovers aren't as widely
used as passive crossovers, however, because the equipment is
much more expensive and you need multiple amplifier outputs
for your speakers.
Crossovers and drivers can be installed as separate
components in a sound system, but most people end up buying
speaker units that house the crossover and multiple drivers in
one box. In the next section, we'll find out what these
speaker enclosures do and how they affect the speaker's
Boxes of Sound In most loudspeaker systems,
the drivers and the crossover are housed in some sort of
speaker enclosure. These enclosures serve a number of
functions. On their most basic level, they make it much easier
to set up the speakers. Everything's in one unit and the
drivers are kept in the right position, so they work together
to produce the best sound. Enclosures are usually built with
heavy wood or another solid material that will effectively
absorb the driver's vibration. If you simply placed a driver
on a table, the table would vibrate so much it would drown out
a lot of the speaker's sound.
Additionally, the speaker enclosure affects how sound is
produced. When we looked at speaker drivers, we focused on how
the vibrating diaphragm emitted sound waves in front of the
cone. But, since the diaphragm is moving back and forth, it's
actually producing sound waves behind the cone as well.
Different enclosure types have different ways of handling
these "backward" waves.
A typical sealed speaker enclosure that holds
a tweeter, a woofer and a midrange
The most common type of enclosure is the sealed
enclosure, also called acoustic suspension
enclosure. These enclosures are completely sealed, so no
air can escape. This means the forward wave travels outward
into the room, while the backward wave travels only into the
box. Of course, since no air can escape, the internal air
pressure is constantly changing -- when the driver moves in,
the pressure is increased and when the driver moves out, it is
decreased. Both movements create pressure differences between
the air inside the box and the air outside the box. The air
will always move to equalize pressure levels, so the driver is
constantly being pushed toward its "resting" state -- the
position at which internal and external air pressure are the
In a sealed speaker setup, the driver
diaphragm compresses air in the enclosure when it moves
in and rarefies air when it moves out.
These enclosures are less efficient than other designs
because the amplifier has to boost the electrical signal to
overcome the force of air pressure. The force serves a
valuable function, however -- it acts like a spring to keep
the driver in the right position. This makes for tighter, more
precise sound production.
Other enclosure designs redirect the inward pressure
outward, using it to supplement the forward sound wave. The
most common way to do this is to build a small port
into the speaker. In these bass reflex speakers, the
backward motion of the diaphragm pushes sound waves out of the
port, boosting the overall sound level. The main advantage of
bass reflex enclosures is efficiency. The power moving the
driver is used to emit two sound waves rather than one. The
disadvantage is that there is no air pressure difference to
spring the driver back into place, so the sound production is
not as precise.
A bass reflex speaker produces two sound
waves by moving one driver. When the driver compresses
air forward, it rarefies it backward, and vice versa.
The second sound wave is emitted from a port at the base
of the speaker
Passive radiator enclosures are very similar to bass
reflex units, but in passive radiator enclosures, the backward
wave moves an additional, passive driver, instead of
escaping out of the port. The passive driver is just like the
main, active drivers except it doesn't have an
electromagnet voice coil, and it isn't connected to the
amplifier. It is moved only by the sound waves coming from the
active drivers. This type of enclosure is more efficient than
sealed designs and more precise than bass reflex models.
Some enclosure designs have an active driver facing one way
and a passive driver facing the other way. This dipole
design diffuses the sound in all directions, making it a good
choice for the rear channels in a home theater system.
The backward air compression and rarefaction
caused by the active driver push and pull on the passive
driver. A speaker with a dipole design emits sound waves
These are just a few of the many enclosure types available.
There are a huge range of speaker units on the market, with a
variety of unique structures and driver arrangements. Check
page to learn about some of these designs.
Alternative Speaker Designs Most
loudspeakers produce sound with traditional drivers. But there
are a few other technologies on the market. These designs have
some advantages over traditional dynamic speakers, but
they fall short in other areas. For this reason, they are
often used in conjunction with driver units.
The most popular alternative is the electrostatic
speaker. These speakers vibrate air with a large, thin,
conductive diaphragm panel. This diaphragm panel is suspended
between two stationary conductive panels that are charged with
electrical current from a wall outlet. These panels create an
electrical field with a positive end and a negative end. The
audio signal runs a current through the suspended panel,
rapidly switching between a positive charge and a negative
charge. When the charge is positive, the panel is drawn toward
the negative end of the field, and when the charge is
negative, it moves toward the positive end in the field.
The diaphragm is alternately charged with a
positive current and a negative current, based on the
varying electrical audio signal. When the diaphragm is
positively charged, it fluctuates toward the front
plate, and when it is negatively charged it fluctuates
toward the rear plate. In this way, it precisely
reproduces the recorded pattern of air
In this way, the diaphragm rapidly vibrates the air in
front of it. Because the panel has such a low mass, it
responds very quickly and precisely to changes in the audio
signal. This makes for clear, extremely accurate sound
reproduction. The panel doesn't move a great distance,
however, so it is not very effective at producing lower
frequency sounds. For this reason, electrostatic speakers are
often paired with a woofer that boosts the low frequency
range. The other problem with electrostatic speakers is that
they must be plugged into the wall and so are more difficult
to place in a room.
Another alternative is the planar magnetic speaker.
These units use a long, metal ribbon suspended between
two magnetic panels. They basically work the same way as
electrostatic speakers, except that the alternating positive
and negative current moves the diaphragm in a magnetic field
rather than an electric field. Like electrostatic speakers,
they produce high-frequency sound with extraordinary
precision, but low frequency sounds are less defined. For this
reason, the planar magnetic speaker is usually used only as a
Both of these designs are becoming more popular with audio
enthusiasts, but traditional dynamic drivers are still the
most prevalent technology, far and away. You'll find them
everywhere you go -- not only in stereo setups, but in alarm
clocks, public address systems, televisions, computers,
headphones and tons of other devices. It's amazing how such a
simple concept has revolutionized the modern world!
For more information, check out the links on the next