Loudspeakers
Last updated: August 22, 2009.
How many times every day do you hear
recorded music on the radios 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 earbud headphones.
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!
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, or
paper 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 iron coil (orange) that sits just in front of a permanent magnet (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.
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.
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: 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.
How to improve the quality of sound that speakers make
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 travelling 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 cancelling 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 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 blast from the past: This wonderfully preserved Amplion speaker was produced in 1928. It's an exhibit at Think Tank, the science museum in Birmingham, England.
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 and there is no sense of sound moving
around.
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 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.
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
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!
What about headphones?
Essentially, they're just loudspeakers in miniature.
To see what headphones look like when you take them apart,
take a look at our separate article on headphones.
