How A Speaker Works: From Your iPod to Your Ears
Jason B. Morris
ENGL 202C
Section 33
Photo From: www.gadgetgrid.com
Audience and Scope
The purpose of this document is to provide the audience with a general
understanding of how a speaker works, and the physical concepts employed in
speaker design. This document will focus on the function of a general, dynamic
speaker rather than a specific product (such as an iHome). This is the most
commonly used type of speaker and almost all other types of speakers function
under the same principles.
Audio companies simply utilize these principles and tweak their speaker design so
as to have their own product line. Company ownership also makes it very difficult to
gain insight as to how a specific product is designed and functions. However, after
reading this document, the audience will understand exactly how the speakers work
in almost all of their everyday electronics such as computer speakers, headphones,
and even cell phones.
The intended audience for this document is upperclassman high school students
with an interest in the Science, Technology, Engineering and Math (STEM) fields.
This document will make clear connections between the concepts that may be
learned in STEM related courses and how you can use these concepts to create
technical products. This understanding of creating advanced technology using
fundamental concepts will give the audience a better idea of the things they can
accomplish with a career in the STEM fields.
An Introduction to Speakers
A speaker is an electro-mechanical device that converts an electrical signal (voltage
and current) to sound. The electrical signal can come from almost any electronic
device: a computer, an iPod, a television, or even a phone. Figure 1 illustrates the
role of a speaker in the creation of sound.
Electrical Signal
(From iPod)
Speaker Converts Electrical
Signal to Sound
We Hear Music
Figure 1: The Role of the Speaker
(Photos from www.kpsec.freeuk.com/acdc and www.europe-audio.com)
Fundamentals of Sound
In order to understand how a speaker works, we need to discuss a few basic
concepts of sound. Sound is a wave (the displacement of a medium), much like the
type that is created in water when you throw a stone into a lake. However, instead
of traveling through water, sound waves travels through air. Sound waves differ
from most other waves in that they are longitudinal waves. This means that the
displacement of the medium is parallel to the direction that the wave is traveling.
This is unlike a transverse wave that is created when a stone is thrown in water,
where the displacement is perpendicular to the direction of wave travel. The
difference can be seen in Figure 2.
1) Longitudinal
Wave
(Sound)
2) Transverse
Wave
(Water)
Figure 2: Longitudinal and Transverse Waves
(Image from www.mediacollege.com)
For a Better Understanding:
Wave motion is much better understood through visual demonstrations. The Penn
State Acoustics Program has put together an excellent visual demonstration of the
different wave motions at:
http://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html
As you can see in Figure 2, sound waves consist of the rarefaction (spreading out)
and condensation (squishing together) of air molecules. In order to accomplish this,
we need something to “push” the air particles, which is exactly what speakers do.
Speaker Composition
Speakers are composed of multiple parts that are specifically designed and
constructed for the purpose of pushing air particles. Although there are many types
of speakers now due to advancements in materials and technology, the most
commonly used is the dynamic speaker. The dynamic speaker is characterized by its
use of a driver (a magnet and copper-coil combination), which controls a diaphragm
that “pushes” the air. All of the speaker components are broken down and explained
in figures 3 and 4.
Figure 3: External Speaker Components
(Image courtesy of www.howstuffworks.com)
1) Basket: Provides the
framework for mounting all the
speaker components
2) Suspension: Connects the
diaphragm edge to the basket
and allows for controlled
diaphragm motion
3) Diaphragm: Rigid material
that pushes the air to create
the sound wave
4) Dust Cap: Completes the seal
of the diaphragm and prevents
particles from getting inside
the speaker
Figure 4: Internal Speaker Components
(Image courtesy of www.howstuffworks.com)
5) Spider: Provides a restoring
force for the diaphragm and
centers the voice coil
6) Driver (Voice Coil/Magnet):
These two components
combine to essentially make a
motor that drives the speaker.
The magnet is stationary and
Why it Works:
The driver (voice coil/magnet combination) can be a tricky concept to grasp. It works
because of a physics concept called the Lorentz Force Law. This states that when an
electric current moves through a wire that is placed within a magnetic field, a force is
created. Therefore, when our electrical signal (current) runs through the voice coil, a
force is generated and the voice coil moves the diaphragm. The diaphragm in turn
“pushes” the air particles creating sound. Figure 5 illustrates the Lorentz Force. Note
that since our magnet is stationary, only the direction of the current dictates the
direction of the force. This means that the voice coil and diaphragm will move in exact
coordination with our electrical signal.
Figure 5: Lorentz Force (F) Explanation. L represents the length of the wire.
(Image from http://hyperphysics.phy-astr.gsu.edu)
the voice coil moves the
diaphragm.
With a basic understanding of the concepts of sound waves and the Lorentz force,
you’re already equipped with all the tools necessary to build speakers. These are the
critical elements of speaker design. However, with technology advancements and
innovative thinking, engineers and scientist have learned that there are quite a few
things to keep in mind when making speakers.
Size Does Matter, But That’s Not All
In general, the size of the speaker usually dictates the types of sounds that we hear
from that speaker. For example, a very large speaker will generate very low
frequencies (bass). Conversely, a very small speaker is best for high frequencies
(high pitches). However, we can compensate for this in many ways. One way is by
altering the location of the speaker. This can mean simply moving the location of the
speakers relative to each other, or even altering the distance from our ears to the
speakers altogether. For example, if we want really small speakers for mobile
applications, we can simply bring them closer to our ears and we will get more
frequency content (a wider range of pitches). This is what allows us to have
earphones and cell phones. We make miniature speakers and stick them right in
your ears.
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Figure 6: Earphones Are Speakers in Your Ears!
(Image courtesy of www.shopmedacou-stics.com)
Another compensation method is by the use of software. With the miniaturization of
circuit elements entering the nano-scale, we can now have entire Integrated Circuit
(IC) chips that are so small, they fit in our ears. These ICs are programmable through
computer software. What this means is that we can make our speakers act almost
like a computer and add missing elements. In other words, if a song doesn’t have too
much bass, we can program the speaker to add more. Speakers can be programmed
to remove sound as well. This can be used in loud environments such as busses or
airplanes, where the speakers will remove the excess noise and you will only hear
the sound of your music. Speakers that add or remove content are called active
speakers. We can have active speakers for an entire range of frequencies, which
allows us to make high quality speakers of all shapes and sizes.
Conclusion
Speakers are designed to take an electrical signal and convert it to sound by pushing
air particles. They do this by using a driver that consists of a moving copper coil
(voice coil) and a stationary magnet. By utilizing the Lorentz Force, the coil moves in
exact coordination with the signal, and pushes a rigid diaphragm accordingly. This
diaphragm physically pushes the air and the result is audible sound. Speakers can be
found in many of our electronics today and the vast majority of them utilize these
same principles.
References
Most of the knowledge in this paper has come from my years of involvement with
the Audio Engineering Society (AES).
Other references are below:
http://electronics.howstuffworks.com/speaker.htm
http://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html
http://www.kpsec.freeuk.com/acdc.htm
http://hyperphysics.phy-astr.gsu.edu/hbase/audio/spk.html