How does a speaker work?

Note

As we already learned, sound is created when there are changes in air pressure (or sound pressure).

These changes happen in specific pattern, created by vibrating object, or object moves in specific patter (oscillator).

To create sound, speakers have to move in some ways in order to create sound as it is the rule.

Which part of speaker move? How does it move? We are going to find out!

Prediction

Simulation of a speaker: bottle speaker!

Procedure

Prepare an empty bottle without a lid (open bottle).

Prepare a stack of strong magnet.

Turn an empty bottle upside down and let it cover a stack of strong magnets. Make sure magnets stand at the center of bottleneck.

Coil a copper wire around bottleneck. Then connect it to a speaker amplifier, which is currently connected to audio source (music player).

Play pure tone on music player and see what happen to the bottle.

See the picture on the right to have an overview on this setup

Figure 1. Picture showing an overview of setup (by Hung Vu)

What will happen?

The two videos below are how the setup look like in reality. Watch the videos and see what happen to the bottle when a tone is play. These two videos are from two different runs of this setup. Try not to scroll down before completing the video.

Video 1. Motion of bottle speaker (by Hung Vu)

Video 1.1. Motion of bottle speaker (by Hung Vu)

Phenomenon

The bottle vibrates.
Very small sound is created from this vibration.

Explanation

Since the bottle start vibrating (moving), there must be a force applied on it.

This force is created from the reaction between magnetic field from magnets inside bottle and current which passes through copper wire around bottleneck when a tone is play on laptop.

Audio signal from laptop in the form of alternating current will be amplified by the amplifier and going to the wire.

The current in wire will be an alternating current, which constantly changes its direction.

Magnetic field in this case will go from the magnets to the outside, with direction perpendicular to surface of bottleneck ( and wire ).

Because this force is caused by reaction between changes in current and magnetic field, it is called electromagnetic force, and the force created by this process is based on right hand rule. In fact, the force is applied on the coil. However, because the bottle is attached to wire, the bottle moves along with the coil too.

Ideally, the bottle should moving up and down instead of vibrating according the forces applied on it. However, in this setup, the wire is not properly coiled and the force cannot is not enough to make wire and bottle move upward (too heavy), so it vibrates instead.

Watch the video on the right to see explanation and demonstration on electromagnetic force and right hand rule.

Video 2. Demonstration on electromagnetic force and right hand rule (by James Dann - Youtube channel: James Dann)

Figure 2. Motion of bottle when current is into to the page on the left, out of page on the right (by Hung Vu)

Figure 2.1. Motion of bottle when current is into to the page on the right, out of page on the left (by Hung Vu)

Motion of speaker cone

How does a speaker produce sound

Figure 3 : Detail structure of a speaker driver (by Jacob O'Neal - Animagraffs)

Video 3. Reaction of voice coil when audio signal is fed into it (by Jacob O'Neal - Animagraffs)

Although a speaker driver contains many small parts inside it, we only need to focus on speaker cone and voice coil (copper wire in our speaker simulation).

From the simulation, we know that a bottle moved duo to the force applied on it by copper wire.

Similarly, in a speaker, speaker cone (bottle in our case) is connected to voice coil.

Since a voice coil will move in a specific pattern based on audio signal it receives. This motion of moving back and forth is simply the same to the vibration of tuning fork, which will then create region of high and low air pressure and move air molecules turn by turn. As a result, sound is created. This is how a speaker produce sound.

Video 4. Simulation on motion of air molecules created by motion of speaker driver (by University of Colorado Boulder - PhET simulation: Wave Inteference)

Summary

To create sound, it is necessary to create a repeated impact on the surrounding air molecules. In a speaker, the speaker cone will do that job. It is connected to the voice coil, and the voice coil surrounded a magnet. When an audio signal is transmitted into the voice coil (in the form of alternating current). It will react with the magnetic field generated from the magnet and thereby create repulsive force on the voice coil and move it. All of this follow the right hand rule. As mentioned, since the signal is transmitted in the form of alternating current, there will be a time when the speaker cone will move inward or outward, which is equivalent to tuning a fork oscillation. And this is how speaker creates sound.

Forces and Energy transformation in the process

Assume that there is no energy lost in the process.

Potential energy is relative. Since the polarity of current is constantly switching so we can assume electric energy is always positive with its respective polarity.

We can divide this process into 2 smaller stages.

The first stage is when all initial electrical energy from audio signal (U_e_i) , in the form of alternating current transfer to kinetic energy (K_f) for voice coil motion. Speaker cone will move along with voice coil as a result, so we can consider voice coil and speaker cone as an uniform object.

The second stage is when all initial kinetic energy from speaker cone (K_i) transfer into sound energy (U_sound).

When ever audio signal is fed into speaker driver, electromagnetic force will appear.

Basically, there is only 1 kind of force (electromagnetic force), but it appears in 2 opposite directions duo to the change in direction of current. We will call the force in F_push when it is in +x direction and F_pull when it is in -x direction.

Although, there are many different configurations for voice coil and magnet in speaker drivers, so the directions of current and magnetic field will vary. Ultimately, what changes the direction of electromagnetic force in these speaker is audio signal since magnetic field has permanent direction, and their purpose is to create F_push and F_pull on x-axis for speaker cone.

Figure 3. Free body diagram for speaker cone movement