Depiction of a helium atom based on the Bohr model. Image courtesy of UC Davis.
An atom has a center nucleus that is made of neutral charges called neutrons, and positive charges called protons. Moving around the nucleus are negative charges called electrons.
Opposite charges attract, so electrons are attracted to the protons in the nucleus. At the same time, similar charges repel, so too many electrons in one area tend to push one or more electrons to leave.
Electrons are in constant motion around an atom.
When multiple atoms are brought near each other, the electrons can move between neighboring atoms. The electrons must follow paths that take into account their constant motion, the force of attraction to the protons, and the force of repulsion to fellow electrons. Balancing all those rules for many atoms brought together sometimes results in regular patterns or crystalline shapes.
NaCl crystals (left) and representation of atoms in a crystalline structure (right). NaCl image courtesy of NASA.
Certain materials will generate a measurable potential difference when they are made to expand or shrink in a particular direction.
Increasing or decreasing the space between the atoms by squeezing, hitting, or bending the crystal can cause the electrons to redistribute themselves and cause electrons to leave the crystal, or create room for electrons to enter the crystal. A physical force on the crystal creates the electromotive force that moves charges around a circuit.
The opposite is true as well: Applying an electric field to a piezoelectric crystal leads to the addition or removal of electrons, and this in turn causes the crystal to deform and thereby generate a small physical force.
Representation of a compressed (left) and stretched (right) crystalline structure.
How Piezoelectric Speakers Move
The piezoelectric effect can be employed in the construction of thin-form-factor speakers that are valuable alternatives to traditional electrodynamic speakers in space-constrained applications. These devices are referred to as both piezo speakers and ceramic speakers.
Apply an electric field to a piezoelectric material and it will change size. The piezoelectric material will shrink or grow as charges are introduced or removed, but the base material will not.
This causes elastic deformation of the material toward or away from a direction that is perpendicular to the surface of the speaker. As soon as the electric field is removed from the piezoelectric material, it will return to its original shape.
As the speaker flexes and strikes air molecules, it causes a chain reaction of collisions that eventually reaches your ear. If enough air molecules strike your ear, the nerve cells send a signal to your brain that you interpret as sound.
How Disturbances Travel
An unimaginable number of atoms and molecules surround us and are in constant motion. These particles move in straight lines until they hit other atoms and their direction changes. A single particle will never move far before a collision, but the effects of the collision can travel great distances as new particles collide with their neighbors.
Imagine adding a single drop of food coloring to the center of a swimming pool. The particles of food coloring might take minutes or hours to reach the edge, but the waves generated by the drop would be at the pools edge in seconds.
Air particles strike our bodies constantly and randomly all the time. When the collisions stop being less constant and less random, and start being more regular and patterned, we are hit with more particles at specific times. Certain nerve cells in our ears can detect these increased, patterned collisions and send signals to our brains, and our brain interprets the pattern as sound.
Representation of the motion of air molecules that create sound.
More on the Properties of Piezoelectric Speakers
Now that we have a basic understanding of how piezoelectric speakers create sound, we can dive a bit deeper into how they work. But we're only going to dip our toe in what is a very deep swimming pool of knowledge.
The electric field established in the piezoelectric material by a potential difference applied across a piezo speaker propagates effectively instantly, which means that the entire material experiences an instant force and begins to flex immediately.
The amount of deflection and the time it takes to get to its final position are determined by the material properties of the chosen materials; and what movement it does make, it doesn't make instantly. A change in any of a number of different factors will affect how the piezoelectric speaker will respond when a potential difference is established across the inputs, and those same properties will determine what electromotive force is generated when it returns to its rest position.
In short—applying a potential difference across the terminals of a piezoelectric speaker will cause it to move and create sound. When the potential difference is removed, the piezoelectric material will return to its rest position and create a potential difference across the terminals in the opposite direction.
In this manner, as a first order approximation, a piezoelectric speaker can be considered to be a capacitor for amplification and driving purposes.
The TPA2100 from Texas Instruments is a class-D audio amplifier that can be used to drive piezoelectric speakers.
Piezoelectric vs. Electrodynamic Speakers
Piezoelectric speakers are similar to electrodynamic speakers in that they are able to convert a potential difference into motion. But they have other characteristics that make them valuable in modern electronics.
Traditional electrodynamic speakers are made by passing electrical current through coils of wire in the presence of a magnetic field. The resulting attractive or repulsive force between the wire and magnet are then used to move a speaker cone that creates the disturbances in the air. Magnets, wires, and cones have practical physical limitations on their thinness before they are no longer effective at delivering sound at an acceptable quality or volume. Piezoelectric speakers can be manufactured to be extremely thin and still have acceptable sound characteristics.
The nature of the wire coil used to generate magnetic fields in the electrodynamic speaker means that these speakers must be treated as inductive loads when designing driving circuits. Piezoelectric speakers are treated more as capacitors when designing driving circuits.
Piezoelectric speakers are more environmentally robust than electrodynamic speakers and can function in direct contact with substances other than air; this property is useful, for example, in underwater applications. Traditional electrodynamic speakers must be protected from the elements.
Compared to their electrodynamic counterparts, piezoelectric speakers require higher drive voltages, and their low impedance at higher frequencies means that large amounts of drive current are required for high-frequency audio signals. But if a little extra voltage and current means we can purchase a speaker that is as thin as a playing card, I think it's a fair compromise.