Piezoelectric Microphone

Part of Telephony

A microphone that generates voltage through mechanical deformation of a piezoelectric crystal or ceramic element.

Why This Matters

Piezoelectric microphones generate their own voltage when deformed — no battery bias is required. A thin crystal (originally Rochelle salt or quartz, later barium titanate ceramic) generates tens to hundreds of millivolts when flexed by sound pressure. This self-generating property makes piezoelectric microphones simpler than carbon types (no bias circuit) and capable of very high output levels compared to dynamic types.

Crystal microphones were widely used in consumer telephones and communications equipment from the 1930s through the 1970s. Their high output voltage (100-200 mV at normal voice levels) drove vacuum tube amplifier grids directly without preamplification, simplifying circuit design. The tradeoff was poor performance at temperature extremes and high humidity, which limits their use in outdoor or industrial environments.

Understanding piezoelectric transduction opens access to a range of sensors beyond microphones: pressure sensors, vibration detectors, ultrasonic transducers, and accelerometers all use the same physical principle.

Piezoelectric Effect

The piezoelectric effect is a property of certain crystals and ceramics: when mechanically stressed, they develop an electric charge on their surfaces. Conversely, when voltage is applied, they deform mechanically. Both effects arise from the same microscopic cause: in a non-centrosymmetric crystal structure, mechanical deformation shifts positive and negative ion charges relative to each other, creating a net electric dipole in each unit cell. The sum of all these dipoles produces a measurable surface charge.

Natural piezoelectric materials include quartz (silicon dioxide), Rochelle salt (potassium sodium tartrate), and tourmaline. Rochelle salt has the highest piezoelectric coefficients of any natural material — it generates far more voltage per unit stress than quartz — but it dissolves in water above 55°C and loses its piezoelectric properties above 45°C.

Synthetic piezoelectric ceramics (barium titanate, lead zirconate titanate — PZT) are stronger, more water-resistant, and can be fabricated in any shape. They are produced by forming the ceramic in a strong electric field at elevated temperature (poling) to align the microscopic crystal domains in a preferred direction. Once poled, a PZT disk maintains its piezoelectric properties indefinitely unless heated above the Curie temperature (~120°C for barium titanate, ~350°C for PZT).

Crystal Microphone Construction

The classic crystal microphone uses a thin slice of Rochelle salt crystal cut along the optimal crystallographic plane to maximize the voltage output for bending stress. The slice is clamped between two metal foil electrodes and attached to a thin diaphragm.

When sound pressure flexes the diaphragm, it bends the crystal slice. Bending generates a charge at the foil electrodes proportional to the curvature. This charge appears as a voltage across the high-impedance electrodes — typically 50-500 mV at normal voice levels.

The critical requirement is maintaining dry conditions. Rochelle salt absorbs moisture from humid air and eventually dissolves. Store crystal microphone cartridges in a sealed container with desiccant when not in use. Avoid breathing directly on the crystal element — exhaled air is humid enough to damage it over time. For tropical or maritime environments, barium titanate ceramic elements are far more practical.

Barium Titanate Ceramic Elements

Ceramic piezoelectric elements are formed by pressing barium titanate powder into disks or other shapes, sintering at 1,300-1,400°C, and poling in a 3-10 kV/cm electric field at 100-120°C. The poled disk behaves piezoelectrically: pressure on the flat faces generates voltage; voltage applied to the faces creates thickness changes.

For use as a microphone diaphragm, bond the ceramic disk to a thin metal plate (brass, aluminum, or steel, about 0.2-0.4 mm thick). This bimorph or unimorph structure bends when the ceramic expands or contracts in response to voltage — or generates voltage when the assembly is forced to bend by sound pressure.

Mount the bimorph by clamping its rim in a housing with a sound port facing the sound field. The resonant frequency of the bimorph depends on its diameter, thickness, and boundary conditions. For a telephone microphone, the bimorph resonance should be above 3,400 Hz so it operates in its well-behaved piston range across the voice band.

Electrical Characteristics

A piezoelectric element is electrically equivalent to a capacitor (typically 1-50 nF for telephone-sized elements) in series with a voltage source. The high capacitive impedance means that a high-impedance load is required to avoid signal loading. At 1,000 Hz, a 10 nF element has impedance of 1/(2π × 1000 × 10×10⁻⁹) = 16,000 ohms. Connecting this to a 600-ohm telephone line directly would severely attenuate the signal.

Solutions: a transformer steps up the impedance from 600 ohms to match the crystal’s 10,000-50,000 ohm source impedance, or a vacuum tube or FET amplifier with very high input impedance buffers the crystal output. Historically, the vacuum tube’s grid input impedance (megaohms) matched the crystal source impedance well — one reason crystal microphones were the standard for tube-era consumer electronics.

For a telephone instrument using transistor amplifiers, use a JFET source follower as the first stage — it presents hundreds of megaohms input impedance to the crystal element and drives the following 600-ohm transformer with low output impedance.

Humidity Sensitivity Testing

Before deploying a crystal (Rochelle salt) microphone system, test sensitivity to ambient humidity. Measure the output voltage at a standard sound level in dry conditions. Then expose the microphone to high humidity (breathe on it, or place it in a humid environment for one hour). Re-measure output voltage. A drop exceeding 6 dB indicates the crystal is absorbing moisture and sensitivity is degrading.

If humidity sensitivity is a problem, replace Rochelle salt elements with barium titanate ceramic. BaTiO₃ is essentially immune to humidity and performs identically wet or dry. The output voltage is somewhat lower (40-100 mV vs. 100-200 mV for Rochelle salt) but the reliability improvement in real-world conditions is worth the tradeoff.