Electronics Guide

Wave Properties

Acoustic waves in fluids (gases and liquids) are longitudinal waves, with particle motion parallel to the direction of wave propagation. In solids, both longitudinal and transverse (shear) waves can exist, and additional wave types occur at surfaces and interfaces. The velocity of sound depends on the medium's elastic properties and density, approximately 343 m/s in air at room temperature and 1500 m/s in water.

Wavelength, frequency, and velocity are related by the fundamental wave equation: wavelength equals velocity divided by frequency. At audible frequencies (20 Hz to 20 kHz), wavelengths in air range from about 17 meters to 17 millimeters. Ultrasonic frequencies, above human hearing, have correspondingly shorter wavelengths that enable fine spatial resolution in imaging and sensing applications.

Acoustic impedance, defined as the product of density and sound velocity, characterizes how a medium responds to acoustic waves. Impedance mismatches at interfaces cause reflection, with the reflection coefficient depending on the impedance ratio. Matching acoustic impedances maximizes energy transfer between media, an important consideration in transducer design.

Frequency Ranges and Applications

The acoustic frequency spectrum spans from infrasound (below 20 Hz) through the audible range to ultrasound (above 20 kHz). Each frequency range has characteristic applications determined by wavelength, propagation characteristics, and interaction with materials.

Audio frequencies (20 Hz to 20 kHz) encompass human hearing and are central to communication, music, and entertainment technologies. Microphones, speakers, and audio processing systems operate in this range. Acoustic properties in this range determine how spaces sound and how audio systems perform.

Ultrasonic frequencies (20 kHz to several hundred MHz) enable applications impossible in the audio range. Lower ultrasonic frequencies (20-100 kHz) are used for industrial cleaning, welding, and distance sensing. Medical ultrasound typically operates from 1-20 MHz, providing the wavelengths appropriate for imaging human tissue. Higher frequencies (above 100 MHz) find application in acoustic microscopy and surface acoustic wave devices.

Propagation and Attenuation

Acoustic waves lose energy as they propagate through absorption in the medium and spreading of the wave front. Absorption increases with frequency, limiting the practical range of high-frequency ultrasound. In air, attenuation at ultrasonic frequencies severely limits range, while water and biological tissues allow much longer propagation distances at diagnostic imaging frequencies.

Reflection, refraction, and diffraction affect acoustic wave propagation similarly to electromagnetic waves. Reflection from impedance discontinuities is the basis for ultrasonic imaging and flaw detection. Refraction at interfaces bends waves toward regions of lower velocity. Diffraction around obstacles limits resolution to dimensions comparable to the wavelength.

Piezoelectric Transducers

Piezoelectric materials generate electric charge when mechanically stressed and conversely deform when electric fields are applied. This bidirectional conversion makes piezoelectric materials ideal for both sensing and actuation. Common piezoelectric materials include quartz crystals, lead zirconate titanate (PZT) ceramics, and polymers such as PVDF.

Piezoelectric transducers dominate ultrasonic applications due to their efficiency, bandwidth, and ability to operate at high frequencies. Transducer design involves selecting materials, determining dimensions for desired frequency response, and providing acoustic matching layers to optimize energy transfer. Arrays of piezoelectric elements enable beam steering and focusing in imaging applications.

The piezoelectric effect also finds application in non-acoustic devices including crystal oscillators for timing, piezoelectric actuators for precise positioning, and energy harvesting from mechanical vibration. Understanding piezoelectric physics supports design across these diverse applications.

Electrodynamic Transducers

Electrodynamic transducers use the interaction between current-carrying conductors and magnetic fields to produce mechanical motion, or conversely generate voltage from motion. The moving-coil loudspeaker, where current through a voice coil in a magnetic field produces cone motion, is the most common example. Dynamic microphones use the same principle in reverse.

Electrodynamic transducers excel at audio frequencies where their large excursions can move substantial air volumes. Efficiency, frequency response, and distortion depend on motor design, suspension characteristics, and cone materials. Understanding the physics of electromagnetic force generation and mechanical resonance supports speaker and microphone design.

Electrostatic Transducers

Electrostatic (capacitive) transducers use the force between charged plates or the capacitance change from plate displacement for acoustic-electric conversion. Condenser microphones use this principle, with a thin membrane forming one plate of a capacitor. Applied bias voltage creates a standing electric field, and membrane motion modulates the capacitance and generates signal voltage.

Electrostatic transducers can achieve very flat frequency response and low distortion due to the simplicity of their mechanical systems. However, they require bias voltages and typically have high output impedance requiring careful interface design. MEMS (microelectromechanical systems) microphones are miniaturized electrostatic transducers that dominate modern consumer electronics.

Magnetostrictive Transducers

Magnetostrictive materials change dimensions when magnetized, providing an alternative mechanism for acoustic transduction. While less common than piezoelectric devices, magnetostrictive transducers find application in high-power industrial ultrasound and specialized sensing applications. The physics involves coupling between magnetic and elastic properties in ferromagnetic materials.

Ultrasonic Sensing and Measurement

Ultrasonic distance measurement uses time-of-flight of acoustic pulses to determine range. A transducer emits a short burst and measures the time until echoes return from target surfaces. This principle is used in automotive parking sensors, robotic navigation, and level sensing in tanks and containers. Resolution depends on pulse duration and timing accuracy, while range is limited by acoustic attenuation and target reflectivity.

Flow measurement using ultrasonic transit time exploits the effect of fluid velocity on sound propagation speed. Upstream and downstream travel times differ by an amount proportional to flow velocity, enabling non-invasive flow measurement without obstructing the flow path. Doppler-based flow measurement detects frequency shifts from moving particles or bubbles.

Non-destructive testing (NDT) uses ultrasound to detect internal flaws in materials without damaging them. Pulse-echo techniques locate defects by their reflections, while through-transmission methods detect attenuation from internal discontinuities. NDT applications range from weld inspection to aerospace component testing.

Medical Ultrasound

Medical ultrasound imaging creates pictures of internal body structures using reflected ultrasonic pulses. Frequencies from 1-20 MHz provide wavelengths appropriate for imaging soft tissues with resolution from millimeters to fractions of a millimeter. Higher frequencies provide better resolution but have limited penetration depth due to tissue absorption.

B-mode (brightness mode) imaging displays two-dimensional cross-sections by scanning the ultrasound beam and mapping echo intensity to image brightness. Doppler ultrasound detects blood flow by measuring frequency shifts from moving red blood cells. Advanced techniques including 3D imaging, contrast-enhanced imaging, and elastography extend diagnostic capabilities.

Therapeutic ultrasound uses focused high-intensity beams to deliver mechanical and thermal energy to target tissues. Applications include lithotripsy (breaking kidney stones), tumor ablation, and physical therapy. Safety considerations limit exposure in diagnostic applications while therapeutic applications intentionally deliver substantial energy to tissue.

Industrial Ultrasound

Industrial ultrasonic applications harness high-intensity ultrasound for material processing. Ultrasonic cleaning uses cavitation, the formation and collapse of tiny bubbles, to remove contaminants from surfaces. The mechanical action of collapsing bubbles provides cleaning action without harsh chemicals and reaches into fine crevices inaccessible to other methods.

Ultrasonic welding joins plastics and metals using high-frequency vibration that generates frictional heat at the interface. This technique provides fast, clean joints without adhesives or filler materials. Applications range from electronic packaging to automotive assembly.

Ultrasonic machining uses abrasive slurry driven by ultrasonic vibration to machine hard and brittle materials. The process can create complex shapes in materials like ceramics and glass that are difficult to machine conventionally.

Surface Acoustic Wave Devices

Surface acoustic wave (SAW) devices confine acoustic energy to a thin layer at the surface of a piezoelectric substrate. Interdigital transducers (IDTs), consisting of interlaced metal fingers on the substrate surface, convert between electrical signals and surface acoustic waves. The wave velocity of about 3000-4000 m/s, much slower than electromagnetic waves, enables compact delay lines and filters.

SAW filters provide sharp frequency selectivity in a compact form factor, finding widespread application in RF systems including cell phones and wireless communication equipment. The filter characteristics are determined by the IDT geometry and substrate properties, set during fabrication. SAW resonators provide stable frequency references for oscillators.

SAW sensors detect changes in surface conditions by their effect on wave propagation. Mass loading from adsorbed materials changes the wave velocity, enabling chemical and biological sensing. Temperature, pressure, and strain also affect SAW devices, supporting diverse sensing applications.

Bulk Acoustic Wave Devices

Bulk acoustic wave (BAW) devices use acoustic waves propagating through the thickness of a piezoelectric layer rather than along its surface. Film bulk acoustic resonators (FBARs) and solidly mounted resonators (SMRs) provide high-frequency filtering with excellent performance and small size. BAW technology enables the RF filters essential for modern smartphones supporting multiple frequency bands.

BAW devices operate at higher frequencies than SAW devices for a given die size, since frequency depends on film thickness rather than lateral dimensions. This advantage becomes more significant as wireless systems move to higher frequency bands. The physics of thin-film piezoelectric materials and acoustic reflection at layer interfaces determines BAW device design.

Quartz Crystal Devices

Quartz crystal resonators provide stable frequency references for electronic systems. The exceptional frequency stability of quartz derives from its consistent elastic properties and high mechanical quality factor. Crystal cuts with particular orientations provide temperature compensation or specific temperature characteristics.

Crystal oscillators use quartz resonators to set oscillation frequency with stability far exceeding LC or RC oscillators. Temperature-compensated (TCXO) and oven-controlled (OCXO) oscillators achieve stability from parts per million to parts per billion. Understanding the physics of quartz resonance supports crystal selection and oscillator design.

Microphones and Sound Capture

Microphone design involves trade-offs among sensitivity, frequency response, directionality, size, and noise performance. Dynamic microphones offer durability and good performance at moderate cost. Condenser microphones achieve superior frequency response and transient performance. MEMS microphones provide the miniaturization required for mobile devices. Understanding acoustic principles supports microphone selection and placement for optimal sound capture.

Loudspeakers and Sound Reproduction

Loudspeaker systems convert electrical audio signals to acoustic waves that recreate the original sound. Multi-way systems divide the frequency range among drivers optimized for different bands. Enclosure design affects low-frequency response through interaction between driver and enclosed air volume. Understanding the physics of radiation, resonance, and room interaction enables effective loudspeaker system design.

Room Acoustics

The acoustic properties of rooms significantly affect sound reproduction and capture. Reflections, absorption, and resonances modify the sound reaching listeners or microphones. Understanding room acoustic physics supports both room treatment to control acoustic properties and signal processing to compensate for room effects.

Acoustic Emission Sensing

Acoustic emission (AE) sensing detects sound generated by materials under stress, including crack propagation, corrosion, and deformation. AE monitoring provides early warning of structural failure in bridges, pressure vessels, and other critical structures. The physics of stress wave generation and propagation in solids underlies AE sensing techniques.

Vibration Sensing

Accelerometers and vibration sensors detect mechanical motion through various transduction mechanisms. Piezoelectric accelerometers are common for industrial vibration monitoring. MEMS accelerometers dominate consumer electronics applications. Understanding the physics of mechanical resonance and transduction supports sensor selection and interpretation of vibration data.

Acoustic Fingerprinting

Acoustic properties can characterize materials and structures for identification and quality control. The sound of a struck object depends on its material, geometry, and internal structure. Analysis of acoustic responses enables applications from fruit ripeness testing to counterfeit detection.