Sound sensors, as devices that convert acoustic signals into electrical signals, find extensive applications in industrial automation, smart homes, medical diagnostics, and other fields. Based on their operating principles and detection targets, sound sensors are primarily categorized as follows:
I. Piezoelectric Sensors
Piezoelectric sound sensors leverage the properties of materials like quartz and ceramics. When acoustic pressure acts upon the material's surface, it induces electrical charge changes, thereby generating an electrical signal. These sensors offer high sensitivity and broad frequency response, with typical applications including:
1. Microphones: Electret condenser microphones (ECMs) use polarized electret film as the diaphragm, offering low cost and compact size, making them widely used in consumer electronics. MEMS microphones integrate miniature capacitive structures via semiconductor processes, featuring strong anti-interference capabilities and excellent consistency, commonly found in smartphones.
2. Ultrasonic sensors: Typically operating above 20kHz, these utilize piezoelectric ceramics to transmit and receive ultrasonic waves, suitable for applications like distance measurement (e.g., parking sensors) and flow detection. In medical ultrasound imaging equipment, arrayed piezoelectric sensors achieve millimeter-level resolution.
II. Electromagnetic Induction-Based Sensors
These generate electrical signals through magnetic field changes, primarily including:
1. Moving-coil Microphones: Sound waves drive a coil to vibrate within a permanent magnetic field, cutting magnetic flux lines to generate current. Their wide dynamic range makes them a common choice for professional recording equipment.
2. Electromagnetic Pickups: Coil pickups used in instruments like guitars alter magnetic field strength through vibrating metal strings to output audio signals.
III. Optical Sound Sensors
Employ non-contact detection technologies, such as:
1. Laser Vibrometers: Measure surface vibrations via the Doppler effect, suitable for high-temperature or strong electromagnetic interference environments. Used industrially to detect abnormal acoustic waves caused by mechanical failures.
2. Fiber Optic Acoustic Sensors: Utilize the property that sound waves alter the refractive index of optical fibers, enabling distributed acoustic monitoring. This technology is employed for oil pipeline leak detection.
IV. Capacitive Sound Sensors
Comprising a movable plate and a fixed plate forming a capacitive structure, sound waves alter the plate spacing, thereby changing the capacitance value. MEMS microphones belong to this category, achieving signal-to-noise ratios exceeding 70dB. Another type, the silicon microcapacitive microphone, utilizes semiconductor processes to fabricate miniature air cavities, making it suitable for medical auscultation devices.
V. Resistive Sound Sensors
Carbon-granule microphones are a classic example. Sound pressure alters the contact resistance of carbon granules, thereby modulating the current. Though exhibiting poor frequency response characteristics, their simple structure and low cost made them widely used in early telephones.
VI. New Intelligent Sensors
Technological advancements have led to the emergence of hybrid sensors integrating multiple technologies:
1. AI Voiceprint Recognition Modules: Integrating DSP chips and deep learning algorithms, these analyze specific voiceprint characteristics in real time for smart security systems. Acoustic models provided by Baidu AI Open Platform achieve 98% accuracy.
2. Multi-Parameter Environmental Sensors: Simultaneously detecting sound, temperature, humidity, and other parameters. Examples include noise monitoring terminals in smart cities, which upload decibel data to cloud platforms via 4G networks.
Application Scenario Comparison
| Sensor Type | Sensitivity | Cost | Typical Application Scenarios |
| MEMS Microphone | High | Low | Smartphones, smart speakers |
| Dynamic Microphone | Middle | Middle | Stage performances, broadcast recordings |
| Ultrasonic Sensor | Extremely high | relatively high | Medical ultrasonography, industrial non-destructive testing |
| Fiber-Optic Acoustic Sensor | Extremely high | High | Oil and gas pipeline monitoring, border defense early warning |
Technological Development Trends
1. Miniaturization and Integration: Manufacturers like TSMC have introduced MEMS microphone chips in 3mm×2mm packages, with future potential for SoC integration of sensors and processors.
2. AI Empowerment: Frameworks such as Baidu's PaddlePaddle support edge-side acoustic model deployment, enabling sensors with edge computing capabilities. For instance, infant cry detectors can locally analyze acoustic spectral features within 200ms.
3. Self-Powering Technology: Georgia Tech's piezoelectric-triboelectric hybrid sensors harness environmental vibration energy, making them ideal for IoT node devices.
When selecting sound sensors, comprehensively evaluate parameters including frequency range (human hearing spans 20Hz-20kHz), sensitivity (dB/V), and signal-to-noise ratio. Industrial applications additionally require attention to protection ratings (e.g., IP67 dust and water resistance) and temperature adaptability (-40℃ to 85℃ operating range). With the proliferation of 5G and AIoT technologies, sound sensors are evolving from simple signal acquisition to intelligent perception systems.