From the outside, a measurement microphone looks deceptively simple. But in real-world engineering, its interface options are surprisingly diverse: Lemo, BNC, Microdot, 10-32 UNF, M5, SMB… Many newcomers to acoustics ask questions like:

• Why can’t microphone interfaces be standardized?
• Why are cables often not interchangeable between microphones?
• What power and signal schemes are hidden behind different connectors?

This article provides a structured overview of common measurement microphone interfaces, looking at physical connectors, powering methods, cable characteristics, and typical application-driven selection.

Main Physical Interfaces for Measurement Microphones

Below is a connector-by-connector summary, including the typical powering approach for each.

Lemo (5-pin, 7-pin): The Classic Solution for Externally Polarized Microphones

Lemo is a precision circular multi-pin connector and is the most common choice for externally polarized measurement microphones. The Lemo B series is widely used (e.g., 0B, 1B, 2B), and most standard measurement microphones adopt the Lemo 1B interface.

Key Characteristics

A multi-pin connector can carry multiple signals simultaneously, such as:

• Microphone output (analog signal)
• External polarization high voltage (typically 200 V)
• Preamplifier power supply
• Calibration/identification signals

Additional benefits:

• Very reliable mechanical locking
• Well-suited for lab environments, metrology, and semi-anechoic chamber measurements where stability and traceability matter

Notes on External Polarization

• Common polarization voltage is 200 V; some systems support switching between 0 V / 200 V
• Polarization voltage stability affects microphone sensitivity; in engineering practice, sensitivity variation is often treated as approximately proportional to voltage variation
• The preamplifier is typically powered separately (up to 120 V) but delivered via the same multi-pin connector
• Maximum output voltage can reach 50 Vp
• Includes pins for charge injection methods
• Separate output and ground paths help achieve lower noise

In metrology labs, type testing, acoustic calibration, and high-precision semi-anechoic chamber work, the combination of “externally polarized microphone + Lemo multi-pin connector” is essentially a standard configuration.

BNC: The Most Common External Connector for IEPE Microphones

Names like IEPE / ICP / CCP refer to the same general technology route: constant-current powering, where power and signal are transmitted on the same line (Constant Current Powering). In this system, the most common physical connector is the coaxial BNC.

Interface and Powering Characteristics

• Coaxial structure, ideal for analog voltage transmission
• Bayonet lock (quick and reliable plug/unplug)
• Supports longer cable runs with good noise immunity
• Low cost and highly universal

Typical IEPE Powering Parameters

• Constant current: 2–20 mA (common settings include 2 mA, 4 mA, 8 mA, etc.)
• Compliance voltage (supply capability): typically 18–24 V
• Maximum output voltage: generally around 8 Vp

If the constant current is too low or the compliance voltage is insufficient, the maximum output signal swing is limited—directly affecting the maximum measurable SPL and the linear measurement range.

In everyday testing such as engineering noise measurements, NVH, and environmental noise work, “IEPE microphone + BNC” has become the de facto standard.

Microdot (10-32 UNF / M5): Lightweight Connectivity for Small Microphones

Microdot is a threaded miniature coax connector widely used for small sensors (compact measurement microphones, accelerometers, etc.). It commonly uses a 10-32 UNF thread.

What “10-32 UNF” Really Means

This is simply an imperial fine-thread standard:

• Nominal diameter: 0.19 inch ≈ 4.826 mm
• Pitch: 1/32 inch ≈ 0.7938 mm

Because 10-32 UNF is the typical thread used on Microdot connectors, the term “10-32 UNF” is often used informally to refer to the Microdot interface itself.

What about M5?

M5 is a metric thread standard:

• Nominal diameter: 5 mm
• Pitch: 0.8 mm

Its dimensions are close to 10-32 UNF, and when tolerances are not extremely strict it can serve as a substitute—commonly seen in accelerometers or vibration microphones.

Interface Characteristics

• Very compact; ideal for lightweight setups
• Threaded locking provides strong mechanical stability
• Commonly paired with IEPE powering
• Best for short runs and high-speed signal transmission

When microphones must be placed in tight spaces, or where sensor mass/size is critical, Microdot is a common choice for compact, high-density installations.

SMB (SubMiniature B): For High-Density Multi-Channel or Internal Connections

SMB is a small “push-on” coaxial connector.

Interface Characteristics

• Compact size supports high channel density
• Push-on structure enables fast connection
• Better high-frequency performance than BNC
• More suitable for semi-permanent internal wiring

SMB is often best viewed as an engineering connector used inside equipment, rather than a field-plugging standard.

Extended Interface Function: TEDS and Smart Identification

In multi-channel and integrated systems, TEDS (Transducer Electronic Data Sheet) is increasingly common.

By integrating a small memory chip into the sensor or cable, TEDS can store:

• Model and serial number
• Sensitivity
• Calibration date and other parameters

Compatible front-end hardware or acquisition software can automatically read TEDS to:

• Identify the sensor type on each channel
• Load sensitivity and calibration coefficients automatically
• Reduce manual entry errors
• Save calibration time and labor

At the connector level, TEDS is typically implemented by using certain pins in multi-pin Lemo connectors, or via overlay methods in specific BNC-based solutions. When planning an interface system, it’s wise to consider early on whether TEDS support is required.

Why Are There So Many Interfaces?

Connector diversity is best explained from three perspectives:

Different Polarization and Powering Schemes

• Externally polarized microphones (≈ 200 V polarization) → better suited to multi-pin connectors like Lemo
• Prepolarized + IEPE systems → better suited to coaxial connectors like BNC / Microdot / SMB

Different Scenarios and Priorities

• Laboratory / metrology: high stability, multiple signals in one cable, secure locking → Lemo
• Field engineering / environmental measurement: convenient wiring, strong universality → BNC + IEPE
• Miniaturization / high-density arrays: size and channel density first → Microdot / SMB

Long Product Lifecycles and Backward Compatibility

• Measurement systems often have lifecycles of 10–20 years or more
• To avoid forcing users to replace large numbers of cables and front-end systems, manufacturers typically continue existing interface ecosystems
• Under long lifecycle constraints, “full unification” is often impractical and offers limited engineering return

Typical Application Mapping (Quick Reference)

• Engineering noise, NVH, vibration/noise tests: BNC / Microdot
  Easy wiring, many channels, low maintenance cost
• Precision lab measurement, type testing, metrology calibration: Lemo 7-pin / 5-pin
  Supports polarization HV and multiple signals; suitable for traceable high-precision measurement
• Acoustic arrays, multi-channel acquisition card systems: Microdot / SMB
  High channel density, compact wiring, easier system integration
• Long-term environmental noise monitoring systems: BNC / customized protected connectors
  Focus on weather resistance, waterproofing, salt fog resistance, and stable long-distance transmission

Conclusion

The variety of measurement microphone interfaces is mainly the result of trade-offs between technology routes, application requirements, and historical compatibility—not simply a “lack of standards”.

You are welcome to learn more about microphone functions and hardware solutions on our website and use the “Get in touch” form to contact the CRYSOUND team.

How does your phone instantly and accurately connect to your earphones instead of someone else’s in a room full of Bluetooth devices? Why does your smart fitness band sync data exclusively to your phone app after a workout? This dedicated “one-to-one” connection relies on the Bluetooth 5.0 unicast mechanism. Its intelligence goes far beyond simple pairing—it lies in how it maintains a stable, efficient, and private wireless link with extremely low power consumption.

Core Philosophy of Connection Strategy: Precision and Energy Efficiency

Unlike Classic Bluetooth, which focuses on establishing a persistently online data channel, the Bluetooth 5.0 Low Energy (LE) unicast mode adopts a “wake-on-demand, instantaneous communication” design philosophy. It no longer maintains a continuous connection link but instead achieves efficient communication through a precise timing synchronization mechanism.

After devices pair (e.g., a phone and a fitness band), they do not stay in a constantly connected state. Instead, they negotiate and establish a “connection interval,” waking up synchronously only at predetermined moments to complete microsecond-level data exchange before immediately entering a deep sleep state. This mechanism allows devices to remain in an ultra-low power state for over 99% of the time, providing the core support for the long battery life (months to years) of IoT devices.

Connection: Dynamic Coordination Under Precise Timing

The establishment and maintenance of a Bluetooth 5.0 unicast connection rely on a precise timing coordination mechanism. The connection establishment process is as follows:

• Advertising and Scanning Phase: The peripheral device (e.g., earphone) sends advertising packets containing identity information at fixed intervals. The central device (e.g., phone) continuously scans on the advertising channels, searching for the target device.
• Connection Initiation Phase: The central device sends a connection request to the peripheral, which includes initial communication timing and suggested connection interval parameters.
• Connection Parameter Negotiation: This is the core of connection optimization. Beyond the connection interval, two other key parameters are negotiated:
  • Slave Latency: When the slave device (e.g., fitness band) has no data to send, it can skip waking up for a specified number of connection interval cycles, thereby extending its sleep time.
  • Supervision Timeout: A threshold for judging the connection state. If no valid communication occurs within this timeout period, the connection is considered lost, triggering reconnection or disconnection procedures.
• Connection Establishment and Maintenance: The master and slave devices switch to data channels, synchronizing their sleep and wake cycles according to the previously negotiated timing. This enables ultra-low power consumption while ensuring stable communication.

New Product: CRY578 Bluetooth LE Audio Interface Empowering BLE Testing

With the introduction of the new high-performance, low-complexity LC3 codec by the LE Audio standard, Bluetooth Low Energy (BLE) technology can now achieve stable transmission of high-quality stereo audio while maintaining its ultra-low power characteristics. Compared to traditional solutions, the LC3 codec can reduce bandwidth requirements by approximately 50% at the same audio quality or improve audio quality at the same bandwidth, effectively addressing the pain point of balancing low power consumption with high audio fidelity.

In response to this technological trend, our newly launched CRY578 Bluetooth LE Audio Interface comprehensively supports audio performance testing for both Classic Bluetooth (BR/EDR) and Bluetooth Low Energy (BLE), covering core metrics such as frequency response, distortion, and audio latency. It is suitable for the R&D and quality inspection phases of various Bluetooth audio products, including TWS earphones, smart speakers, and wearable devices.

For detailed specifications, application cases, or to inquire about trial opportunities for the CRY578, please fill out the “Get in touch” form below.

Under regulations such as the EU Machinery Noise Directive, more and more products—from toys and power tools to IT equipment—are required to declare their sound power level on labels and in documentation, rather than simply claiming they are “quiet enough.”

For typical office devices like notebook computers, idle noise is often around 30 dB(A), while full-load operation can approach 40 dB(A). These figures are usually obtained from sound power measurements performed in accordance with ISO 3744 and related standards.

Sound Pressure vs. Sound Power

A noise source emits sound power, while what we measure with a microphone is sound pressure.

Sound pressure varies with room size, reverberation, and microphone distance, whereas sound power is the source’s own “noise energy” and does not change with installation or environment. That makes sound power a better metric for external product noise specification.

In simple terms:

• Sound power is the cause – the energy emitted by the source (unit: W / dB);
• Sound pressure is the effect – the sound pressure level we hear and measure (unit: Pa / dB).

ISO 3744 defines how to do this in an “essentially free field over a reflecting plane”: arrange microphones around the source on an enveloping measurement surface, measure the sound pressure levels on that surface, then apply specified corrections and calculations to obtain stable, comparable sound power levels.

Device Under Test: An Everyday Notebook Computer

Assume our DUT is a 17-inch office notebook. The goal is to determine its A-weighted sound power level under different operating conditions (idle, office load, full load), in order to:

• Compare different cooling designs and fan control strategies;
• Provide standardized data for product documentation or compliance;
• Supply baseline data for sound quality engineering (for example, whether the fan noise is annoying).

The test environment is a semi-anechoic room with a reflecting floor. The notebook is placed on the reflective plane, and multiple microphone positions are arranged around it (using a hemispherical frame or a regular grid). Overall, the setup satisfies ISO 3744 requirements for the measurement surface and environment.

Measurement System: SonoDAQ Pro + OpenTest Sound Power Module

On the hardware side, we use SonoDAQ Pro together with measurement microphones, arranged around the notebook according to the standard.

OpenTest connects to SonoDAQ via the openDAQ protocol. In the channel setup interface, you select the channels to be used and configure parameters such as sensitivity and sampling rate.

From Standard to Platform: Why Use OpenTest for Sound Power?

OpenTest is CRYSOUND’s next-generation platform for acoustic and vibration testing. It supports three modes—Measure, Analysis, and Sequence—covering both R&D laboratories and repetitive production testing.

For sound power applications, OpenTest implements a sound-pressure-based solution fully compliant with ISO 3744 (engineering method), and also covering ISO 3745 (precision method) and ISO 3746 (survey method). You can flexibly select the test grade according to the test environment and accuracy requirements. The platform includes dedicated sound power report templates that generate standards-compliant reports directly, avoiding repeated manual work in Excel.

On the hardware side, OpenTest connects to multi-brand DAQ devices via openDAQ, ASIO, WASAPI, and NI-DAQmx, enabling unified management of CRYSOUND SonoDAQ, RME, NI and other systems. From a few channels for verification to large microphone arrays, everything can be handled within a single software platform.

Three Steps: Running a Standardized ISO 3744 Sound Power Workflow

Step 1: Parameter Setup and Environment Preparation

After creating a new project in OpenTest:

1. In the channel setup view, select the microphone channels to be used and configure sensitivity, sampling rate, frequency weighting, and other parameters.
2. Switch to Measure > Sound Power and set the measurement parameters:
   • Test method and measurement-surface-related parameters;
   • Microphone position layout;
   • Measurement time;
   • Other parameters corresponding to ISO 3744.

This step effectively turns the standard’s clauses into a reusable OpenTest scenario template.

Step 2: Measure Background Noise First, Then Operating Noise

According to ISO 3744, you must measure sound pressure levels on the same measurement surface with the device switched off and device running, in order to perform background noise corrections.

In OpenTest, this is implemented as two clear operations:

• Acquire background noise
  Click the background-noise acquisition icon in the toolbar. OpenTest records ambient noise for the preset duration.
  • In the survey method, OpenTest updates LAeq for each channel once per second;In the engineering and precision methods, it updates the LAeq of each 1/3-octave band once per second.
• Acquire operating noise
  After background acquisition, click the Test icon. OpenTest will:
  a. Record notebook operating noise for the preset duration;
  b. Update real-time sound pressure levels once per second;
  c. Automatically store the run as a data set for later replay and comparison.

Step 3: From Multiple Measurements to One Standardized Report

After completing multiple operating conditions (for example: idle, typical office work, full-load stress):

• In the data set view, select the records you want to compare and overlay them to observe sound power differences under different conditions;
• In the Data Selector, click the save icon to export the corresponding waveform files and CSV data tables for further processing or archiving;
• Click Report in the toolbar, fill in project and device information, select the data sets to include, adjust charts and tables, and export an Excel report with one click.

The report includes measurement conditions, measurement surface, band or A-weighted sound power levels, background corrections, and other key information. It can be used directly for internal review or regulatory/customer submissions, following the same idea as other standardized sound power reporting solutions.

From a Single Notebook Test to a Reusable Sound Power Platform

Running an ISO 3744 sound power test on a notebook is just one example. More importantly:

• The standardized OpenTest scenario can be cloned for printers, home appliances, power tools, and many other products;
• Multi-channel microphone arrays and SonoDAQ hardware can be reused across projects within the same platform;
• The test workflow and report format are “locked in” by the software, making it easier to hand over, review, and audit across teams.

If you are building or upgrading sound power testing capability, consider using ISO 3744 as the backbone and OpenTest as the platform that links environment, acquisition, analysis, and reporting into a repeatable chain—so each test is clearly traceable and more easily transformed from a one-off experiment into a lasting engineering asset.

Visit www.opentest.com to learn more about OpenTest features and hardware solutions, or contact the CRYSOUND team by filling out the “Get in touch” form below.