arrow_back_ios

Main Menu

See All Acoustic End-of-Line Test Systems See All DAQ and instruments See All Electroacoustics See All Software See All Sensors See All Vibration Testing Equipment See All リソースセンター See All アプリケーション See All 各産業 See All インサイト See All サービス See All サポート See All 当社の事業 See All 私たちの歴史 See All サステナビリティ(持続可能性) See All グローバルな事業展開
arrow_back_ios

Main Menu

See All Actuators See All Combustion Engines See All Durability See All eDrive See All Transmission Gearboxes See All Turbo Charger See All DAQ Systems See All High Precision and Calibration Systems See All Industrial electronics See All Power Analyser See All S&V Hand-held devices See All S&V Signal conditioner See All Wireless DAQ Systems See All Pinnae See All Accessories See All DAQ See All Drivers API See All nCode - Durability and Fatigue Analysis See All ReliaSoft - Reliability Analysis and Management See All Test Data Management See All Utility See All Vibration Control See All Inertial Sensor Software See All Acoustic See All Current / voltage See All Displacement See All Force See All Inertial Sensors See All Load Cells See All Pressure See All Strain See All Torque See All Vibration See All Exciters See All Smart Sensors IO-Link See All Shaker Systems See All Power Amplifiers See All Vibration Controllers See All 音響 See All 設備とプロセスの監視 See All カスタムセンサ See All データの取得と分析 See All 耐久性および疲労 See All Electric Power Testing(電力テスト) See All NVH See All 信頼性 See All スマートセンサ See All 振動 See All 計量 See All 自動車および地上輸送 See All 校正 See All インストール、メンテナンス、修理 See All サポート :ブリュエル・ケアー製品 See All Release Notes See All コンプライアンス (Compliance) See All Our People
arrow_back_ios

Main Menu

See All CANHEAD See All GenHS See All LAN-XI See All MGCplus See All Optical Interrogators See All QuantumX See All SomatXR See All Fusion-LN See All Accessories See All Hand-held Software See All Accessories See All Wireless Gateways See All Wireless Nodes See All BK Connect Pulse See All API See All Microphone sets See All Cartridges See All Special Microphones See All Acoustic Calibrators See All Microphone Pre-amplifiers See All Sound Sources See All Acoustic Transducer Accessories See All Accessories See All Digital load cells See All Accessories See All Experimental testing See All Transducer Manufacturing (OEM) See All Accessories See All Rotary Torque Sensors See All CCLD (IEPE) accelerometers See All Charge Accelerometers See All Impulse hammers / impedance heads See All Cables See All Accessories See All Accessories See All Power amplifier See All Accessories for exciters See All 電気音響 See All 音源探査(Noise source identification, NSI) See All 環境騒音 See All 音響パワーと音圧 See All 騒音認証 See All 産業用プロセスコントロール See All 構造ヘルスモニタリング See All 電気デバイス試験 See All 電気システム試験 See All グリッド試験 See All 高電圧試験 See All 導電加振機による振動試験 See All 構造力学 See All 機械分析と診断 See All プロセス計量 See All 車両の電動化 See All トランスデューサの校正サービス See All ハンドヘルド測定器の校正サービス See All 機器およびDAQの校正サービス See All 現地での校正 See All リソース See All ソフトウェアライセンス管理

Brüel & Kjær (now HBK) has developed and produced high-end measurement microphones since 1945. We have uninterruptedly worked on research and development and on calibration methodology to ensure continuous improvements in microphone accuracy and performance.

Today, this approach contributes to a wide portfolio of measurement microphones covering all kinds of applications from acoustic seismic detection systems to airbag measurement and ultrasonic applications.

Microphone Physics

Measurement microphones are based on a very simple physical principle: Capacitance.

The capacity of a microphone is inversely proportional to the distance between the backplate (a stiff plate), and the diaphragm (a thin, highly tensioned metal foil). When exposed to sound pressure, the diaphragm deforms and moves closer to or further away from the backplate, changing the capacity of the system. These variations in capacitance are converted to voltage variation. Most measurement microphones are based on this principle.

Microphone Diaphragm

Measurement microphones come in diameters of 1-, ½-, ¼- and 1/8 inch. The bigger the diaphragm, the less stiff it is and better to detect smaller sound pressure variations. On the other hand, the size of the diaphragm limits detecting frequencies with wavelengths that are in the same range or bigger than the diaphragm.

knowledge, resource center, articles, measurement microphones

The displacement of the diaphragm for a typical ½-inch microphone is only in the magnitude order of 5 nanometres for excitation of one pascal. For comparison, a diaphragm with a diameter the size of planet earth would barely move 5 meters

knowledge, resource center, articles, measurement microphones

The wavelength of a 20 kHz sound wave (the upper limit of the audio range) is 1.7 cm. That is approximately the width of the index finger

knowledge, resource center, articles, measurement microphones

Microphone frequency response chart.

Microphone Sensitivity

The sensitivity is the voltage produced by microphones under defined sound pressure. It is defined in V/Pa and is frequency dependent. The sound pressure of one pascal corresponds to a level of 94 dB SPL. That is why most microphone calibrators (for example our Sound Calibrator Type 4231) produce 94 dB SPL or 1 Pa.

Because sensitivity is frequency-dependent, the frequency response is defined as an expression of the sensitivity in the entire frequency range. This is generally expressed in decibels relative to the 250 Hz sensitivity.

knowledge, resource center, articles, measurement microphones

Microphone sensitivity chart showing the sensitivity of Type 4160 and Type 4180 laboratory standard microphones from 1984 to 2018.

Microphone Dynamic Range

The dynamic range of measurement microphones (for example, 16 dBA - 143 dB) is the range where the microphone acts as a perfect linear transducer.

The first number is the inherent noise. Both condenser microphones and pre-amplifiers have an inherent noise caused, among other things, by limitations in the electronics and Brownian movements.

This number indicates the sound pressure level that would create the same voltage as the noise generated by the microphone itself. The noise is measured in the third-octave band, A-weighted in the human hearing range (between the 22.4 Hz and 22.4 kHz band), if nothing else is specified. The second number is the highest sound pressure level measurable with less than 3% total harmonic distortion.

microphone sensitivity specifications
Microphone Dynamic Range

Microphone Sound Field

Measurement microphones, by their size and shape, influence the sound pressure. This influence, which depends on the type of sound field, is considered in each microphone’s design and the microphone responses are optimized to compensate for the influence. This allows us to always give a flat response to the chosen sound field.

Measurement microphones are divided into three main types, each type optimized for one of the three main types of sounds field, and it is important to choose the microphone that is best suited for the sound field.

Free-field

Free-field microphones are typically used to measure loudspeakers or outdoor sound.

 

knowledge, resource center, articles, measurement microphones

In order to make measurements in a free-field, totally without reflecting objects, the measurements must be made outdoors at the top of a flagpole (or equivalent) or in an anechoic chamber.

In an anechoic chamber the ceiling, floor and all the walls are covered by a highly absorptive material which eliminates reflections. Thus, the sound pressure level in any given direction from the noise source may be measured without the presence of interfering reflections. 

Diffuse-field

The opposite of an anechoic chamber is the reverberation chamber where all surfaces are made as hard and reflective as possible and where no parallel surfaces exist. This causes a diffuse-field because all the sound waves are arriving simultaneously from all directions with equal probability and level.

Sound fields with a close resemblance to diffuse-field can be achieved in environments such as buildings with hard walls, where many simultaneous sound or noise sources exist, such as with in-cabin measurements and in churches.

If you are not sure about the sound field you are in, it is best to assume that it is diffuse to minimize measurement error. Diffuse-field microphones are typically used to measure vehicle interior noise or building acoustics.

knowledge, resource center, articles, measurement microphones

Pressure-field

A pressure-field is sound pressure that has the same magnitude and phase at any position within the field. Sound fields with a close resemblance to the pressure field can be achieved in small cavities (small compared to the wavelength) such as artificial ears.

Pressure-field microphones are typically used in couplers, wind tunnels or in flush-mounted measurements.

knowledge, resource center, articles, measurement microphones

Microphone Stability

Brüel & Kjær's measurement microphones are designed to be extremely stable, especially over time, temperature, humidity, and ambient pressure.

To achieve maximum stability, we use carefully selected, high-quality material; we apply controlled heat treatment to artificially age and release any tension in the cartridge and we continuously test each microphone at every step of the production process.

The microphones are exposed to multiple cleaning processes during fabrication in a class 10 cleanroom. In class 10 clean room, there are less than 10 particles larger than 0.5 µm and less than 2 particles between 1  and 5 µm per cubic foot. As a comparison, ordinary room air is approximate ‘class 1 million’.

Considering the distance between the backplate and the diaphragm being generally around 20 µm, any particles bigger than this that are present in the cavity would cause stability issues – especially when condensation or changes in temperature occur. 

If the membrane is punctured, contamination will occur, and particles and residue will enter the cartridge. Achieving the same level of cleanliness during a repair may not guarantee a microphone with an optimal response. This is the reason why Brüel & Kjær does not offer microphone repair.

knowledge, resource center, articles, measurement microphones

More articles by Dr Rémi Guastavino

Dr Remi Guastavino

Article 1
What to expect when using lower quality measurement microphones

Article 2
What are the implications of using lower-quality measurement microphones

Article 3
Setting the bar for microphone excellence

Article 4
Measurement Microphones Explained

Article 5
High-frequency HATS, why?

Support Content