G.R.A.S. 50AI-B 1/2" LEMO Intensity Probe

Freq range: Ref. IEC 61043 Class 1
Dyn range: 25 dB(A) to 152 dB
Sensitivity: 25 mV/Pa
  • 50AI-B 1/2
  • 50AI-B 1/2

Typical applications

  • Sound-intensity measurements
  • Sound-source location
  • Sound-power measurements
  • IEC 61043 standard measurements
  • Sound-source ranking
  • Sound-intensity mapping


All components are made specifically for sound intensity applications. Each small 1⁄4-inch diameter by 40 mm long microphone preamplifier is housed in a robust stainless steel casing, which enables novel probe designs for reducing disturbances to the soundfield otherwise brought about by the effects of shadows and diffraction, and a symmetry which enables reliable calibrations as described in the proposed standard (ISO/DIS 9614-2) for sound power measurements using sound-intensity measurements.

The Sound-intensity Probe Type 50AI comprises: G.R.A.S. 40AK Intensity Microphone Set, two G.R.A.S. 26AA Microphone Preamplifiers, four solid spacers of various lengths and remote-control handle.

The microphones are high sensitivity, free-field 1⁄2-inch condenser microphones with a uniquely designed pressure equalisation system which ensures extremely well defined phase characteristics. The microphones and preamplifiers are mounted on a swivel head on the telescopic arm of the Remote control handle. To cover the full frequency range from 50 Hz to 10 kHz, the 50AI probes are delivered with four solid spacers for spacing the microphones at 12 mm, 25 mm, 50 mm and 100 mm. These spacers can be easily interchanged without dismantling the probe.

The 50AI can also be supplied with a pair of phase-matched G.R.A.S. 40BI 1/4" microphones instead. The 40BI is for measuring very high intensity levels which lie above the dynamic range of the 40AK or in situations where the smaller size of the 40BI is necessary.

Different versions of the G.R.A.S. 50AI are available for connecting directly to a wide range of general purpose frequency analysers as well as specialised sound intensity analysers.

Version B
has built-in remote control functions for direct connection to, and control of, sound-intensity measuring systems from a wide range of suppliers such as 01 dB, Müller-BBM and Neutrix-Cortex

Version C
can be connected to any analyser with standard 7-pin LEMO microphone inputs

Version D
has built-in remote control functions enabled by internal batteries for direct connection to, and control of, sound-intensity measuring systems from a wide range of suppliers such as 01 dB, Müller-BBM and Neutrix-Cortex 

All versions are delivered in a carrying case, complete with microphones, preamplifiers, mechanical parts, windscreen and remote control handle. Special configurations can also be made to order.

Frequency range (±2 dB) Hz Ref. IEC 61043 Class 1
Dynamic range lower limit (microphone thermal noise) dB(A) 25
Dynamic range upper limit dB 152
Set sensitivity @ 250 Hz (±2 dB) mV/Pa 25
Polarization voltage V 200V
Microphone venting Rear
Temperature range, operation °C / °F 5 to 40 / 41 to 104
Temperature coefficient @250 Hz dB/°C / dB/°F -0.01
Humidity range non condensing % RH 0 - 100
Humidity coefficient @250 Hz dB/% RH -0.001
Influence of axial vibration @1 m/s² dB re 20 µPa 66
Connector type 12-pin LEMO
CE/RoHS compliant/WEEE registered Yes/Yes/Yes
Weight g / oz 400 / 14.110

Frequency ranges for different microphone spacers

Included items

G.R.A.S. AI0001 Windscreen for Intensity Probes

Optional accessories

G.R.A.S. 12AB 2-Channel Power Module for G.R.A.S. Intensity Probes (for versions B, C, D)