TYPE 3923

Rotating Boom Stand

Microphone Rotating Boom Stand designed for use in sound power measurements to ISO 3741 and in building acoustics, and complements the range of acoustic measurement equipment available from Brüel & Kjær.

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The Rotating Microphone Boom Type 3923 is designed for use in sound power measurements to ISO 3741 and in building acoustic measurements.


  • Sound power measurements
  • Building acoustic measurements


  • Fulfills ISO 3741
  • Length of boom adjustable between 50 cm and 200 cm
  • Battery operation with built-in NiCd-cells or line operation
  • Three switch-selectable rotation times
  • Plane of rotation adjustable in steps of 10 degrees
  • Synchronization facility
  • Power to and signal from microphone via slip rings
  • Typical emitted sound power equal to 26 dB re 1 pW (A-weighted)

Type 3923 consists essentially of three parts: A boom, the length of which can be adjusted to any value between 50 and 200 cm; a motor assembly to which the boom is attached and which contains power supply and input and output sockets, and a trunnion mount which allows the rotation plane of the b boom to be varied in steps of 10°.

Length of Boom Stand

The boom consists of 6 aluminium tubes which can be screwed together to provide effective lengths up to 200 cm. A ball joint with rubber suspension for the microphone accepting both 1/2″ and 1″ microphone preamplifiers is mounted at one end of the boom. This suspension attenuates possible vibrations transmitted through the boom to the microphone. At the other end of the boom, counterweights (two of which are included) can be fitted to balance the assembly. The load capability of the boom is 1 kg at its maximum length of 200 cm

Remote Controlled Motor and Rotation

The Boom Stand has three electronically controlled, switch-selectable rotation times of 16, 32 and 64 seconds, which are accurate within ± 5%. The stop/start of the boom can be remotely controlled and a built-in micro-switch allows synchronization with external equipment.

The motor assembly is suspended in a trunnion mount which
allows the rotation plane of the boom to be varied in steps of 10°.

Designed for Low-Noise Operation

The noise emitted from the 3923 is very low (see Fig. 2), typical 24 dB (A) re 2 × 10–5 Pa at a distance of 33 cm from the instrument, under free field conditions for a rotational speed of 16 sec./rev., which assuming a perfectly spherical radiation pattern corresponds to a sound power level of typical 26 dB (A) re 1 pW. Even less noise is produced at lower rotational speeds.

The boom stand is mounted providing a 3/8″ whitworth thread allowing it to be directly mounted on tripods. External power supplies, such as the ZG 0283, are connected to the 7-pin EXT. POWER socket on the motor assembly.