GRAS 47HC 1/2” CCP Low-noise Microphone System

Introduction

Low noise emission becomes increasingly important for all kinds of machines, equipment and consumer products, and quiet products are often preferred over more noisy ones. It therefore becomes increasingly important to develop quiet products, and for this measurements close to the threshold of human hearing are needed.

A standard ½” microphone with high sensitivity (50 mV) has a noise floor about 16 dB(A), and the useful measurement range therefore begins at about 19 dB(A). To measure below this level, a special low-noise system is needed.

The 47HC is a prepolarized low-noise microphone set which builds on GRAS’ more than 20 years’ of experience with state of the art low-noise measurement microphone systems. With this set, the performance previously reserved for externally polarized setups is now available for CCP powered systems.

With a sensitivity of 450 mV/pa and a noise floor at 6.5 dB(A) or lower, it opens up for measurement levels down to about 10 dB(A). Its upper dynamic limit is 100 dB below 8 kHz, 85dB from 8 kHz to 20 kHz.

The microphone set has the same form factor as standard GRAS ½” microphone sets, and therefore it can be used with standard mounting accessories.

The 47HC is delivered with Generation II TEDS. The calibration data is programmed into the built-in TEDS according to IEEE 1451.4 using UDID I27-0-0-0U. If your measurement platform supports Transducer Electronic Data Sheets you will be able to read and write data like properties and calibration data.

Generation II TEDS chip (DS2431) may require updated system software.

Typical applications and use

• Measurements at very low sound pressure levels
• Sound-power measurements at low levels
• Measurements in quiet rooms and anechoic rooms
• Measurements on, for example, hard-disk drives, computer products, fans, electric cars, fluorescent lamps and other quiet products

Design

The 47HC comprises a special high-sensitive 1/2″ free-field measurement microphone and an integrated 1/2″ low-noise preamplifier. In combination, they connect to high-quality CCP input modules with BNC connector.

Preamplifier

The preamplifier is a special 1/2″ amplifier with BNC connector optimized for low-noise. It provides amplification and has a built-in compensation filter that ensures linear free-field response up to the upper limit at 20 kHz.

Microphone

The 1/2″ microphone is a pre-polarized free field microphone with a specially reduced inherent noise floor in order to achieve a large dynamic range and a wide frequency response. Its diaphragm is specially tuned for high sensitivity coupled with low internal noise.

Compatibility

The microphone set has the same form factor as a standard GRAS microphone set, and therefore it can be used with the standard mounting accessories available for GRAS standard microphone sets. It has the same dimensions as the 46AE, an advantage if it shall be mounted in a GRAS hemisphere.

Calibration

For level calibration you need a 42AG Multifunction Sound Calibrator which can generate a signal of 94 dB re. 20 µPa.

The level normally used for sound level calibration (114 dB) will overload the 47HC and therefore the lower level is needed.

For frequency calibration you must send the set for calibration at GRAS.

See the tab Ordering info.