BL-21994-000 Knowles Acoustics, BL-21994-000 Datasheet - Page 26

• Manufacturer Part Number: BL-21994-000
• Description: MIC ASSY PIEZO INSTR W/38" CABLE
• Manufacturer: Knowles Acoustics
• Series: BLr
• Type: Piezo Condenserr

Specifications of BL-21994-000

• Sensitivity: -69dB ±3dB
• Voltage - Rated: 3V
• Current: 160µA
• Size / Dimension: Circular - 12.7mm Dia x 25.4mm H
• Impedance: 4K Ohm
• Frequency: 20Hz ~ 10kHz
• Termination: Wire Leads
• Direction: Omnidirectional
• Port Location: Top
• Diameter: 12.7 mm
• Transducer Function: Microphone
• Device Type: Microphone
• Directivity: Omni
• External Diameter: 12.7mm
• Output Impedance: 4000ohm
• Length: 25.4mm
• Outer Diameter: 12.7mm
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Filter Type: -
• S/n Ratio: -
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant
• Other names: 423-1002; BL-1994; BL-21994; BL1994

26
MICROPHONES
Microphones measure sound pressure. Inside a Knowles microphone is a thin flexible diaphragm, an electrically
charged plate, and an amplifier (Fig. 1). The output voltage is proportional to changes in the small separation
between the diaphragm and the charged plate (Fig. 2).
As sound pressure inside the front cavity increases, the diaphragm is pushed closer to the plate. As the
pressure decreases, it moves further away. The motion of the diaphragm produces a small electrical signal that
is amplified by a miniature circuit inside the microphone.
The sensitivity of a typical Knowles’ microphone is measured in units of dB relative to 1 Volt per 0.1 Pascal.
Fig 1: Cross section of a Knowles EM microphone
FIG. 2: EM diaphragm and electret
MICROPHONE AND SPEAKER BASICS
MICROPHONE AND SPEAKER BASICS
AMPLIFIER
OUTPUT TERMINAL
CHARGE PLATE
CHARGE PLATE
DIAPHRAGM
DIAPHRAGM
www.knowles.com
FRONT CAVITY
AIR GAP SEPARATION
WIRE
SOUND PRESSURE