BL-21994-000 Knowles Acoustics, BL-21994-000 Datasheet - Page 27

• 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

MICROPHONE AND SPEAKER BASICS
SPEAKERS
The speaker converts an electrical signal into sound. A cross section of a typical Knowles speaker is shown in Fig. 3.
The basic components of the speaker are: a coil of wire, a metal U-shaped reed called the armature, a pair of permanent
magnets, a drive rod, and a diaphragm.
The coil and armature act as an electromagnet. An alternating current in the coil causes the polarity of the armature to
switch back and forth from north to south. The free end of the armature bends slightly up and down as it is attracted
alternately to the top and bottom magnets (Fig. 4). The diaphragm, pulled along by the drive rod, pumps air in and out of
the speaker. The mechanical motion of the armature is thus converted into sound.
The sound output of a typical Knowles’ speaker is measured in units of dB SPL (sound pressure level) relative to 20 µPa.
Fig. 3: Cross section of a Knowles EH speaker
DIAPHRAGM
DRIVE ROD
OUTPUT TERMINAL
COIL
MAGNETS
ARMATURE
Fig. 4: The motor of the speaker has a coil, an armature, and a pair of permanent magnets.
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Note: All performance curves are typical