Sennheiser HMEC 25-KA - Manual (Page 7)

13 12
Noise compensation HMEC 25
Reduction/dB
Frequency/Hz
20 100 1000 10000 20000
0
10
20
30
40
active
passive
ON/OFF
LED
HMEC 25-KA / HMEC 25-CA
A headset is comprised of two units, a headphone and a microphone. The HMEC
25-KA/-CA headset is mostly used for communication in aircraft cockpits.
̈ The microphone is a high quality back-electret condenser microphone. As per
ARINC it receives its operating voltage via a connector from the aircraft´s
internal power supply.
̈ The headphone is a Sennheiser NoiseGard ™ system. This is a dynamic
headphone system which, in addition to reproducing the original audio signal,
electronically cancels the low frequencies of the ambient noise. This active
noise compensation operates on the principle that sound and ”anti-sound“ (in
phase opposition) cancel each other out. The
NoiseGard ™ compensation
circuitry in the headphone requires an extra power supply, so a battery com-
partment has been integrated into the headset cable (see illustration on the right).
Clearly intelligible communication is ensured, and the pilot no longer has to
turn the volume up to overcome ambient noise.
NoiseGard ™ increases your comfort,
but it is not a medical hearing protection!
Active noise compensation with NoiseGard ™
Inserting the batteries into the battery box
The NoiseGard ™ compensation circuitry is powered via a battery pack integrated
into the headset cable. Insert two AA size batteries into the box as shown in the
digram below.
Switching the power supply on/off
The on/off switch for the power supply of the compensation circuitry is at the front
of the battery box.
Pilot light
An LED on the battery box indicates the battery status:
̈ When the LED lights up green, the power supply for the compensation
circuitry is sufficient.
̈ When the LED is red, the batteries are almost flat and should be changed
immediately. The
NoiseGard ™ system is still operational.
̈ If the LED does not light up at all when the power supply is switched on, the
batteries are completely flat. The headset is still operational but without the
noise compensation.
The microphone is powered by the aircraft‘s internal power supply. Thus, it
is independent of the batteries and always remains operational.