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For older models of telephones, the headset microphone impedance is different from that of the original handset, requiring a telephone amplifier for the telephone headset. A telephone amplifier provides basic pin-alignment similar to a telephone headset adaptor, but it also offers sound amplification for the microphone as well as the loudspeakers. Most models of telephone amplifiers offer volume control for loudspeaker as well as microphone, mute function and switching between headset and handset. Telephone amplifiers are powered by batteries or AC adaptors.
Ambient noise reduction
Interference from external sound can be reduced either by active noise cancellation, or by attenuating noise getting into the ear. The two headphone types that do this attenuation best are in-ear canal headphones and closed-back headphones (both circumaural and supraural). Open-back and earbud headphones provide some noise isolation as well, but to a much lesser extent than the closed-back and in-ear. Typically closed-back headphones block 8 to 12 dB and in-ears anywhere from 10 to 15 dB.