Pioneer SE-NC21M - Manual (Page 7)

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When to replace battery
When battery becomes expended, the power indicator will
become darker, and sound will appear distorted, with greater
than normal amounts of noise. Always replace a new battery.
Nominal battery life is as follows when noise-canceling function
is used continuously:
AAA Alkaline battery: About 120 hours
AAA Manganese battery: About 60 hours
(Battery life may vary depending on surrounding temperatures
and other conditions of use.)
Using the Headphone
1 Connection to AV devices
When connecting the headphones to an airline onboard audio
system, use the supplied plug adapter.
In flight plug adapter