Audio-Technica ATH-CKS50TW Noise-Canceling True Wireless In-Ear Headphones (Black) - Manual (Page 7)

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About the active noise-cancelling function
The product is designed to use its built-in microphone to pick up noise from the surrounding environment (noise from
vehicles, noise from air conditioners, etc.) and to cancel out that noise by producing sounds in negative phase. This
results in a reduction in the sounds in your environment.
The product’s noise-cancelling function is mainly effective against noises in the lower frequency range, but not all
noises are cancelled.
Depending on the type of noise (or in quiet locations), you may not notice the noise-cancelling effect.
The noise-cancelling effect may vary or a bleep (howling) may occur depending on how you wear the product. In
these cases, take off the product and put the product in your ears again correctly.
To maximize the noise-cancelling effect, select the eartips that offer the best sound insulation for each side to block
surrounding sound, and insert the headphones into the ears correctly.
When you use the product in a train or a vehicle such as bus or truck, noise may be heard depending on rail/street
conditions.