Sound isolation, in the headphone and audio field, refers to the ability to physically block external environmental noise from entering the ear canal. It primarily relies on the structural design of the headphones, such as the silicone/foam ear tips of in-ear monitors, the fully enclosing ear cups of over-ear headphones, and the anti-phase sound waves generated by active noise cancellation (ANC) technology. Unlike "noise reduction," sound isolation emphasizes passive physical blocking, does not rely on batteries, and is particularly effective at attenuating high-frequency noise (such as voices and keyboard sounds).
Sound isolation performance directly determines your listening experience in noisy environments. Good sound isolation allows you to listen to music at lower volumes, thereby protecting your hearing, while also avoiding distortion caused by turning up the volume to overpower background noise. When making a purchase, if you frequently use headphones on the subway, on airplanes, or in the office, strong sound isolation (such as closed-back over-ear headphones or in-ear models with foam tips) is a must-have; if you are in a quiet indoor environment, there is no need to over-pursue it, so as not to impair your awareness of your surroundings (such as safety when crossing the street).
Practical tip: When trying on headphones, play a recording of noisy street sounds. If you can clearly hear dialogue at a moderate volume without needing to turn it up, the sound isolation of those headphones is sufficient for daily commuting.