何謂cut-off frequency

The parapgraph below is an excerpt from the web-page, which gives a concise explanation of "cut-off frequency" in general:

http://www.tech-faq.com/cutoff-frequency.html

Cutoff frequency (corner or break frequency) is commonly used in the physics and electrical engineering fields. This frequency causes a device to stop operating in an efficient manner, resulting in it shutting down or being cut off. More technically, the cutoff frequency reduces energy flow through the device and prevents it from working as designed. That is to say, insufficient power prevents the device from operating efficiently.

Q:另外 square wave 同line spectrum有什麼關係?
A: No physical relation between the two.