Here are a variety of techniques: https://en.m.wikipedia.org/wiki/Analog_delay_line https://en.m.wikipedia.org/wiki/Analog_delay_line I think a better word tha

Here are a variety of techniques: https://en.m.wikipedia.org/wiki/Analog_delay_line https://en.m.wikipedia.org/wiki/Analog_delay_line I think a better word than "physical" domain is "mechanical" domain. Mechanical waves propagate much more slowly than electromagnetic waves.