[a-l]

"Let me add an interesting "rule of thumb" for limited frequency range:
Multiplying the LF and HF -3dB numbers should come out to about 600,000.
For example 30Hz to 20KHz = 600,000. In the days of 5KHz Telco loops, we
would use a 120Hz high-pass. The result? A natural sounding bass-treble
balance. Small box radios and TVs that can't produce much below 100Hz
benefit from a 6KHz low pass.
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????

please explain how you go from 600,000 to 120 hz