The Shure A15HP filter rolls off low audio frequencies to reduce 60 Hz electrical hum, low-frequency wind noise, air conditioner noise, or stand- and floor-transmitted low-frequency vibrations.
When loaded with a low-impedance microphone input (800 ohms or more), the rolloff is 12 decibels per octave below 100 Hz. When loaded with 150 ohms, the rolloff is 3 decibels at 100 Hz. The output is flat above the rolloff point (with a maximum loss of 0.5 decibels at 1 kHz).