Check the listing for details. Harrison Labs Subsonic Filter FMOD 37Hz High Pass @18dB Electronic Crossover. Color: Red, Condition: New. Listed at 29.99 USD. Harrison Labs Subsonic Filter FMOD 37Hz High Pass @18dB Electronic Crossover - Dual female RA adapter includedInsert this in the RCA line to your amplifier to help control excessive woofer flapping. This will provide better bass detail and higher output potential with lower distortion while remaining virtually inaudible (picture is generally representative of the item for sale but not the exact item, the one you will get is red and says 37Hz subsonic on the card)Preamp level inline RCA crossover - acts like an "electronic" crossover by filtering before the amplifier but requires no power This model is unique in the inline FMOD series as it's a higher 18dB slope. This means that it will cut off the low frequencies faster below it's cutoff frequency at a rate of 18dB per octave instead of the typical 12dB. This allows a lower cutoff but still good protection from over-excursion, This unit works especially well since many speakers are tuned around 35-40Hz and most music does not go below that nor do most speakers have much capability below that range. Subsonic filtering will minimize excessive excursions Minimizing excess excursion helps keep the woofer voice coil within the linear operating range and minimal effect to the sound other than making your system seem more dynamic and able to play at much higher level without woofer flapping sounds and distortion. It can also reduce flutter from turntables