Tonearm skating forces and other perturbations are also picked up by the stylus. This is a form of frequency multiplexing as the control signal (restoring force) used to keep the stylus in the groove is carried by the same mechanism as the sound itself. Subsonic frequencies below about 20 Hz in the audio signal are dominated by tracking effects, which is one form of unwanted rumble ("tracking noise") and merges with audible frequencies in the deep bass range up to about 100 Hz. High fidelity sound equipment can reproduce tracking noise and rumble. During a quiet passage, woofer speaker cones can sometimes be seen to vibrate with the subsonic tracking of the stylus, at frequencies as low as just above 0.5 Hz (the frequency at which a 33 1⁄3 rpm record turns on the turntable; 5⁄9 Hz exactly on an ideal turntable). Another reason for very low frequency material can be a warped disk: its undulations produce frequencies of only a few hertz and present day amplifiers have large power bandwidths. For this reason, many stereo receivers contained a switchable subsonic filter. Some subsonic content is directly out of phase in each channel. If played back on a mono subwoofer system, the noise will cancel, significantly reducing the amount of rumble that is reproduced.