Doppler effect: You experience the Doppler effect (almost) every day: If you hear an ambulance with a siren sounding far away, it will sound Ton brighter. When the car finally drives level with you for a short time and then passes you, the tone sounds lower than normal. The reason for this is the compression or expansion of the sound waves by the moving object.
The Doppler effect plays in HiFirange with loudspeakers: The more broadband they are designed, the greater the risk that it will happen, because: If a converter transmits a 50 Hz and a 5.000 Hz tone at the same time, then a "virtual" one moves Tweeter back and forth on a bass chassis. The membrane excursion of the 50 Hz tone causes the 5.000 Hz tone to be compressed / stretched 50 times per second. The tone sounds rough.