"reflected" at equal amplitude in the frequency domain, on the other side of the Nyquist limit (half the sampling frequency), causing an unacceptably high level of ultrasonics to accompany the desired output. No workable scheme has been found to adequately deal with this. The term "digital" or "digital-ready" is often used for marketing purposes on speakers or headphones, but these systems are not digital in the sense described above. Rather, they are conventional speakers that can be used with digital sound sources (e.g., optical media, MP3 players, etc.), as can any conventional speaker. See also Audio power Audiophile Bandwidth extension Directional sound Earphone Echo cancellation Electronics Ferrofluid#Heat transfer Guitar speaker Headphones High-end audio Isobaric loudspeaker List of loudspeaker manufacturers Loudspeaker acoustics Long Range Acoustic Device(LRAD) Music centre Parabolic loudspeaker Phase plug Planephones Rotary woofer Shelf stereo Sound Sound from ultrasound Soundbar Speaker driver Speaker stands Speaker wire Speakerphone Studio monitor Super tweeter Surround sound References External links ALMA – A Forum for the Global Loudspeaker Industry Audio & Loudspeaker Technologies International (ALTI) Conversion of sensitivity to energy efficiency in percent for passive loudspeakers Article on sensitivity and efficiency of loudspeakers Speaker Principles Illustrated guide to loudspeaker design and practice Types of Loudspeakers American inventions Audiovisual introductions in 1924 Audio engineering Music technology Consumer electronics