Loudspeaker Distortion with Low-Frequency Signals
Three differing forms of distortion, which are associated with low-frequency signals in loudspeakers, are investigated. It is shown that distortion due to the Doppler effect can be compared with that due to wow and flutter in recording machines, and subjective data obtained for this purpose can be applied to loudspeakers. Generalized design limits for loudspeakers are calculated. In loudspeakers designed to reproduce low frequencies, the voice coil is made longer than the magnetic field. At low frequencies, when the amplitude of vibration of the cone exceeds the difference in length, it is shown that instead of the peaks of the waveform being clipped, expansion of the input-output curve takes place. This effect, with its associated distortion, can be compensated by employing an appropriate nonlinear suspension, and thus a much greater useful output can be obtained than by using a linear suspension. Finally, a vented cabinet is often used to reduce the magnitude of the undesirable effects previously mentioned as well as to extend the bass response. However, a vented cabinet is a resonant system and high sound pressures and particle velocities are produced in the vent. These are liable to give rise to distortion from the inherent nonlinearity in the air and from turbulence at the orifice and in the pipe. Existing data ae used to estimate the sound levels which may be generated in a typical listening room before distortions from any of these causes are audible. It is also shown that this form of distortion is not a troublesome factor in the design of studio monitoring loudspeakers.
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