Physical Distortions created by the movement of the loudspeaker membrane
During recording of sound the condenser microphone membrane moves up to ±1 µm at low frequencies and high sound pressure levels.
During play back a loudspeaker membrane moves up to ±10 mm to reproduce that sound. In practical terms: We record at a fixed position in space, but we reproduce at a variable position in space. This 1 to 10000 relation of the membrane amplitudes leads to significant distortions even for an ideal chassis, which are independent from further distortions a real chassis may add because of its non-linearity.
In order to get an imagination, the following short animation may help. It shows the movement of the loudspeaker membrane as blue line parallel to the x-axis (elevated for better visibility).
The pure phase modulation, which is created by the varying distance to the receiver – listener or measurement microphone – is displayed by a green line.
The sound reaches – depending on the actual distance – the receiver at other times and the resulting phase is proportional to that and the frequency of the sound. This effect (a.k.a. Doppler effect) create the unwanted phase modulation.
The sound pressure is in practical terms proportional to the double differentiated membrane movement. It is represented by the purple line. As the sound pressure follows the inverse proportional law (1/distance law), an additional nonlinear amplitude modulation is created at the receiver. On top of the phase modulation a nonlinear amplitude modulation (NLAM) is added The length of the line varies in addition to its angle. For illustration purposes the movement of the membrane has been exaggerated for the animation.
For single frequencies both modulations generate mainly a second harmonic spectral line with further harmonic content at lower level. For two or more frequencies both modulations create additionally intermodulation products, which are not harmonic to their fundamental frequencies and significantly disturb a good listening experience.
The generated phase modulation (PM) is independent from the distance to the loudspeaker. The generated nonlinear amplitude modulation (NLAM) decays with increasing distance to the loudspeaker. Both effects show levels up to the 10% range at large membrane amplitudes.
For more information see the respective articles on NLAM and Doppler.
AudioChiemgau developed Analog and Digital AudioProcessors which are able to compensate these two physical effects allowing an undistorted listening experience.
All measurments are done with our ModeCompensator