Loudspeaker Doppler Distortion

The Doppler effect can affect loudspeakers causing distortion on high frequency tones in the presence of low frequency notes.

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Under some circumstances the Doppler effect can have a noticeable affect on moving coil loudspeakers.

When designing high quality loudspeaker systems this is an issue that needs to be taken account of.

Loudspeaker Doppler distortion: the cause

Doppler effect can give rise to distortion - Doppler distortion in moving coil loudspeakers. The distortion is noticed when a high frequency tone is being reproduced at the same time as the cone is moving in and out to accommodate a low frequency tone.The Doppler effect causes the pitch of the high tone increases as the cone moves forwards with the bass tone, and then falls as the cone moves towards the back.

The level of modulation of the high pitched tone is proportional to the amplitude of the low frequency tone. This generates a form of frequency modulation.

It is found that loudspeakers with smaller cones are more affected by the Doppler effect. The reason for this is that they need larger excursions to produce the same audio output. In fact the FM distortion due to the Doppler effect is proportional to the square of the decrease in cone diameter. It is also found to be at its maximum along the axis of the loudspeaker, falling either side.

Minimising loudspeaker Doppler distortion

There are two main methods of reducing FM distortion arising from the Doppler effect on moving coil loudspeakers.

  • Use speaker with large cone diameter:   As the level of frequency modulated distortion decreases as the square of the increase in cone size, larger speakers will exhibit the Doppler effect much less.
  • Use multi-speaker system:   The best method of reducing the effects of Doppler distortion on a loudspeaker is to use a multi-speaker system. When this is done, low frequencies are diverted to the bass unit or woofer and higher frequencies are diverted to a tweeter or in three units systems to a squawker and tweeter. In this way the high frequency sounds are separated from the low frequency sounds and the problem does not arise.

The Doppler effect is not one that might be not thought to affect moving coil loudspeakers, and yet for high quality systems Doppler distortion is an issue that needs to be taken into consideration.

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