- Intermodulation distortion (IM)
- Amplitude modulation (AM)
- Phase shift
The first hurdle
I am reminded by the Rio Olympic games how problems can be taken one hurdle at a time. Designers face challenges like runners face obstacles. If you want to design a full range single-driver loudspeaker, there are some seriously tall challenges to face. First and foremost, at least in my mind, is what we commonly refer to as the Doppler Effect (or distortion). Doppler distortion is what causes train whistles to change frequency: higher pitched when approaching, lower when moving away from you. Doppler occurs because the wavefront is 'compressed' as the object approaches the listener, and 'stretched' as it heads away. This happens because the phase of the signal is constantly changing, resulting in the frequency shift (higher to lower). If you take a single driver to cover all frequencies, large slow moving excursions of the cone compress and expand smaller, faster moving, higher frequencies in a type of distortion that has long been associated with the Doppler Effect. It turns out, this isn't completely accurate. Doppler distortion means the object generating the sound is physically moving towards or away from you—which is clearly not the case with a speaker. The cone's movement is moving closer and further from the listener, the speaker itself is not. Yet distortion remains. What’s actually happening when the single cone moves at slow and fast frequencies at the same time is three forms of audible distortion:
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