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:

• Intermodulation distortion (IM)
• Amplitude modulation (AM)
• Phase shift

Of these, the most audible is phase shift, caused by the position of the cone at any instant in time relative to the listener. Low frequencies signals can move the cone quite a lot, as much as half an inch forward and the same back, for a total of one inch. Movements of this size cause quite audible IM distortion of the high frequencies. IM distortion is a term we hear a lot of, typically it is ganged together in spec sheets with THD. THD (Total Harmonic Distortion) refers to the addition of harmonics that weren't in the original signal. IM refers to the effect of one frequency modulating (making louder or softer) another. In this case, the 'one' frequency is the slow moving woofer as it makes bass notes, causing higher frequencies to get louder and softer in synch - something not part of the original signal. More tomorrow.