The initial purposes of the analysis on this set of pages was to try and decide what mechanism (or mechanisms) might be the main cause, or causes, of tweeter damage in domestic loudspeakers. The main hypotheses being compared being:
- Harmonic generation due to clipping altering the waveform shape. Thus damage occurs due to HF which would not be present at all in an unclipped waveform.
- Differential compression where the clipping limits the LF level, but permits the HF to rise by an amount comparable with what would be possible in the absence of clipping. Thus damage occurs due to the HF which would be present in the waveform even if its shape was unaltered by clipping
On the basis of the analysis carried out, and the examples used, the amount of HF/LF differential clipping which occurred did not seem to be very great. When the example waveform was increased in amplitude and clipped, the HF power level rose by an amount that was broadly similar to the rise in the LF power level. Hence the compression, as such, did not seem to behave as implied in the above hypothesis. However, despite being compressed by about as much as the LF, the HF power level was seen in the example to become high enough for it to be plausible that it might be enough to cause damage in some cases. It is also possible that a larger HF/LF differential in compression would be evident with other examples of musical waveforms, and the one chosen here may not be ‘typical’ of most or all situations.
On the basis of the analysis of the effects of clipping an essentially sinusoidal waveform, it seems quite possible that, in some cases at least, the resulting HF power generated might be high enough to lead to a risk of tweeter damage if clipping were severe enough and sustained for long enough. Hence it seems plausible that HF generation by clipping may be a mechanism for such damage in some cases.
To the above we must add the possibility that some of the LF power may also ‘leak through’ the crossover to the tweeter, adding to the total power it must deal with. This may add to any overheating, and also produce displacements large enough to create high levels of mechanical stress.
In general, therefore, it seems quite plausible to assume that damage may sometimes occur due to any one of the following mechanisms, or various combination of them:
- HF creation by clipping of the LF.
- Rise in the HF level of the original signal which – although limited by clipping by an amount similar to that which occurs at LF – rises to a high level.
- Leakage of LF power.
- Non-themal effects due to an increase in the peak displacement or accelerations.
The main conclusion is that there is no obvious reason indicated by the analysis why we should either rule out any of the above factors entirely, or to assume that only one of them is always (or usually) a sole or primary cause. Which is in practice a more common cause, or whether they tend usually to act together, would be hard to say without more information on typical systems and musical waveforms in order to do a relevant statistical analysis, etc.
Jim Lesurf
March 2003
