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Power compression Vs Thermal distortion
Power compression is the common term for one of the audible effects of voice coil heating / overheating, i.e. when the voice coil reaches its maximum power handling limit trying to ram more power in is sonic train crash…It does not get any louder just compressed and grossly distorted.
This effect is better described as “Thermal Distortion” (TD) and is much more serious than just a maximum power handling limitation or side effect…..TD is a huge problem, overlooked or deliberately ignored by most manufacturers as there is no easy (low cost) solution and TD is audible and measurable most of the time at most power levels…!
TD is caused by the conductive metal (aluminium, copper or silver) voice coil getting hotter when you pass electrical energy through it….The more power you pass through it the hotter the metal gets….The hotter the metal gets the more the electrical resistance increases….The efficiency goes down and you need to ram in more and more power for smaller and smaller increases in SPL….a very vicious circle.
The vast majority of loudspeaker drivers are severely “under voice coiled” and nearly all suffer from massive audible and measurable TD even at medium power handling levels, long before maximum power where audible “power compression” kicks in.
Taking a look at the relationship between voice coil power handling and voice coil heating reveals that long before audible power compression becomes an issue, TD is a serious problem….So widespread that the entire audio industry accepts it as standard and never questions it.
TD creeps into sound reproduction just like harmonic or inter-modulation distortion from low power levels and gets worse the higher the SPL’s….Far more serious than the simple “loss of sensitivity” at max power as loudspeaker driver describe it.
TD causes severe program dependent tonal imbalances in multi-way active speakers and the effect is even worse in passive loudspeakers.
Room temp to 120 degrees C
Taking an example of a voice coil using 10 meters of 0.2mm Copper with a typical resistance of 5.5 Ohm at 20 degrees C.
Apply power (music signal) & the voice coil jumps to 120°C and the resistance rises to approx. 7.5 Ohm….Now the resulting current flow through the voice coil drops by approx. 35% ….So does the SPL….This is a 35% distortion!
Orders of magnitude worse than the usual suspects of THD or IMD which loudspeaker manufacturers like to quote.
With passive crossovers in multi way systems thermal power compression will shift XO frequencies when the voice coil resistance changes – resulting in summation errors between the two drivers involved, depending on program and listening level.
The same holds true for notching out resonances or compensating impedance….The more complex the passive crossover the worse the TD effect becomes.
Why 120 degrees C?
This is a safe or medium range voice coil temperature, many voice coils hit double or triple this temperature on a regular basis.
The energy involved is as follows:
The weight of our VC wire is slightly below 3g if we calculate length multiplied by cross sectional area multiplied by specific mass of copper.
We require approx. 1 Watt for 1 sec for each Kelvin temperature increase (no cooling assumed here), looking at the thermal material constants for copper
To heat up this < 3g of copper wire our voice coil is made from, we only need 10 sec of 10W input to finally arrive at a 100°C increase – or – if we have fortissimo playing – a very short 1 sec of 100W input.
Well within the realms of average….
Now if you want to enjoy your fortissimo at 115dB SPL and given your speaker would be 95dB / W / m - well - after one (!) single second you have reached 35% of distortion – given your speaker to be 105dB sensitive you can enjoy your fortissimo of 115db SPL a fantastic long 10 (!) seconds until you end up at reaching 35% distortion.
Obviously the higher the loudspeaker sensitivity the lower the TD effects, and the better the voice coil cooling the lower the TD effects. Most “raw” audiophile loudspeaker drivers struggle to exceed 89dB over their working range, Pro audio drivers average out around 94 dB to 95dB SPL for 1 watt at 1 meter over their working bandwidth.
Real world voice coil cooling inside loudspeaker cabinets is very limited and varies from almost zero in a small sealed box with small magnet / motor to reasonable in large vented cabinets with huge heavy ferrite magnets.
But still cooling down from a 100Ws input will take much longer than the near instantaneous heating up….
Using a single layer 50mm voice coil with a height of 13mm with no isolating former (Kapton or Nomex...) is the most optimistic scenario.
This will provide a heat dissipation area of around 40 square cm. A two layer voice coils will half the effective radiating area & a four layer voice coil reduces effective radiating area by a factor of 4!
Again, being optimistic & assuming the surrounding magnet structure remains at 20°C while the voice coil already has reached 120°C the heat dissipation will be approx. 3W.
This means the decay from our 35% distortion fortissimo would last around 30 sec. Unfortunately this does not hold true - because – if the voice coil is cooled down to 70°C (half way down) then there is only about 1W cooling through radiation left (again optimistically assuming the surrounding magnet structure still to be at 20°C), further stretching the time of distortion decay.
Using an aluminium former would help to enlarge the radiating area of the coil itself but the need to minimise moving mass forces designers to use ultra-thin aluminium which cannot support massive heat transport
Forced air / convection cooling will help but pianissimo follows fortissimo - you only have forced cooling due to voice coil and membrane movement during fortissimo…
The bottom line is TD is a massive problem….The elephant in the room.
Currently the best ways to minimise the effects are:
(1) Use an active crossover.
(2) Select loudspeaker drivers with the largest voice coils…Bigger is better.
(3) Single layer voice coils are best, two layers bad, 4 layers worst.
(4) Aluminium voice coil formers may help, but they have other sonic downsides compared to Kapton or Nomex.