Low Sensitivity Speakers Compression Issue.
Main stream Hi-end producers, audio reviewers and sellers push low sensitivity tower speakers and high power amplifier solution to audiophile society as "reference" low distortion solution.
My experience shows me that this solution doesn't work.
I had myself Dynaudio Audience 60 speakers. These speakers was my worth purchase in audio.
These speakers sound horribly compressed. They sounded completely different on different volumes.
I have listened many different low sensitivity tower speakers and high power amplifier in audio shows, dealers and people houses. This kind speakers ALWAYS sound compressed even with 1000w amplifiers.
I heard about TD in speakers. This distortion are not linear, because they have a time inertia.
Here is I found a very good article written by Derek Alexander-Wilson
that enplanes this issue: https://www.linkedin.com/pulse/power-compression-vs-thermal-distortion-loudspeaker-alexander-wilson/
"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
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
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
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
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
(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