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 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."

 


alexberger

Showing 5 responses by bdp24

@tomic601. Small world; the SP-6 you got from Brooks came from me, and before me it belonged to Steve McCormack. I bought it from him when he was closing his retail store (named Music By The Sea, iirc) to focus on The Mod Squad and McCormack products.

Duke, Brooks Berdan Ltd. is in Monrovia, a 15 minute drive east of Pasadena. He had a fair number of regular customers who worked at either the Jet Propulsion Laboratory or Caltech, smart guys with good educations. A goodly number of them bought either Jadis or VTL pre and power amps from Brooks, but often chose Wilson speakers, for reasons that eluded me. No offense, Wilson lovers! It was Brooks who turned me on to the Eminent Technology LFT-8 loudspeaker and Music Reference electronics, both of which he sold. It was from his wealthy customers that I learned sound quality was often perceived in relation to how much it cost. I place a fair amount of blame for that on Harry Pearson.

Bruce Edgar would come in the shop frequently, always updating the regular Saturday group of hi-fi nerds with news of his loudspeaker progress. I liked the idea of his horns being carved out of wood, as I associated the "horn sound" with that of ringing metal. I really wanted to hear a pair, but Brooks was a funny guy, very opinionated and somewhat ornery. Really good at what he did, though. Bill Johnson had him drive out to his winter home in Indian Wells (next door to Palm Desert) to set up his turntable. Brooks could find the optimum location for a pair of loudspeakers in a room faster than anyone else I’ve seen do it. Really good ears.

Damn @audiokinesis, I gotta get out more. ;-) The Edgar of Edgarhorn came into Brooks Berdan Ltd. regularly, but Brooks didn't seem to take him seriously, and never had a pair of the horn speakers in his shop to listen to. Near the end of his life, Brooks took a pair of Klipsch K-Horns in on trade (and a pair of Wilson WAMM"s!), but took them home for his own use.

His main speaker at home was the Jadis Horns, but when I was over at his place it was usually to bring him a drumset (I was his "dealer" ;-). His wife Sheila (who now manages the shop) hired me to put a band together to play at his 50th birthday party, at which he got up on my set and played "Wipe Out". Brooks had been in a high school garage band, and hadn't played since. It was the happiest I ever saw him. 

Performing on stages with Altec Voice-Of-The-Theater PA speakers starting at a young age (before I heard my first audiophile speakers), I had a bad first impression and opinion of horn loudspeakers. But the VOTT was designed for a different purpose than are hi-fi speakers. When I did started hearing hi-fi models (late 60’s), I still heard in varying degrees the colored sound (what J. Gordon Holt coined "cupped hands vowel colorations") of PA speakers. One glaring example was the JBL Century L-100 (recently reintroduced), which I found absolutely unlistenable.

Hearing my first ESL’s in 1971 (the Infinity Servo-Static, and the RTR tweeters in the ESS Transtatic) set my standards for "uncolored" (think color temperature in a video monitor) sound pretty high. Same for the quality Gordon called "liquid transparency"; grain-free, like Kodak 25 ASA film. I haven’t heard many high-sensitivity dynamic (cone) speakers, so I’m very interested in any of them that are free of the colorations of horns. Or in any modern horns that are free of the classic horn colorations. Any loudspeaker that changes the timbre of a vocalist is immediately crossed off my list; for me, that is the one mortal sin in a loudspeaker!

I have tapes I recorded myself using a pair of omni small diaphragm condenser mics plugged into a Revox A-77, both of spoken voice and a Jump Blues Band I was a member of. Comparing the sound of my son’s voice to it reproduced by a loudspeaker is a brutal test! Same for the sound of my Gretsch drumset and Paiste 602 cymbals, our two sax players’ tenor and baritone Selmers, the upright piano, the Fender Telecaster Bass, the Gibson 1-pickup hollowbody of the other players, and the vocalists (a white man and black woman).

Highly-dynamic and un-dynamic are relative terms. Anyone care to name speakers of either type he considers prime examples? Klipsch and other horn-designs are sure to be first on the highly-dynamic side, but how about non-horn speakers? I imagine Maggies will be high on the other side, though lovers of that design might not totally agree.

I find that ESL's make concerns about dynamics disappear; the speakers sound so "quick", their abilities at micro-dynamics are SO much better than with any other type, that macro-dynamics become less important. Just my point-of-view.