i also miss Brooks
i stop in there now and then
i have his SP-6C and MC250
i stop in there now and then
i have his SP-6C and MC250
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/
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."
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- 80 posts total
Trytone wrote: "Perhaps it is not that big a problem, Google "thermal compression" loudspeakers and there is a Stereophile article from 2006, complete with measurements." I remember that article and it dismissed thermal effects as being inconsequential. Since then I’ve had relevant conversations with two of the top researchers in the field, Earl Geddes and Floyd Toole. I asked Earl to look at the article and he told me what he found to be wrong with the methodology, the details of which I do not remember. But he felt the measurement method was fatally flawed. (In another conversation he specifically mentioned large voice coil diameter as a characteristic which minimizes thermal compression.) Later I had a conversation with Floyd Toole on the subject of "thermal modulation", which is rapid-onset (short time constant) thermal compression (as opposed to thermal compression based primarily on the loss of magnet strength as the magnet heats up over time). He said that it is a real phenomenon which has not been adequately studied, and went on to give the example of a three-way speaker he had tested whose midrange driver was compressing peaks on normal program material at normal home audio sound pressure levels by 7 dB! Finally, some years ago a recording engineer shared with me details of measurements that he had been recording for many years on many loudspeakers, looking specifically at how much they compressed a 20 dB peak. It was not unusual for a loudspeaker to compress a 20 dB peak by 3 or 4 dB, but some exhibited no compression (I don’t recall what SPLs he was using for his tests). He found a general correlation between efficiency and lack of compression on the peaks. There were some moderate-efficiency speakers that defied the trend, and these had unusually robust motors which presumably had very high thermal capacity. I’m not posting the details of what he told me here because he asked me to keep the details confidential, nor have I used some of the terminology that he developed, as he intends to write a paper based on his findings at some point. Most of my work is in the prosound world, and there the awareness of thermal compression is alive and well. If a prosound speaker is pushed hard, it compresses. My point being, thermal compression and thermal modulation (short-term thermal compression) are real things, but unfortunately that information is not widely known in the home audio world. It’s almost like a dirty little secret that speaker manufacturers don’t want to talk about. No manufacturer wants to admit that his megabuck exotic-diaphragm tweeters have thermal compression issues that a thirty dollar compression driver doesn’t. Duke |
My point being, thermal compression and thermal modulation (short-term thermal compression) are real things, but unfortunately that information is not widely known in the home audio world. It’s almost like a dirty little secret that speaker manufacturers don’t want to talk about. the stereophile article says an 8% increase in resistance corresponds to a temperature increase of just under 20°C (36°F), and corresponds to a thermal compression of just 0.67dB https://www.stereophile.com/content/hot-stuff-loudspeaker-voice-coil-temperatures-page-2#R4eshWmRTxL... Its negligible. Most of my work is in the prosound world, and there the awareness of thermal compression is alive and well. If you put a $1k prosound speaker that does 5kw against a Magico, the prosound speaker would go louder but it wouldn't be as refined or high quality as the magico. If you were playing at such high levels in a home environment the main concern would be hearing damage rather than thermal compression. |
There is a lot of mis conception and misunderstanding about dynamic range, The reason that many older audio components used loud controls and is now the common practice for recording technicians to compress the dynamics of recordings, is because human hearing is not equally sensitive to all frequencies at mid to low volume levels. I know we each, as individuals, like to believe that we possess some kind of incredible hearing; the truth is, regardless of how good we think our hearing is, at low volume levels (at your listening position) both high and low frequencies will be diminished and at volume levels below 30db, will seem almost non existent. Just because any given speaker measures flat at 1W/1M doesn't necessarily mean it will be more musical, eight or ten feet away, at your listening position. There is a lot to be said for Michael Greens idea of tuning. Regardless of the speaker type or design and the associated electronics and how they measure, what's important is how well your speakers and equipment produce an enjoyable musical experience in your environment at a volume level acceptable to you....Jim |
- 80 posts total