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In Reply to: Euphonic Compression posted by RandyB on November 11, 2002 at 18:24:44:
All speakers exhibit compression as a result of heating.The copper of the voice coil heats up as a result of power dissipation (don't forget, we're only talking 1-2% efficiency here for normal non horn drivers.) When it heats up, the resistance causes less current for the same voltage drive from the amp. The time constant is a thermal one, rather slow, not cycle to cycle.
If the different drivers are not matched in this respect, the frequency response will change with time at higher power levels.
The magnet structure will have some hysteresis, but that is to be avoided, it will cause all sorts of harmonics.
Follow Ups:
Hi, John:Thanks for the background info . ..
The line of speakers I'm developing is designed to be used with low-powered SET amps from 1.5 - 10 watts, although they will handle upwards of 100 watts rms. With the low wattage of SET's, will there still be heat-related compression?
If so, how do you determine heat-compression compatibility between woofer and tweeter, so that they stay "in-synch", compression-wise?
Also, what if there is a whole lot of bass going on, and little up on top, wouldn't this make it very difficult to maintain both drivers in heat-compression synch?
Thanks, rdb :-)
##The line of speakers I'm developing is designed to be used with low-powered SET amps from 1.5 - 10 watts, although they will handle upwards of 100 watts rms. With the low wattage of SET's, will there still be heat-related compression? ##That would be driver dependent. Some manuf's give the info.
##If so, how do you determine heat-compression compatibility between woofer and tweeter, so that they stay "in-synch", compression-wise?##
I would assume matching compression parameters. Unfortunately, different musical programs will give different power spectra.
##Also, what if there is a whole lot of bass going on, and little up on top, wouldn't this make it very difficult to maintain both drivers in heat-compression synch? ##
Same answer as last...But, I run a mobile rig for hobby/party stuff... Never too much bass. Gotta shake the martini's without bartender intervention.
I remember some application using light bulbs for compression. Don't remember what app, though. Light bulbs have about a ten to one resistance ratio from cold to hot. In series with a speaker, they'll be low resistance for low levels, and at high levels, will warm up and increase resistance. By playing with different wattage/voltage bulbs, you could conceivably tailor the "compression" in this fashion.
Keep this in mind: I run 300w/per channel for subs, my light rail attaches to the sub cabinets. When I run "hot", the 120 volt, 300 watt bulbs tend to blow out; they don't like vibration. I had to design an isolation mount for the lighting effects fixture. The 12 volt bulbs, with thicker filaments, didn't show that sensitivity.
TTFN, John
PS. IF you patent something using this, I want credit too.
Hi, John:What driver parameter provides heat-compression data? If two drivers have different coefficients of heat-compression, what can be done to put them in synch?
I guess this is where ferro-fluid comes in handy, to help reduce voice coil heating and subequent increase in resistance. If the woofer doesn't have adequate cooling, and a ferro-fluid tweeter is used, I guess this means that over 1/2 of all consumer loudspeakers have heat-compressive woofers and non-compressive tweeters, so after 1/2 hour of play, the woofer is compressive and the tweeter is non-compressive.
This phenomenon might mean that designers of loudspeakers should take this into account, and take measurements after the voice coil has had a chance to warm up. If so, this would seem to indicate that like electronics, speakers need to "warm up" in order to achieve their steady-state performance equilibrium, at least where the woofer and tweeter have different coefficients of heat-compression.
I imagine the average audio consumer wouldn't want to buy a loudspeaker if he thought that its performance would change depending on how loud it is played, how many watts are being used, what the temperature of the voice coil is, etc.
If a design could be implemented that would take heat-compression into account as between woofer and tweeter, which would maintain synchronized compression modes, this might be considered to be a value-added benefit, i.e., it might have marketing value to differentiate this type of design from everthing else. Or is this just too esoteric to matter to a typical audio consumer? It would have to be explained in accessible terms such as "iso-comp" or "equisterisis" or similar. The benefit would be less coloration at loud SPL's, so while dynamic compression can't be eliminated, it can be managed to prevent tonal anomalies.
Anybody out there working on anything like this, or has it already been implemented in a consumer loudspeaker? Or is this new turf, ready for some enterprising individual to plum the depths of the technology and come up with a new design paradigm?
Best regards, rdb :-)
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