Exlibris wrote: "Thanks Duke. I’ve been thinking of doing that, at your suggestion, and have been looking at the Townshend maximum supertweeters."
I’d look primarily at adding some rear-firing energy from about 12 kHz to about 20 kHz... the Fostex FT17H is imo a fairly low-cost way of exploring this idea.
"Sometimes it’s as if my system has even less jump after 2 or 3 hours of listening. I’ve often wondered if this a subjective thing or if it was something in my amplification chain. Reading a post on another forum, the poster said this could be caused by hot speaker voice coils."
That would be my guess.
A 50-watt peak is like touching a 50-watt soldering iron to your woofer’s voice coil. It heats up instantly but cools down slowly. The voice coil’s resistance goes up with temperature, and the motor structure is the primary "heat sink" that the voice coil radiates into (the air inside the box is also heated). As the motor gets pretty hot there is a reduction of magnetic strength, which comes back once the magnet cools off. This is one of the downsides of going with drivers that have relatively low thermal capacity.
Thermal compression is a big consideration in prosound, and imo it is relevant but under-appreciated in home audio. Floyd Toole told me of some of the rapid-onset thermal effects he observed during his tests, and said that this is an area which has not yet been adequately investigated.
As a ballpark rule of thumb, thermal compression is typically about 1 dB at 10% of a driver’s rated AES power handling, and typically 3-4 dB at its AES rated power handling (which in turn is 1/2 its "music program" or "continuous" rating, and typically 1/4 of its "peak" rating.) This refers to the long-term compression effect once the voice coil and motor temperature have stabilized, so it takes a while set in. There are also the short-term effects that I spoke with Toole about, and my assumption is that there’s some correlation.
Not surprisingly, prosound-type drivers tend to have negligible thermal compression at quite loud home audio levels because they have both high efficiency and high thermal capacity. The challenge is, beating prosound drivers into submission so that they don’t have any distracting colorations. That’s what I try to do, because imo they have enough inherent advantages that it’s worth the effort. Obviously most speaker designers feel otherwise!
Duke
