Higher sensitivity - more dynamic sound?

@inscrutable wrote:

"if the amplifier has a very large - let’s say infinite - current capability, so can maintain voltage to be applied as the speakers impedance changes with signal frequency changes …?"

A constant-voltage amplifier will put out reduced wattage into a higher impedance.  That's just the way that type of amplifier behaves. 

However a constant-power amplifier will put out essentially the same wattage into the speaker's impedance even as it changes, at least within a realistic range of change. 

Most tube amplifiers have a constant-power characteristic, and therefore the system's dynamic contrast would be less sensitive to heat-induced changes in the drivers' impedances. 

Duke

@deludedaudiophile,

There are multiple mechanisms by which powert compression can occur, and minimizing them just about always favors high-efficiency designs. This will not be a textbook-comprehensive look at the topic, and I will make some generalizations along the way.

There are two main types of thermal compression, both originating with voice coil heating, and we’ll come back to them in a minute.

Flux modulation is a compression mechanism which occurs because the permanent magnet’s field strength is modulated - temporarily weakened - by interaction with the magnetic field induced in the voice coil by the amplifier signal. For a given SPL, in general the more powerful the motor (and therefore in general the more efficient the speaker) the less the magnetic flux is modulated. Faraday rings in the motor can reduce flux modulation; alnico motors are inherently relatively immune to flux modulation (and neodymium drivers less so); and field-coil motors (electromagnets) are effectively immune to flux modulation (because the perpetual current from the power supply instantaneously restores the magnetic flux).

Mechanical compression can also occur as the suspension system becomes non-linear at long excursions, and/or the voice coil exceeds its linear excursion limits. Big drivers are less likely to get into mechanical compression than are small drivers, and since big drivers tend to be more efficient, there is again a correlation between efficiency and low compression.

And now for the two types of thermal compression: As the voice coil heats up its resistance rises, and the voice coil in turn heats up the magnet over time, which reduces the magnet’s strength. When the magnet cools back down, its strength returns. Alnico magnets are relatively immune to thermal compression UNLESS they are overheated, and THEN they unfortunately will permanently lose strength. The greater the thermal mass of the voice coil and magnet, the more gradual this heating. And the less wattage required to reach a given SPL, the less heat there is in the first place. Compression due to the magnet heating up and losing strength takes a while to set in, and it takes a while to go away. This topic has been studied because it matters a lot in prosound.

Imo the most interesting and audibly significant thermal compression effect arises from the rapid heating of the voice coil due to high-power music transients. This heating is instantaneous. So a 100-watt peak is (to a crude first approximation) like touching the voice coil with a 100-watt soldering iron that transfers heat instantly. So there will be an instantaneous spike in the voice coil’s temperature and resistance, which dissipates fairly quickly to the surrounding air and motor assembly, but the onset is still much faster than the dissipation. What can happen is this: Since the PEAKS are where that instantaneous heating occurs, it is the PEAKS which are compressed the most. We might call this effect "thermal modulation", to distinguish it from the more long-term "thermal compression" which includes the aforementioned reduction in motor strength due to the magnet heating up over time.

Unlike thermal compression, "thermal modulation" has not been studied because it’s not a critical factor in prosound (where the financing for such studies usually comes from), and because there is insufficient incentive to finance such a study coming from the consumer audio side.

Floyd Toole had this to say on the topic, in a conversation with me on another forum:

"The audibility of power compression in its many variations probably could use some more research to define what is audible and what is tolerable. The magnet heating that you describe is important in pro audio sound reinforcement systems where the loudspeakers are required to work at or close to their design limits for long periods. Such heating and cooling has a very long time constant. This is not the case in most home systems. Although the modification of motor strength through magnet heating is a factor, most of the audible effects are from voice coil heating, which has a much shorter time constant. I just saw a test of a high-end audiophile speaker that in going from an average level of 70 dB (loud conversation, background music) to 90 dB (a moderate crescendo, or foreground rock listening) lost about 4 dB in output over about 3 octaves in the mid-high-frequency range. It became a different loudspeaker at different listening levels." [emphasis Duke’s]

In other words, the speaker Toole measured had a midrange driver which was subject to significant thermal modulation effects, while the woofer and tweeter were not. So 20 dB crescendos were compressed by 4 dB in the midrange region. Yuck!

Years ago Stereophile published an article which supposedly "debunked" the "myth" of thermal modulation. Their measurement procedure was flawed in that they did not sample the voice coil temperature at the instant of peak power, but rather they sampled voice coil temperature at regular time intervals and then averaged the results. So whatever was happening at the peaks was not captured.

Anyway as you can probably see, the less wattage required to reach a given sound pressure level, the less thermal modulation effect on the peaks. And also the greater the thermal mass of the voice coil, the less thermal modulation for a given power level. Therefore in general, big, high-efficiency prosound-style drivers have a significant advantage over small, low-efficiency high-end audio drivers in this area.

If you have ever heard a speaker whose tonal balance is different at different volume levels, this is probably because the thermal modulation effects are different for the various drivers. The designer has probably "voiced" the speaker to sound best at a particular volume level. Any driver will eventually run into thermal (or mechanical or magnetic) compression issues, but big efficient prosound-style drivers will have similar thermal characteristics over a much wider SPL range, so they are more likely to retain the same tonal balance from very low to very high volume levels.

Duke

so-called hardcore technical speaker designer

@lonemountain , I don’t follow your reasoning. You said:

"So your 102dB 1w/1m speaker may not have such good dynamics with a 20W amp if 1W= 102dB SPL then .2W=105, 4W =108, 8W =111, 16W=114dB SPL and that’s it! That’s 12dB of dynamic range.

"86dB 1w/1m, 89dB 2w, 92dB 4w. 95dB 8w, 98dB 16W, 101dB 32W, 104dB 64W, 107dB 128W, 110dB SPL at 256W/1m. 86dB 1/1m speaker on a 250W/ch amp = 24dB of dynamic range! 12dB dynamics is better than 24dB dynamics?"

The dynamic range of a system is the difference between the softest and the loudest sounds the system can produce. Assuming both systems are limited by the same noise floor, the 102 dB system is capable of 114 dB peaks while the 86 dB system is capable of 110 dB peaks. So in your example, the 102 dB system has 4 dB more dynamic range.

But that’s not the whole story. Making another comparison and using 110 dB SPL for both, the 102 dB system will have negligible compression effects from the 7 watts it needs (this may not be true if it’s a single fullrange driver), while at 110 dB/256 watts the 86 dB speaker is far more likely to have thermal compression effects. And since musicians use dynamic contrast to convey emotion, compression effects tend to rob the music of emotion and "life". The use of very high-quality drivers (and/or many drivers) in the 86 dB speaker does make a worthwhile improvement in this area.

Let’s revisit noise floor for a moment: To a crude first approximation the in-room reflection field can be thought of as a "noise floor" which can mask low-level sounds. For a given room, the narrower the loudspeaker’s radiation pattern, the higher the direct-to-reverberant sound ratio, and therefore the lower the "noise floor" imposed by the in-room reflection field. So IF our 102 dB speaker is also highly directional (which is likely), it will probably result in a lower effective "noise floor" and correspondingly greater dynamic range and may result in better retrieval of low-level details.

Edit: Looks like several of us were typing at the same time, and I was the slowest.

Duke

@deludedaudiophile wrote:

"You are convincing me more and more that active is the way to go long term."

Going active will have no direct impact on driver compression effects, but if you are "rolling your own", going active may make it easier for you to use drivers which have inherently low compression characteristics.

Duke

@deludedaudiophile, to quote my favorite line from The Princess Bride, with poetic license invoked:

"Speaker design is tradeoffs, Highness. Anyone who says differently is in marketing."

Duke

@lonemountain wrote:

"my point about dynamic range was "system dynamics" not one system vs another. The 86dB efficiency speaker on a 250W amp has more total dynamics to cover incoming source material of a wider dynamic content than the 102dB/20w amp system. "

In your example upthread, the 86 dB/250 watt system has a maximum SPL of 110 dB, while the 102 dB/20 watt system has a maximum SPL of 114 dB (actually the math says 115 dB).

How is 110 dB max SPL "more total dynamics" than 114 dB max SPL?

The only way I can see that happening is IF the system noise floor is at least 5 dB lower for the 86 dB/250 watt system, and that’s not something you have included in your example.

(Dynamic range does not start where the amp is producing 1 watt; dynamic range starts at the system noise floor. I mention this because, upon re-reading, one of your posts above seems to make that assumption.)

And here’s another real-world effect which may come into play: If the 20 watt amp is a tube amp (which is likely), and if the 250 watt amp is a solid state amp (also likely), clipping will become audible and objectionable about 3 dB sooner on the 250 watt solid-state amp than on the 20 watt tube amp. In other words, you can push the AVERAGE SPL 3 dB higher on the tube amp than on the solid state amp before clipping becomes audible and objectionable. So instead of the 102 dB/20 watt system having 4 (or more precisely 5) dB more dynamic range from the raw math, real-world that difference may be more like 7 (or 8) dB.

Duke

@deludedaudiophile wrote:

"I was more thinking smart people, i.e. like what I see with Kii, as a start, will figure out some way to compensate for these effects in real time."

I think you are right, though there is a limit to how much compression "active gain riding" can compensate for. More wattage to compensate = more heat = more compression = even MORE wattage needed to compensate...

"From a simpler aspect, would not an active cross-over system be less [do you mean MORE?] immune to the effects of variable voice coil resistance on speaker response including critical crossover points?"

Yes, but if you start out with drivers that don’t compress significantly anyway, you have already addressed the crossover issue. Also, second-order and higher passive crossovers are much less sensitive to driver DC resistance variations than are first-order passive crossovers.

Duke

@deludedaudiophile wrote:

"can you comment on room response of line arrays?

"... I expect that electrostatic speakers and large planar speakers must be fairly immune to these power compression / thermal modulation effects within limits?"

The room response of a given line-array system depends on the specifics. Obviously at frequencies where they approximate a line source the sound pressure level is falling off at about 3 dB per doubling of distance rather than the normal 6 dB per doubling of distance, not counting the contribution of reflections. There are of course tradeoffs to be juggled.

If small fullrange drivers are used, they beam moreso than most tweeters in the horizontal plane, resulting in a pretty big spectral discrepancy between the direct sound and the reflections. If tweeters are stacked alongside midwoofers, the frequency response in the crossover region changes significantly with the horizontal angle. If there is a central tweeter flanked north and south by stacked midwoofers, the tweeter may not approximate line source behavior as well as the midwoofer array.

All of that being said, with a line-source-approximating speaker the direct sound tends to be more dominant than with a point-source-approximating speaker, so I’m not sure how audible the aforementioned off-axis anomalies tend to be in practice.

Electrostatic speakers can have compression from transformer saturation at high power levels, and in extreme cases the transformer can overheat and melt. My impression is that in general they are less prone to compression than cone-n-dome speakers, but also less efficient and/or more difficult to drive, and in general will not go as loud as comparably-priced cone-n-dome floorstander speakers.

Single-ended planar magnetic speakers, those having magnets only on one side of the diaphragm, have a compression-ike mechanism because of the non-linear motor; that is, the motor strength decreases when the panel is further from the magnet, and increases when the panel is nearer the magnet, such that motor strength available for the higher frequencies is modulated by the lower frequencies. A push-pull motor structure eliminates this effect. I don’t know much else about compression mechanisms specific to planar magnetic speakers, but I am not under the impression that they are champions in the realm of dynamic contrast.

Loudspeaker/room interaction happens to be something that I give high priority to. The two types of speakers I sell are large, curved, line-source-approximating fullrange electrostats, and hybrid horn systems. I think full-range horn systems can be superb but they are inevitably larger than what I want to work with.

Duke

@jhills wrote:

"... it seems a bit irrelevant unless you are listening at live concert SPLs (104 - 115+ db)."

In my experience freedom from compression effects is audible at pretty much all volume levels, including low levels, which is where I normally listen.  I never listen at anything approaching "live concert levels", and only rarely listen "loud". 

Duke