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Old 31st May 2008, 03:33 PM   #3831
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Default Sensitivity and Efficiency

Quote:
Originally posted by gedlee
Beyond about 96-98 dB / watt the improvement will become insignificant. As the efficiency drops below about 92-94 the improvement is significant.
I am surprised to hear you say this, Earl. dB/W/M figures describe sensitivity, not efficiency. Without also knowing directivity, you can't translate SPL/W/M into conversion efficiency. But I think you are assuming omnidirectional radiation in this statement.

Even so, assuming omnidirectional radiation, 96dB/W/M is about 2.5% efficient which means most power is converted to heat. If the speaker is more directional, like a horn, then 96dB/W/M describes an even less efficient power conversion, since the sound is concentrated in a smaller field, and so not as much total power is radiated. If efficiency is fixed and the radiation angle is reduced, sound energy is more focused and sound pressure level within the coverage angle increases. So all that said, 96dB/W/M is 2.5% efficiency at best, maybe less depending on radiation pattern.

A speaker that generates 92dB/W/M in an omnidirectional pattern is 1% efficient. So 96dB/W/M is 2.5x as efficient. Any time you double the efficiency, you can expect 3dB increase and better thermal performance. But still, in either case, we're talking about single digit conversion efficiency, so most power is still converted to heat. One would hope for a good cooling system in the speaker, else the temperature local to the voice coil will rise. Voice coil venting may be useful at low frequencies where the cone can act as a pump and move air through the gap, across the coil. But at midrange frequencies, cone excursion is small and the size, shape and geometry of the vents isn't optimal for airflow at those frequencies. The cooling vents are stalled above bass frequencies, so the only way to get heat out of the motor is with other measures, like the heat wicking mechanism that Nick and I have been talking about.

This isn't limited to prosound applications, as Lynn said. Even if you're just putting a few dozen watts in your speaker, that power is converted into heat. At 2.5% efficiency, most of the power is converted into heat. Think of the heat of a 25 watt soldering iron contained inside a thermos bottle. That's pretty much what you have in a loudspeaker, with the voice coil on the pole piece surrounded by the magnet. It gets pretty hot in there.
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Old 31st May 2008, 04:06 PM   #3832
gedlee is offline gedlee  United States
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Wayne - I know the difference between sensitivity and efficiency. I should have said "sensitivity drops below ..."

But it is the sensitivity that matters, not efficiency, regardless of directivity, because we are interested in a specific SPL at the listening point (ignoring the reverbereant field for the moment). So 6 dB more sensitivity results in 1/2 the heat generated for an equivalent SPL - thats 100% improvement. AT best a cooling system might increase the heat flow by 30-50% and that is high. So my point stands that efficiency/sensitivity is far more effective at thermal managment than any cooling techniques. One need only resort to cooling techniques when the efficiency has maxed out AND you still haven't reached the desired SPL. This is NOT going to happen in a home situation. Any speaker with sensitivity above about 96 dB/watt/meter is going to be fine in a home. Anything less than about 92 dB is going to have trouble with higher SPLs and anything less than say 88 dB is going to have thermal problems at almost any listening level.
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Old 31st May 2008, 04:07 PM   #3833
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"Even so, assuming omnidirectional radiation, 96dB/W/M is about 2.5% efficient which means most power is converted to heat."

Hello Wayne

I think the real issue is about amount power going into VC in the first place. Thats what the higher sensitivity really tell us depending of course on how that was measured. Averaged over the bandwidth of the drivers vs. measure at a peak.

If you work the numbers from 92db vs 98db that's a 4 fold power increase into 92db driver. Assuming thats at 1 watt you get 4 into the other driver. If you go up 10db in level that becomes 10 vs 40 watts. 40 watt's is a lot of power and 10 watts seems a much more reasonable number for the driver to heatsink.

As you go up in level is where you see significant power level diferences. That's where I think the advantage lies.

Rob
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Old 31st May 2008, 05:06 PM   #3834
KSTR is offline KSTR  Germany
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Default Re: Black is alwawy in fashion...

Quote:
Originally posted by moray james
I forget the percentage (but 10% sticks in my mind) you can increase your thermal disipation by a good factor just by making your parts black. Obviously thermally concuctive colour like oxide is better than paint but even a black paint job will increase your thermal disipation. So this is almost as easy and free as it gets and it all adds up. Some heat sink expert can supply the fine details.
From the OnSemi "Rectifier Applications Handbook", Chapter "Cooling Principles", a table about heat radiation emissivity (vs. the Black Body):

Interesstingly the color of anodization and paints isn't specified...

- Klaus
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Old 31st May 2008, 05:14 PM   #3835
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Quote:
Originally posted by gedlee
But it is the sensitivity that matters, not efficiency, regardless of directivity, because we are interested in a specific SPL at the listening point (ignoring the reverbereant field for the moment). So 6 dB more sensitivity results in 1/2 the heat generated for an equivalent SPL - thats 100% improvement. AT best a cooling system might increase the heat flow by 30-50% and that is high. So my point stands that efficiency/sensitivity is far more effective at thermal managment than any cooling techniques. One need only resort to cooling techniques when the efficiency has maxed out AND you still haven't reached the desired SPL. This is NOT going to happen in a home situation. Any speaker with sensitivity above about 96 dB/watt/meter is going to be fine in a home. Anything less than about 92 dB is going to have trouble with higher SPLs and anything less than say 88 dB is going to have thermal problems at almost any listening level.
I agree with you to a large degree, it's better to increase efficiency than to improve heat management. But the two don't have to be mutually exclusive and I think it is important to look at the real numbers in any case. On one hand we can agree that high efficiency speakers convert more electrical energy to kinetic energy and ultimately into acoustic energy. On the other hand, I think it is important to remember that even a speaker that generates 96dB/W/M omnidirectionally is only 2.5% efficient.

When we're sending only 10 watts to a prosound woofer designed for 600 watts, heat isn't an issue. But once you start increasing power to 50 watts, 100 watts, 200 watts, these thermal issues start coming into play. Our speakers are certainly capable of those levels and they perform very well. But there is room for improvement, and one place to do it is with motor core heat management.

Quote:
Originally posted by Robh3606
I think the real issue is about amount power going into VC in the first place. Thats what the higher sensitivity really tell us depending of course on how that was measured. Averaged over the bandwidth of the drivers vs. measure at a peak.

If you work the numbers from 92db vs 98db that's a 4 fold power increase into 92db driver. Assuming thats at 1 watt you get 4 into the other driver. If you go up 10db in level that becomes 10 vs 40 watts. 40 watt's is a lot of power and 10 watts seems a much more reasonable number for the driver to heatsink.

As you go up in level is where you see significant power level diferences. That's where I think the advantage lies.
That's very true. It's really a matter of scale. What matters is the driver be used in a range where it has good thermal performance. I prefer high efficiency speakers over low efficiency speakers and do not make anything with on-axis sensitivity under 95dB/2.83v/M. Still, even a high efficiency speaker converts most input power to heat, and if the listening environment is large, a lot of power is required. In those situations, both high efficiency and heat management are important. Even in home hifi use, I think there is a benefit in having both. It never hurts, that's for sure.
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Old 31st May 2008, 06:00 PM   #3836
gedlee is offline gedlee  United States
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Quote:
Originally posted by Wayne Parham


Still, even a high efficiency speaker converts most input power to heat, and if the listening environment is large, a lot of power is required. In those situations, both high efficiency and heat management are important. Even in home hifi use, I think there is a benefit in having both. It never hurts, that's for sure.

In my listening room I could cause physical pain and never clip my 100 watt amp. The room would have to be huge for there to be a thermal issue with Hi-eff speakers. Since all "fixes" cost money, fixing a problem that doesn't exist is not good engineering. Even if "It never hurts".
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Old 31st May 2008, 06:14 PM   #3837
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Quote:
Originally posted by gedlee
In my listening room I could cause physical pain and never clip my 100 watt amp. The room would have to be huge for there to be a thermal issue with Hi-eff speakers. Since all "fixes" cost money, fixing a problem that doesn't exist is not good engineering. Even if "It never hurts".
My four Pi loudspeakers are very similar to your Summa's, so I am quite aware of the SPL they'll generate. 100 watts isn't a lot for them, but it is enough to start introducing thermal and magnetic issues. If it weren't for gap venting and other thermal management already built-in to the drivers we use, they would be in serious trouble at 100 watts. As it is, they are pretty good for another 3dB or so above that. After that, they're getting pretty hot.

You may not care how speakers perform above 100 watts, and that's fine. But I do.
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Old 31st May 2008, 06:30 PM   #3838
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Default A bit about Lowthers

I was off at VSAC, and thus am a bit late getting into your discussion about Lowthers.

The aluminum voice coils have several sharp bends in them. The bends have resulted in work hardening of the coils. This is a potential point of failure. Warming up the voice coil before playing it will result in these areas becoming less brittle, and therefore less likely to fail. I have seen a few failures of this type - but they are rare.

Lowther does have a Hi-Ferric coating on the voice coils, which consist of baked on iron particles. It does help transient response. It is applied as standard, but can be left off by request. My personal preference is to leave it off of the Alnico drivers, and have it applied to the others. This is due to the warmer, richer nature of the Alnico sound, which does not come through as well with Hi-Ferric applied. On the other drivers it helps the transient response without any ill effect.

The voice coil on all Lowthers (except the 4 ohm versions) are wound on both the inside and the outside of the voice coil.

A little bit about VSAC. I used Tone Tubby 12 Alnico drivers with PM6A's on a 18" x 40" open baffle. VSAC information You can see a bit here. Thanks for the information about those Tone Tubby's Lynn, they really did sound great. Anyway, lots of positive comments about their sound. They hung out to about 60 hz or so, and my Hartley 18" in the corner picked up around 50 hz.
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Old 31st May 2008, 06:53 PM   #3839
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Default Re: Re: Black is alwawy in fashion...

Quote:
Originally posted by KSTR
From the OnSemi "Rectifier Applications Handbook", Chapter "Cooling Principles", a table about heat radiation emissivity (vs. the Black Body):

Interestingly the color of anodization and paints isn't specified...
I would assume for paint that they used black. But the interesting thing is that it doesn't matter for anodization. Any color has pretty much the same emissivity at the IR wavelengths of interest.
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Old 31st May 2008, 09:05 PM   #3840
mige0 is offline mige0  Austria
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Default Re: Sensitivity and Efficiency

Quote:
Originally posted by Wayne Parham

This isn't limited to prosound applications, as Lynn said. Even if you're just putting a few dozen watts in your speaker, that power is converted into heat. At 2.5% efficiency, most of the power is converted into heat. Think of the heat of a 25 watt soldering iron contained inside a thermos bottle. That's pretty much what you have in a loudspeaker, with the voice coil on the pole piece surrounded by the magnet. It gets pretty hot in there.
Wayne, lets not mix up two very different effects of heat in a speaker which easily happens for those from the pro department.
- quality of sound
- reliability

Second one - wont be a problem to most home gear not heavily abused even if its low sensitive. At least I'm not that much interested into improving that unless the glue dribbles on my carpet.


First one yes that's the one I am really interested in. The solution to improve "distortion" due to the heat up of the VC wire and to shorten temp decay DRASTICALLY in real world can't be provided by improved cooling techiques (at least not passive = without refrigerator) regardless of how big your cooling fins or heat pipes are nor by pushing sensitivity of a speaker.


The ONLY ways to get rid of the big sonic impacts of heat in a speaker - somehow sloppy called power compression - is either using something similar behaving like a current amp or using a VC suitable wire with close to zero tempco.


Figures of power compression are good to have (though seldom seen) but don't tell the whole story.
Thermal transients have to be better specified than by a random noise measurement. The rise and decay and to what degree the cooling mechanism is program dependant should be reflected.


Greetings
Michael
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