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Old 8th January 2013, 02:43 PM   #1501
gedlee is offline gedlee  United States
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Quote:
Originally Posted by Omholt View Post
@gedlee
How important is high efficieny? Is efffiency related to dynamics?
I absolutely believe this is the case.

When current flows through a copper wire it heats up and its resistance changes (this change happens instantly). As the resistance changes the signals current through the wire gets modulated by this change. The larger the resistance change the more the "dynamics" get crushed.

There are two factors in the actual amount of resistance change that occurs - the amount of copper in the voice coil and the amount of current through it. The more copper there is the more thermal capacity it has and hence the less the temperature change for a given amount of heat. The more current there is the greater the heat.

The higher the efficiency the less current is required to generate a given SPL so the lower the heat generation, but high efficiency requires bigger voice coils so there is also more copper. Hence simply put, thermal modulation is proportional to the inverse square of the efficiency.

By my calculations the difference between a 1" dome tweeter and a 1" compression driver is somewhere in the thousands. I have measured this difference as well, being on the order of several dB - as much as 6 in some cases. It is NOT an insignificant effect. For woofers it is not as great, but it is still a factor. The real issue is that to have high efficiency at the high frequencies where it is a major effect and most audible you also have to have it at the lower frequencies as well.
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Old 8th January 2013, 03:13 PM   #1502
Omholt is offline Omholt  Norway
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Thanks gedlee.
What about the size of drivers for bass reproduction?
Does bigger come at a cost?
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Old 8th January 2013, 03:25 PM   #1503
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Inhibiting my first impulse to consider those comments on efficiency crazy out of well-earned respect for the author (not to mention the possibly casual use of the word "instantly" in an otherwise rigorous presentation).... For sure, I have never heard this idea or that distortion, compression, or whatever results in copper wired devices connected to low-impedance driving sources.

Without necessarily agreeing with a word of the theory, I like to hear more about the tests. Can you please tell us more about your tests?

Ben
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Old 8th January 2013, 03:48 PM   #1504
jcx is online now jcx  United States
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dynamic thermal compression distortion from voice coil heating is well known, copper has high resistance tempco, coils get hot, enough to change Rdc by several x10%
Rdc is a significant electroacoustic parameter

Introduction site is very good on speaker measurement, modeling, lots of papers

http://www.klippel.de/uploads/media/...ransfer_01.pdf
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Old 8th January 2013, 03:57 PM   #1505
Moonfly is offline Moonfly  Spain
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Quote:
Originally Posted by gedlee View Post

The higher the efficiency the less current is required to generate a given SPL so the lower the heat generation, but high efficiency requires bigger voice coils so there is also more copper. Hence simply put, thermal modulation is proportional to the inverse square of the efficiency.
Interesting post. The thing is, efficiency rating of a driver doesnt account for the cabinet its in, or the desired frequency range you want to use. If you want a good subwoofer that goes low, then high efficiency at 80hz isnt that big of a deal if the driver is rolling off quite a bit below this point. On paper the driver may look to be highly efficient, but if your frequency extension is very limited then this highly efficient driver may actually be less desirable than an apparently less efficient driver that peaks at say 50hz and rolls of less steeply at lower frequencies. Often, the desired application and the cabinet requirements can dictate the driver choice, and a higher efficiency figure may not automatically mean a particular driver is a better choice.

There is an article I was reading on the subject here which could be worth discussing.
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Old 8th January 2013, 04:08 PM   #1506
Moonfly is offline Moonfly  Spain
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Quote:
Originally Posted by Omholt View Post
Thanks gedlee.
What about the size of drivers for bass reproduction?
Does bigger come at a cost?
The frequency produced by a driver is down to the time it takes to move from still, to full out, to full in, then back to rest again. This cycle is occurs a number of time per second, and this is what frequency is, the frequency at which this cycle occurs per second is what results in the frequency produced (or attempted to be produced). As such, in theory, any driver can produce any frequency. How much air the driver moves is what determines how loud it produces the frequency being produced. More air equals louder. Lower frequencies need to be a lot louder to be perceived as equal to higher frequencies, so to go low you need to go loud at those low frequencies, which in basic terms means you need to move lots of air. The best way to do that is to use a bigger driver with a larger surface area that has a greater swept volume of air.

So, bigger drivers are better for subs. There is one argument floated around that bigger drivers are heavier so sloppier and less accurate, but this simply isnt the case. To begin with, to produce a set output spl at a set frequency, you need to move a specific amount of air, regardless of driver size. A larger driver will do this with less movement, so even if a larger driver was slower, it would be countered by the driver not having to move as far anyway. A heavier driver will also have a lower natural resonance frequency, and lower is also better for subwoofers where your trying to produce low frequencies, so in this case a heavier driver can also be an advantage. All that is really need is a motor that is powerful enough to control the larger driver, and as long as you have that then your golden.

The only real advantages of using smaller drivers are when they are used in multiples, or you want a smaller cabinet. There are no real disadvantages to using larger drivers beyond the larger cabinets they generally require.
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Old 8th January 2013, 04:12 PM   #1507
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Originally Posted by gedlee View Post
The higher the efficiency the less current is required to generate a given SPL so the lower the heat generation, but high efficiency requires bigger voice coils so there is also more copper. Hence simply put, thermal modulation is proportional to the inverse square of the efficiency.
Thermal modulation is not always proportional to the inverse square of the efficiency.
In testing a pair of Lab 12" compared to a BC 18SW115-4 (18 inch) in a tapped horn, the Lab 12" were almost 3 dB more efficient, but the 18SW115 was able to dissipate the heat from the voice coil much better, so it had much better ultimate dynamic range.

At high power, the 3 dB advantage the Lab 12"s had succumbed to thermal compression, and the 18SW115 was able to put out more SPL, showing virtually no thermal change from one watt to 1500 watts of sine wave, double the AES rating.
The pair of Lab 12"s, each getting about 400 watts sine wave, were quite hot to the touch, while the 18SW115 barely rose over the ambient temperature with 1500 watts.

Had I not done the high power tests, I would have concluded the B&C was overpriced compared to the Lab 12".
Having done the tests, I consider the 18SW115 a bargain, at least it was before the price of Neodymium went through the roof. Fortunately, B&C has come up with the ceramic magnet 18TBW100, nearly the equivalent of the 18SW115, kind of like a giant big brother to the 12TBX100 you are using.

Art
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Old 8th January 2013, 04:28 PM   #1508
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Quote:
Originally Posted by jcx View Post
dynamic thermal compression distortion from voice coil heating is well known, copper has high resistance tempco, coils get hot, enough to change Rdc by several x10%
Rdc is a significant electroacoustic parameter
Let's not talk about changes over the course of a Mahler symphony. That's imperceptible to human senses.

Let's talk about changes of due to heating and cooling when driven by a low impedance source during a half-cycle... say 15 milliseconds for half a LF wave. Any effect?

Ben
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Old 8th January 2013, 04:36 PM   #1509
jcx is online now jcx  United States
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too lazy to skim the thermal distortion modeling paper, even just the figures at the end?
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Old 8th January 2013, 05:18 PM   #1510
gedlee is offline gedlee  United States
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As expected people are confusing thermal compression with thermal modulation. they are related but distinctly different effects.

"Thermal compression" is well understood, documented and sometimes even measured. It is a long term effect that happens after a long exposure to a signal and the entire motor structure heats and eventually reaches a thermal equilibrium. In this effect the heat dissipation is critical as it determines the long term temperature.

Not so for the short term "thermal modulation". The copper does begin to heat "instantaneously" as the current passes through it. I had this discussion with my physics advisor and he agreed that the heating would occur on a time scale of that of the speed of light. Of course the "temperature" lags the "heating" by a time rate that depends on the thermal capacity of the material. This is exactly why more copper yields a lower rate of temperature change than less copper for a given current.

Yes, "thermal compression" does not necessarily go as the inverse square because it is affected by the thermal dissipation of the motor. But thermal modulation happens at time rates too short to be affected by the thermal dissipation and hence are independent of this factor and depend only on two things - amount of copper and level of current. Thermal compression is also, for the most part, independent of the current voice coil temperature, while thermal compression is completely dependent on the current voice coil temperature. Thermal modulation acts in addition to thermal compression and is essentially independent of it.

As to tests, I continue to evolve these concepts in my head and as I do so the tests that need to be done are different. I am planning on doing an audibility test to find the thresholds at which this modulation can be heard. In the past I did a test where the frequency response was tested from ever increasing bursts of noise. These tests showed that the frequency response of less efficient systems changed far more than those of more efficient systems at the same SPL level. And I mean a lot more!! This was early confirmation of the effects, but these tests still did not yield an easy quantification such that it could be used to scale the perception.

That this effect is real, I would stake my reputation on, the level of its significance is still only a guess, based on the fact that everyone agrees that my speakers have "dynamics" that are unchallenged in any other speaker. I attribute this to thermal modulation - I could be wrong, but the math, existing data and experience all add up to say that I am correct.

This discussion should not really be in Subwoofers. But I was asked here.

Last edited by gedlee; 8th January 2013 at 05:35 PM.
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