Crashman :
the nerd 389 :
I know how you get the numbers. That's not the issue at all, and it never has been.
The issue is that the numbers are presented as acoustic efficiency, and simply do not reflect the relationship between cooling power and noise output in practice. The end result is that these numbers are often cited to say product A is more acoustically efficient than product B. The problem as I see it is that in many cases, product B will provide equal cooling at a lower noise level.
This is entirely counter intuitive to many readers, and it takes some effort on our part to determine which product will actually give lower noise in any given application. I would greatly appreciate it if I could read a review and know what to expect out of a product. As it is, I have to hunt through the charts to find what the delta T and SPL values are, and then figure out the relationship between them on my own.
It's the actual measurement of how much cooling you get per unit of noise. Or how much noise you get per unit of cooling. Actually it's both of those. It's like measuring how many work units you get per unit of electricity...so yes...a system that produces twice the data while using the same energy is twice as efficient, same deal using acoustical energy instead of electrical energy...
We get the same feedback occasionally when a system that uses 100W to produce 100 work units is rated less efficient than one that uses 150W to produce 200 work units. "The 100W unit [must] be more efficient because it uses less power". Nah, it's just more miserly.
I understand the logic behind that. Unfortunately, acoustics rarely works like that. The units for, say, a motor's efficiency are based on two similar units, namely mechanical vs electrical watts. Since any decibel value is a unitless ratio, you can't simply use it like you would other units.
Acoustic efficiency of a fan (and yes, there is an official definition) is very different. Because the noise level produced by a specific fan scales with 10*log( RPM^5), airflow scales with RPM, and heat transfer scales with e^(-k*cfm) you cannot simply look at the difference in cooling vs the difference in noise. I can't even begin to cover all of the places where the math simply does not work.
The official definition of acoustic efficiency is actually a simple dB offset that's added into the fan noise prediction equations, and is defined in terms of RPM, not delta T. No where will you find a dB/watt unit, a dB/C unit, or a dB/RPM unit in the field of acoustics. The closest you actually see is dB (SPL) - dB (watts). That's a minus, not a division. For fans, this comes into play as a specific 50*log(RPM1/RPM2) term in the noise equations.
Mind you, there's still the exponential decay involved with the amount of cooling a fin array can give you with increasing airflow. That's probably the most annoying factor to account for due to modern heatsinks inducing turbulence as a means of increasing heat transfer. I'm unfortunately not in a position that I could test that, but TH reviewers might be. I understand the time limitations you guys face, though. I don't know of an efficient way to measure that off the top of my head, so it probably isn't practical to implement a measurement method without further research.
I'd be happy to link in some reference material for you to go over if you're interested. I have some documents that may be worth going through, as they outline the key parameters used to describe fans in terms of acoustics. They're far more concise and understandable than I can hope to be.
Minor Update:
I feel like it's worth mentioning that I wouldn't bother the reviewers at TH with this if I hadn't gone through and triple checked both the methods that I'm suggesting and the methods currently used in the reviews. I don't care to waste anyone's time on trivialities or even minor errors. I only bring this up because it presents significant issues when selecting products, and can easily lead consumers to purchase products that are not what they want/need.