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Power consumption
We measured total system power consumption at the wall socket using an Extech power analyzer model 380803. The monitor was plugged into a separate outlet, so its power draw was not part of our measurement. The cards were plugged into a motherboard on an open test bench.

The idle measurements were taken at the Windows Vista desktop with the Aero theme enabled. The cards were tested under load running Left 4 Dead at 2560x1600 resolution, using the same settings we did for performance testing.

This power thing isn't making my job any easier. The Radeon HD 4890 OC draws quite a bit less power when running a game, but the situation is almost exactly reversed at idle, continuing the dizzying asymmetrical parity of our test results.

Noise levels
We measured noise levels on our test system, sitting on an open test bench, using an Extech model 407738 digital sound level meter. The meter was mounted on a tripod approximately 8" from the test system at a height even with the top of the video card. We used the OSHA-standard weighting and speed for these measurements.

You can think of these noise level measurements much like our system power consumption tests, because the entire systems' noise levels were measured. Of course, noise levels will vary greatly in the real world along with the acoustic properties of the PC enclosure used, whether the enclosure provides adequate cooling to avoid a card's highest fan speeds, placement of the enclosure in the room, and a whole range of other variables. These results should give a reasonably good picture of comparative fan noise, though.

At full tilt, the Radeon HD 4890 OC's cooler makes more racket than the GTX 275's. That's pretty clear, and it matches with my subjective impressions.

GPU temperatures
I used GPU-Z to log temperatures during our load testing. In the case of multi-GPU setups, I recorded temperatures on the primary card.

Here's one possible reason why the 4890 OC's cooler is louder: it's working to keep GPU temperatures relatively low. This is something of a reversal for AMD's stock coolers in the Radeon HD 4800 series. The 4870 512MB above, at 84°C, is more typical. Meanwhile, the GTX 275 is one hot chip. Only the two Asus Radeons in CrossFire, with their plainly broken coolers, produce higher temperatures—just before they crash. The GTX 275 isn't in the same boat, but it does appear Nvidia has chosen to favor good acoustics over lower GPU temperatures, while AMD has taken a step in the opposite direction.