Eight to ten hours of use may seem short for wireless use compared to other--often less-expensive--headphones. One reason higher-end models have shorter battery life is that the CSR chipset includes higher-quality AptX Bluetooth audio encoding, which requires more power than standard A2DP stereo-audio encoding. But given that AptX's benefits are, to some extent, lost when using low-bit-rate audio files, it would be nice to see a more power-efficient version of the MM 450-X.
Dulling the roar
Sennheiser isn't new to noise cancelation, as its NoiseGard technology was introduced in headsets designed for Lufthansa back in 1987, and the company's first pair of consumer noise-canceling headphones was released in 1992. The MM 450-X uses the latest iteration of Sennheiser's NoiseGard, version 2.0.
I managed to test the MM 450-X's noise-canceling performance in a variety of conditions, with external noise levels ranging between 50 to 95 dB. Overall, I came away impressed with the reduction in audible noise level, which I found comparable to what a good pair of generic foam earplugs would provide.
On the other hand, unlike Sennheiser's larger MM 550-X, which uses full-size earpieces that completely surround your ears, the MM 450-X uses an on-ear design that doesn't block as much high-frequency noise as I had hoped. That said, this is a tradeoff you make for portability, and the MM 450-X is comparable in this respect to other on-ear headphones I've used.
Another minor issue I experienced was that the noise-canceling circuitry would occasionally generate clicking or thumping noises--the former with lower ambient noise levels (roughly less than 55 dB), the latter with transient/intermittent noises. For most environments where you'll likely use noise-canceling headphones--say, on a plane or train, or in a noisy office or crowd--chances are these minor issues won't affect you, but they're worth noting.
The MM 450-X lets you to listen to music in any of four ways: wired with and without noise cancelation, and wireless with and without noise cancelation. I spent most of my listening time using the MM 450-X with a Bluetooth connection.
My first impression of the MM 450-X's audio performance, tested with the noise-canceling circuitry disabled, was how bright the headphones sound: The treble and upper midrange are somewhat over-emphasized in the presentation. On the other hand, while low bass is a little lighter than what some listeners might like, the MM 450-X does a good job with bass guitar and kick drums, and it produces crisp bass without muddying more-dynamic passages.
A common gripe with closed headphones is a limited soundstage, but even though the MM 450-X doesn't produce the most expansive soundstage I've heard, it does a good job of placing instruments in the correct spacial positions.
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