Pros: None whatsoever.
Cons: Poor isolation, even though that's their whole purpose. Even poorer sound.
Description: The Beyerdynamic DT770M is an alternate version of the popular DT770, with supposedly strong isolation for use by drummers or monitoring in noisy environments. Physically, it is the same as the other DT770s, only with black cups, pleather earpads, and an in-line volume control. Concerning the pads, they are much shallower than the normal velour ones. My ears pressed up right against the drivers, and I noticed that it got hot and sweaty in there pretty quickly. Build quality is good, exactly the same as you'd expect from any other DT770, only the in-line volume control seemed unnecessarily bulky, and was made of cheap and flimsily put together plastic. The DT770M has an 80 ohm impedance and 105 dB sensitivity, so it is very easy to drive from just about anything.
Functionality: Beyerdynamic claims that the DT770M have a noise attenuation of 35 dB. That's higher than the quoted attenuation of many earplugs, and matches the lower range of what Etymotic claims for their IEMs. Well, it is an outright lie. Either that or it's an intentionally misleading spec based on a meaninglessly high frequency. While playing drums, I compared them to Ety mc5s and some regular foam earplugs. I wouldn't even say the DT770M isolated half as well as either. In fact, they didn't really isolate better than any other typical set of closed headphones, which is to say they are completely useless for the drumming or monitoring purposes they were intended for. Is it possible I just didn't get a good seal? Perhaps, but quite frankly I don't see how. They fit very snugly on my head, and I have pretty small ears which had no problems getting a circumaural fit inside the cups. There are some reviews on retail sites where people claim they have excellent isolation as advertised, but I'm not sure I believe in those peoples' credibility. I've known many drummers and musicians over the years, and I'm one of the only ones I know who takes even the slightest precautions to protect their hearing. (Yet I'm the one with ear troubles. Thanks irony!) The DT770M might seem like they isolate some if you've never worn earplugs before and you are blasting music through them. Of course, I could just be crazy.
Sound: But what about the sound? They are DT770s after all. How does it compare to the regular versions? It doesn't. Unfortunately, whatever it is that they did inside the cups to try and make them more isolating killed the drivers. There is absolutely no extension in either the bass or treble, and I don't mean just deep sub-bass or really high treble. I mean even the bass or treble you would expect from an already rolled off headphone is missing. Grados would be bass monsters in comparison. All that's left are some mids, and what's there is extremely muddy, smeared, and compressed. If I had to describe them in two words, those words would be "Immanently Unlistenable." A $20 pair of earbuds from Walmart would probably be a big improvement over these, and they cost $200! I know that they were meant for more utilitarian purposes and high fidelity sound isn't their main selling point, but for crying out loud!
Conclusion: So all in all, the DT770M fail miserably in every way. They don't even partially fulfill their main purpose which is isolation, and on top of that they are probably the worst sounding headphones I've ever heard. They don't even deserve to be used as a paperweight. Thankfully I was able to return them right away.