Is it ethical to buy noise canceling headphones with research funds? Noise canceling headphones are quite expensive (up to 300 USD) compared to normal headphones, but can boost productivity by creating a sound isolation in noisy environments. Would it be ethically justifiable to buy those for non-research purpose (i.e., it's not needed for experiments)?

On one hand, I feel like good office furniture like chairs and desks are fully justifiable for the well-being and productivity of the research team members. But, this argument seems like a slippery slope, since I could also say that having a high-end espresso machine or special dark chocolate bars also could boost research productivity. In a corperate environment, I would have no problem in purchasing some of these, and I would consider them to be ethical, but in an academic environment, I find it difficult to find the ethical boundary since the funds are intended for research.

The fact that noise canceling headphones will likely be used for listening to music both while working on research and not, especially during travel, also adds to my ambivalence. But then research purpose laptops are also used this way often, and I don't have a problem with that.

EDIT: I'm not looking for alternate solutions. Just an ethical evaluation. It seems there's no real answer to this question. I appreciate everybody's response. I will choose the most balanced response as an answer but you should really read all the answers.
Disclosure: we actually bought pretty expensive Bose Noise Cancelling headphones for our students recently, for the reasons you mentioned.

I think there is a very easy answer to this - will your funds cover the expenses if the purpose is reported truthfully, or will some "creative" cost reporting be required?

In the first case, I have a hard time seeing an ethical issue - as you say, equipment is equipment, and if whoever funds your research is ok with spending some of it on "potentially productivity-increasing even if ultimately not absolutely necessary" equipment, then why should you be ethically required to not make use of this option? The only case I would see where this would become an issue if you, for instance, would not actually be planning to be using the headphones much at all in the office, and are mainly looking to use them for a private purpose (but then again, if you wrote this in your expense report, whoever funds your research would probably not be ok with this expense anyway, making this more similar to the second case).

In the second case, it is pretty clearly not ethical to hack around whatever the criteria given out by your funding agency (e.g., pretending like you will need noise cancelling for a specific experiment so that it counts as costs for conducting a specific experiment rather than equipment costs, etc.).