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.