The best place to start is the Wired for Sound article on headphones and earphones for ear training.
This article explains the headphones vs earphones choice, which mostly boils down to personal preference and the environments you’ll be using them in. You may want a nice pair of over-the-ear headphones for long periods of listening at home, but in-canal earphones with good sound isolation for listening when out and about.
There are three types of company which produce headphones:
- Professional audio companies (e.g. Shure, Sennheiser, Behringer, Etymotic)
- Electronics companies (e.g. Sony, Philips, Panasonic)
- Fashion brands (e.g. Skullcandy, Beats By Dre)
Choosing the right category goes a long way to ensuring your headphones will have good audio quality, and be suitable for ear training.
Category 1 should be your first choice. Pretty much any set of headphones from this kind of company will be well-made and have very high quality (i.e. accurate, clear) sound which lets you hear detail more easily. Expensive, but can last you decades.
Category 2 can be good, but you have to very carefully choose a suitable model. If you’re paying $75+ you’re probably better off just sticking with category 1.
Category 3… It’s possible some of the headphones aren’t terrible. But if sound quality is your top priority, just steer clear of these.
There are two more things to consider beyond choosing a suitable manufacturer, which are the frequency response and the impedance.
If you’re going to be doing audio ear training (frequencies, timbre, etc) as well as musical, you’ll want a set of headphones with “flat” frequency response, which means they most accurately reproduce the frequencies of the sound source (rather than e.g. boosting the bass).
The impedance matters mostly for what equipment you can easily just plug the headphones into – this is covered in the Wired for Sound article.
There are some review sites which lean towards audio quality over fashion. Try these (with a pinch of salt!):
Hopefully that helps you pick the best ear training headphones. If you have a pair to recommend or warn against, please leave a comment below!