There are a lot of factors that determine the final sound quality of headphones. Although nothing beats trying them in person, these specs can offer some guidance.
Impedance and Sensitivity
Impedance and sensitivity go hand in hand and represent the amount of power required to achieve a certain volume level. Lower impedance and higher sensitivities are better with lower power devices such as MP3 players or smartphones. The Sennheiser HD25-SP II has a high impedance which may require an amp for best use.
The frequency response indicates the range of pitches the headphones can produce. Human hearing range is usually quoted at 20 Hz – 20kHz, with most music falling well within those bounds. With a closed operating principle and a solid low end frequency response, these headphones can handle a strong bass and would be great for listening to pop or rock music.
These headphones use a neodymium magnet which is the most common magnet material for headphones.
Weight is simply a user preference and metric of portability. It does not indicate higher quality headphones as some brands add unnecessary weight to their models. Keep this in mind when evaluating the slightly lighter than average Sennheiser HD25-SP II.
3.5mm is the most common input for headphones and is what you see connecting with MP3 players, smartphones, and laptops.