I'm generally a fan of open-back headphones. The design prevents cabinet colouration and affords the drivers with a more open, natural sound signature. However, headphones aren't quite portable enough for outdoor usage, where you need smaller earphones for convenience. Unfortunately, due to the limitation of physics, it's impossible to achieve the same natural fidelity from smaller drivers. The fact is, as the transducer (speaker) gets smaller, it costs more to produce the same sound accuracy as its larger equivalent. Therefore, it costs much more to obtain the same level of sound quality from headphones as you'd get from a decent pair of bookshelf speakers. Due to the smaller planar surface of IEMs (In-Ear Monitors), it's still more challenging and expensive for them to match the performance of headphones with larger drivers.
Tragically, since urban India has spread out like a cancer without much in the name of city planning, we have been cursed to spend anywhere between one to three hours of our waking lives commuting to and from work. Despite all of its disadvantages then, it makes perfect sense to invest in a good pair of IEMs that will block out the city's chaos and transport you to your own personal space right in the middle of a crowded locomotive. Mind you, this guide won't merely throw random brands and models. The main objective here is not just to learn which IEMs to purchase, but rather how to go about finding the right one for your needs.
Choosing the correct driver type
There aren't many choices available in IEMs when it comes to the drivers, with dynamic and balanced armature (BA) drivers being the two dominant types. Dynamic drivers are most common because they are the cheapest to manufacture. These drivers are pretty much miniaturised versions of your regular loudspeakers employing a magnet and moving voice coil to oscillate a diaphragm, in order to reproduce sound. Balanced armature drivers employ a similar setup, but the voice coil is stationary. The diaphragm is moved by an armature that's suspended frictionless between two opposing magnets and the voice coil.
A dynamic driver gives excellent bass response, but at the cost of high-frequency detail
Technical mumbo-jumbo aside, this configuration allows the BA driver to have phenomenal electrical efficiency, which allows it to produce a more detailed sound. However, since it doesn't push a large volume of air like dynamic drivers, a BA driver lacks the ability to produce natural bass. To put it in a nutshell, dynamic drivers are cheap and produce great bass, whereas BA drivers are expensive but deliver great high-frequency detail at the cost of a stunted low-end performance.
Having said that, you have examples where multiple BA drivers are combined to produce a greater impact, or even instances where passive crossovers are employed to split high, mid and low frequency ranges across multiple BA drivers. Some expensive IEMs even use a combination of BA and dynamic drivers to deliver the best of both worlds—detailed highs and deep lows.
Getting the specifications right
That technical mumbo-jumbo at the back of the product packaging actually does account for something. If you can read it right, it should allow you to match the IEMs well with your PMP and decide if you should invest in a headphone amplifier. As a thumb rule, most IEMs have nominal impedance between 16-32 ohms, which makes them easy to drive without the need for separate amplification.
Balanced armature drivers have a terrific high-end response and detail, but at the cost of bass
If that sounded Greek and Latin to you, here's an easy example to understand what impedance really is. Imagine sipping a glass of milkshake through a straw, where your lungs are the amplifier, the glass of milkshake the IEM and the straw representing the concept of impedance. The impedance is higher when the straw diameter is small, whereas it gets lower as the diameter increases. It's easier to sip milkshake through a thinner straw, but the volume of milkshake coming through is miniscule. However, you may be able to suck in copious amounts of milkshake through a two-inch PVC pipe, but that will also put a tremendous strain on your lungs.
IEM impedance works in the same way. A high-impedance driver won't put much strain on your PMP, but the maximum volume will be limited. Conversely, a low-impedance IEM will sound louder, but it will put a lot of stress on your PMP's amplifier. Certain balanced armature drivers can be tough on amplifiers due to their very low impedance, whereas some dynamic drivers may sport too high an impedance to provide sufficient volume without amplification. As a rule of thumb, these outliers are better auditioned with and without headphone amplification to see if there's a noticeable improvement in sound quality.