As with any expensive electronic product, doing research and reading reviews on headphones is a good way to reduce the risk of buyer's remorse. Physical characteristics such as the build quality, comfort, and accessories are easily described and understood, but when it comes to describing how something sounds - that's a whole other ball game. Understanding the frequency range of sound and how it affects the music is a great running start towards understanding your own preferences and what sound signature of headphone is best for you.
When deciding what headphones are best, we have to start with frequency response.
The Spectrum of Sound
The deep rumbling of a bass guitar. The lively tweeting of a piccolo. Both of these sounds begin as vibrations, carried through the air from the instrument to your ears. As the speed of vibrations is directly correlated to pitch, the low notes on the bass guitar vibrate the air slowly as compared to the higher notes on a piccolo. Measured in Hertz (named after the German scientist who proved the existence of electromagnetic waves), both the bass guitar and the piccolo fall within the audible range of human hearing, approximately 20 Hz to 20 kHz. This spectrum makes up the desirable range of sound for a headphone/IEM/speaker to electronically reproduce.
A headphone is only as good as its parts. Make sure you look beyond the exterior and dive into the engineering of it all.
Instruments on the Frequency Range
Independent Recording Network has created a fantastic, interactive chart (http://www.independentrecording.net/irn/resources/freqchart/main_display.htm) that breaks out and describes the spectrum. The breakout into each range is somewhat subjective and varies based on who is asked, but this serves as a good general guideline.
Sub bass: <60 Hz
Sub bass is a powerful rumbling that is more felt than heard. Here are the chest thumping vibrations that give songs impact. Sub bass is difficult to hear at low volume levels, but too much of a boost results in a muddy-sounding lower end.
Words to describe this range: Rumble, Thump, Power, Weighty, Muddy
Bass: 60 Hz – 250 Hz
Rhythm driving instruments such as the kick drum, toms, snares and bass guitar can be tweaked in this range. Most mainstream music today heavily uses the 90 Hz-200Hz area to color the lower end. A boost here adds warmth and thickness, but too much of a good thing makes music sound boomy and drowns out the rest of the frequency ranges.
Words to describe this range: Tight, Warm, Slam, Thick, Boomy, Bloated, Thick, Thin
Midrange: 250 Hz – 2,000 Hz
250Hz to 500 Hz is where low order harmonics of most instruments lie. Boosting the lower midrange can make bass and lower-stringed instruments sound clearer, but too much can cause higher frequency instruments to sound muffled. 500 Hz onwards has a significant impact on how human vocals sound. Done correctly, male and female vocals should sound lush. Boosting too aggressively turns music syrupy and/or shouty, whereas a recessed midrange causes vocals and instruments to sound distant and far.
Words to describe this range: Fullness, Hard, Honky, Lush, Tinny, Recessed, Muffled, Syrupy, Shouty
Upper midrange: 2,000 Hz – 6,000 Hz
The attack on percussive and rhythm instruments like cymbals are dependent on this range. A well-executed upper midrange adds clarity and has a positive effect on the sound timbre. Excessive boosting causes glare and sibilance, but not enough and music sounds dull and dark.
Words to describe this range: Clarity, Sibilance, Glare, Bright, Edge, Crunch, Dark, Dull
High frequencies: 6,000 Hz – 20,000 Hz
At this range, harmonics are all that's left. High frequencies done right make songs sound airy and open. Over boosting can make music overly bright and fatigue listeners, not enough makes music sound closed-in and soft.
Words to describe this range: Airy, Open, Definition, Sibilance, Bright, Closed-in, Soft
Words of Caution
It's worth noting that reviews online should be taken with a grain of salt. While we've described the spectrum as well as the words most associated with them in reviews, it's an individual's subjective opinion. People have different tastes - just because a respected reviewer showers a headphone with praise doesn't necessarily mean you'll like it. Similarly, just because people speak scathingly about a headphone doesn't mean it can't be a great product to your ears. In the end, reviews should only be used as an initial guide on your road to finding your perfect (headphone) match.
Other Cool Sites to Check Out
We've only just touched on the frequency range and words you might see in a typical audio review. For more information, visit the websites below.
http://www.headphone.com/pages/evaluating-headphones - A dive into understanding headphone measurements, what is desirable, and other helpful graphs.
http://www.audiocheck.net/soundtests_headphones.php - Offers several interesting headphone tests ranging from frequency response to dynamic range to a bass shaker.
http://www.zytrax.com/tech/audio/audio.html - Read up on technical information regarding frequency ranges.
http://www.stereophile.com/reference/50/ - A glossary of audio review terms according to Stereophile.