Etymotic Research ER6i

Summary
In-ear headphones that fit right into the ear canal have a reputation for providing great sound insulation, and the Etymotic ER6i shows this is justified; these small headphones do an awesome job blocking out sound from the outside world. The sound quality of the ER6i headphones is also pretty awesome; in our tests, we found a tight, smooth frequency response curve, minimal distortion, and the ability to handle plenty of volume if you want it.

All of these tests are carried out using a high-end electroacoustic audio analysis system consisting of a Head and Torso Simulator (HATS) and a professional audio analysis program called SoundCheck, produced by Listen, Inc. The HATS simulates the human head and ears, and has two microphones in the ears that respond to sounds in the same way your ears do. The Soundcheck system produces a series of test signals and captures the output from the headphones using these microphones, allowing us to do very precise, scientific testing of headphones. For more information on how we do our headphone quality audio testing see this article.

Frequency Response (6.70)

One of the fundamental aspects of performance in headphones is frequency response; how accurately do the headphones reproduce the many frequencies in music? Our testing system analyzes this by sending a frequency sweep to the headphones that goes from the very low (about 80Hz, which sounds like an earthquake) to the very high (20kHz, which is higher than most people can hear) and analyzing the sound the headphone outputs. How well the headphones reproduce these frequencies is shown on the graph below: low frequencies are on the left, high on the right. The green line is for the left channel, the red is for the right. 

Distortion (4.31)
The evil gremlin waiting to wreck your pleasant audio experience is distortion, where the headphones clip or otherwise mangle the sound because they can't accurately reproduce it. We did see some minor distortion on the ER6i headphones.

Our system examines the distortion that headphones introduce by playing back a frequency sweep of  sounds across the frequency range at a listening level of around 90 dBSPL (which is fairly loud, but highlights the distortion) and examining the results for any differences from the original waveform. The graph below shows what's called the Total Harmonic Distortion (THD), which is the amount of distortion (as a percentage) of the basic frequency, as well as the important harmonics that provide the texture and subtleties of musical instruments. If these are distorted, the instrument won't sound right; think of a clear guitar note and a distorted one from a heavy metal solo.  The graph starts with low, bassy frequencies at the left and goes up to high frequencies at the right. Again, the green line is the left channel and the red line is the right.
 

Tracking (9.52)
Headphones have one channel for each ear, so it is vital for good performance that both sides are identical. That's what we test in the tracking test, and the ER6i headphones did well here; we found a maximum difference between the two channels of just more than one percent, which most people won't even notice.

Our test system produces the graph below, which shows the tracking across the frequency range of 80 to 2kHz. If the line is at 0 percent, that indicates the sound at that frequency is identical in both channels. If it goes above that line, the left channel is stronger, and if it goes below, the right channel is is putting out more.
 

Maximum Usable Volume (10.0)
Most people listen to headphones at reasonable volume levels, but some people like it loud. So, we also test how loud headphones can go without the sound turning into a distorted mess. We test this by how high we can take the volume until the distortion in the sound reaches 3 percent, where it becomes audible and annoying. The ER6Is keeps the distortion below this level until we push the volume to a maximum level of 120dB, which is loud enough to cause hearing damage. And remember; our testing system has replaceable ears; yours are not. If you crank the volume up that high, you're likely to permanently damage them.


Isolation (10.01)
The Er6i headphones are effectively ear plugs with built-in speakers, so it's no surprise they do an excellent job blocking sound from the outside world. We measured the average sound reduction across the board at an impressive 30db, which is a lot.

The graph below shows the same frequency range of 80 to 10,000Hz, with the green line indicating how much sound is blocked at that frequency. Higher is better.

This shows the headphones are most effective at blocking high frequency sounds, such as screaming children or dental drills. They are less effective at blocking low frequencies, though, as these can still get through the earplug effect of the headphones.

It's also worth comparing these against the performance of headphones that include an active noise canceling circuit, like the Bose QuietComfort 2 and 3. The ER6is are more effective overall, but the active noise cancellation of the QuietComfort headphones does a slightly better job at low frequencies, so they block more noise similar to the engine inside an airplane or the dull rumbling of your stomach before lunch. If you're looking for a set of headphones to specifically block noise, the choice comes down to how you feel about sticking things in your ears; if you don't like the idea, the Bose QuietComforts are a better choice, as they fit on (with the QuietComfort 3) or over (the QuietComfort 2) the ear.

Leakage (10.0)
As well as blocking outside sounds, the Etymotic ER6i headphones do a good job keeping your music private; we found in our tests that only a very very small amount of sound escapes from the headphones when they are correctly fitted. So your choice of music remains a private one, even on a crowded bus or train on the way into work.