I listen to headphones every day and I'm picky about sound quality. I've listened to a lot of headphones over the years and found very few that provide the clarity and neutrality that I like. First are the Sennheiser series HD580, HD600, HD650. These 3 cans are very similar - the frequency response, phase response and distortion are almost identical. The only audible difference (and it is subtle) is that the HD600s and 650s have slightly better response in the extreme bass and treble. The second are the Etymotic series ER-4S, 4P, HF5, MC5. The Senns are large over-the-ear open baffle cans. The Etys are small, in-the-ear plugs with about 30dB of noise isolation.

First I'll discuss the Sennheiser HD-600s since those are my main headphones both for casual daily and for serious music listening sessions.

The frequency response of these cans compensates for the ear canal's natural response rise from 2 to 4 kHz. This gives them a more laid back warm sound than many other cans, but it's necessary to reproduce the natural sounds heard without headphones. It does not mean any lack of detail. In fact, detail is where it's at with these headphones. They have a spooky real sound with all the detail but it doesn't unnaturally jump out into your face like it does with many other headphones. This makes for enjoyable listening for extended periods of time without fatigue.

Within their rated output level their distortion never goes above 0.03% (-70 dB) which is virtually unheard of for any transducer. This is lower distortion than some solid state electronics! For most of their bandwidth the total distortion is under 0.01% (-80 dB) which is incredibly low. This low distortion is measured at about 90dB SPL output, a realistic and somewhat loud listening level. This makes it quite impressive since transducer distortion usually rises with output level. The frequency response is wide: 20 Hz and 20 kHz are both audible. The impedance curve is smooth and reasonably flat, suggesting good phase response.

The only shortcoming of these headphones is the linearity of their frequency response. And I'm not talking about their 6 dB dip from 2 to 4 kHz, because this is necessary to compensate for ear canal resonance. They have a bump in the bass, +3 to +6 dB from about 80 to 320 Hz, and they are weak in the extreme low bass, -6 dB or so at 20 Hz. They have a dip in the treble, about -6 dB from 10 to 20 kHz, though response is climbing back to neutral at 20 kHz. This bass boost and treble cut give them an artificially warm, laid back sound.

However, their distortion is so incredibly low they have a spooky real sound that can raise the hair on my neck on good recordings. They are not artificially bright or detailed like many transducers are. They have incredible detail without brightness and without any harshness or grain. They have that liquid smooth transparency of sounds we hear in the real world. Non-linearities in frequency response seem a small price to pay for such incredible sound quality.

These headphones are not efficient. Their 300 ohm load is easy to drive but they require more voltage than many other headphones to achieve a decent listening level. Most portable devices at full blast will drive these headphones to SPLs into the low to mid 80 dB range. That would be fine, but turn the volume down a notch to avoid clipping and distortion and the SPL is a bit too low for serious listening. Besides, these cans don't mate well with portables simply because they are so big.

HEADPHONE AMP

I use a Headroom Maxed Out Home amp for my headphones. It is cleaner than the headphone jack or line stage of most devices. And it has the power and impedance matching to drive these inefficient headphones properly. You can read more about it here.

PORTABLE HEADPHONES

I use a portable player (phone or tablet) when traveling - mostly sitting on airplanes. In this environment noise reduction is crucial for enjoyable listening. But I don't like the electronic noise reduction of Bose and other headphones. Not only is it ineffective - providing only about 10 dB of noise reduction - but it also requires batteries. Instead, I use in-ear speakers that seal against the ear canal like an earplug. They offer the same noise reduction as earplugs - 30 dB or more - and they don't use batteries.

There are several good choices for this kind of plug transducer: Etymotic, Shure, and Ultimate Ears are perhaps the top 3. Of these, Etymotic has the most neutral frequency response. All these plugs share one common drawback compared to the HD600: limited frequency response. None of them go much beyond 16 kHz. This limitation is not a big issue with most music. First, it's beyond the range of hearing of many people, and due to the logarithmic frequency response of the human ear, it's less than 1/3 octave (about 3 half step notes) below the highest audible frequency. Also, few musical instruments have much energy in that frequency range. But it becomes apparent on musical test signals and high quality recordings of demanding source material. It can be heard as lacking "air", "space" and 3 dimensionality to the sound. However, the ruler flat response of the Etymotics from the low bass (below 20 Hz) to 10 kHz is amazingly realistic.

Etymotic makes 4 high quality plug transducers: ER-4S & ER-4P, and HF5 & MC5. The first is the most accurate while the last three have higher efficiency. The HF5 and MC5 are a more recent variants of the ER-4P that achieve similar sonic accuracy for about half the price. This is what I use with my portable. The HF5 has a balanced armature, the MC5 has a moving coil. I opted for the MC5 because it has the same response curve and low distortion as the HF5, but costs less.

PORTABLE PLAYERS

As for my player: I use my Samsung Galaxy Note 2 phone. I tested it myself (using Audacity and Stereophile test CD signals) and found it to be sonically accurate and tonally neutral. It goes from below 20 Hz to 20 kHz +/- 0.5 dB and is +/- 0.1 dB for most of the audible range. Harmonic distortion is around -90 dB, IM distortion (worst case, 19+20 kHz @ 0 dB) is around -60 dB. These numbers are high, because they include the distortion added by my sound board. These specs - surprisingly good and acoustically transparent - are typical of many phones and tablets.

HD600 VERSUS MC5 COMPARISON

Both compensate for ear canal resonance with a 0-6dB cut from 2 to 4 kHz. The HD600 has a dip in extreme low bass, a rise in the mid bass and a dip in the extreme high treble. The MC5 is almost perfectly flat, within 1 dB from 20Hz to 10kHz, at which point its reponse rolls off sharply. This makes the HD600 sound a touch on the warm side, though with no less detail. Compared to the HD600, the MC5 sounds just as clean but more neutral and realistic, but lacks the "air" or "space" that the HD600 has in the extreme highs. 20 Hz bass test tones are easily audible (if you can hear that low) and the extreme low bass through lower midrange is crisp and natural but not exaggerated.

Overall, the MC5 is comparable to the HD600. The tradeoff is linearity versus HF response. The MC5 is more linear and neutral but doesn't have the extreme HF response of the HD600.

The MC5 is a vast improvement over any of the OEM earbuds that come with a portable player. In fact it's one of the most accurate and neutral in-ear transducers that money can buy. And it is efficient enough to be driven well by most portable players.

However, for serious listening I prefer the HD600 as its better HF response makes up for its uneven frequency response, by providing air and space to the sound that makes it more dimensional.