Sennheiser HD 660S2  Headphones Review

The Sennheiser HD 660S2 have a flat sound profile with a slightly warm treble. Although they lack low bass, like most open-back headphones, they deliver an analytical tuning that more closely follows the SoundGuys.com Studio Curve than our target. They can reproduce vocals and instruments very naturally, but a narrow peak in the mid-treble followed by dips in the mid-treble can cause cymbals to sound a bit piercing, but not especially airy or brilliant.

The frequency response consistency is good. Once you get a good fit, you'll achieve consistent bass and treble delivery between wears. There are small variations in the mids and bass across people with different physical attributes, but these differences aren't drastic.

The bass profile target compliance is satisfactory. Their bass response is underemphasized in the low to mid-bass, which is common in open-backs. While mixes lack thump and weight, the high bass adheres to our target well for decent boom on kicks.

These demonstrate exceptional mid-range target compliance. The range is very even and flat, which results in present and natural-sounding vocals and instruments. In songs like Glassworks: I. Opening by Philip Glass, the piano sounds smooth and detailed throughout the entire track.

These headphones have good treble profile target compliance. Most of the treble region is evenly de-emphasized for a warmer sound on vocals and upper harmonics, but with the bass roll-off, it doesn't sound overly dull. That said, if brighter cymbal harmonics and transients suit your tastes better, the Beyerdynamic DT 1990 PRO MKII may be worth a look instead.

The peaks and dips performance is great. Most of the deviations are somewhat minor. The L/R drivers exhibit stereo mismatch in the bass and lower mid-range, but it's otherwise relatively smooth through the mids. A peak in the low-treble affects the left driver a little more than the right, emphasizing the presence of vocals and leads. However, a narrow dip following the peak can sound a bit veiled in detail. The rest of the treble region has a series of smaller peaks and dips that can interchangeably cause sharp or closed-off sounding upper harmonics on elements like cymbals.

These headphones have mediocre stereo mismatch results. Their weighted amplitude, frequency response, and phase mismatch results tilt the stereo image more to the left. The bass range differences, in particular, are where these headphones exhibit their mismatches. The mids and treble are better matched between the L/R, though. That said, these results are only applicable to our unit, and your pair's results may vary.

The group delay is reasonable. Considering that these headphones don't output much deep bass, the timing delay isn't as detrimental to reproducing tight bass as it would be with bass-heavy headphones. Through the upper frequencies, the group delay yields a transparent treble.

When these headphones output audio, they don't interact with your outer ear (pinna) very much. As a result, they don't mimic the same immersive PRTF effect of an angled reference monitor.

These headphones have remarkably low harmonic distortion. Frequencies tested at 94 dB and 104 dB/SPL playback with overall low distortion, especially in the treble and mids. Consequently, your audio remains unaffected by impurities in the signal.

These headphones have a high impedance and need an amplifier to drive them to useful listening levels.

These are the settings used to test these headphones. Our results are only valid when used with these settings.

The Sennheiser HD660S2 look nearly identical to the Sennheiser HD 660 S but are differentiated by the color of the manufacturer's label and nameplate on the ear cups and yokes, respectively. These headphones have bronze detailing, rather than silver, like their predecessor. That said, the frame is still largely the same, as it's made from plastic with a black satin finish, and the oval ear cups are well-padded with a microfibre cloth. Like most open-back headphones, they have a stiff mesh grille covering their ear cups. They only come in one color variant: 'Black.'

These over-ears have a decently comfortable fit. They're similar in design to the Sennheiser HD 660 S and have the same headband design. Overall, they're lightweight, and their padding feels soft against the skin, although they're not as plush as the 'Producing' pads that come with the Sennheiser HD 490 PRO. However, they have a high clamping force out of the box, which can be uncomfortable to wear for long periods. You'll especially feel their high clamping force if you have a large, wide head or wear glasses.

These headphones aren't very portable, which is normal for audiophile headphones. They're not designed to be used on the go and can't fold into a smaller format.

Unlike the Sennheiser HD 660 S, these headphones come with a dustbag to help protect the headphones from dust accumulation when not in use. There's a drawstring to keep the pouch closed, but you can't fully seal it. Since it's made of fabric, it won't protect the headphones from water exposure or impact damage.

The build quality is good. These headphones have a similar design to the Sennheiser HD 660 S. They're partially made of plastic, with a metal driver shell and a thin metal headband that's solid and has some give for it to pivot. The ear cups are spacious, and the microfiber velour padding is plush and soft. The audio cable is also detachable. However, the point where the headband meets the yoke doesn't feel very solid, and it requires a bit of strength to adjust the headband to fit your head.

These over-ears have good stability thanks to their high clamping force. They won't move around if you're listening to audio casually at your desk or studio. However, if you like to headbang to tunes, these can move around and even fall off your head.

• Sennheiser HD 660S2 headphones
• 6.35 mm (1/4") audio cable 1.8 m (5.9 ft)
• 4.4 mm (0.17") balanced audio cable 1.8 m (5.9 ft)
• 1/4" to 1/8" adapter
• Carrying pouch
• Manuals

The Sennheiser HD 660S2 aren't meant to block out background noise. They have an open-back design, allowing ambient sound to pass through the ear cups to create a natural sound that interacts with your space's acoustics. They don't block out any of the low rumble of car engines from an open window or ambient chatter. While they can partially muffle voices, you can still make out conversation.

These open-back headphones don't meaningfully isolate you from the noises in common scenarios. They let in nearly all of the sounds of your environment, including airplane cabin din, traffic, and chit-chat.

The leakage performance is terrible, but this is also by design. Open-backs are meant to leak audio, as they can interact with your environment and create a more immersive listening experience. It isn't a problem if you're in an isolated place like a studio or room. However, if you want to use these in a noisy environment, people will hear your audio.

These headphones come with a 6.35 mm (1/4") TRS cable and a 4.4mm balanced cable, which are the same length. They also come with a 1/4" to 1/8" adapter (22 cm or 8.6 inches) so that you can connect them to other devices with this port.

These over-ears can connect to your PC using their audio cable with the included adapter. However, since they don't have a mic, you'll only receive audio. Since they need extra drive to push them, your experience can vary with different headphone output ports.

You can connect these over-ears to your PlayStation console via the controller's AUX port. However, you'll need to use their adapter, and they can only receive audio.

These over-ears support audio-only when you use the adapter to connect them to your console controller's AUX port. Since these headphones lack a mic, you'll need an outboard for co-op gameplay.