The Sennheiser HD 660 S Headphones: An Audiophile's Dream Come True

Have you ever had a ghost speak to you through a song? I don’t mean a literal phantom, but a note, a breath, a whisper of a sound you’d never heard before in a piece of music you could hum in your sleep. I had that experience recently with Fleetwood Mac’s “Dreams.” I’ve heard it a thousand times, but on this day, for the first time, I heard the subtle, almost imperceptible scrape of Stevie Nicks’s fingernail against the tambourine’s skin just before the chorus. It was a tiny, fleeting, utterly human detail. The song hadn’t changed. My ears hadn’t magically improved. So, where did this new sliver of reality come from?

It came from the space between the recording and my consciousness—a space that a new generation of audio instruments, like the Sennheiser HD 660 S I was using, is designed to make disappear. This isn’t about magic; it’s about physics, engineering, and a decades-long quest for auditory transparency. Let’s take a journey into that space and dissect the science that uncovers these beautiful ghosts.

The Shoulders of Giants

To understand the HD 660 S, you must first know its ancestry. It comes from a legendary lineage, the Sennheiser 600 series, which is to headphones what the Fender Stratocaster is to guitars—an icon. The family story begins in 1997 with the HD 600, a headphone that became a benchmark for its neutrality and analytical precision. It was like a perfectly calibrated scientific instrument, trusted by sound engineers worldwide to reveal the unvarnished truth of a recording.

A few years later, its sibling, the HD 650, arrived. It was warmer, richer, and arguably more “musical.” Where the HD 600 was the dispassionate analyst, the HD 650 was the passionate artist, wrapping the listener in a velvety, forgiving embrace. It became a darling of music lovers.

The HD 660 S stands on the shoulders of these two giants. It isn’t just a simple update; it’s an adaptation to a new ecosystem. In a world of high-resolution digital files and a wider array of audio sources, from dedicated amplifiers to high-quality portable players, a new kind of balance was needed. It was engineered to be the bridge between the analytical clarity of its forebear and the engaging musicality of its brother, all while being more adaptable to the modern listener’s world.

The Open-Air Theatre: The Physics of Space

The most striking visual feature of these headphones is that you can see right through their earcups. This is the hallmark of their open-back acoustic design, and it is arguably the most critical factor in their sound.

Imagine singing in a small, tiled bathroom. Your voice bounces off every hard surface, creating a chaotic mess of echoes and resonance. The sound is loud, jumbled, and trapped inside a tiny space. This is, in essence, what can happen inside a closed-back headphone. The sound waves produced by the driver have nowhere to go, so they reflect off the inside of the earcup, interfering with the new waves being created. This can lead to standing waves—peaks and nulls at certain frequencies—that color and distort the sound, making it feel congested and “stuck in your head.”

Now, imagine singing in an open field or an amphitheater. Your voice radiates outwards, dissipating naturally into the air. There are no nearby walls to create early reflections. This is the principle of open-back headphones. They allow the sound waves to escape to the outside world, preventing the buildup of acoustic pressure and internal reflections. The steel mesh on the HD 660 S acts as a carefully tuned acoustic impedance material—it doesn’t just let sound out haphazardly, but controls its escape to prevent turbulence and ensure a smooth response.

The result is a listening experience that begins to mimic how we hear in the real world. This is where psychoacoustics—the study of how our brain perceives sound—comes in. Our brain determines the location of a sound source by interpreting the tiny time and volume differences between what our two ears hear. By eliminating the confusing reflections of a closed space, the open-backed HD 660 S allows these subtle spatial cues in the recording to reach our ears intact. This creates a wide, three-dimensional soundstage, where you can perceive the guitarist to your left, the drummer behind the singer, and the vastness of the recording studio around them.

Anatomy of a Moment: The Engine of Fidelity

If the open design is the theater, the transducer is the star performer. This is the tiny engine that translates electrical signals into physical sound waves, and its performance dictates the ultimate clarity of the music. Think of it like a hummingbird’s wing; its ability to start, stop, and change direction with near-instantaneous speed is paramount.

This agility is called transient response. In a piece of music, a transient is a high-amplitude, short-duration sound, like the initial hit of a drumstick on a cymbal. A slow, heavy transducer will blur this moment, turning the crisp “tick” into a duller “thud.” The HD 660 S uses a very light but rigid aluminum voice coil to drive its 38mm diaphragm. This is a direct application of Newton’s second law (Force = Mass × Acceleration). Less mass means the same electrical force can create a much faster acceleration, allowing the diaphragm to track the musical signal with breathtaking precision. It can reproduce that cymbal hit perfectly, then stop on a dime, ready for the next note.

This process is governed by Faraday’s Law of Induction, where the audio signal (an alternating current) flows through the voice coil, creating a fluctuating magnetic field that interacts with a fixed magnet, causing the coil and attached diaphragm to vibrate. Any imperfection in this dance introduces harmonic distortion—unwanted frequencies that weren’t in the original signal. You can think of it as the difference between a pure, clean guitar note and the same note played through a fuzzy distortion pedal. The new transducer in the HD 660 S is meticulously engineered to minimize these distortions, keeping the sound exceptionally clean.

Finally, to create a stable, believable stereo image, both the left and right transducers must perform identically. Sennheiser hand-matches the drivers in each pair of HD 660 S to a tolerance of just ±1 decibel (dB). To put that in perspective, 1dB is widely considered the Just-Noticeable Difference (JND) in volume for the human ear. This incredible precision ensures that when a sound is mixed to be dead center, it appears in your mind as a stable, solid point in space, not a vague image drifting slightly to one side.

The Quiet Highway: Power, Purity, and the Path of Least Resistance

The final piece of the puzzle is how we get the electrical signal to the transducer cleanly and efficiently. This brings us to impedance and cables.

The HD 660 S has an impedance of 150 ohms. You can think of impedance as a measure of how much “push” (voltage) is needed to get the “flow” (current) of electricity moving. The 150-ohm rating is a brilliant compromise. It’s low enough that high-quality portable players and laptops can drive it effectively, but high enough to benefit significantly from the greater power and control of a dedicated headphone amplifier. It’s a versatile design that can grow with your audio setup.

More fascinating is the inclusion of a 4.4mm Pentaconn balanced cable. Most standard headphone connections are “unbalanced.” They use a single ground wire for both the left and right channels. Imagine a busy two-way country road where cars going in opposite directions can interfere with each other. A balanced connection is like a private, two-lane highway for each channel. It uses separate signal and return paths for left and right, and a clever principle called Common Mode Rejection. Any external electronic noise (from your phone, Wi-Fi, etc.) that gets picked up by the cable will be induced on both signal wires equally. The balanced amplifier at the other end can identify and cancel out this identical “common” noise, leaving only the pure, original music signal. The result is a “blacker,” quieter background, from which the faintest musical details—like that ghostly finger-scrape on a tambourine—can emerge.

Erasing the Glass

Let’s return to that Fleetwood Mac song. The ghost in the machine was never a ghost. It was a fragment of reality, pressed into the vinyl or encoded in the digital file, waiting to be resolved. For years, it was obscured by the acoustic equivalent of a cloudy pane of glass—the subtle resonances of a closed design, the fractional blurring of a slower transducer, the faint haze of noise in the signal path.

Every engineering principle we’ve discussed is a polishing cloth for that glass. The open-back design prevents the fog of internal reflections. The lightning-fast transducer brings the image into sharp focus. The balanced cable wipes away the last smudges of electrical noise.

The ultimate goal of this obsessive engineering isn’t to boast about specs or celebrate the technology for its own sake. It is to dismantle the barrier between the artist’s moment of creation and our own consciousness. It’s about erasing the machinery of reproduction so completely that all that’s left is the music, the space in which it was born, and the raw, unvarnished emotion that propelled it. It’s about building a bridge of pure science that leads directly to the heart of art.