Breaking the Box: The Physics of Open-Back Headphones and E.A.R. Technology

For the uninitiated, the first experience with high-end headphones can be confusing. You put them on, expecting the music to be pumped directly into your brain, isolated from the world. Instead, you hear the hum of the refrigerator, the traffic outside, and when the music starts, it leaks out for everyone to hear. Is it broken? No, it is Open-Back, and it is the gateway to audiophile sound.

The Sennheiser HD 559 is a classic example of this design philosophy. It rejects the isolation of consumer headphones in favor of something more elusive: Soundstage. But how does “letting sound out” result in a better listening experience? And what is the “E.A.R.” technology stamped on the box?

This article explores the Physics of Enclosures, the psychoacoustics of Angled Drivers, and why tearing down the walls of the headphone cup creates a sonic reality that closed-back models can rarely match.

The Physics of the Enclosure: Why Open is Better

To understand open-back headphones, we must first look at the problem with closed-back ones. A speaker driver vibrates forward and backward. The forward motion creates the sound you hear. The backward motion creates an equal but opposite sound wave inside the earcup.

The Standing Wave Problem

In a closed-back headphone, this rear wave is trapped. It bounces off the hard plastic shell and reflects back towards your ear, mixing with the fresh sound coming from the driver. This creates Standing Waves and Resonance. It can make bass sound “boomy” or muddy and obscure fine details. Engineers use damping materials to fight this, but it’s a constant battle against physics.

The Sennheiser HD 559 solves this by removing the back wall entirely. The metal grille allows the rear sound wave to escape into the room. * No Reflections: Without a back wall, there are no standing waves to muddy the sound. * Natural Decay: Notes fade away naturally into the air, rather than being abruptly cut off by a plastic shell.

This results in a sound that is “airy” and “transparent.” It mimics the behavior of loudspeakers in a room, rather than tiny speakers trapped in a cup.

E.A.R. Technology: Hacking the Outer Ear

The HD 559 features Sennheiser’s “Ergonomic Acoustic Refinement” (E.A.R.) design. This is marketing speak for a very specific engineering choice: Angled Transducers.

In most headphones, the driver is parallel to your ear. It fires sound directly down the ear canal. This is efficient, but it’s unnatural. In the real world, sounds rarely come from directly inside our ears. They come from in front of us. Our outer ear (the Pinna) catches these sounds and funnels them into the canal. The reflections off the folds of the Pinna give our brain cues about the location of the sound. This is part of our Head-Related Transfer Function (HRTF).

The Angled Advantage

Sennheiser angles the drivers inside the HD 559 to fire slightly from the front, parallel to the cheekbone rather than the ear.
1. Pinna Activation: The sound waves hit the outer ear at an angle, interacting with the Pinna’s unique geometry before entering the canal.
2. Out-of-Head Localization: This interaction tricks the brain. Instead of perceiving the sound as originating “inside the skull” (the typical headphone experience), the brain places the soundstage slightly in front of the listener.

This creates a holographic effect. Instruments have separation and specific locations in 3D space. It transforms listening from a claustrophobic experience into a cinematic one.

The Impedance Factor: 50 Ohms

The HD 559 is rated at 50 Ohms impedance. In the world of headphones, this is a “Goldilocks” number. * Low Impedance (<32 Ohms): Easy to drive with a phone, but susceptible to background hiss (noise floor) and often lacks damping control. * High Impedance (>100 Ohms): Requires a dedicated amplifier to get enough volume, but offers tighter control over the driver movement.

At 50 Ohms, the HD 559 sits in the middle. It is efficient enough to be driven by a decent laptop or tablet without an external amp, but it has enough electrical resistance to benefit from the higher voltage of a dedicated home theater receiver or desktop amp. It creates a Damping Factor that tightens the bass response, preventing the “woolly” sound that plagues cheaper, low-impedance headphones.

Conclusion: The Audiophile’s First Step

The Sennheiser HD 559 is a masterclass in acoustic fundamentals. By stripping away the enclosure and angling the drivers, it leverages the physics of air and the anatomy of the ear to create a superior listening experience.

It asks the user to make a trade-off: give up isolation and privacy in exchange for soundstage and naturalism. For those willing to make that trade, it opens the door to a world where music isn’t just heard, but placed in a living, breathing space.