Closed-Back Headphones Are Detuning Your Hearing, According to Legendary Headphone Engineer

What closed-back headphones really do to your ears goes far beyond isolation.

Your closed-back headphones might literally be detuning your hearing. That’s the striking revelation from Axel Grell, the legendary engineer behind Sennheiser’s HD 600, HD 800, and HE 1 headphones.

In a recent Grell Sound Lab video, Grell explains how the very design that makes closed-backs popular for isolation could be fundamentally altering how your ears function.

The Hidden Impact of Closed-Back Headphones

Many listeners notice a slight “pressure” and an in-head presentation when they seal the ears with closed-back headphones. A major reason is the occlusion effect: once the ear canal is sealed, low-frequency energy that would normally escape gets trapped, which can make the same volume feel denser and more fatiguing.

Axel Grell argues there can be more going on than occlusion alone. In his view, strong internal reflections in sealed designs can nudge your hearing system away from its most natural operating point, or “detune” it.

At higher levels and for some listeners, the middle-ear muscle reflex (tiny muscles that tighten to protect the ear from loud sounds) may also engage and subtly change how sound is transmitted.

In plain terms, “detuned” can mean changes you actually notice while listening:

• Loudness & balance: sensitivity can dip a little, especially in the lows, so you might nudge the volume up; tonal balance can feel uneven or slightly boomy.
• Clarity & imaging: reflections add faint time-delayed copies of the sound, which can smear fine detail and pull the image inside your head.
• Fatigue & pressure: the sealed canal and altered mechanics can increase the sense of pressure and lead to listening fatigue over time, even at modest volumes.

On the other hand, open-back designs often feel more natural and “transparent” because unused sound energy can leave the system instead of bouncing around the cup.

The best results come when that openness is managed with acoustically resistive materials that let energy pass without creating strong reflections.

The Science Behind Sound Reflections

Openness in headphones isn’t a simple binary of “open” versus “closed.” It’s about how easily the ear-side acoustic energy can leave or be absorbed instead of bouncing back toward the eardrum.

In acoustic terms, higher “openness” means the path from the driver and ear canal to the outside world has higher effective admittance (lower net reflectance).

However, there are degrees of openness, and even open designs vary widely.

Why reflections happen even in “open” designs

At any boundary in the ear-side path, including the grills, pads, vents, or the final opening to free air acoustic impedance changes. And, whenever the impedance seen by the wave changes abruptly, part of the energy reflects and part transmits.

This is true at both closed ends and open ends of a duct; the difference is the phase and amount of the reflection. That’s why even very open headphones can still throw small, time-delayed copies of the signal back toward the ear if the openings aren’t shaped and terminated well.

To understand this better, Grell provided a quick analogy:

In short, the ear-side pathway behaves like a short duct whose termination must be matched to the outside.

How Headphone Designs Can Address This Issue

According to Grell, the goal is a large open area that’s controlled with acoustically resistive material so energy can pass through without strongly reflecting back toward the ear. As he puts it:

In practice, that means covering generous openings with porous, flow-resistive layers that dissipate part of the wave as heat, lowering the strength of reflections that bounce back toward the eardrum.

Many designs also add targeted absorbers inside the cup or on the ear-side baffle to knock down specific resonances. Done well, this kind of resistive openness vents the excess energy that contributes to the “detuned” feeling, all while keeping enough acoustic impedance at the ear to preserve low-frequency pressure.

The bass trade-off

Unfortunately, as you increase the size or effectiveness of those vents and porous layers, you also change the low-frequency load the driver “sees.” The ear-side acoustic impedance drops so far that the driver “short-circuits” at low frequencies.

Basically, the pressure can’t build at the eardrum, so bass rolls off.

This is why tiny leaks from glasses, hair, or pad gaps can cause large, listener-to-listener LF swings, even tens of decibels in severe cases.

The flip side is that sealing everything aggressively tends to raise reflections again.

That’s why well-behaved designs live in the middle: open enough to vent excess energy and curb the “detuned” feel Grell warns about, yet resistive enough to keep bass intact.

At the extreme end are “ear speakers” such as the AKG K1000, which suspended drivers in front of the ear with no pads at all. They produced stunning clarity in the mids and highs. However, they had minimal bass, since the drivers couldn’t build pressure.

Can We Measure Headphone Openness?

There’s no single, agreed-upon spec today that measures openness.

The standards and rigs we use for headphones, like IEC 60318-4 ear simulators (the “711” coupler), IEC 60318-7 HATS manikins, and IEC 60268-7 measurement procedures, are excellent for frequency response, distortion, isolation, etc. But, they don’t report a direct metric for how much ear-side energy reflects back toward the eardrum (the heart of “openness”).

According to Grell, it’s hard to measure “openness” as it’s fundamentally a reflection/impedance problem at the ear side, not just an outward-leakage or isolation issue.

Standard FR curves on couplers and HATS mostly capture driver → eardrum transfer. They don’t quantify the return path (eardrum/ear canal → reflections back from the headphone opening/cup). So, you don’t get a single “openness” number out of a normal lab session, even though the physics matters for the “detuned” feeling Grell warns about.

What’s the closest we can measure right now?

There are five things we can refer to:

• Ear-canal reflectance / absorbance (Wideband Acoustic Immittance): In audiology, WAI directly measures how much acoustic energy is reflected vs absorbed in the ear canal across frequency, which is conceptually aligned with what we care about for “openness.” It’s proven for middle-ear assessment, but there’s no headphone “openness score” defined from WAI yet.
• Ear-side acoustic-impedance methods: Work led by Roman Schlieper compares Pressure Division Ratio (PDR), Headphone Selection Criterion (HPC), and Acoustic Impedance Curve (AIC) using a custom impedance tube + KEMAR to rank relative openness among headphones.
  These align tightly with Grell’s claim, but they’re research-grade methods and not industry specs you’ll see on product sheets (yet).
• Material-level “resistive openness:” Engineers can quantify the flow resistivity of meshes/foams that cover large openings (the stuff Grell champions) using standards like ISO 9053-1. Useful to design resistive (not purely open) vents that let energy out while damping reflections. But it’s a material property, not a headphone-level openness metric.
• Outward leakage / passive isolation on a HATS. You can measure how much sound escapes to the room or how much outside noise is blocked (like hearing-protector tests). Those numbers correlate with how “open” a headphone might feel, but they’re indirect; they don’t tell you the ear-side reflectance that matters for detuning.
• Leak sensitivity at low frequencies. Classic leakage studies show that tiny pad gaps (glasses, hair) can swing bass response by tens of dB, which is an evidence that ear-side impedance is pivotal. This is a useful proxy for how “too open” (or too leaky) paths affect LF behavior. But again, it’s not a reflection metric.

What This Means for You

All the discussions above don’t mean closed-backs are bad. They remain essential for commuting, travel, and studio tracking where isolation is non-negotiable. But it does mean you should be aware of the trade-offs.

In the same way, not all open headphones are equal. Designs that combine a large open area with resistive materials tend to manage reflections more effectively than bare grills alone.

When you need isolation, pick closed-backs that hint at resistive openness: large vented areas with mesh or foam, absorber features, or designs that mention internal damping.

Be cautious with “DIY openness” tricks like creating leaks, though. While they can relieve some pressure, even tiny pad gaps from glasses or hair can wipe out bass by tens of decibels.

If you already own closed-backs, check pad condition and fit. Small leaks from glasses or worn pads can swing bass dramatically, while pad material trades off bass vs. openness. Keep listening at moderate levels, since the ear’s reflexes are level-dependent.

Until there’s a standard “openness” spec, these cues, like design language, quick checks, and fit awareness, are the best way to avoid the detuned feeling Grell describes while still getting the isolation you need.