Earpads are the second most important factor of a headphone’s sonic profile. They're not just a matter of comfort— they’re the final and most aggressive filter between the energy propagating from the driver (the third most important factor) and your ear (the most important factor). Due to the particular size constraints of over-ear headphones, every dimension and material selection within an earpad dramatically changes what you hear in the region where your hearing is most sensitive. It’s a direct high-stakes parallel for the thoroughly-explored interplay between room acoustics and loudspeaker performance. Given the lack of recent innovation and exploration in the realm of earpad design in the high-end headphone market, it’s no wonder earpad research and optimization has been fundamental to ETA’s mission from the very beginning:
During the brainstorming phase for ULI, we knew we wanted an extremely large cavity for the listener’s ear, and a low acoustic impedance target to maintain the spaciousness and clarity we sought out to achieve with an earspeaker-like headphone design. Some of the earlier ULI prototypes borrowed generic earpads designed for large planar magnetic headphones with a reputation for their ‘openness’. The samples we favored from this selection generally had a few things in common - they kept the driver close to the ear, and they used a hybrid selection of materials that allowed some degree of airflow to escape from the ear cavity. However, they consistently yielded narrow peaks and dips in the upper midrange and low treble area regardless of how we positioned the driver within the ear cavity space.
In the field of room acoustics, it is widely understood that parallel walls create room modes - peaks in the low-end response where the room resonates in accordance with how its dimensions align with various wavelengths being reproduced from the sound source. This principle remains true in headphones - but these resonances are usually in the upper midrange and low treble, due to the much smaller size of an earpad’s cavity compared to a listening room. The vast majority of readily-available earpads feature ear cavities with flat parallel walls, highly-impeding leather-like materials, and/or high-density internal foam - a worst-case scenario for the development of peaks and dips in the most critical tuning region, like a concrete box for a loudspeaker listening room.
We explored solutions to this issue with a few variables: casing material, foam specs, and geometry. The thinking went this way - given earpad resonance is an unavoidable issue, either we can 1. Use absorptive material on inner walls to prevent reflections. 2. Allow so much airflow through the inner walls that the shape becomes slightly closer to acoustically invisible. Or 3. Eliminate parallel surfaces altogether to level the modes across a wider bandwidth. Given the heavy degree of venting and angling built into the ULI chassis, we were free to explore these variables in isolation without concern for seal-related bass performance that force most manufacturers into materials like suede or leather.
Finding the ideal recipe involved real-world iteration as well as acoustic modeling, factoring in material absorption coefficients and geometry-related modes - here's a simplified data-driven interactive visualization of how these elements played together to inform ULI's earpad specs:
Optimizing each of these three categories of earpad design (internal foam, casing material, and wall geometry) individually leads to substantial improvements in high-frequency linearity, but stacking them together created an undeniable synergy with the ULI driver's raw performance and enclosure response. Fast forward a few years from initially tuning around earpads borrowed from other headphones, and our first satisfactory samples arrived - featuring the ultra-soft and breathable bamboo silk casing with ultra-compliant open cell foam. Later, we added an aggressive chamfer across the inner wall of the earpad that we could only mimic using hand-stitching on our earliest samples:
We’re continuing to expand ULI’s earpad selection to further tailor comfort and tuning - as we begin to ship our first units, we’re proud to include the JET pad (top left) we developed in collaboration with ZMF headphones (featuring our thickest ultra-soft inner foam with bamboo silk casing as a refined version of the spacious, mid-forward ULI sound our listeners have loved from our recent expos) as well as our newest INK pad (bottom right), with a slightly firmer feel and nylon casing for a denser, more immediate tone:
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