Why Ultra Low (Acoustic) Impedance?

Same setup, different sound - what gives?

Have your ears ever felt stuffy? For me, when I have a cold or flare-up of seasonal allergies, I’m prone to that feeling of pressure inside my ears. Sometimes the act of drinking some water - or perhaps, in more stubborn cases - the valsalva maneuver (shown below) - can help one or both of my ears “pop” back into place. But when I can’t neutralize the pressure inside my ears, truly weird stuff starts to happen with my hearing. 

File:Valsalva maneuver.jpg. (2025, October 9). Wikimedia Commons. Retrieved December 4, 2025, from wikimedia.

When the pressure is different between my ears, I hear a distinct channel imbalance (particularly in the bass) when listening to music that drives me nuts with anxiety about whether my gear is broken. Or when a difference of pressure between my ears isn’t the issue but I’m generally congested, sometimes I’ll have one of those days where I think to myself “why does everything sound so sharp, grainy, and generally terrible when mere days ago I had zero grievances about the sound of my system”?

On the opposite end, sometimes after strenuous physical activity when all of my airways feel open, my hearing becomes temporarily muted or like I’m underwater - an opposite effect to the above. Suddenly everything sounds soft and lacking in detail.

If you’ve had an experience like either of these, the following is probably an obvious statement to you - differences of pressure inside and outside our ears relative to atmospheric air pressure change our perception of sound. 

That’s all (literally) in my head - aren’t we talking about headphones?

If you’ve read this far, perhaps you’ve connected the dots about where this is going regarding the design of headphones. The vast majority of circumaural headphones are designed to sound best with a partial or complete degree of seal - or, in other words, with some amount of trapped air between the driver and our ears. Many headphones utilize impeding meshes or paper filters to allow a partial amount of pressure release, but very few will allow the air pressure to truly neutralize to ambient atmospheric pressure with near-zero resistance.

We can visualize this effect clearly with frequency response and impedance response measurements. When the seal of a headphone is broken (for example, when a listener will wear thick-armed glasses or have thick curly hair), bass will begin to roll off at a drastically higher frequency. On an impedance response graph, this sometimes accompanies differences in the observed Fs (or low frequency resonant point) of the driver when the headphone is worn versus in open air, with a huge change in the peak electromechanical impedance at that resonant point. 

(As an example, here’s one of the ‘most open’ over ear headphones on the market) green = open air impedance, yellow = on-head impedance:

Our ears: a reactive transducer -

Considering these measurable correlates to the anecdotal experiences I started with, the following stands to reason: if acoustic seal can radically change the behavior of a large headphone diaphragm, wouldn’t that same degree of seal also radically change the behavior of the much smaller and more sensitive anatomical features involved in our hearing? And wouldn’t the nature of this change to hearing be non-linear relative to the change of diaphragm behavior considering our sensory organs are not identical to the active diaphragm?

With these questions in mind, I’d like to pose two hypotheses and explore their potential ramifications:

1. Any performance target for a headphone should consider the degree of seal inherent to the design of that headphone, given that air pressure likely changes our hearing perception via the same mechanism that it changes the behavior of a headphone driver.
2. An ideal acoustic environment for listening is one where the air pressure at the ear is neutralized to atmospheric air pressure (a statement which is always true when using loudspeakers).

The problem with universal performance targets -

If the first hypothesis is true, the idea of a universal frequency response target becomes incredibly messy. With the vast amount of acoustic impedance variations in baffle materials, earpad / tip materials, venting schemes, and air volumes across open and closed back headphones as well as IEMs - all affecting the behavior of the active diaphragm as well as our sensory organs to varying non-linear degrees -  one could argue that it is impossible to create a universal target. If a fully sealed headphone with zero front or rear airflow - one which clearly creates a feeling of pressure in the ears when worn with a high degree of clamp as well as audible driver flex/crinkle - affects the response of our hearing, why should it be held to the same standard as an open baffle earspeaker with zero front or rear damping affecting the driver or the ear?

Balancing non-neutral pressures - 

Perhaps the second hypothesis is more intuitive than the first, though its ramifications are equally problematic with regards to headphone design. In most headphones, air pressure is manipulated with changes in the rear air volume (behind the driver), front air volume (in front of the driver), and according to driver specs.

In simplified terms, the formula goes like this:

More rear air pressure = less bass. 
More front air pressure (ie seal) = more bass. 
Therefore a balanced response is one where the front and rear air pressures are proportional to each other, relative to the optimization of driver specs.

Principles of driver-level damping -

As I mentioned before, most headphone drivers are designed for some degree of seal - hence, when that seal is broken and baseline air pressure at the ear is neutralized, bass and acoustic efficiency are lost. In other words, when a common driver is forced to operate without a seal, there is too much energy loss (also known as damping) for adequate bass response. 

In the world of driver specifications, damping is expressed by two separate values, Qms and Qes. For both of these specs, higher values mean less loss/damping. Qms describes mechanical damping - informed primarily by the material properties of the diaphragm, suspension, former, and voice coil mass. Qes describes electrical damping - informed primarily by the voice coil impedance and properties of the magnetic assembly. Both Qms and Qes combine into the composite value Qts. 

Optimizing specs for performance at atmospheric pressure -

Looping back around to the second hypothesis, prioritizing a design with near-zero front and rear seal means starting from scratch with driver specifications - this is where AlNiCo comes in. Swapping an AlNiCo magnet in place of a neodymium (NdFeB) one led to drastic increases in Qts. Dropping the coil impedance increased the Qts even more.

With these adjustments, the material properties of the active portion of the driver system became paramount to the total damping of the headphone (which is why we developed the world's-first underhung silver-plated voice coil, with a traditional paper former and diaphragm, plus a high-compliance suspension) - as opposed to relying on accumulations of air pressure to control the behavior of the driver that may negatively affect the listening experience. In other words, you could say that the ULI driver is critically damped at normal atmospheric air pressure - which you can visualize with the near-zero difference of the headphone’s electromechanical impedance when worn versus in free air:

ULI: begging important questions -

In short, ULI is a headphone that leverages our observations - both objective and subjective - about hearing, measurement metrics, and material science. In a sense, it presents a few questions to our listeners and the personal audio market at large - how much of our understanding of performance is based on phenomena that are not fully understood or taken out of the context of our ears? Can a radical new approach to design change how we understand what makes a headphone pleasurable to listen to?