There seem to be some misunderstandings around all this. The resonance you're observing with your broken-seal measurements is specifically the
Helmholtz resonance within the front volume (dependent on not only the driver, but the headphone enclosure, pads and artificial ear canal), with the specific degree of broken seal you have artificially introduced. Oratory explains the Helmoltz resonance
here (in the context of an IEM, but the same acoustic principles also apply to headphones):
With the key point at the end:
This is what you are effectively doing by introducing a broken seal - changing the (total) dimensions of the front volume 'vents'/'ports', and so the Helmholtz resonance frequency will change accordingly. In what way does it change?
https://www.reddit.com/r/oratory1990/comments/m6vwgc/_/gr9flc6
In more detail, the Helmholtz resonance frequency
f_
H is
defined as:
where
v is the speed of sound,
V_0 is the volume of the air cavity, and
A and
L_
eq relate to the (total) dimensions of the 'neck' (vents) of the resonator (cross-sectional area and length respectively). When breaking the seal of a headphone on the rig, the total cross-sectional area
A of the resonator vent increases (but
L_
eq doesn't). As the Helmholtz resonance frequency
f_
H is proportional to the square root of
A, as you increase the area of the vent by increasing the amount of seal breach, the resonance frequency will increase, and vice versa as I stated in my previous post ("smaller vent => lower resonance frequency"). Your own measurements actually show this e.g. the DCA Stealth (green=small leak, orange=large leak):
Now, these measurements are not even relevant to how a headphone will sound if these seal breaches do not occur on the listener's head. It's like boring out a bigger port in a subwoofer, measuring the sub's frequency response and concluding that can explain subjective perceptions of its original sound before you made this 'mod'. In the case of the DCA Stealth, for you they do seal well and do not show the above bass drop-off, as you've measured with in-ear mics. Yet you still perceive them to be lacking in 'slam'. So Helmholtz resonance frequency is not the explanation, and something else must be the cause. (Note I am specifically talking about
bass slam/impact here, as other descriptions of 'slam' at higher frequencies are just too nebulous and likely conflating bass slam with distinct other perceptions).
As I said before, the actual cause is front volume seal. The greater the front volume is sealed by the pads and headphone enclosure, the greater the bass slam/impact. The same effect is found with IEMs and
explains why balanced-armature models can be perceived as having less bass slam than dynamic models - simply because the former are less 'leakage tolerant', and so their bass extension decreases more readily with a front volume leakage. As Oratory says in that post, IEMs rely on pressure-chamber conditions and:
Sound familiar?
Note he doesn't say every single LCD model, and that comment was originally made
5 years ago, since which time I seem to remember Audeze have introduced some front volume venting to some models to reduce static pressure / suction when putting on / taking off the headphones. Every headphone model from every manufacturer will likely have a different degree of front volume seal due to various design constraints, but Oratory's point is clear: greater front volume seal => greater bass extension => greater perceived bass impact.
Of course any rejections of the hypothesis that bass slam correlates with extension below 20 Hz would require comparisons to be done with everything else being equal i.e. all headphones EQed to the same target (e.g. Harman) down to 20 Hz. Also note I am not really talking about sub-20 Hz frequencies being more audible here (although sub-20 Hz content is more common than you'd think, even in genres you might not
expect, and could be relevant to perceived bass slam). What I'm really talking about is measuring low frequency extension down to ~0 Hz as a
proxy measurement for degree of seal and so how much and how long pressure is maintained in the front volume before leaking out. This measurement can be
done with REW (which I believe you use for your tests) by setting the measurement sweep starting frequency below 20 Hz:
If you really want to correlate perceived bass slam to its actual physical cause, this is the most promising avenue to explore with your headphone measurements.