Yet again, with the help of my friend, Inks, Heir Audio 3.Ai and 4.Ai (generic versions) have been analyzed thoroughly. Initially I requested a review sample too to the manufacturer, but somehow my emails did not make it to their inbox. Hmm.
I don't know much about Heir Audio, but it is known at Head-Fi.org that they make fine custom IEMs. Since their generics have same internal components as the customs do, users who are currently using custom IEMs from Heir Audio might find this analysis quite useful as well.
CON: Inverted polarity. A mid frequency deep-null is present due to the crossover network design error. Even assuming the IEM's design goal was to achieve an user-tweakable electric linearity, just like with the case of JH Audio, a steep notch at 4 kHz is not something that can be easily restored with a conventional equalizer. On top of that, the frequency bandwidth is still too short.
ON SECOND THOUGHT #3: Heir Audio sells an upgrade cable, Magnus 1, for $149. Since the manufacturer says it has more silver content, it should lower the overall impedance of the IEM, and increase the output SPL due to improved efficiency in electroacoustic transduction.
And the plots on the left shows Magnus 1 surely does what it is supposed to do, but my random $25 furutech cable from Ebay performs better than Magnus 1. How odd.
ON SECOND THOUGHT #5: Heir Audio started directly responding to my article. Well, they even failed to properly identify their own data, even though legends are clearly shown on the right. Anyway, let us first take a look at this:
The cyan curve is an unaided real ear gain, referenced to free-field @ 0-degree. Magenta is the aided real ear response of 4.Ai. Red is my ear simulator gain, raw. There are a few things that need to be addressed.
First, although deviating below(or above) 2 kHz, possibly due to leakage cause by the probe microphone or to the actual RECD characteristic of the individual, existence of the same 4 kHz notch is evident.
Second, free-field reference introduces linear distortion, according to Theile's study published in the late 80's.
Third, 3 kHz peak is a natural ear canal resonance, which defines the mid-range fidelity, not something to be cut off.
Their arguments definitely do not prove anything related to what 4.Ai was rated for, and contradict the basic principles of human HRTF. But then again, why would basics matter if the manufacturer do not care for them?