A special thanx goes to three of Sony MH1 units, as they sacrificed themselves for this crazy experiment.
Sony MH1 has so much potential in terms of acoustic modifiability. Previously, various modification techniques for fine-tuning MH1 have been presented by the designer, Mr. Sead Smailagic and me.
- Front-damper modification from here, presented by Sead Smailagic
- Top-vent modification presented by udauda and Sead Smailagic
- Front-vent modification presented by udauda
Although above techniques do change MH1's tonality greatly, it is also true that the low frequency range is little too emphasized, compared to the conventional diffuse-field target. As Sony MH1 has great fidelity in the frequency range over 1kHz, if the bass can be tamed down with a very low Q factor, MH1 will surely become one of the flattest IEM in the market. Thus, tutorials, reversible and irreversible, will be presented to help users further fine-tune their MH1's low frequency response according to individual preferences.
What you need to know beforehand
The rear cavity volume of Sony MH1 is equivalent to acoustic compliance of 9e-14 m4s2/kg
First of all, it is vital to understand the rear cavity and the vent together behave like a RLC filter. By decreasing the rear cavity volume(acoustic compliance), its lowpass cut-off frequency increases. While changing the diameter(acoustic resistance) of the vent increases/decreases the effect of such filter, changing the length(intertance) modifies its highpass cut-off frequency. Sony MH1 is tuned with such filter as as well, and if the filter is tweaked to suit our individual needs, a perfect IEM will be born.
Upon a complete vacuum seal of the rear cavity and the vent, the low frequency of MH1 attenuates up to 10 dB at 20 Hz, turning the tonality of MH1 close to that of Etymotic Research ER-4 series. Although such seal nicely tames the bass, it should be more appropriate to retain some sort of lowpass filter at least for depressurization, as the diaphragm will be permanently damaged otherwise.
2. Irreversible method: For advanced users. Try at your own risk!
Materials needed: A 25-gauge dull 1cc syringe, low-viscosity polymer, a hand drill with a drill bit smaller than 2 mm in diameter, a pair of acoustic dampers (Yellow 4700 Ohm for the tutorial)
First, inject your polymer into the strain relief, as it works as an acoustic vent & rear cavity. Make sure you reach to the very end of the strain relief, and completely seal off so that there is no residual cavity left inside of the relief.
Second, widen the top vent to approximately 2 mm in diameter, so that Knowles dampers can fit in. Or you may seal off the stock vent with your polymer, and drill a hole somewhere else. However, do not drill a hole from behind, since the cable is in the way. You may also seal off any residual leak around the damper with polymer.
Finally, the result. As mentioned above, the mid-bass can be further cut down by decreasing the rear cavity volume. Moreover, upon changing the size of the rear volume and/or the value of acoustic impedance, various sound signature can be achieved. IOW, there are numerous possibilities!
Materials needed: A low-viscosity polymer, an electrical tape, a needle
Cover the stock vent, block the opening (the opening only!) at the strain relief with polymer and puncture an extremely tiny hole, which is barely visible to your eyes, on the blocked vent. A hole bigger than 0.25 mm will revert the sound back to stock, as the rear cavity volume is already too large due to the strain relief.
The effect is rather minimized, since the rear cavity volume size & acoustic impedance can't be modified this way. Still, if it is easy to do and totally reversible, why not have a shot? There is nothing to lose!