Fosi I5 – Quick Update After the Review!

I’ve got a quick but important update on the Fosi I5, their first planar headphone. If you haven’t seen the full review yet, I recommend checking it out first, link below.
Long story short: both myself and other reviewers noticed a sub-bass roll-off in the I5’s frequency response. Fosi investigated and traced the issue back to sound leakage caused by the Velcro-mounted earpads.

They tested several fixes, and the best solution was the addition of a sound-isolating ring around the diaphragm. This fix significantly reduced leakage and restored the sub-bass performance we were all expecting.

First of all, credit to Fosi:

Not just for sending out the I5 for review, but also for the way they’ve responded to feedback. They didn’t get defensive, they listened, learned, and acted. That’s exactly the kind of mindset we love to see in a brand that wants to grow with its community.

They sent me the new isolation ring mod, and here are my impressions.

Installation:

The installation is super simple, you just remove the earpads and carefully unscrew the four small Torx screws using the included screwdriver, making sure not to drop them onto the delicate diaphragm as they are tiny and could easily cause damage, then you replace the old mounting plate with the new one that includes the isolation ring, screw everything back in place, and you're done in about five minutes.

First Listening Impressions (Before Measuring):

As soon as I installed the isolation rings, I jumped straight into a few sub-heavy tracks like Alva Noto and Dominic Miller’s London Paris Cardiff , and oh yes, the sub-bass is finally there. You can instantly feel a much fuller, deeper foundation that gives the music more impact and weight. The low-end isn’t just present, it’s controlled and authoritative. Interestingly, the mids also felt slightly more open and engaging, likely benefiting from the improved balance, while the top end struck me as a touch smoother and less sharp than before, still detailed, but more refined. The overall sound feels better grounded and more cohesive. But of course, let’s move on to the measurements to see if the graphs confirm what I’m hearing.

Measurements with MiniDSP EARS:

Using the MiniDSP EARS, I measured the left and right channels with the new isolation ring installed and found a difference of about 1dB in the mids and around 2 to 3dB in the lows, which is noticeable but not a cause for concern.

Comparing it with previous measurements confirmed that this variation was already present before the mod.

I then performed multiple sweeps while repositioning the I5 on the MiniDSP EARS, and the low-end response remained consistently stable, confirming that the new modification does not negatively affect channel balance or bass performance.

Frequency Response Consistency L

Frequency Response Consistency R

When comparing the old setup to the new one with the isolation ring installed, the difference is immediately clear: the sub-bass roll-off is completely gone, the overly punchy peak around 80Hz has been smoothed out and sounds more natural, and the treble above 6kHz is now slightly more refined, with around a 2dB reduction that makes it less sharp while retaining clarity. Overall, the isolation ring proves to be a very effective improvement.

Updated Sound Impressions:

With the new isolation rings installed, the sub-bass finally comes to life, delivering a powerful and well-extended foundation that was previously missing, adding real weight and presence to the overall sound; the mids now feel slightly more forward and engaging, offering a clearer sense of body and presence especially in vocals and acoustic instruments; while the treble still carries a hint of sharpness in the uppermost registers, it no longer comes across as aggressive or fatiguing, making long listening sessions much more enjoyable; overall, the I5 retains its open and energetic character, with a lively and detailed presentation that will definitely appeal to listeners who appreciate an exciting, fast, and articulate sound signature.

Final Thoughts:

For a first headphone, Fosi did a really solid job.
The sub-bass issue is now fixed, and the I5 becomes now a strong recommendation, especially for those who like an exciting, transparent sound.

It would be great to see a lighter, more ergonomic version in the future.
But as it stands, for the price (about €380 via Kickstarter), this headphone is now very competitive.

If you’re interested, you can use my affiliate Kickstarter link below.
Thanks again to Fosi, and to all of you for the feedback and support!

Kickstarter campaign affiliate link: https://fosiaudio-inc.kckb.me/2ed96a7f