How We Research
We compare products from their published specs and features; the verdicts below are our independent editorial opinions.
Updated: 2026-10-02 · 6 min read · Independent comparison
If you want a single pair of open-ear headphones that runs all day on one charge without a case, choose Fenpos Bone Conduction Headphones; if you need a lighter, more compact earbud setup with a rotating charging case and newer Bluetooth, choose Monster Open Ear AC390.
• Fenpos Bone Conduction Headphones — biggest strength: a true bone conduction driver with a single 10-hour battery, so there's no case to manage and no charging anxiety mid-ride.
• Fenpos Bone Conduction Headphones — biggest trade-off: it's a one-piece neckband-style design, so it's less pocketable than a case-based earbud, and there's no charging case as a backup top-up.
• Monster Open Ear AC390 — biggest strength: ultra-light 5g-per-bud ear-hook design with a rotating charging case that pushes total listening to 24 hours, plus Bluetooth 6.0.
• Monster Open Ear AC390 — biggest trade-off: each bud only lasts 8 hours on its own, so you're dependent on the case for a full day, and you have two separate pieces to keep track of.
• Both share: IPX5 water resistance and an open-ear fit that keeps you aware of traffic, cyclists and gym surroundings.
• Pick Fenpos if you want one cable-free piece with maximum single-charge runtime. Pick Monster if you want featherweight buds, touch controls and a case you can clip on.
The Fenpos pair leans hard into the bone conduction approach. Instead of firing sound into your ear canal, the driver vibrates against the bone in front of your ear, which is exactly why the open-ear design here doubles as a safety feature — you stay aware of cars, other runners and whatever is happening around you. For anyone who runs on roads, cycles in traffic, or works out in a busy gym, that situational awareness is the whole point, not a bonus.
On the spec sheet, the standout number is the battery: a built-in cell rated for up to 10 hours of average life, charged over Type-C. That's a single charge covering a long training block, a full commute both ways, or a whole workday of background listening. There's no case in the box to top it up mid-day, which is a genuine trade-off — but it also means there's nothing extra to remember, lose or plug in.
Who should buy it: runners, cyclists and commuters who want one piece, one charge, and maximum awareness of their surroundings. Who should skip it: anyone who wants a compact case they can slip into a jeans pocket, or who prefers touch controls over physical buttons.
Monster Open Ear AC390: Who It's For
The Monster Open Ear AC390 takes the opposite approach: two separate ear-hook buds, each weighing just 5g — the kind of weight you forget about within minutes. The soft silicone hooks are designed to sit around the ear rather than press into it, which matters if you wear headphones for hours at a desk or on long calls and don't want that dull ache behind the ear.
The audio hardware is the other headline. There's a 13mm PU titanium diaphragm driver listed in the features (the spec sheet lists an 18mm driver size, so treat the exact figure as manufacturer-stated rather than something I can verify), tuned for crisp highs and deep bass. Realistically, a larger dynamic driver in a 5g bud is a different physical proposition from a bone conduction driver, and it usually shows up as more low-end punch. For music-first listeners, that's a meaningful difference.
Battery is where you need to read carefully. Each bud gives 8 hours, and the rotating charging case adds another 16, for 24 hours total. So the case isn't optional garnish — it's half your runtime. Charge time is listed at 8 hours. Bluetooth 6.0 is the newest version in this comparison, which should mean faster pairing and lower latency for gaming or video, and the case itself rotates and includes a lanyard, so it's designed to be clipped to a bag rather than buried in a pocket. Controls are touch, voice and Siri, with environmental noise cancellation for calls.
Who should buy it: people who want the lightest possible fit, listen to bass-heavy music, take a lot of calls, and don't mind carrying a case. Who should skip it: anyone who wants a single self-contained unit, or who dislikes touch controls.
Side-by-Side: Which Should You Choose?
• Best for total runtime with a case → Monster Open Ear AC390 (8 hours per charge plus 16 from the case, 24 total).
• Best for the lightest possible fit → Monster Open Ear AC390 (5g per bud versus a 26g one-piece band).
• Best for road and trail safety → Fenpos Bone Conduction Headphones (bone conduction open-ear design built around situational awareness).
• Best for bass-forward music → Monster Open Ear AC390 (large diaphragm dynamic driver tuned for deep bass).
• Best for glove-friendly, tactile controls → Fenpos Bone Conduction Headphones (push-button control method).
• Best for the newest wireless standard → Monster Open Ear AC390 (Bluetooth 6.0 versus Bluetooth 5.4).
• Best for calling in noisy places → Monster Open Ear AC390 (Dynamic Noise Cancellation), though Fenpos counters with front-and-rear dual-mic noise reduction, so both are credible for calls.
In my opinion, if you're buying for outdoor training, road running or cycling, I'd take the Fenpos Bone Conduction Headphones. The 10-hour single charge and the bone conduction open-ear design line up perfectly with what that kind of buyer actually needs, and the push-button controls are the right call for sweaty or gloved hands.
If your priority is comfort, music quality and getting the most total hours out of a compact kit, I'd take the Monster Open Ear AC390. The 5g buds, the bass-forward driver and the 24-hour case total are a better fit for desk work, commuting and long call days.
Neither is objectively "better" — they solve different problems. Decide whether you want one piece with one long charge, or two tiny buds with a case, and the choice basically makes itself.
are headphones waterproof
Not in this comparison — both are rated IPX5, which means water resistant rather than waterproof. IPX5 covers sweat, splashes and light rain, so both are fine for workouts and drizzly commutes, but neither should be submerged or worn for swimming.
are headphones water resistant
Yes, both the Fenpos Bone Conduction Headphones and the Monster Open Ear AC390 carry an IPX5 water resistance rating. That protects the internal components from sweat and light rain during runs, gym sessions and daily commutes. It is not a rating for swimming or full immersion, so keep them out of the pool.
The Fenpos Bone Conduction Headphones, at up to 10 hours of battery average life. The Monster Open Ear AC390 gives 8 hours per charge, but its rotating charging case adds 16 more hours, bringing the total to 24 — so Monster wins on total runtime while Fenpos wins on a single uninterrupted charge.
For outdoor running, I'd point to the Fenpos Bone Conduction Headphones. Its bone conduction open-ear design is built specifically around staying aware of your surroundings, and the 10-hour charge covers long runs and races without a case. The Monster AC390 is also IPX5 and very light, so it works fine too — it just leans more toward everyday listening than pure outdoor safety.
As an Amazon Associate I earn from qualifying purchases.
We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon and affiliated sites.
When you buy through links on this page, we may earn a commission from qualifying purchases at no extra cost to you.
Amazon and the Amazon logo are trademarks of Amazon or its affiliates. This site is an independent third-party review and is not affiliated with, endorsed by, or sponsored by Amazon or the product brands mentioned.
See also: Perfcaze Hard Headphone Case vs ANCAMITA Hybrid Active Noise, YouthWhisper Bone Conduction Headphones vs Xmenha Low Profile Mini, JLab Studio vs Ozeino 2.4GHz, Dr.meter Noise Cancelling Headphones vs eKids Mario Kids Headphones