Underwater Audio: Bone Conduction Swimming Headphones

For an athlete training in the water, silence has long been the only companion. Runners and cyclists can stream playlists from a phone on the deck, but swimmers hit a physical wall: water is a different medium than air, both acoustically and electromagnetically. Getting usable sound beneath the surface means solving two problems at once: how the audio reaches the ear, and how the data crosses the water line at all.

The devices that solve this problem are bone conduction swimming headphones, and they force a genuine split in consumer electronics between a splash-proof earbud and a tool that is engineered to live under the surface. One concrete example, the Relxhome AS19 Bone Conduction Headphones, makes the underlying engineering easier to see.

Why Standard Bluetooth Drowns in a Pool

The first frustration for anyone who has tried to swim with ordinary wireless earbuds is the moment the water reaches the phone on the deck. The buds lose their signal and go quiet. The water is swallowing the radio link, for reasons that sit in the physics of the electromagnetic spectrum.

Bluetooth runs on the 2.4 GHz band. Water molecules are polar, meaning they carry a charge that is off-balance, and they soak up electromagnetic energy in exactly that frequency range. The same physics that lets a microwave oven heat food is what a pool does to a wireless link. A few inches of water attenuate the signal to the point where it cannot carry audio data. The result is a dead zone that no amount of receiver gain can punch through.

Because a phone cannot beam music to a submerged head, the only workable architecture is local storage on the device itself. A headphone that carries its own memory becomes a standalone player, and the radio link is switched to MP3 mode before the head goes under. That shift, from streaming to on-board playback, is the design decision the whole category turns on. It is also why the category spec sheet always lists a storage figure alongside the waterproof class.

The Cranial Vibration Path, Step by Step

In a normal air-conduction design, a driver pushes sound waves down the ear canal to the eardrum. Underwater, the canal is filled with water or blocked by an earplug, and the mismatch between an air driver and a wet canal muffles the result. Bone conduction sidesteps that pathway entirely.

The transduction step happens where the transducer sits on the zygomatic arch, the cheekbone, and the device converts the audio signal into a mechanical vibration. In the transmission step, that vibration travels through the dense bone of the skull. In the reception step, the inner ear, the cochlea, picks it up the same way it always has. The water itself becomes a coupler. Because the acoustic impedance of water sits closer to bone than air does, the low end tends to feel more present underwater than it does in open air, where bass energy leaks out of the open canal.

That is why bone conduction swimming headphones are tuned for bass and clarity rather than raw high-frequency sparkle. The frequency response of a bone conduction driver is limited by the range the skull can carry, and the ear-hook form that rests on the cheekbone is what keeps the transducer sealed against the water while leaving the ear open. A wider, more flexible headband distributes that pressure, and the soft pads are what most buyers notice in the first few minutes of a session. I first noticed the low-end pressure on a bone conduction band, and it is the tell that the vibration path is working.

The IP Rating Ladder and What IPX8 Means

Water resistance labels are precise only if you know the ladder behind them. The ingress protection system, or IP code, splits out two axes, dust and water, and the consumer units in this category live on the water half.

IPX4 is splash proof. It holds up against rain and sweat, and it is the floor for any open-ear running unit. IPX7 means the device can be immersed up to one meter for thirty minutes, enough for an accidental splash but not a swim. IPX8 is the class that allows continuous immersion beyond one meter, under conditions the manufacturer sets, which is what an intentional, sustained training session needs.

Hitting IPX8 in practice means sealing the chassis so there is no open port for water to enter. That usually replaces a physical charging port with a magnetic charging interface, and it coats or pots the circuit board so that even microscopic ingress does not corrode the traces. If a listing only carries IPX4, treat it as a sweatproof runner and not as pool gear. If I were buying for a pool, I would treat the IP code as the first filter and the brand as the last.

The Relxhome AS19 as a Concrete Example

The AS19 is a useful lens because it packs the core category features into a single, reasonably priced unit.

What the AS19 Does in the Water

The AS19 carries an IPX8 rating and is specified for immersion to two meters, with the listing description claiming up to three hours against two hours in the summary bullets, which is enough for a real training block rather than a single lap. In the water it runs in MP3 mode off the internal 32GB of storage, enough for roughly 8,000 tracks when the files are standard MP3s, so the phone stays on the deck and the whole swim runs device-free. The mode switch has to happen before the head goes under, because the radio link cannot survive submersion.

Fit, Battery, and the Open-Ear Trade

The form factor is an over-ear, open-ear band that rests on the cheekbones, and the unit weighs about 1.13 ounces, light enough to sit under a swim cap without pulling. The battery is rated at up to 10 hours of playback, and the wireless link on land is Bluetooth 5.3. It is sold in Carbon Black and positions itself as a 2023 upgrade of the bone conduction line.

In a buyer rating, it sits at 4.3 out of 5 stars across 275 ratings, which is solid for the category but not a runaway. The recurring strength in the reviews is the battery life and the secure fit, and the recurring complaint is that the open-ear sound, while clear, does not stack up against a premium air-conduction earbud on pure musical detail. I read the review mix the way the star average suggests: strong on fit and battery, softer on musical detail.

That trade is inherent to the category and worth naming out loud before anyone buys. Open ear keeps the canal open so the swimmer hears the pool. In exchange, some high-frequency detail and bass depth are traded away relative to a sealed design. The AS19 is a clean, mainstream answer to that trade, sitting at $45.99. I would flag that open-ear trade before you commit; I only had a short listen, so treat the detail comparison as provisional.

Dual-Mode Architecture, Open-Ear Safety, and Where the Category Is Heading

The dual-mode split is the load-bearing feature of every serious unit in the category. On land, the Bluetooth link handles streaming, calls, and the podcast app. In the water, the player switches to MP3 mode and plays files that were copied ahead of time through the magnetic charging connector, which doubles as the data link to a computer. The user must make that switch before the head goes under, because the radio link cannot survive submersion. Streaming services only work on the land side, in Bluetooth mode, and the files that play underwater are the ones already sitting in the internal memory.

Open ear doubles as a safety layer for the land sessions between swims. A runner or cyclist on a street cannot afford the full isolation that a noise-canceling headphone creates. An approaching electric vehicle is quiet and easy to miss, and a siren or a voice calling out is what keeps a head off the curb. The open-ear band lets the swimmer process both the music track and the ambient track at the same time, without toggling a mode.

Across the budget band the tracked competitors cluster close to the AS19. The PSIER units pair a 14mm driver, 32GB of storage and 8 hours of playback with IPX8 sealing that lists one hour of submersion at two meters. The SAMVEK option is the lightest at 0.9 ounces, with IP68 sealing, 32GB storage and a 220 mAh battery good for 8 hours. The VLOP unit takes a dual-driver hybrid that blends bone conduction with directional air conduction, and lands in the same 8 hour, 32GB envelope. At the premium pole, the SHOKZ OpenSwim Pro at $229.95 swaps the budget spec sheet for triple-sealed IP68 construction and a 9 hour battery, while its submersion window holds at two meters for two hours. The shared budget envelope comes out to 32GB of storage, eight to ten hours of battery, IPX8 or IP68 sealing, and Bluetooth plus MP3 dual mode.

Because the medium is the hard constraint, a diver and a sprinter gear up differently, and a swimmer needs a headphone that treats water as a conductor for vibration rather than as a barrier for radio. The units that combine IPX8 sealing, local storage, and the cranial vibration path are an engineering answer to a physical limit, and the buyer is better off reading the spec against that limit than against a brand name.