If you want a compact Bluetooth adapter with clearly stated TX/RX battery life for running, choose SYENGKKY 5.4; if you need longer listed runtime and want to share audio with two headphone pairs, choose Nulaxy Air in-Flight. For running, neither is itself a pair of headphones: each connects through a 3.5 mm audio jack, so the right choice depends on the wired source you want to make wireless.
SYENGKKY 5.4 vs Nulaxy Air in-Flight at a Glance
• SYENGKKY 5.4’s biggest strength: Bluetooth 5.4 and up to 14 hours in receiver mode or 11 hours in transmitter mode. Trade-off: Nulaxy lists longer runtimes in both modes.
• SYENGKKY 5.4 suits someone who wants a 3.5 mm adapter for both transmitting and receiving, with a simple button-based setup.
• Nulaxy Air in-Flight suits travelers or workout partners who want to share audio, especially from a plane’s 3.5 mm jack.
• For running, choose based on whether your source has a 3.5 mm port and whether the adapter’s TX or RX mode fits how you plan to listen.
SYENGKKY 5.4 is a two-way 3.5 mm Bluetooth audio adapter. In TX mode, it sends sound from a non-Bluetooth source—such as an airplane audio jack, TV, or gym equipment—to Bluetooth headphones or speakers. In RX mode, it receives audio from a Bluetooth source such as a phone and passes it to a stereo system through a 3.5 mm cable. That flexibility is its clearest advantage: one adapter can serve different audio setups.
Its listed battery life is up to 11 hours in TX mode and 14 hours in RX mode. It can pair with two Bluetooth devices in either mode, and it uses Bluetooth 5.4. The operation described is straightforward: plug it into an auxiliary port, turn it on with the multifunction button, select the mode, and pair the Bluetooth device. Those details make it a reasonable fit for someone who wants one small adapter for a gym machine, an airplane, or a home audio setup.
For running, the key question is what you are connecting. If you want to send audio from a treadmill or other equipment with a 3.5 mm output to Bluetooth headphones, TX mode is relevant. If your running audio already comes from a phone, the adapter is not automatically a better choice than using the phone’s existing Bluetooth connection; its RX role is mainly for sending phone audio to a wired stereo. Skip it if you need headphones themselves, a standalone music player, or an adapter that works without a 3.5 mm audio connection.
How We Research
We compare products from their published specs and features; the verdicts below are our independent editorial opinions.
Nulaxy Air in-Flight also works as a TX/RX Bluetooth adapter for 3.5 mm audio connections. Its strongest practical case is transmitting sound to headphones, particularly when two people want to listen to the same source. The product information says it can connect two pairs of Bluetooth headphones at once, making it a useful arrangement for sharing a movie or other audio on a flight. It also lists compatibility with airplane audio, TVs, gym equipment, and other devices with a 3.5 mm interface.
Battery life is listed as up to 15 hours in transmitter mode and 20 hours in receiver mode—longer than the SYENGKKY figures in both modes. It also supports fast charging with an included Type-C cable. Pairing may need a little attention: the guidance recommends turning off nearby Bluetooth devices, and AirPods need to be in their case to enter pairing mode. These aren’t major barriers, but they matter if you want a setup that requires as little fiddling as possible.
For running, Nulaxy can make sense if the equipment you use has a 3.5 mm output and you want to transmit its audio to Bluetooth headphones. Its two-headphone feature is more compelling for a shared listening situation than a solo run. There are also explicit limits: it is not suitable for piano, drums, guitar, or live music, and it cannot be used in a car in TX mode. If those use cases matter to you, this is not the right adapter.
• Best for longer listed battery life → Nulaxy Air in-Flight. It specifies up to 15 hours in TX and 20 hours in RX, compared with SYENGKKY’s 11 and 14 hours.
• Best for two people sharing a source → Nulaxy Air in-Flight. It is explicitly described as connecting two pairs of Bluetooth headphones at the same time.
• Best if Bluetooth 5.4 is a priority → SYENGKKY 5.4. Its stated connection technology is Bluetooth 5.4; the Nulaxy sheet does not specify a Bluetooth version.
• Best for a simple button-led setup → SYENGKKY 5.4. Its setup instructions describe plugging in, powering on, selecting the mode, and pairing.
• Best for a flight’s headphone jack → Either, with a small edge to Nulaxy for sharing. Both are described as transmitters for airplane audio; Nulaxy specifically highlights two-headphone listening.
• Best for a solo run using gym equipment with a 3.5 mm output → Either. Both can transmit audio from compatible equipment to Bluetooth headphones. Choose by battery runtime, pairing needs, and whether you value the stated Bluetooth version.
• Best for sending phone audio to a wired stereo → Either. Both offer RX functionality for receiving Bluetooth audio and outputting it through a 3.5 mm connection.
• Best if you might use it in a car in TX mode → SYENGKKY 5.4. Nulaxy explicitly says its TX mode cannot be used in a car; SYENGKKY lists car audio among compatible setups, though the intended connection should still match the mode and port.
Neither adapter is a direct answer to the search for the best open ear headphones for running or the best noise cancelling headphones for running. Those are headphone types; these products are adapters that connect a 3.5 mm audio source to Bluetooth devices. They may help with a treadmill or airplane jack, but they do not supply the headphones’ fit or listening features. If your real need is the best Bluetooth headset for running, select the headset separately and treat either adapter as an optional accessory for a compatible source.
My Verdict
Nulaxy lists up to 15 hours in TX mode, while SYENGKKY lists up to 11 hours. Those are the manufacturers’ stated figures; actual runtime can vary with use.
Both are described as transmitters for audio sources with a 3.5 mm connection, including gym equipment. Check that the equipment has the appropriate output and use TX mode to send its audio to Bluetooth headphones.
Both product sheets say the adapter can connect with two Bluetooth devices simultaneously. Nulaxy specifically describes connecting two pairs of headphones to share audio, so that use is stated more directly for that model.
For a run using compatible gym equipment, either can transmit audio from a 3.5 mm jack to Bluetooth headphones. I’d lean toward SYENGKKY for a solo setup and its stated Bluetooth 5.4; Nulaxy is more appealing if longer listed runtime or shared listening is the priority.
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