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We compare products from their published specs and features; the verdicts below are our independent editorial opinions.
If you want a sealed, isolating studio monitor headphone for tracking and noisy rooms, choose Superlux HD661; if you need a semi-open, software-paired mixing system built around a big 50mm driver, choose Fluid Audio Focus.
Flat frequency response is the whole point of this matchup. Both of these are wired studio tools that try to hand you an honest, uncolored version of your mix instead of a flattering one. But they get there in completely different ways, and that difference decides which one belongs on your desk.
• Superlux HD661 — biggest strength: A sealed, airtight closed-back design with strong 102 dB sensitivity, so it isolates you from outside noise and gets loud without much power. Great for tracking, live monitoring and broadcast.
• Superlux HD661 — biggest trade-off: A closed back and a higher 68Ω impedance means it wants a decent source, and the sealed cup trades some openness for isolation — great in a loud room, less airy when you're mixing.
• Fluid Audio Focus — biggest strength: A 50mm dynamic driver with a lightweight diaphragm and a low 32Ω impedance, paired with dSONIQ Realphones software that simulates mixing rooms and monitor speakers.
• Fluid Audio Focus — biggest trade-off: Semi-open back leaks sound in and out, so it's a poor fit for loud rooms and live tracking, and it leans on the bundled software to deliver its full value.
• For the isolation-first engineer → Superlux HD661. For the mix-and-simulate workflow → Fluid Audio Focus.
The HD661 is a sealed studio monitor headphone, and "sealed" is the headline. Superlux designed an enclosed, airtight cup that gives you a deep, contained sound field so you can work in places with heavy external noise without fighting the room. That's why it shows up in live monitoring, studio tracking and broadcast — environments where you want your ears locked to the source and nothing else bleeding in.
The specs back that role up. A 40mm dynamic driver with a neodymium magnet, 102 dB SPL sensitivity at 1 kHz, and a 10 Hz–20 kHz frequency band. That sensitivity figure matters: it means the HD661 gets loud easily, which is exactly what you want when you're checking a take on a modest interface or a field recorder. Impedance sits at 68Ω, so it's not a featherweight load — a phone or a weak dongle won't drive it as forcefully as a proper headphone amp or interface will.
Buy it if you track, monitor live, or record in noisy spaces and need isolation plus high sensitivity. Skip it if you want a wide-open, airy mix environment — a sealed cup by definition won't give you that, and the higher impedance asks more of your gear.
Fluid Audio Focus: Who It's For
The Focus is a different philosophy entirely. Fluid Audio built its reputation on studio monitors, and the Focus is that thinking turned into a headphone: a semi-open, over-ear design aimed at tracking, mixing, mastering and casual listening. The centerpiece is a 50mm dynamic driver — the largest in this comparison — with a lightweight diaphragm tuned for a detailed, responsive performance, and it's comfortable enough to wear for long sessions.
The low 32Ω impedance is the practical standout. It's an easy load, so the Focus plays nice with interfaces, laptops and mobile setups without demanding a dedicated amp. That's a real advantage if your rig is simple. Frequency range is 20 Hz–20 kHz, and the semi-open back gives you a more open, speaker-like feel than a fully sealed cup.
Then there's the part that makes the Focus unusual: it ships as a system, not just a headphone. dSONIQ Realphones software is included, and it provides virtual mixing rooms, several professional monitor models — including Fluid's own FX Series — plus EQ and sonic adjustment options. In plain terms, you can hear your mix as if you were sitting in a treated room in front of real monitors, which is a genuinely useful crutch for anyone mixing on headphones in an untreated bedroom.
Buy it if you mix, master, or want a headphone-plus-software system that simulates monitor speakers, and you value an easy-to-drive 32Ω load. Skip it if you need isolation — a semi-open back leaks both ways, so loud rooms and live tracking are the wrong job for it.
Side-by-Side: Which Should You Choose?
• Best for mixing with simulated monitor rooms → Fluid Audio Focus. The bundled dSONIQ Realphones software is the deciding feature.
• Best for easy driving from modest gear → Fluid Audio Focus. 32Ω is a lighter load than 68Ω.
• Best for high-sensitivity monitoring on the move → Superlux HD661. 102 dB SPL sensitivity means it gets loud with little power.
• Best for long, comfortable sessions → Fluid Audio Focus. The over-ear, semi-open design is pitched for hours of listening.
• Best for tracking and broadcast → Superlux HD661. Sealed monitoring and the included cables and adapter fit that workflow.
• Best for a speaker-like, open listening feel → Fluid Audio Focus. Semi-open beats closed-back for airiness.
• Best for a self-contained hardware-only kit → Superlux HD661. Two cables, adapter, clip and pouch; nothing to install.
In my opinion, the Fluid Audio Focus is the stronger pick for most people reading a "flat frequency response" comparison, because the 50mm driver, low 32Ω impedance and the Realphones software together give you a portable mixing environment that's hard to match with a plain headphone. That said, I'd buy the Superlux HD661 without hesitation if isolation is the priority — if you track in loud rooms or monitor live, the sealed design and 102 dB sensitivity are exactly what you need, and the Focus's semi-open back simply can't do that job. Match the tool to the room and you'll be happy either way.
does frequency response matter in headphones
Yes, but it's only part of the picture. Both of these cover a wide range — the HD661 spans 10 Hz–20 kHz and the Focus 20 Hz–20 kHz — yet the driver size, impedance and open-vs-closed design affect how usable that range feels in a real room. A wide range tells you the reach; the design tells you whether it's flat and honest in practice.
Is the Fluid Audio Focus good for mixing?
Yes — that's its strong suit. The sealed, airtight enclosure blocks external noise so you can monitor in loud environments, and its 102 dB SPL sensitivity means it gets loud easily. Just note the 68Ω impedance wants a capable source, so pair it with an interface or amp rather than a weak output.
The Superlux HD661, clearly. It's a sealed, closed-back design, while the Fluid Audio Focus is semi-open and lets sound pass both ways. If you're tracking next to a microphone or working in a shared space, the sealed HD661 is the safer choice.
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