The 64 Audio U4s is 11Ω at 107.0 dB/mW; the 64 Audio Volur is 6.3Ω at 103.0 dB/mW. This page is only about what they ask of an amplifier — not how they sound, which no specification on either sheet can tell you.
The Volur needs 4.0 dB more than the U4s to reach the same loudness — 0.50 mW against 0.20 mW.
| Specification | U4s | Volur |
|---|---|---|
| Impedance | 11 Ω | 6.3 Ω |
| Sensitivity | 107.0 dB/mW | 103.0 dB/mW |
| Sensitivity (voltage) | 126.6 dB/V | 125.0 dB/V |
| Power for 100 dB | 0.20 mW | 0.50 mW |
| Voltage for 100 dB | 47 mV | 56 mV |
| Current for 100 dB | 4.3 mA | 8.9 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 1.38 Ω | 0.79 Ω |
| Sources that reach 100 dB out of 280 | 280 | 280 |
Every source in the database that drives one of them drives the other. On power, the choice between these two changes nothing about what amplifier you need.
41 sources are low enough in output impedance for the U4s under the one-eighth rule but not for the Volur (FiiO SnowSky Tiny B, FiiO KA15, FiiO BTR15, 38 more).
7 sources stay below the audible-hiss threshold with the Volur but cross it with the U4s (Grace Design m900, FiiO M33 R2R, FiiO K17, 4 more).
280 of the 280 sources here reach 100 dB on both. That count is about power only — the output-impedance and hiss lines above are separate questions, and a source can pass this one and fail those.
The gap between these two in decibels does not change with volume — it is a property of the two headphones. What changes is the size of the numbers, and it changes fast: every 10 dB costs ten times the power. The row that matters is the one nearest the level you listen at, and for most people that is nearer the top of this table than the bottom.
The Volur is more demanding than 14% of the 567 non-electrostatic models in this database; the U4s than 10%. Both figures use the same measure as the rest of the site: whichever runs out first against a 2 V, 100 mW dongle, voltage or power.
| dB SPL | U4s power | U4s voltage | Volur power | Volur voltage |
|---|---|---|---|---|
| 80 | 2.0 µW | 5 mV | 5.0 µW | 6 mV |
| 85 | 6.3 µW | 8 mV | 0.02 mW | 10 mV |
| 90 | 0.02 mW | 15 mV | 0.05 mW | 18 mV |
| 95 | 0.06 mW | 26 mV | 0.16 mW | 32 mV |
| 100 | 0.20 mW | 47 mV | 0.50 mW | 56 mV |
| 105 | 0.63 mW | 83 mV | 1.6 mW | 100 mV |
105 dB is included because forum power figures are usually quoted there or higher. It is also a level that damages hearing in minutes — how many minutes is its own calculator.
| Source | Price | Headroom, U4s | Headroom, Volur | Output impedance | Noise |
|---|---|---|---|---|---|
| JadeAudio JA11 | $20 | 17.1 dB | 10.7 dB | 0.5 Ω | 1.7 µV |
| FiiO KA11 | $29 | 29.3 dB | 22.8 dB | 0.7 Ω | 1.7 µV |
| FiiO BTR13 | $59 | 26.2 dB | 19.7 dB | 0.6 Ω | 1.0 µV |
Of the 280 sources here, 280 reach 100 dB on both. That is the easy axis. 32 of them also publish an output impedance low enough for both under the one-eighth rule and a noise figure that stays below the audible-hiss threshold with both — the rest either fail one of those or never printed the number at all, and 208 of them are in the second group.
The cheapest of the 32 is the JadeAudio JA11, at $20. It leaves 17.1 dB of headroom on the U4s and 10.7 dB on the Volur, its output impedance is 0.5 Ω against a one-eighth limit of 1.38 Ω and 0.79 Ω, and its noise figure puts the hiss floor 9 dB and 10 dB below the 20 dB SPL this site treats as the point where a quiet room stops hiding it.
Cheapest that satisfies three published numbers is not the same as best, and none of it is about how either headphone sounds. To put either model against a source this page does not mention, or to change the target loudness, use the amplifier matcher.
| Model | As printed | Tolerance | Standard |
|---|---|---|---|
| U4s | 10hz – 20kHz | not printed | not printed |
| Volur | 5hz – 22kHz | not printed | not printed |
64 Audio prints 10hz – 20kHz for the U4s and 5hz – 22kHz for the Volur.
Neither range has a tolerance printed beside it, and that is the whole of what this section can tell you. A wider range on one sheet is not deeper bass or more treble on that headphone: without a tolerance, neither number says how loud either model is at the ends of its own range, so the difference between the two is a difference between two pieces of marketing copy. A ±dB figure is what turns a range into a specification, and of the 490 models whose maker prints a range, 58 print it.
None of this is a measurement. What each maker publishes, and what it is worth, is set out in the frequency response audit.
Each model has a page of its own with the full source list, the hiss estimate and the provenance of every figure above: 64 Audio U4s and 64 Audio Volur. Every figure on this page comes from what 64 Audio published, or is derived from it — what that maker publishes, counted.