The Etymotic ER4XR is 45Ω at 98.0 dB/mW; the Truthear Zero:RED is 17.5Ω at 99.9 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 Etymotic ER4XR needs 1.9 dB more than the Truthear Zero:RED to reach the same loudness — 1.6 mW against 1.0 mW.
| Specification | Etymotic ER4XR | Truthear Zero:RED |
|---|---|---|
| Impedance | 45 Ω | 17.5 Ω |
| Sensitivity | 98.0 dB/mW | 99.9 dB/mW |
| Sensitivity (voltage) | 111.5 dB/V | 117.5 dB/V |
| Power for 100 dB | 1.6 mW | 1.0 mW |
| Voltage for 100 dB | 0.27 V | 0.13 V |
| Current for 100 dB | 5.9 mA | 7.6 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 5.62 Ω | 2.19 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.
1 source is low enough in output impedance for the Etymotic ER4XR under the one-eighth rule but not for the Truthear Zero:RED (Shanling H5).
1 source stays below the audible-hiss threshold with the Etymotic ER4XR but crosses it with the Truthear Zero:RED (Schiit Mjolnir 3).
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 Etymotic ER4XR is more demanding than 69% of the 567 non-electrostatic models in this database; the Truthear Zero:RED than 43%. 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 | Etymotic ER4XR power | Etymotic ER4XR voltage | Truthear Zero:RED power | Truthear Zero:RED voltage |
|---|---|---|---|---|
| 80 | 0.02 mW | 27 mV | 0.01 mW | 13 mV |
| 85 | 0.05 mW | 47 mV | 0.03 mW | 24 mV |
| 90 | 0.16 mW | 84 mV | 0.10 mW | 42 mV |
| 95 | 0.50 mW | 0.15 V | 0.32 mW | 75 mV |
| 100 | 1.6 mW | 0.27 V | 1.0 mW | 0.13 V |
| 105 | 5.0 mW | 0.47 V | 3.2 mW | 0.24 V |
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, Etymotic ER4XR | Headroom, Truthear Zero:RED | Output impedance | Noise |
|---|---|---|---|---|---|
| JadeAudio JA11 | $20 | 11.3 dB | 12.1 dB | 0.5 Ω | 1.7 µV |
| FiiO KA11 | $29 | 19.5 dB | 23.6 dB | 0.7 Ω | 1.7 µV |
| FiiO BTR11 | $36 | 11.3 dB | 16.2 dB | 0.5 Ω | 8.0 µV |
Of the 280 sources here, 280 reach 100 dB on both. That is the easy axis. 69 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 69 is the JadeAudio JA11, at $20. It leaves 11.3 dB of headroom on the Etymotic ER4XR and 12.1 dB on the Truthear Zero:RED, its output impedance is 0.5 Ω against a one-eighth limit of 5.62 Ω and 2.19 Ω, and its noise figure puts the hiss floor 24 dB and 18 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 |
|---|---|---|---|
| Etymotic ER4XR | 20 Hz – 16 kHz | not printed | not printed |
| Truthear Zero:RED | 20-40.5kHz (IEC61094, Free Field) | not printed | IEC61094 (free field) |
| 20-20kHz (IEC60318-4, -3dB) | -3 dB | IEC60318-4 |
Etymotic prints 20 Hz – 16 kHz for the ER4XR. Truthear prints two ranges, “20-40.5kHz (IEC61094, Free Field)” and “20-20kHz (IEC60318-4, -3dB)”, for the Zero:RED.
Only one of the two is a specification. The range printed for the Truthear Zero:RED comes with a tolerance; the one printed for the Etymotic ER4XR does not, and a range without a ±dB figure says nothing about how loud that model is at either end of it. The two cannot be compared number against number, however similar they look in the table. Of the 490 models whose maker prints a range, 58 print the tolerance.
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: Etymotic ER4XR and Truthear Zero:RED. Every figure on this page comes from what Etymotic and Truthear published, or is derived from it — what Etymotic publishes and what Truthear publishes, both counted.