The Etymotic ER2SE is 15Ω at 96.0 dB/mW; the Thieaudio Monarch MK3 is 20Ω at 99.0 dB (no reference unit printed). 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 ER2SE needs 3.0 dB more than the Thieaudio Monarch MK3 to reach the same loudness — 2.5 mW against 1.3 mW.
| Specification | Etymotic ER2SE | Thieaudio Monarch MK3 |
|---|---|---|
| Impedance | 15 Ω | 20 Ω |
| Sensitivity | 96.0 dB/mW | 99.0 dB |
| Sensitivity (voltage) | 114.2 dB/V | 116.0 dB/V |
| Power for 100 dB | 2.5 mW | 1.3 mW |
| Voltage for 100 dB | 0.19 V | 0.16 V |
| Current for 100 dB | 12.9 mA | 7.9 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 1.88 Ω | 2.50 Ω |
| Sources that reach 100 dB out of 280 | 280 | 280 |
One caution before reading the rows. Thieaudio Monarch MK3 printed a decibel figure with no reference unit, so this page reads it as per milliwatt. If it is really per volt, that model needs far more than the table says. Why the two readings differ by so much.
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.
3 sources are low enough in output impedance for the Thieaudio Monarch MK3 under the one-eighth rule but not for the Etymotic ER2SE (Astell&Kern HC3, RME Fireface UFX II, RME Fireface UFX III).
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 ER2SE is more demanding than 56% of the 567 non-electrostatic models in this database; the Thieaudio Monarch MK3 than 49%. 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 ER2SE power | Etymotic ER2SE voltage | Thieaudio Monarch MK3 power | Thieaudio Monarch MK3 voltage |
|---|---|---|---|---|
| 80 | 0.03 mW | 19 mV | 0.01 mW | 16 mV |
| 85 | 0.08 mW | 35 mV | 0.04 mW | 28 mV |
| 90 | 0.25 mW | 61 mV | 0.13 mW | 50 mV |
| 95 | 0.79 mW | 0.11 V | 0.40 mW | 89 mV |
| 100 | 2.5 mW | 0.19 V | 1.3 mW | 0.16 V |
| 105 | 7.9 mW | 0.35 V | 4.0 mW | 0.28 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 ER2SE | Headroom, Thieaudio Monarch MK3 | Output impedance | Noise |
|---|---|---|---|---|---|
| JadeAudio JA11 | $20 | 7.5 dB | 11.7 dB | 0.5 Ω | 1.7 µV |
| FiiO KA11 | $29 | 19.6 dB | 22.5 dB | 0.7 Ω | 1.7 µV |
| FiiO BTR11 | $36 | 12.3 dB | 14.9 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 7.5 dB of headroom on the Etymotic ER2SE and 11.7 dB on the Thieaudio Monarch MK3, its output impedance is 0.5 Ω against a one-eighth limit of 1.88 Ω and 2.50 Ω, and its noise figure puts the hiss floor 21 dB and 19 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 ER2SE | 20 Hz - 16 kHz | not printed | not printed |
| Thieaudio Monarch MK3 | nothing printed | — | — |
Etymotic prints 20 Hz - 16 kHz for the ER2SE. Thieaudio prints nothing at all for the Monarch MK3.
So one side of this comparison has a frequency-response number and the other does not — which changes less than it looks. The range printed for the Etymotic ER2SE comes with no tolerance beside it, and a range without a ±dB figure says nothing about how loud the headphone is at either end of it: thirty decibels down at the bottom and the sentence would still be true. On the only reading that would have made it a specification, the two sheets say the same amount: nothing.
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 ER2SE and Thieaudio Monarch MK3. Each figure on this page is labelled on those pages with where it came from, including the ones Etymotic and Thieaudio never published — what Etymotic publishes and what Thieaudio publishes, both counted.