The Sennheiser HD 400 PRO is 120Ω with a sensitivity of 100.8 dB/mW, which is 110.0 dB/V. To reach 100 dB SPL — genuinely loud, louder than most people listen — it needs 0.83 mW, or 0.32 V across the drivers.
That puts it in the easy category. A phone, a laptop headphone jack or the cheapest USB dongle will drive it to painful levels. Of the 280 sources in the database, 270 have at least 10 dB of headroom and another 10 have enough. The cheapest portable source that does the job is the Apple USB-C dongle (US) at $9.
Sensitivity here is in dB per milliwatt. Why the unit matters.
Sennheiser publishes this as 110 dB (1 kHz / 1 V RMS) (1 kHz at 1 V RMS) — a figure per volt, not per milliwatt. At 120Ω the two differ by 9.2 dB, so the 100.8 dB/mW above is that published number converted, not a different measurement.
This is the conversion that online calculators skip. Quote the per-volt number as if it were per-milliwatt and this headphone looks 9 dB easier to drive than it is — which is roughly the difference between a dongle being enough and not being enough.
Source: sennheiser.com. How this data was built.
| As it is labelled | As printed | Tolerance | Standard |
|---|---|---|---|
| Audio frequency response | 6 - 38,000 Hz (-10 dB) | -10 dB | not printed |
Sennheiser prints 6 - 38,000 Hz (-10 dB) for the Sennheiser HD 400 PRO, under the heading “Audio frequency response”. Source: sennheiser.com.
This is one of only 58 models — out of the 490 whose maker prints a range at all — that also print the tolerance. That is what makes it a specification rather than a headline.
Only what the maker printed; maker by maker.
| Level | What it is | Power | Voltage | Current |
|---|---|---|---|---|
| 85 dB SPL | quiet background listening | 0.03 mW | 56 mV | 0.5 mA |
| 95 dB SPL | normal listening | 0.26 mW | 0.18 V | 1.5 mA |
| 100 dB SPL | loud listening | 0.83 mW | 0.32 V | 2.6 mA |
| 105 dB SPL | very loud | 2.6 mW | 0.56 V | 4.7 mA |
| 110 dB SPL | peaks on dynamic music | 8.3 mW | 1.00 V | 8.3 mA |
Most people listen between 75 and 85 dB; the 100 dB row leaves room for peaks.
The 15 cheapest of the 280 sources here that reach 100 dB SPL into 120Ω with at least 10 dB of headroom and no known hiss or output-impedance problem with these headphones. All 280 sources, judged for these headphones.
| Source | Type | Price | Power into 120Ω | Headroom | Verdict |
|---|---|---|---|---|---|
| FiiO SnowSky Tiny A | USB dongle | $22 | 9.1 mW | +10 dB | Comfortable |
| FiiO SnowSky Tiny B | USB dongle | $22 | 9.1 mW | +10 dB | Comfortable |
| FiiO KA11 | USB dongle | $29 | 54 mW | +18 dB | Comfortable |
| Tanchjim Stargate II | USB dongle | $40 | 20 mW | +14 dB | Comfortable |
| ddHiFi TC35AL | USB dongle | $46 | 56 mW | +18 dB | Comfortable |
| ddHiFi TC35Air | USB dongle | $46 | 56 mW | +18 dB | Comfortable |
| Fosi Audio N3 | Bluetooth receiver | $50 | 37 mW | +16 dB | Comfortable |
| Hidizs SD2 | USB dongle | $50 | 19 mW | +14 dB | Comfortable |
| FiiO BTR13 | Bluetooth receiver | $59 | 26 mW | +15 dB | Comfortable |
| iFi GO link | USB dongle | $59 | 23 mW | +15 dB | Comfortable |
| Kiwi Ears Allegro Pro | USB dongle | $60 | 18 mW | +13 dB | Comfortable |
| ddHiFi TC35Pro (Mountain2) | USB dongle | $66 | 17 mW | +13 dB | Comfortable |
| iBasso DC03 Pro | USB dongle | $69 | 26 mW | +15 dB | Comfortable |
| DUNU DTC 480 | USB dongle | $70 | 17 mW | +13 dB | Comfortable |
| EPZ TP35 Pro | USB dongle | $70 | 30 mW | +16 dB | Comfortable |
Hiss. At 110.0 dB/V these are sensitive enough to reveal a source's own noise floor. The noisiest source in the database, the Schiit Mjolnir 3, would put its hiss at roughly 18 dB SPL in these headphones — clearly audible in quiet passages. Check your own source.
Output impedance. At 120Ω these want a source output impedance below 15.00Ω to satisfy the one-eighth rule; 17 of the 192 sources that publish one miss it. Check yours here.
120Ω · 100.8 dB/mW · Easy
See requirements →120Ω · 98.7 dB/mW · Easy
See requirements →130Ω · 96.0 dB/mW · Moderate
See requirements →25Ω · 94.0 dB/mW · Easy
See requirements →25Ω · 94.0 dB/mW · Easy
See requirements →25Ω · 94.0 dB/mW · Easy
See requirements →Compared with its stablemates: vs HD 280 Pro · vs HD 58X Jubilee (Drop) · vs HD 600.