The Sennheiser HD 550 is 150Ω with a sensitivity of 98.5 dB/mW, which is 106.7 dB/V. To reach 100 dB SPL — genuinely loud, louder than most people listen — it needs 1.4 mW, or 0.46 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, 257 have at least 10 dB of headroom and another 23 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 106.7 dB / 1 V RMS (1 V RMS (Sennheiser gives no frequency for this row; the THD row separately cites 1 kHz)) — a figure per volt, not per milliwatt. At 150Ω the two differ by 8.2 dB, so the 98.5 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 8 dB easier to drive than it is — which is roughly the difference between a dongle being enough and not being enough.
Source: support.sennheiser-hearing.com, us.sennheiser-hearing.com, eu.sennheiser-hearing.com. How this data was built.
| As it is labelled | As printed | Tolerance | Standard |
|---|---|---|---|
| Frequency response | 6 Hz – 39.5 kHz | not printed | not printed |
Sennheiser prints 6 Hz – 39.5 kHz for the Sennheiser HD 550, under the heading “Frequency response”. Source: support.sennheiser-hearing.com.
No tolerance is printed with it, so “6 Hz – 39.5 kHz” says nothing about how loud this headphone actually is at 6 Hz.
Only what the maker printed; maker by maker.
| Level | What it is | Power | Voltage | Current |
|---|---|---|---|---|
| 85 dB SPL | quiet background listening | 0.04 mW | 82 mV | 0.5 mA |
| 95 dB SPL | normal listening | 0.45 mW | 0.26 V | 1.7 mA |
| 100 dB SPL | loud listening | 1.4 mW | 0.46 V | 3.1 mA |
| 105 dB SPL | very loud | 4.5 mW | 0.82 V | 5.5 mA |
| 110 dB SPL | peaks on dynamic music | 14 mW | 1.46 V | 9.7 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 150Ω 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 150Ω | Headroom | Verdict |
|---|---|---|---|---|---|
| FiiO KA11 | USB dongle | $29 | 43 mW | +15 dB | Comfortable |
| Tanchjim Stargate II | USB dongle | $40 | 16 mW | +11 dB | Comfortable |
| ddHiFi TC35AL | USB dongle | $46 | 45 mW | +15 dB | Comfortable |
| ddHiFi TC35Air | USB dongle | $46 | 45 mW | +15 dB | Comfortable |
| Fosi Audio N3 | Bluetooth receiver | $50 | 30 mW | +13 dB | Comfortable |
| Hidizs SD2 | USB dongle | $50 | 15 mW | +10 dB | Comfortable |
| FiiO BTR13 | Bluetooth receiver | $59 | 21 mW | +12 dB | Comfortable |
| iFi GO link | USB dongle | $59 | 20 mW | +12 dB | Comfortable |
| Kiwi Ears Allegro Pro | USB dongle | $60 | 15 mW | +10 dB | Comfortable |
| Tempotec Sonata BHD Pro | USB dongle | $60 | 60 mW | +16 dB | Comfortable |
| iBasso DC03 Pro | USB dongle | $69 | 21 mW | +12 dB | Comfortable |
| EPZ TP35 Pro | USB dongle | $70 | 24 mW | +12 dB | Comfortable |
| Tanchjim Space | USB dongle | $70 | 27 mW | +13 dB | Comfortable |
| Truthear SHIO | USB dongle | $70 | 27 mW | +13 dB | Comfortable |
| Aiyima A2 Pro | desktop DAC/amp | $75 | 32 mW | +14 dB | Comfortable |
Hiss. At 106.7 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 15 dB SPL in these headphones — clearly audible in quiet passages. Check your own source.
Output impedance. At 150Ω these want a source output impedance below 18.75Ω to satisfy the one-eighth rule; 17 of the 192 sources that publish one miss it. Check yours here.
150Ω · 95.8 dB/mW · Moderate
See requirements →150Ω · 96.8 dB/mW · Moderate
See requirements →150Ω · 104.0 dB · Easy
See requirements →32Ω · 92.0 dB · Easy
See requirements →80Ω · 96.0 dB · Easy
See requirements →80Ω · 96.0 dB · Easy
See requirements →Compared with its stablemates: vs HD 280 Pro · vs HD 58X Jubilee (Drop) · vs HD 600.