Impedance and sensitivity: can your phone handle it?

Almost every box lists two: impedance and sensitivity. Together they tell you how easily your IEMs or headphones get loud. On their own they say surprisingly little.

• Impedance, in ohms (Ω): the resistance your IEM presents to the signal. The number on the box is a nominal value, usually measured at 1 kHz. At other frequencies it can be very different, which we'll get to.
• Sensitivity in dB/mW: how loud it gets with 1 milliwatt of power.
• Sensitivity in dB/V: how loud it gets with 1 volt.

Watch the unit. 105 dB/mW and 105 dB/V are not the same, and if the box just says '105 dB' you can't tell which one is meant. You can convert between them:

Why impedance alone says little: FiiO's JT3 is 80 Ω and the JT7 only 18 Ω. Yet the JT3 needs less power for the same volume, because it's more sensitive. Our headphone guide puts them side by side.

Two formulas tell you how much power and voltage a given volume takes. Pick a target first. We use 100 dB: that's already very loud, fine as a peak in the music, not for hours of listening. The WHO uses 80 dB for 40 hours a week. Our safe listening guide has more on that.

• Power in milliwatts = 10 ^ ((target level − sensitivity in dB/mW) ÷ 10)
• Voltage in volts = √(power in watts × impedance)

An example with the FiiO JT7, the least sensitive headphone we sell: 18 Ω and 92 dB/mW.

1. 100 − 92 = 8 dB short of what 1 milliwatt gives.
2. 10 ^ (8 ÷ 10) = 6.3 milliwatts.
3. √(0.0063 × 18) = 0.34 volts.
4. Check with the other spec: FiiO also lists 109 dB/V. 10 ^ ((100 − 109) ÷ 20) = 0.35 volts. The same, give or take rounding.

Every extra 10 dB takes ten times the power and just over three times the voltage. For peaks of 110 dB on the JT7 you'd need 63 milliwatts and 1.1 volts. That's still no problem for the dongles further down this guide.

Two things stand out. Every IEM here needs less than half a milliwatt, so volume is almost never the problem with IEMs. And even the planar headphones, known as hard to drive, reach 100 dB on a few milliwatts.

Apple and most other phone makers don't publish the output power of their headphone jack or adapter. Dongle makers do, usually in milliwatts into 32 Ω. A few examples from our range, on the 3.5 mm output:

Put that next to the table above and you see how much headroom there is: hundreds of milliwatts against a few tenths for an IEM. Two caveats. The listed power applies at 32 Ω; at a lower impedance a source may deliver less, because more current is needed. And if your phone has a volume limit switched on, the ceiling is lower. Our safe listening guide explains how to set it.

This is where it gets interesting if your IEMs have lots of drivers. Every amplifier has an impedance of its own, the output impedance. It sits in series with your IEM and forms a voltage divider with it: part of the voltage stays behind in the source.

As long as your IEM's impedance is the same at every frequency, that only costs a little volume. But IEMs with several balanced armatures and a filter (crossover) between them rarely work like that. The well-known audio blogger NwAvGuy used the Ultimate Ears SuperFi 5 as an example: rated at 21 Ω, but anywhere between 10 and 90 Ω depending on frequency. On a 10 Ω source he saw a 6 dB variation in its response, on a 43 Ω source as much as 12 dB.

His rule of thumb: keep the output impedance below an eighth of your headphones' or IEMs' impedance. That's why the last column of the table shows that limit. A single dynamic driver or a planar usually has a flatter impedance and is less affected, but the rule never hurts.

• In our range, the FiiO FA19 (10 BAs) has the lowest listed impedance at 10 Ω. Limit by the rule: 1.25 Ω.
• The KZ ASF has 5 BAs and 18 Ω. Limit: 2.25 Ω.
• According to iFi, the hip-dac3 has an output impedance of at most 1 Ω. Switch iEMatch on and it becomes at most 4.5 Ω. So with the FA19, better leave iEMatch off.

A sensitive IEM makes not just the music loud but also the source's noise. You hear it as a soft hiss in quiet passages. You can work that out too: noise level = sensitivity in dB/V + 20 × log10(noise voltage in volts).

Say a source has 10 microvolts of noise. On the KZ EDX Pro (24 Ω, 112 dB/mW, so 128 dB/V) that becomes 128 − 100 = 28 dB. In a quiet room that can be just audible. On the FiiO JT7 (109 dB/V) it's 9 dB, which you won't hear. So the same source can hiss on your IEMs and be silent on your headphones.

• Look at the noise in microvolts if the maker lists it. A high signal-to-noise ratio (SNR) is measured relative to maximum output. A powerful amplifier with a great SNR can therefore hiss more in absolute terms than a weaker one.
• Use the lowest gain setting that gives you enough volume.
• An attenuator such as the iFi iEMatch+ sits between source and IEMs and lowers the signal, so you can turn the source up. iFi quotes 12.24 dB less noise.

• Your phone has no headphone jack. Then you need a DAC either way, and a good dongle costs little more than a basic adapter.
• You have to turn the volume almost all the way up and it's still too quiet. Check what you need with the table and pick a dongle with plenty of headroom.
• You hear hiss in quiet passages. Then a quieter source or an attenuator helps more than extra power.
• Your IEMs have many BAs and a low impedance. Pick a source with a listed output impedance under 1 Ω.
• You want to use a balanced 4.4 mm cable.

Everything sounds fine at a normal volume, without hiss? Then you don't need anything extra. To find the dongle that suits you, read our USB-C dongle comparison. The basics of DACs and amplifiers are in the guide 'Do you need a DAC or amplifier?'.