Headphone Impedance, Sensitivity, Voltage, and Power Explained

When shopping for professional headphones, you will often see specifications such as 38 Ω, 80 Ω, or 250 Ω. These numbers describe the headphone's impedance. But what does impedance actually mean? And why can one pair of headphones get extremely loud from a portable recorder while another pair needs a more powerful headphone amplifier?

To make this easier to understand, instead of starting with electrical theory, we're going to use three pieces of real production equipment: Audio-Technica ATH-M50x headphones, Beyerdynamic DT 770 Pro 80 Ω headphones, and a Sound Devices MixPre-10 II mixer/recorder.

• ATH-M50x → 38 Ω impedance, 99 dB/mW sensitivity, 1,600 mW maximum input power
• DT 770 Pro 80 Ω → 80 Ω impedance (also available in 32 Ω and 250 Ω versions)
• MixPre-10 II → stereo headphone amplifier rated at 300 mW + 300 mW, specified for use with headphones of any impedance

Those specifications give us everything we need to start understanding how headphones and headphone amplifiers work together.

What Is Headphone Impedance?

Headphone impedance is measured in ohms (Ω). In simplified terms, impedance describes how the headphones oppose the alternating electrical current coming from the headphone amplifier. Our two headphones have different impedances — the DT 770 Pro's 80 Ω is more than twice the ATH-M50x's 38 Ω. That does not mean the DT 770 Pro is better, and it does not mean the M50x is better. It simply means they present different electrical loads to the headphone amplifier.

The Headphone Amplifier Is the Source of the Power

When you plug your headphones into the MixPre-10 II, you are not connecting them directly to the recorded audio. There is a small amplifier inside the recorder specifically designed to drive headphones:

Recorded Audio → Headphone Amplifier → Headphones → Your Ears

The headphone amplifier supplies electrical energy to the headphones. That electrical energy involves three things we need to understand: voltage, current, and power. The headphones then convert that electrical energy into sound.

Voltage and Current

Voltage and current are easiest to understand together. You can think of voltage as the electrical pressure supplied by the amplifier, and current as the amount of electrical charge flowing through the headphones. The headphone's impedance affects the relationship between the two: lower impedance means more current for the same voltage, and higher impedance means less current for the same voltage. This relationship comes from Ohm's Law:

• Current = Voltage ÷ Impedance (I = V ÷ R)

Let's use our actual headphones. Imagine the MixPre's headphone amplifier sends exactly 1 volt to our ATH-M50x. The M50x has an impedance of 38 Ω, so:

• 1 ÷ 38 = 0.0263 amps, or approximately 26 mA

Now send that same 1 volt to our DT 770 Pro 80 Ω:

• 1 ÷ 80 = 0.0125 amps, or 12.5 mA

So at the same voltage, the lower-impedance M50x draws more current. That is the first important relationship: lower impedance generally creates greater current demand.

What About Power?

The electrical power delivered to headphones is usually measured in milliwatts (mW) — one watt contains 1,000 milliwatts. Headphones generally need only a relatively small amount of power to become loud. The easiest power equation for our purposes is:

• Power = Voltage² ÷ Impedance (P = V² ÷ R)

Again, let's give both headphones exactly 1 volt:

• ATH-M50x: 1² ÷ 38 = 0.0263 watts ≈ 26.3 mW
• DT 770 Pro 80 Ω: 1² ÷ 80 = 0.0125 watts = 12.5 mW

So with the same 1-volt signal, the lower-impedance M50x receives about twice as much electrical power as the DT 770 Pro.

Why Higher-Impedance Headphones Need More Voltage

Now let's reverse the problem. Instead of giving both headphones the same voltage, suppose we want to give both exactly 10 mW. The equation becomes:

• Voltage = √(Power × Impedance)

• ATH-M50x: √(0.01 × 38) ≈ 0.62 V
• DT 770 Pro 80 Ω: √(0.01 × 80) ≈ 0.89 V

So to deliver the same 10 mW of power, the higher-impedance headphone needs more voltage. That gives us our second important relationship: higher impedance generally creates greater voltage demand for the same amount of power.

So Is the M50x Louder?

Not necessarily. This is where sensitivity enters the picture. Impedance tells us about the electrical relationship between the headphone and amplifier — it does not tell us how efficiently the headphone turns that electricity into sound. For that we need sensitivity.

What Is Headphone Sensitivity?

Sensitivity tells us approximately how much sound a headphone produces from a specified electrical input. Audio-Technica rates the ATH-M50x at 99 dB/mW, meaning under the manufacturer's test conditions, approximately 1 milliwatt of electrical power produces 99 dB SPL. dB SPL means decibels sound pressure level — it describes acoustic sound level, the actual sound coming from the headphones, which is different from dBFS, the unit we use to describe digital recording levels.

From Electrical Power to Sound

The MixPre supplies electrical power, and the M50x turns that electrical power into sound: approximately 1 mW produces 99 dB SPL. What happens if we increase the power? Because decibels are logarithmic, doubling power adds approximately 3 dB:

• 1 mW → 99 dB
• 2 mW → 102 dB
• 4 mW → 105 dB
• 8 mW → 108 dB

And multiplying power by ten adds approximately 10 dB, so 10 mW → approximately 109 dB. These are theoretical calculations based on the sensitivity specification, not recommended listening levels — the important point is that headphones can become very loud with surprisingly little electrical power.

Impedance and Sensitivity Do Different Jobs

This distinction is the most important part of the entire article. Impedance tells us how the headphones interact electrically with the amplifier. Sensitivity tells us how efficiently the headphones turn that electrical input into sound. You need both pieces of information to understand how easy a headphone is to drive. A low-impedance headphone is not automatically louder. A high-impedance headphone is not automatically quieter. Sensitivity matters too.

Now Let's Bring in the MixPre-10 II

Sound Devices lists the MixPre-10 II headphone output as 3.5 mm TRS stereo, 300 mW + 300 mW, for use with headphones of any impedance. But what does 300 mW + 300 mW actually mean? The answer is much simpler than it looks.

Why Does It Say 300 mW + 300 mW?

Your headphones contain two drivers — left ear and right ear. The MixPre-10 II is a stereo headphone amplifier, so it has to drive both sides. The specification means approximately 300 mW available for the left channel and 300 mW available for the right channel. The plus sign separates the two stereo channels — it does not mean that one headphone driver receives 600 mW. Instead, the left amplifier channel feeds the left headphone driver and the right amplifier channel feeds the right headphone driver. That is why Sound Devices writes the specification as 300 mW + 300 mW rather than simply 600 mW.

Your Stereo Headphone Connector

The MixPre-10 II uses a 3.5 mm TRS headphone connector. TRS stands for Tip, Ring, Sleeve. For a conventional stereo headphone connection, the tip carries left, the ring carries right, and the sleeve is the common return. So the MixPre sends separate left and right audio signals through the same headphone connector.

Does the MixPre Constantly Send 300 mW?

No. This is another important distinction. The 300 mW + 300 mW specification describes the headphone amplifier's available capability under its specified conditions. Your M50x is not sitting there receiving 300 mW continuously whenever you plug it into the MixPre. The actual electrical output depends on the recorded signal, your headphone-volume setting, the headphone impedance, and the amplifier itself. At ordinary monitoring levels, your headphones may be using only a small fraction of the amplifier's available power.

Why Have So Much Available Power?

Because the MixPre needs to work with many different headphones — some with low impedance, some with high impedance, some very sensitive, others less sensitive. A capable headphone amplifier gives the recorder enough flexibility to drive a wide variety of headphones. It also provides headroom.

What Is Headphone-Amplifier Headroom?

Imagine your normal headphone level requires only a small amount of the amplifier's capability. Then an actor suddenly becomes louder, or you move from a quiet interior to a noisy exterior and need a little more monitoring level. The amplifier still has additional capability available — that extra capability is useful headroom. You do not want normal monitoring to require the headphone amplifier to operate at its absolute limit all the time.

M50x vs. DT 770 Pro on the MixPre-10 II

The ATH-M50x (38 Ω, 99 dB/mW) has relatively low impedance and good sensitivity. It does not require much voltage or power to reach substantial monitoring levels, and the MixPre-10 II should have no difficulty driving it.

The DT 770 Pro 80 Ω has a higher impedance than the M50x, so for the same electrical power it requires more voltage. But 80 ohms is still a completely reasonable headphone load for a professional recorder such as the MixPre-10 II. Beyerdynamic currently describes the 80-ohm DT 770 Pro as its "Universal Studio" version, while its 32-ohm model is aimed more toward mobile devices and its 250-ohm version toward professional devices.

Why Does Beyerdynamic Make Different Impedance Versions?

This gives us a useful real-world example of why impedance exists as a consideration. The DT 770 Pro is available in 32 Ω, 80 Ω, and 250 Ω versions. The 32-ohm version requires less amplifier voltage and is therefore easier to use with portable devices that have limited voltage output. The 250-ohm version requires more voltage and is better suited to more capable stationary or professional headphone amplification. The 80-ohm version sits between them. Beyerdynamic specifically says its 32-ohm version works well with portable recorders and players, while the 250-ohm model is intended for stationary sources such as headphone amplifiers and studio controllers.

Let's Compare 32, 80, and 250 Ohms Mathematically

Suppose we want to deliver 10 mW to each version, using Voltage = √(Power × Impedance):

• 32 Ω: √(0.01 × 32) ≈ 0.57 V
• 80 Ω: √(0.01 × 80) ≈ 0.89 V
• 250 Ω: √(0.01 × 250) ≈ 1.58 V

So to deliver exactly the same 10 mW, the amplifier needs to provide more and more voltage as impedance increases. This makes the concept much easier to see.

Does That Mean Lower Impedance Is Always Better?

No. Lower impedance reduces the voltage requirement, but it increases the current demand on the amplifier — remember, current = voltage ÷ impedance. So there are two sides to the problem: low impedance creates greater current demand, while high impedance creates greater voltage demand. A good headphone amplifier needs to provide an appropriate combination of both. That is why we think of the headphones and amplifier as a system.

What Actually Determines Whether Headphones Are Easy to Drive?

Three specifications matter most:

• Impedance, measured in ohms (Ω) — helps determine the electrical load presented to the amplifier.
• Sensitivity, often specified in dB/mW — tells us how efficiently the headphones turn electrical input into acoustic sound.
• Headphone-amplifier capability — the amplifier needs to provide enough voltage, current, and power for the headphones being used.

So the complete relationship is: Headphone Amplifier + Impedance + Sensitivity → Available Monitoring Level.

The Simplest Way to Remember the Math

You do not need to calculate all of this every time you buy headphones. Remember these relationships:

• Lower impedance generally requires less voltage.
• Higher impedance generally requires more voltage.
• Lower impedance generally draws more current at the same voltage.
• Sensitivity tells you how much sound you get from the electrical input.
• Your headphone amplifier has to supply enough voltage and current to do the job cleanly.

That is really what all of these numbers are describing.

Why Impedance Does Not Determine Sound Quality

A 250 Ω headphone is not automatically better than a 38 Ω headphone. Impedance is an electrical characteristic — it does not directly tell you how accurate the headphones sound, how detailed they are, how comfortable they are, how well they isolate, or how durable they are. The ATH-M50x being 38 Ω does not make it inferior to a 250-ohm DT 770 Pro. Likewise, the DT 770 Pro being available at 250 Ω does not automatically make that version superior. The question is whether the headphones are appropriate for the amplifier and application.

The Practical Production-Sound Question

If you are choosing headphones for a field recorder, the most useful question is not "Which headphone has the best impedance?" — there is no such thing. Ask instead: "Can my mixer/recorder comfortably drive these headphones?" With something like the MixPre-10 II, Sound Devices specifically designed the headphone amplifier to work with a broad range of headphone impedances, so both our 38 Ω ATH-M50x and 80 Ω DT 770 Pro are reasonable examples of headphones that can be used with it.

You Can Also Test This Practically

Specifications are useful, but you can learn a lot by simply plugging the headphones into the recorder. Play or monitor familiar dialogue, increase the headphone level to a comfortable monitoring volume, and ask:

• Is it loud enough?
• Do I still have additional volume available?
• Does it remain clean when the level increases?
• Can I hear subtle problems in the recording?

If the headphones reach a comfortable level while the recorder still has useful amplifier headroom, you probably have a perfectly workable combination. You do not need to solve an equation every morning before going to set — the equations simply explain why the equipment behaves the way it does.

Don't Confuse Headphone Power With Recording Level

One final signal-flow distinction is important. Increasing your MixPre headphone volume does not increase your microphone gain.

Microphone → Preamp / Gain → Recorded Signal → Headphone Amplifier → Headphones

The microphone gain occurs much earlier in the signal path. The headphone amplifier only affects what you hear through your headphones. If your recorded signal is healthy but your headphones are too quiet, increase your headphone level — do not increase microphone gain just to make your headphones louder.

Conclusion

Headphone specifications become much easier to understand when we look at the headphones and amplifier as one system. Our Audio-Technica ATH-M50x (38 Ω) and Beyerdynamic DT 770 Pro (80 Ω) present different electrical loads to the Sound Devices MixPre-10 II headphone amplifier. The M50x requires less voltage for a given amount of power but draws more current at the same voltage. The higher-impedance DT 770 Pro requires more voltage to achieve that same power. But impedance does not tell us how loud either headphone will actually become — for that, we also need sensitivity. And everything depends on whether the headphone amplifier can provide enough voltage, current, and power.

The MixPre-10 II's 300 mW + 300 mW specification describes the available power for its two stereo headphone channels: up to 300 mW for the left channel and up to 300 mW for the right channel. It does not mean one headphone driver receives 600 mW, and it does not mean your headphones constantly receive 300 mW. It describes the capability of the amplifier.

The easiest way to remember the entire relationship is: Amplifier → Electrical Energy. Impedance → Electrical Load. Sensitivity → How Efficiently That Energy Becomes Sound. Once you understand those three ideas, specifications like 38 Ω, 80 Ω, 99 dB/mW, and 300 mW + 300 mW stop looking like random numbers. They begin describing how the entire headphone-monitoring system works together.

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