The Complete Guide To Headphones Sensitivity Ratings

When listening to music, we humans like it loud. Whether it's turning up a volume knob or dial, applying more gain, or choosing a more sensitive audio device, perceived loudness is something to consider when choosing the optimal listening experience.

What is headphone sensitivity? Headphone sensitivity is a specification that refers to the efficiency of a driver to turn an electrical audio signal into sound pressure. It is measured in dB SPL (decibels of sound pressure level) per unit of power (typically 1 milliWatt) or sometimes unit of voltage (typically 1 Volt) from the source at a frequency of 1 kHz.

In this article, we'll discuss headphone sensitivity in greater detail and explore its effect on headphone performance and listener experience.

Related article: What Is Microphone Sensitivity? An In-Depth Description

What Is Headphone Sensitivity?

Simply put, headphone sensitivity measures how loud a pair of headphones will be at a given signal strength.

All else being equal (in the headphones design and the audio signal), headphones with higher sensitivity ratings will produce louder sounds than headphones with lower sensitivity ratings.

Sensitivity is a strange specification because it is calculated differently by different manufacturers.

The sensitivity rating is often rated as the sound pressure level produced at a given audio signal power. However, it could also refer to the sound pressure level at a given audio signal voltage.

Skip ahead to the sections on Headphone Sensitivity Vs. Efficiency and/or How Is Headphone Sensitivity Measured? for more information.

Headphone sensitivity is given as a specification on headphone datasheets and is meant to give us an idea of how loud the headphones are. However, it's difficult to know just how much power and/or voltage the headphone amplifier or audio device will be sending to the headphones at any given time.

Sensitivity rating, then, can be given as dB SPL/mW; dB/mW, or simply as dB for sensitivity relative to power.

For example, the Sony MDR-7506 have a sensitivity rating of 104 dB/mW.

The Sony MDR-7506 is featured in the following My New Microphone articles:
• Top 5 Best Headphones For Podcasting Under $100
• Top 5 Best Closed-Back Headphones Under $100
• Top 5 Best Circumaural (Over-Ear) Headphones Under $100

Conversely, they can be given as dB SPL/V, dB/V, or simply as dB for sensitivity relative to voltage.

For example, the AKG K872 have a sensitivity rating of 112 dB SPL/V.

The fact that some manufacturers give sensitivity ratings in dB only makes it difficult to know which measurement system they are using and makes the sensitivity rating somewhat invalid.

So headphone sensitivity has historically been a good tool for comparing the potential loudness of headphones when paired with certain amplifiers. However, with the two current methods of calculating sensitivity, it is difficult to compare headphone brands since we may not be sure which brands use which system.

In general, earphones tend to have sensitivities in the range of 80 to 125 dB SPL/mW, while headphone sensitivity ratings typically fall between 90 dB SPL and 105 dB SPL/mW. Of course, there are outliers beyond these ranges.

To give some real-world examples, I've assembled the following list of headphones complete with their sensitivity ratings (noted as either dB/mW or dB/V):

Note that the STAX models on the list are electrostatic headphones that require incredibly high voltages compared to other headphone types. For this reason, you'll find their sensitivity ratings as per 100 V RMS (root mean square) rather than per 1 V or 1 mW.

Electrostatic headphones aside, within the randomly chosen headphones above, we have a sensitivity range between 91 and 113 dB SPL/V.

Headphone Sensitivity Vs. Efficiency

Since the beginning of the 21st century, there has been a trend toward measuring sensitivity as a function of voltage rather than power.

There must be a technical distinction between sensitivity as measured with voltage and sensitivity as measured with power. A common distinction is as follows:

The term efficiency has somewhat replaced the old sensitivity term. It refers to power transfer. Efficiency is the measure of the sound pressure level when standard power (1 mW) is supplied. Efficiency ratings can be given as dB SPL per mW (dB/mW).

Sensitivity, then, has, in some cases, become the measure of the sound pressure level produced when a standard voltage level (1 volt)is supplied to reproduce sound. Sensitivity ratings can be given as dB SPL per V (dB/V)

That being said, the term efficiency is rarely used by headphone manufacturers. To make matters worse, the term sensitivity is used differently by different manufacturers.

To convert efficiency (“power sensitivity”) to sensitivity (“voltage sensitivity”), use the following equation:

S_V=S_P+20\log(\sqrt{\frac{1000}{Z}})

Where:
S_V is the voltage sensitivity in dB SPL/V(RMS)
S_P is the power sensitivity (efficiency) in dB SPL/mW
Z is the impedance of the headphones in Ω

Alternatively, we may convert sensitivity (“voltage sensitivity”) to efficiency (“power sensitivity”) by altering the equation to read:

S_P=S_V-20\log(\sqrt{\frac{1000}{Z}})

What Is Electrical Power And Voltage?

Headphones are transducers that convert analog audio signals (electrical energy) into sound waves (mechanical wave energy). The electrical signals have power and voltage factors, and sensitivity is based on one or both of these factors.

Audio Signals Are Electrical Signals

The audio signals that are sent to the headphone drivers are alternating currents. These AC signals mimic the shape of sound waves, and it is the job of the headphones to convert the AC signals into their coinciding sound waves.

Audio signals typically have frequencies within the audible range of human hearing (20 Hz – 20,000 Hz) though they may have more limited or even extended bandwidths.

The accuracy in which the headphones will reproduce the audio signal across all frequencies is given as the headphones' frequency response.

To learn more about electrical audio signals and headphone frequency response, check out my article What Is A Microphone Audio Signal, Electrically Speaking? and What Is Headphone Frequency Response & What Is A Good Range? respectively.

What Is Electrical Power?

Electric power is the rate per unit of time at which an electric circuit transfers electrical energy. The SI unit of power is the watt, which means one joule per second.

Electrical power changes instantaneously along with the amplitude and direction of the current, and so it is typically measured as an average. Sensitivity values, as discussed, are often calculated against a 1 kHz sine wave with a power of 1 mW (1/1000 of a Watt).

But audio signals, as we've discussed, are not only 1 kHz sine waves. Low frequencies require a bit more energy, so headphones will generally require more energy than the sensitivity rating may suggest in the low-end.

Headphone jacks typically provide between 10 and 20 mW of output power, and, of course, this power can be lowered with volume control. Most dedicated headphone amplifiers can provide power between 10 mW and 2 W depending on the specific headphone being used and the amplifier's design.

In general, headphones with higher impedance ratings will require greater amounts of power to produce equal sound pressure levels. However, as we'll see in the Relationship Between Headphone Impedance And Sensitivity section, headphone design overcomes this direct association.

What Is Electrical Voltage?

Voltage is the difference in electric potential between two points. It causes the flow of electrons in a circuit or, in other words, an electrical current.

In a static electric field, voltage is defined as the work needed per unit of charge to move a test charge between the two points.

The SI unit of voltage is the volt, and 1 volt = 1 joule (of work) per 1 coulomb (of charge). The official SI definition for volt actually uses power, mentioned above, and is defined as 1 volt = 1 watt (of power) per 1 ampere (of current).

Audio signal strengths are often measured as AC voltages.

Dedicated headphone amplifiers are typically capable of outputting higher voltage signals than general headphone jacks. The amount of output voltage is dependent not only on the volume control of the output device but also on the impedances of both the source (device) and the load (headphones).

What Is Sound Pressure Level And Decibels?

dB SPL (Decibels sound pressure level) is used in both sensitivity/efficiency calculations.

dB SPL measures the sound pressure and the intensity on a logarithmic scale.

What Is Sound?

Sound is defined as a vibration that propagates as an acoustic wave through a transmission medium such as a gas, liquid or solid. Sound waves cause localized pressure variations from the ambient atmospheric pressure (with maximum compression and maximum rarefaction) in the particles of the medium as they pass through.

As sound waves reach our ears, they cause vibrations in our ears and the air molecules around our ears. Our sense of hearing effectively converts these sound waves into electrical impulses that are sent to and understood by our brains.

What Are Decibels?

Decibels are a unitless expression of the ratio of one value of a power or field quantity to another on a logarithmic scale.

The power/intensity of sound is based on the sound pressure level and can be measured in the SI unit for pressure, known as the Pascal. 1 Pa = kg⋅m⁻¹⋅s⁻².

\text{1 Pa} = \frac{kg}{ms^2}

Sound pressure level, as we know, can also be presented in decibels, which is the standard. For sound in air, which is the most common when using headphones, the dB SPL scale is relative to 20 microPascals (μPa), or 2×10⁻⁵ Pa, which is approximately the quietest sound a human can hear.

Every 6 dB increase effectively doubles the amplitude of the sound wave. A 6 dB decrease, then, effectively halves the amplitude of the sound wave.

As confusing as it may be to understand fully, dB SPL is commonly used to help us better compare different sound pressure levels by turning the logarithmic nature of sound pressure intensity into a linear measurement.

To help us understand, I've included the following chart that relates dB SPL values to their coinciding Pascal values.

To learn more about decibels, sound and audio, check out the following articles:
• What Are Decibels? The Ultimate dB Guide For Audio & Sound
• What Is The Difference Between Sound And Audio?

How Is Headphone Sensitivity Measured?

The standard for measuring headphone sensitivity, like frequency response, is first to attach the headphones to a dummy head. This dummy head has built-in microphones.

Next, the manufacturer applies a sine wave signal (typically at 1 kHz) with 1 milliwatt of power to the headphones. This audio signal is converted to sound by the headphones, and the sound pressure level produced is measured at the driver (with the dummy head microphones).

Sensitivity is then stated as the measured sound pressure level (in dB SPL) produced by the headphones with 1 mW of input. Spec sheets may refer to this as “dB/mW” or simply as dB.

Alternatively, as discussed, some manufacturers will measure their headphones' sensitivity ratings with a voltage rather than power. In this case, the same procedure is followed but with a 1-volt sine wave audio signal (again, typically at 1 kHz).

These ratings are then described as the sound pressure level (in dB SPL) produced by the headphones with 1 V of input. Spec sheets may refer to this as “dB/V” or, confusingly, as dB.

The fact that different manufacturers measure sensitivity differently makes it difficult to compare headphones across different brands. To make matter worse, as I've alluded to, sensitivity ratings are often given simply as “X dB” and do not specify whether the sound pressure level is relative to power or voltage.

Sound Pressure Level Exposure Time And Hearing Loss

Our sense of hearing is a gift and should be treated as such. Exposure to high sound pressure levels will cause permanent damage to our sense of hearing if we are not careful.

Unfortunately, there's no way to get our hearing back naturally once it has been damaged. Hearing aids may help, but there is no way for our bodies to repair themselves after hearing loss has taken place.

Therefore, it's important to limit exposure to loud sound sources. Yes, headphones sound great when cranked up to max volume, but we need to be wary of the amount of time we spend listening at full volume if we decide to expose ourselves to the max volume at all.

Headphones are essentially tiny loudspeakers that are positioned immediately outside our ears. Even worse, earphones are positioned within the ear canal.

Below is a table that describes the recommended maximum time limits for continued exposure to various sound pressure levels.

The NIOSH Standard (National Institute for Occupational Safety and Health) is to the left of the table. To the right of the table is the OSHA Standard (Occupational Safety and Health Administration).

NIOSH is a United States federal agency, while the OSHA is an agency of the United States Department of Labor. Both offer their own guidelines for the amount of time we can safely listen to varying sound pressure level intensities.

My personal recommendation is to follow the NIOSH standard since it keeps levels lower and, therefore, further reduces the risk of hearing loss.

We're all probably guilty of listening to music too loud in our headphones at one point or another. Listening to our headphones at a reasonable level and taking regular breaks will increase the longevity of our hearing, reduce ear fatigue, and improve the quality of our listening experiences now and in the future.

For reference, here is a table of dB SPL values for common situations:

What Is A Good Headphone Sensitivity Rating?

As mentioned previously, headphone sensitivity ratings typically fall between 90 dB SPL/mW and 105 dB SPL/mW. This is a great range for headphone sensitivity.

Headphones with ratings outside this range can still be excellent products, though they would be considered on the very quiet end or the very loud end of the full spectrum.

Lower Sensitivity Vs. Higher Sensitivity

All else being equal, higher sensitivity headphones will be louder than lower sensitivity headphones.

However, sensitivity is only one of many factors that makes up the full audio quality of a pair of headphones and should not be the main factor we look at when determining our choice of headphones.

Can Headphone Sensitivity Ratings Be Changed?

It makes perfect sense that applying a greater voltage to the headphone drivers would cause them to produce higher sound pressure levels but is there any way to increase or decrease the sensitivity of the drivers themselves?

The short answer is: not without physically altering the headphones. Sensitivity ratings are given in the format of output-per-given-input (sound pressure level at a given electrical power). Though there are plenty of ways to adjust the amount of power sent to the headphones, there's no way to alter the sensitivity itself without modifying the headphones.

Modifying the headphones could potentially change the sensitivity rating. These mods may include but are not limited to:

• Swapping the headphone drivers.
• Implanting active noise-cancellation circuits into the headphones.
• Adding an amplifier in the headphones.

Relationship Between Headphone Impedance And Sensitivity

Impedance and sensitivity are often included together in the discussions of headphone specifications. They both have to do with the relationship between the signal strength and the sound pressure level caused by the coinciding driver movement.

Impedance is measured in ohms (Ω) and is a specification that all headphones have. It basically tells us much power the headphones will need to get to a reasonable listening volume. Basically, higher headphone impedances require stronger audio signals to move the driver.

To learn more about headphone impedance, check out my article The Complete Guide To Understanding Headphone Impedance.

As we know from the previous sections in this article, headphone sensitivity also has to do with the audio signal strength required for the headphone drivers to produce sound at a certain volume.

It is in this way that headphone sensitivity and impedance are related.

Headphone (load) impedance values are fixed while the audio source impedances vary from device to device. Source impedance variation may alter the potential power transfer between the source and load and, therefore, affect the volume of the headphones. However, the sensitivity rating (let's call it volume per power) will remain the same.

Note that the impedance of a pair of headphones will affect the relationship between the sensitivity ratings of dB/mW and dB/V since the impedance plays a role in determining the voltage required to transfer electrical power.

Different impedance matching will alter the amount of power at the headphone driver but will not change the sensitivity rating of the headphones.

With all that being said, there is no correlation between impedance and sensitivity in headphone design.

Let’s have a look at some headphone examples to see that impedance and sensitivity are not causally linked:

The headphones in the table above are taken from My New Microphone’s article The Top 13 Best Headphone Brands In The World.

There’s some pretty wild variation in the above chart. Here are a few things to consider about the headphones in the list:

• The Bose QuietComfort 35 Series II; Bang & Olufsen Beoplay H4, and Bowers & Wilkins PX7 are wireless headphones.
• The STAX SR-007A MK2 is a pair of electrostatic headphones.
• The Shure SE215s are in-ear monitors.

Related Questions

How can you tell the quality of headphones? The best way to test the quality of a pair of headphones is to listen for yourself ideally with an audio source you're familiar with and intend on using the headphones with. Headphone specifications such as impedance, sensitivity and frequency response can help to give us an idea of HP quality but are not as effective as physically trying the HPs.

What do headphone specs mean? Headphone specifications refer to concrete and measurable attributes of headphone design. Some specs are electrical (impedance, sensitivity, frequency response, battery life for active headphones and wireless transmission method for wireless headphones). Other specs are physical (weight, driver type and form factor).

Related article: Full List: Headphone/Earphone Specifications w/ Examples.

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