Headphone Impedance and Sensitivity

Why Headphone Impedance and Sensitivity are Important

• Which headphones will work best with my new headphone amplifier?
• Which headphones are best with my portable device?
• Is headphone impedance important?
• How do I compare sensitivity specifications?

This application note addresses these common questions and presents some guidelines for selecting headphones with the proper impedance and sensitivity.

Portable Applications

Portable music players, smart phones, tablets, and laptop computers often require low-impedance headphones. Acoustic output level is a function of input power. Low-impedance headphones can deliver more power from the small voltage swing that is available from battery-operated devices. Unfortunately, the low impedance demands high current from the headphone amplifier, and this often leads to high distortion.

Studio and Hi-Fi Applications

In contrast, many high-quality headphones, such as the Sennheiser HD 650 are intended for studio use and have much higher input impedances. Large voltage swings are always used in professional audio equipment. High voltage signals overcome the low-level noise that is produced by all electronic devices. Large voltage swings make it easier to achieve a very high signal to noise ratio (SNR). When large voltage swings are available, high-impedance headphones can take full advantage of the available dynamic range. High-impedance headphones are easier to drive than low-impedance headphones because they usually draw less current from the headphone amplifier for a given loudness. This often means that distortion will be lower when using high-impedance headphones. High-impedance headphones are a great choice if your headphone amplifier can supply enough voltage to drive them.

What happens if I connect my studio-quality headphones to my iPhone?

The good news is that high-impedance headphones will never overload or damage a portable device. Battery life may be increased, distortion may decrease, noise may be decrease, and the SNR may improve. The bad news is that the maximum playback level may be too low. Bottom line - if you have a pair of high-impedance headphones, try them with your portable devices. If the playback level is loud enough, high-impedance headphones should improve the overall sound quality of your portable device. But beware, the maximum playback level may be too low when driving high-impedance headphones from a portable device.

Can I use my low-impedance portable headphones with studio gear and stand-alone headphone amplifiers?

The answer to this question is somewhat more complicated. Fortunately, high-quality gear usually comes with detailed audio specifications. Check the manufacturers recommendations for "minimum headphone impedance". Do not attempt to use headphones that have a lower impedance than this minimum recommended impedance. Impedances below the manufacturer's recommended minimum may overload and damage the headphone amplifier. Some headphone amplifiers such as those produced by Benchmark, include short-circuit and overload protection. These circuits protect the amplifier from damage caused by incorrect loads. Remember, low-impedance headphones may damage some amplifiers, so use caution.

Low-impedance headphones may degrade the performance of a headphone amplifier. In general, amplifier output stage noise will be reproduced at higher levels as headphone impedance decreases. Distortion may increase due to the higher current required to drive low-impedance headphones. Low-impedance headphones may force the user to operate with the volume control turned down well below normal. Some high-quality headphone amplifiers, such as Benchmark's HPA2™ (included in most Benchmark DAC2 converters), have circuits that avoid these problems. The Benchmark HPA2™ is well-equipped to drive high or low impedance headphones.

Some anecdotal experiences with my Samsung S4 smartphone:

I have a wide selection of headphones here at Benchmark, so I grabbed a few and plugged them into my Samsung S4 Android phone. All of the headphones I tested were loud enough for me while listening in a quiet office environment, but some may not be loud enough for all users in all situations. The headphones with marginal loudness were Sennheiser HD 650, Sennheiser HD 600, Audeze LCD-X, and Grado SR325 . These four are excellent headphones, but they may not provide a high enough playback level in some portable applications. In contrast, I found that I had more than enough loudness from the low-impedance PSB M4U 1, Sony MDR-V6, and Sony MDR-V600 headphones. 

How can I judge loudness from headphone sensitivity specifications?

Headphone manufacturers have different methods for specifying headphone sensitivity. This makes direct comparisons difficult without doing some math.

Sensitivity is often expressed as dB SPL at 1 mW (acoustic output as a function of input power). This "power sensitivity" specification makes comparisons complicated. This specification tells us nothing about the loudness of the headphones at a given voltage unless we also know the impedance of the headphones.

In contrast, Sennheiser specifies sensitivity in terms of dB SPL at 1 Vrms (acoustic output as a function of input voltage). Sennheiser's "voltage sensitivity" method takes impedance out of the equation and allows direct comparisons without making calculations.

"Power sensitivity" must be converted to "voltage sensitivity" before we can directly compare the loudness of our headphones.

To convert from "power sensitivity" to "voltage sensitivity" you will need to do the following:

power_sensitivity + 20*LOG(SQRT(1000/R)) = voltage_sensitivity

Where:

R is the impedance of the headphones in Ohms

power_sensitivity is expressed as dB SPL @ 1 mW

voltage_sensitivity is expressed as dB SPL @ 1 Vrms

I have calculated the voltage sensitivities of each of my headphones and these are shown in the table below:

I have sorted the table according to "Voltage Sensitivity". The loudest headphones are on the bottom of the chart.

The chart shows that the Sony MDR-V600 headphones are about 7.5 dB louder than the Sennheiser HD 650 headphones when driven from 1 Vrms. We never would have guessed this if we compared the published "Power Sensitivity" column on the right-hand side of the chart. The "Voltage Sensitivity" column explains why the PSB and Sony headphones were noticeably louder in my listening tests.

The chart also shows that the first four headphones all have nearly identical voltage sensitivities. This makes sense because all four seemed to have equal loudness when driven from my Samsung S4. Again, note that the "Power Sensitivity" column is misleading. The Sennheiser headphones are not 11 dB louder than the Audeze, but they produce almost exactly the same acoustic output when driven from the same voltage. The Audeze draw more power than the Sennheisers, but they do not require more voltage. 

Conclusions

The first four headphones are only marginally loud enough to work with my portable device. The last three are a better match for the limited output voltage provided by portable devices.

Please note that two of the first four headphones are low-impedance designs but they have the same voltage sensitivity as the high-impedance Sennheiser headphones. The Audeze LCD-X and Grado SR325 are inefficient designs that require both high voltage and high current, making them somewhat unsuitable for portable applications.

Universal Headphone Amplifiers

Benchmark's HPA2™ headphone amplifier is incorporated into many Benchmark products such as the DAC1 and DAC2 converters. It has three gain ranges to allow adaptation to a wide variety of headphone sensitivities. It is capable of driving 22-Ohm headphones (such as the Audeze), and it is capable of delivering the high voltages required by high-impedance headphones. Any of the above headphones will work well with the HPA2™.

"0-Ohm" Headphone Amplifiers

Some high-end headphone amplifiers, such as Benchmark's HPA2™, have very low output impedances. The HPA2™ has an output impedance of about 0.1 Ohms. This very low impedance provides a high damping factor that can improve the control over the headphone drives. This can flatten the frequency response, reduce distortion, and improve damping. See "The 0-Ohm Headphone Amplifier" and "The HPA2™ Headphone Power Amplifier" for more information.

Personal Recommendations

The Sennheiser HD 650 is probably my all-around personal favorite. They sound great and they are comfortable. I find that I can use them with some of my portable devices, but they really shine when driven from a high-quality headphone amplifier.

The Audeze headphones are also outstanding, but only when driven from a high-performance headphone amplifier such as the HPA2™. These headphones are a poor match for portable devices.

When I travel, I carry the PSB headphones. The closed-back design provides excellent isolation, and they have enough sensitivity for my portable devices.

Recommended reading:

More information on headphones and headphone amplifiers can be found here.