How to Measure Sound Quality of Headphones

Ever since the first pair of headphones has been made, the discussions regarding their quality among their users have been arising. The huge growth of the audio industry and consumer market brought endless lists of different headphone models into our lives and caused discussions to grow into disputes between regular users, professionals, and audiophiles.

Their disagreements do not refer only to the headphone overall quality (physical appearance, build quality, comfort level) or to the superiority of a specific model over other models but also to the possibility to objectively measure sound quality. No matter if you are just an ordinary user, a professional musician or simply an audiophile, you will definitely understand what we are talking about.

The whole point of the previously mentioned arguments is to prove that headphone sound quality can or can’t be objectively measured. On the one hand, the professional users claim that there is a list of characteristics that can be objectively measured and expressed in numbers. This information (or most of it) is commonly given in the product specifications and audiophiles and musicians say that they get all the information about headphone sound quality just from them. On the other hand, there is an average citizen who claims that reading the specs list doesn’t guarantee you the quality you are looking for and that the sound quality of every audio device is defined only by the user’s personal taste. 

We have made small research, reviewed all of these statements, and realized that the truth is somewhere in between. In our opinion (and, of course, you don’t have to agree with us) headphone sound quality is determined by both their measurable features (accuracy and all the characteristics that define it, such as frequency response, THD, SNR, etc.) and the level of joy you experience when using them. That being said, let’s start from the beginning and explain our statement.

What Does Sound Accuracy Imply?

Measuring the accuracy of the sound you are listening to isn’t just audiophiles’ imagination. It is quite real as well as the tools used to perform the measurement. Sound accuracy measurement is a process of determining several parameters that significantly affect the sound quality.  

Frequency response (range)

Headphone frequency response is the range of frequencies reproduced by a certain pair of headphones. It varies from model to model and wider ranges are considered more desirable. Most headphones successfully reproduce frequencies within the well-known 20Hz – 20kHz range, which is the hearing range of an average human being (aka audible spectrum). In spite of the fact that most of us can’t hear all of these frequencies, headphones with a wider range are considered more prestigious and the reason for that is the ability of the human body and brain to feel the frequencies it can’t hear. 

However, the frequency response can be measured and relatively accurate data can be received from the process. This requires measuring equipment and so-called dummy heads. Measurement microphones are placed inside each ear canal of the fake head so that the sound coming from the headphones comes straightforward to them in order for its frequency response to be measured. The results of these measurements are usually displayed as graphs with two lines; one for the range of the reproduced frequencies and the other for the relative earphone output level at every single frequency.

Measuring the frequency response of a pair of headphones is a bit easier than measuring the same parameters of a pair of earbuds. The problem is that the buds are inserted inside the ear canal and they become a part of the canal. As the dummies have their ear canal shaped in a specific way, which is not as unique as the shape of the human ear canal (depth, curvatures, and diameters differ from person to person), measuring earbud frequency response using this method is not completely accurate.

Total Harmonic Distortion (THD)

This data is often required by audiophiles and professional musicians, while most of the regular customers can’t understand its true meaning or recognize it when using their audio equipment. THD shows the measure of harmonic distortion (caused by driver imperfections) in a signal received by your headphones.

This information is often hidden from specifications lists but the desirable value is below 1%. The greatest thing would be a complete elimination of distortion but, since this is not possible, you should stick to the rule that lower THD leads to better sound quality and more accurately reproduced audio content.

Signal-to-Noise and Distortion Ratio (SNDR)

SNDR represents a measure of the quality of a signal coming from a communication device. It can be calculated by dividing the sum of signal, noise, and distortion power by the sum of noise and distortion power. SNDR is usually expressed in dB. It can also be defined as the ratio of the total received power (signal) to the sum of noise and distortion power. 

Spurious-free Dynamic Range (SFDR)

SFDR is a measure of the smallest power signal that we can distinguish from one large spurious signal. In other words, it is the ratio of the basic signal to the most powerful interfering signal within the output. SFDR is commonly measured and expressed in dBc (in terms of carrier signal) or dBFS (in terms of a full-scale range).

Signal-to-Noise Ratio (SNR)

SNR is a measurement of sound quality expressed in dB (decibels). It compares the ratio of the levels of signal to background noise power. Higher SNR values imply that your headphones are more accurate and reproduce clearer sound (the sound of higher quality).

We can explain it using a simple example. Let’s say you’re answering a call using your headphones in a crowded café. Your headphones should have SNR high enough to make the signal stronger and louder than the noise in order to enable you to hear the person on the other side. If you owned a pair of headphones with 60 dB SNR, it would mean that the level of the audio signal received by it, is 60 dB higher than the level of the background noise in the café. 

Does Bitrate Matter?

The thing that most users know, even if they are not professionals, is that more kilobits per second mean better sound quality. This is incredibly important for Bluetooth headphones. Depending on the capabilities of the Bluetooth connection and on supported max bitrate, the sound quality will be lower or higher. Since most people don’t know how to explain why higher bitrate means better sound, let’s say a few words about that.

Every song that you have ever heard is recorded in a music studio where the original soundtracks are made. These soundtracks are considered to have the best possible sound quality because they are recorded in the highest possible resolution. They are full and rich due to all the details they contain. Subsequently, as they contain a lot of data, these audio files are extremely large and they occupy a lot of storage space.

As we all tend to use small portable devices that occupy the least possible space in our bag and get all the job done, we can’t store these large files on our smartphones or tablets because their storage space is not big enough. The only way to store numerous music tracks on such a small device is to compress them and create some of the most popular audio file formats that we constantly use such as MP3.

The problem with these files is that they don’t have the same quality as the original ones because compression takes away not only size but most of the details and elements that make the original soundtracks so powerful and great. It simply takes away a great part of the original sound quality. 

The same goes for Bluetooth headphones. Bluetooth connection is simply not capable enough to transfer large amounts of data. So, in order to stream a song, Bluetooth module built inside your phone or some other Bluetooth-enabled device you’re using to stream music wirelessly compresses the audio track and sends it to the Bluetooth module (receiver) built inside your headphones. Different Bluetooth codecs are used for audio compression and some of them are more capable than others. When we say ‘’more capable’’, we are actually talking about their max bitrates (the amount of data they can transfer in a second) and about the quality of the sound. The most common Bluetooth codec is SBC and its max bitrate is 320kbps. For most Bluetooth headphones on the market, this is the max bitrate. However, SBC is not the only Bluetooth codec. The company called Qualcomm developed four different Bluetooth codecs that are more capable than SBC (aptX, aptX LL, aptX HD, and aptX Adaptive). Sony also developed its proprietary LDAC Bluetooth codec. These codecs support much higher bitrates and, as a result, the headphones featuring support for these codecs are capable of delivering better sound quality. So, to put it simply, a higher bitrate leads to better sound quality.  

Impedance

Impedance is not directly related to sound quality but it affects headphone loudness (which is for many people an important characteristic when evaluating sound quality) and it is measurable, so we have decided to mention it in our article.

Impedance represents your device’s resistance to the electric current and it is expressed in Ohms. When you look at your headphones’ specs and read the impedance, you should know how to use that information because the most important thing about impedance is not the value itself – it’s the user’s ability to match his/her headphones with the adequate audio equipment (source device, amplifier, etc.).

For example, when you’re listening to music stored on your phone, you are not supposed to use headphones with high impedance because high impedance will shut the signal out. On the other hand, when using professional music equipment, you need headphones with higher impedance, that usually require an amplifier that will offer them more power and help reproduce louder and more detailed audio.

Sensitivity

Apart from the impedance, sensitivity is also an indicator of headphone loudness and it is measured in dB of SPL/mW (decibels of sound pressure level per milliwatt). In fact, sensitivity is the measure of how effectively a pair of headphones can convert an electrical signal into sound. It also measures the loudness of the headphones for the given source input.

Final Thoughts

In the text above, we have tried to explain the meaning of each spec that can be used to describe the headphone sound quality and express that quality in numbers. These numbers are supposed to reveal some of the secrets about the headphones to certain people who get the meaning of those numbers. Hopefully, we have managed to reveal a part of that secret and made a few things clearer. However, we have said, at the beginning of this article, that the numbers are only one part of a puzzle.

The fact is that every model of headphones is unique and that every single model sounds differently. This happens because different headphones have different sound signatures. In fact, there can only be six sound signatures (extra bass, V-shaped, flat, bright, warm, and balanced) but, when you add physical properties to the equation, you get so many varieties. Some of these signatures are preferable when listening to rock or electronic music, while the others are great for classical or instrumental music but the most important thing is that they suit your ears.

Most of us don’t have perfect hearing, which means that the perfect pair of headphones is the one that compensates our hearing imperfections and makes us feel alive and happy. So, if you’re still not sure about all these objective ways of measuring headphone sound quality, we suggest you to let it go. Just go to a store and try a few pairs. Listen to some of your favorite songs on different headphones and buy the pair that delivers music in a way that makes you dance and you won’t make a mistake. Ignore the numbers and hit play!