Have you ever wondered why your music sounds different, depending on what pair of headphones you're wearing?
For example, listening to the iconic Jaws theme will be a very different experience on a pair of Beyerdynamic over-ear headphones, compared to a pair of Beats in-ear headphones.
The dramatic difference between two pairs of headphones, other than the style and design, is which notes they choose to enhance or downplay. This is called frequency response, which is the most important performance metric that we assess in our headphone testing.
All headphones can be divided into two camps, frequency-wise: those trying to most closely reproduce the sounds as they were recorded, and those trying to ensure that each note sounds equally loud to the listener. But before we jump into the difference between those two ideals, let's break down the definition of "frequency response".
The frequency of any wave is defined as the number of cycles per second (aka “hertz” or “Hz”). When I say “cycles”, I refer to the fact that some waves have a structure or wave shape that repeats over and over throughout the lifespan of the wave. Here are some examples of those types of waves:
As you can see, there are many different types of waves with repeating shapes, but we're going to focus on the so-called “sinusoidal” waves ("sine" in the image above)—waves that have peaks and valleys with curving characteristics.
Because these shapes are identical and repeat throughout the wave, we can isolate one wave shape and look at how long it lasts over a period of time. If a wave repeats many times in one second, that wave is said to be “high frequency.” If a wave repeats very few times per second, it is said to be “low frequency.”
In music, we deal mostly (but not exclusively) with sinusoidal waves. The note middle C—whether it is played on the clarinet, the piano, or the saxophone—is made up of waves that always have the same frequency: 261.6 Hz, or 261.6 cycles of the repeated wave structure in one second.
The "response" of a given frequency refers to its corresponding decibel level. Decibels define the intensity (or the amount of energy transmitted over time over an area) of the frequency wave carrying a given sound.
Decibels (dB) are tricky to understand because they're logarithmic in nature. That means an increase of 10 dB (say, from 25 dB to 35 dB) represents a ten-fold increase in intensity. More confusingly, the intensity of a frequency wave doesn't directly translate into the "volume" that we hear in our ears. In order to change the volume setting on your radio from "1" to "2", the radio has to increase the intensity of that frequency wave by 6-10 dB.
The strange relationship between numerical decibels and the loudness (i.e. the volume) was first quantified by Harvey Fletcher and Wilden A. Munson, in the early 1930s. They asked volunteers to listen to single notes at a range of frequencies and intensities. At each frequency, they adjusted the intensity of the wave until the listeners believed that the loudness matched that of a reference frequency wave with a set decibel level.
In this way, Fletcher and Munson built empirical curves that represented a constant loudness, as perceived by the human ear and defined by a range of frequencies and decibel values. These curves, which have since been revised and updated, are called "equal loudness" (EL) curves. Today, they're a key part of our frequency response testing.
Sounds that have lower pitches have low frequencies. The rumble of traffic or the thrum of an air-conditioning unit are sound waves with fewer cycles per second and lower dB values. They're difficult to perceive because, with fewer cycles, the wave shape (and the sound produced) change very gradually, making it more difficult to observe and recognize the wave.
Conversely, sounds with high pitches have high frequencies. Fire alarms and nails on a chalkboard create sounds that have high frequencies and high dB values. High frequency sounds are more easily distinguished with the human ear because many cycles occur in one second. Our ears more easily pick up on the rapidly changing wave than those with lower frequencies.
With all of this background information, it is perhaps unsurprising that when we discuss the “lows,” “mids,” and “highs” of a piece of music, we are referring to the frequencies of the sound waves. A single note represents a sound wave with a specific frequency; a note with a low frequency of ~100 Hz may be produced by a bass drum, a note with a mid frequency of ~1,000 Hz may be made by an alto saxophone, and a note with a high frequency of ~10,000 Hz may be made by a piccolo.
Our Frequency Response Test
In our frequency response test, we use SoundCheck software to send a range of sound waves with known frequencies through headphones and measure the decibel level the headphones reproduce. When we look at the data for the frequency response test, we're plotting frequency vs. response in decibels.
In general, headphone manufacturers are trying to match one of two popular frequency profiles: the “consumer” response or the “studio” response.
The consumer response is defined by the equal loudness curves mentioned above, which attempt to achieve a constant loudness across a range of frequencies. Because it is easier for us to hear mid- and high-frequency notes than low-frequency notes, EL curves increase the intensity of lower-frequency waves and limit the intensity of mid- and high-frequency waves. With this type of frequency response, you'll hear the hum of an upright bass at the same loudness as the blare of a trumpet.
We call this the “consumer” frequency response because most headphones on the market aim for this frequency profile. It allows you to hear all parts of your music with equal emphasis on the different sounds, vocals, and instruments.
The studio frequency response is the opposite: the manufacturer seeks to reproduce all of the notes, both at high and low frequencies, at the same decibel level.
This doesn’t correlate to the volume as perceived by the human ear; doing so requires a curve, as you saw earlier. On the frequency vs. response in decibels plot, the ideal studio response is a flat horizontal line at any given decibel level. This is appealing to listeners who want to hear the music as it was mixed in the studio. There's no artificial emphasis or de-emphasis on certain notes.
We call this the studio response because these headphones are often used in a studio setting, where producers can clearly hear the sound as it was recorded, and then choose to boost or reduce certain frequencies to achieve the exact sound they want.
The "consumer" and "studio" profiles represent two distinct philosophies, so when we test a pair of headphones, we look at the frequency response and judge it based on which of the two frequency profiles it was seeking to reproduce. As most headphones on the market have a consumer profile, it would be unfair to compare that rapidly changing sound to a studio profile. Similarly, a flat studio profile would be a poor match for an ideal EL curve. Some companies will aim to produce their own "signature sound," but those tend not to deviate very far from either of the two previously mentioned philosophies.
Using our methodology, each pair of headphones is scored on how well it actually mimics the frequency profile that it most closely resembles. The reviewers can then take this information and tell you categorically that, with a given pair of headphones, the violins in the shower scene of Psycho are so muted that all sense of suspense is lost, or that the roar of the waterfall at the dam in The Fugitive will be so loud that you'll feel like you're being pursued by Tommy Lee Jones.
Only you can decide which sound profile you prefer, and that desire has to be balanced with the price tag and the look of a given pair of headphones. For the products that top our list, be sure to check out the Best Right Now: Headphones article, or our constantly updated library of headphone reviews.