The basic principle for frequency response measurements is that they're a visual representation of the headphone's sound pressure level across the full frequency spectrum for human hearing (typically 20-20khz). In other words, these measurements represent the amount of energy (volume) each part of the full frequency range has. This allows us to get an idea of how the headphone may sound. But it's also important to know that looking at measurements is not a substitute for actually listening to the headphone, and at best it's just an indicator or predictor for how it might sound.
When looking at measurements, there are two types of graphs for frequency response:
- Raw graphs - These graphs show how the headphone measures without any compensation or target in mind. Raw measurements for headphones should never be a flat line across. This may seem counter-intuitive, but because of the various gain factors the human ear imparts to a headphone's frequency response, the general shape to a 'normal' sound signature will be substantially elevated from around 2-9khz. So when looking at measurements, make sure you know if it's a raw representation, and then also know what that raw graph should look like for the various sound signature targets you may be looking for.
- Compensated graphs - For most of us, these graphs will be more useful. Compensated measurements show how significantly the frequency response deviates from a specific target. For this measurement, a straight line across is desirable, however it's important to understand which target is being used for the compensation. Frequency response targets are devised in a way that imagines if a headphone's frequency response perfectly matches the target curve, it would sound 'neutral'. However because there's no current scientific consensus on what neutral is for headphones, we have a number of different target curves. If we understand what the compensation target is (and what it might sound like), we can better understand what the deviations from that target mean. So when a headphone doesn't measure as a flat line on a compensated graph, we need to know what the compensation target is in order to know what the various dips or elevations actually mean for the way it will sound.
There are a number of different target curves currently being used to measure headphones, and these are devised in order to determine the ideal frequency response for headphones. The most common target curve is the well-known Harman target, which is based on consumer preference research. However other targets have been devised independently, and it's worth pointing out differences among them.
This is an example of how the Audeze LCD-1 measures with the Reveal+ DSP plugin on the MiniDSP EARS rig, which is not industry standard or comparable to industry standard systems. This measurement uses the HEQ compensation, which is based on the Harman target. Notice that it looks like it measures slightly elevated in the upper frequencies relative to this compensation, however on a compensation that isn't based as strongly on the Harman target, the LCD-1 won't show the same upper frequency elevation.Harman Target - A general preference curve that forms the basis for a number of other target curves. Compensations based on this target follow the general shape of the gain factors for the human ear, but with extra bass emphasis and an overall balanced sound for a wide variety of genres. Headphones generally follow this curve:
Diffuse Field - A diffuse field target aims at a frequency response that's meant to emulate the way flat-measuring speakers would sound in a somewhat 'lively' room (as opposed to an anechoic chamber). This ends up being slightly brighter than compensations based on the Harman target, and doesn't have as much bass emphasis, but it's still warmer and more realistic than a Free Field target developed in a completely 'dead' room. This has been a common target for many years, however more recently headphones have tended to aim for the extra bass emphasis of the consumer preference curve instead.