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The house curve: how we make our target headphone responses
When you browse through our headphone reviews, you’ll see we include charts showing, among other things, the measured frequency response of each product we test. The frequency response displayed on our current charts is an average of the unweighted, raw responses captured by the left and right artificial ears of our B&K 5128 head simulator, as we subject the device under test to a special stimulus signal. To help you interpret this information, we also display a reference curve (colored pink) on the same chart, that represents what we consider to be the ideal frequency response shape for the kind of product you’re looking at. You might see this referred to as the SoundGuys “House Curve” or “Target Curve.” If you’ve ever been curious about this, it’s what we’ll be going over in this article.
What exactly is a target response curve?
A frequency response chart provides a lot of information about how a headphone will sound, because it provides a visual representation of the sound balance. The graph shows the objective quality of the cans. It doesn’t tell the whole story, but does give you an idea of how they actually sound, regardless of perception. But, we all perceive headphones differently. Such is the problem with subjective experience: it is only valuable to you as an individual.
Frequency response graphs show the objective quality... But, we all perceive headphones differently.
We provide a target response curve as a visual reference that represents what we believe is a response shape that most people will enjoy. It is a comparative reference that shows you how the product under test’s measured behavior compares with a theoretical model that has made a positive subjective impression across multiple listeners (i.e. one that sounds good to most people). So basically, the closer the measured product response is to the target, the more confidence we have that people will like the sound.
How do we know what sounds good?
Over the years, we’ve both listened to and measured a lot of headphones (close to 700 at the time of writing)—as well as other products—and amassed a lot of test data. Unfortunately, before we acquired our prized B&K 5128 test head and underwent our measurement system overhaul in early 2021, our test setup was less than ideal, and a lot less accurate in replicating human physiology. But we were still able to do a good job of identifying great sounding products, or separating the wheat from the chaff, to reach for a handy idiom.
Since the upgrade, we revisited some of what we consider to be benchmark products and remeasured them on the new system, once it was all calibrated and up and running. We updated many older product reviews with new data, for consistency, and we also threw the benchmark measurements into a pile labelled “for reference.”
That pile gets added to every time we identify a benchmark product, which are generally standout products that our technical team agree do a great job of catering to the consumer audio market by providing a pleasing, enjoyable sound that we can all get along with. Products we identified don’t necessarily have to appear on our best lists, or be considered popular products based on sales metrics, but by the same token, they may well be.
How was our curve developed?
This is the part where subjective perception and objective measurement meet, and hopefully produce something useful. Once our pile of reference measurements represented a cross section of the current headphone market in terms of headphone styles—including in-ear, over-ear, open and closed designs, plus a variety of different driver types—we loaded those measurement data into a spreadsheet and crunched them down to produce our calculated reference curve.
Our current compendium of reference consumer products includes recent headphones from AKG, Apple, Audeze, Bose, Google, Jabra, KEF, Monoprice, Shure, and Sony. In the chart above you can see the response curve we derived from their measured data.
Although no actual standards exist that are applicable to the frequency response of a pair of headphones, we are by no means the first to consider and attempt to define a target for this particular metric. It’s actually been done with a lot more thought and resources than we at SoundGuys could ever hope for…
How does it compare with the Harman curve?
In the early 2010s, Dr Sean Olive (a senior fellow at Harman International, now part of Samsung) and his colleagues carried out a series of blind listening tests to find out what sound profile most headphone listeners like. The initial work involved just six headphones and ten listeners, but the resulting 2012 paper, titled “The Relationship Between Perception and Measurement of Headphone Sound Quality,” made a big impression. Dr Olive and his colleagues then built on this work and went on to run hundreds more tests all over the world, resulting in a further eighteen published papers.
The culmination of this research is known as the “Harman curve,” a specific target for frequency response, as preferred by most listeners. Admirably, Harman published its findings, allowing others (including their competitors) to benefit from the research that led to the Harman curve. When measuring the response of a new headphone using an ear simulator, headphone designers are therefore able to compare it with the Harman curve, meaning that they can quickly get an idea of how close they are to a “good sound”. This is obviously much faster than running listening tests on prototypes across multiple listeners.
What does the Harman curve sound like? Theoretically, it makes headphones sound much like a good set of stereo speakers in a good listening room, but without any crosstalk or reverberation.
The diffuse field response
If you were to measure the acoustic output from a loudspeaker in an untreated, reflective room at your eardrum, the response shape you would get would include the effects of your head, shoulders and ears. What you’ll measure at the ear can be approximated by a transfer function—shown above—referred to as the diffuse field response. Unsurprisingly, the upper midrange and high frequency boost this response provides is reflected in the shape of the Harman curve. You’ll also notice the Harman curve includes a bass boost—this is both to simulate the low frequency enhancement that a typical room gives to loudspeakers (known as room gain), and a result of the preferences of the listeners in the tests.
Sadly for us, the Harman response curve was developed using a different artificial ear fixture (a modified GRAS 45CA) than the ones in our test head (Bruel & Kjaer Type 4620), so it can’t be compared directly with our data. However, with some rough calculations, it’s possible to derive an approximate theoretical version of this preference curve as it would apply to our test head, allowing us to do a side by side comparison of our curve and the one produced by their research:
As you can see, our curve is a pretty close match to Harman’s findings, which we feel provides validation of both our calculated curve, and the Harman curve derived for our test fixture. Based on this comparison, we decided to modify our curve slightly to be more in keeping with the Harman findings in two key areas of the spectrum: the very lows and the highs.
We felt that we should adopt the slightly extended high frequency response and maintained sub bass output shown by the Harman curve, since our averaged, measured data include the very real mechanical limitations of the headphones we tested, and doesn’t necessarily represent the absolute ideal in terms of performance. After these minor tweaks, we present The 2021 SoundGuys Headphone Target Response Curve (AKA SoundGuys Curve v2.0), shown above.
Why not create separate curves for the different headphone styles?
You may be wondering, if we’re pulling in data that includes measurements of different headphone styles, why not create separate ideal curves for in-ears, or gaming headphones for example? That’s a good question, and our answer here is quite straightforward: It’s what you hear that matters, and therefore what matters is what reaches your eardrums. That statement is definitely going to catch some flak, but we stand by it.
We believe that the type of headphone we're wearing shouldn't impact our sonic preferences or ideal performance characteristics
We measure the acoustic signals at what are effectively the eardrums of our artificial head, and it is that physical location that our curve intends to serve. Just like in a blind listening test with different loudspeakers—the size or type of speaker can’t and won’t influence your choices—we believe that the type of headphone we’re wearing shouldn’t influence our sonic preferences or ideal performance characteristics.
What about headphones intended for professionals?
When it comes to studio headphones—by which we mean those products designed to meet the needs of audio professionals working in sound creation and production—we’ve recognized there might be a need for different target response. Many legacy headphone products that are relied upon by those in the content creation industry have been around for a while, and were designed at a time when the philosophy of headphone design was a little different. There are still plenty of headphones being produced to meet this sound profile, indicating that there’s demand for this approach, so we acknowledge that with our Studio frequency response target curve.
To produce this curve, we used a similar process to the one we used to develop the standard SoundGuys preference curve, detailed above. First we identified benchmark products, which we measured, then averaged together, before applying some smoothing to produce our target. We used data measured using multiple headphone and IEM models from Beyerdynamic, Sennheiser, and more.
Why isn’t the target curve for studio headphones flat?
Our target curve for studio headphones isn’t flat because a headphone that measures flat on our test setup would actually sound very dull, and there’s a reason for this. We know that studio monitor speakers are designed to produce a flat frequency response on-axis, measured in the free field. But consider, as we discussed above, that when you observe the sound pressure at a person’s eardrum when they’re listening to a “flat” speaker, the response is pretty far from flat (see the diffuse field response curve above).
The response at a person's eardrum when they're listening to a loudspeaker with a flat response is actually pretty far from flat.
As you can see from the above chart, it’s actually pretty similar to the diffuse field response. And we’re okay with that, since on the one hand we have our theoretical idea of what should work (and we don’t produce headphones), and on the other hand we have data from actual products by brands that have spent decades developing headphones for professionals and have found massive success doing so.
So we’re going with that, for the time being! We fully intend to revisit our headphone targets periodically, since it’s entirely possible that preferences or philosophies may shift, and technology will advance. As it happens, our targets and our reviews will adapt accordingly. Onwards and upwards!