It wasn’t even that long ago that Dr. Dré shocked the world with his BEATS. €400 Euro for a pair of headphones!? Is he out of his mind! By now we’ve reached a point where it’s okay to just put a ‘0’ after it. And even then, no one is surprised.
Partly because of the above development, the world of head-fi has grown extremely fast in the last ten years (there is money to be made, so every manufacturer jumps right in).
Before all the madness, the choice was easy: you just go for a decent pair of Sennheisers or maybe a a decent Beyerdynamic DT-series. Now you have to filter through dozens of brands. Because all the big brands have jumped in. Think Focal, Bowers & Wilkins, JBL (and of course AKG)….
But it doesn’t stop there. Many new brands have also joined. And quit again, for that matter. But in the end, we can say that head-fi has become a pretty big part of our hobby. And so Alpha Audio also pays the necessary attention to it.
Now you may think that measuring headphones has a similar approach to measuring speakers. After all: it is also a reproducer. Make no mistake, however: measuring headphones requires a totally different approach.
Harry, Harriette… EARS and…?
We’re going to be very, very honest right off the bat: measuring headphones is just hard… very hard. And tricky. We’ve been trying to get solid, reproducible results for about a decade. And while over-ears tend to work pretty well; on-ears and in-ears are less “stable”. That has everything to do with how the headphones seal off.
Our first prototype – Harry – first worked with a Behringer microphone in mono. A couple of years later that became an MB measurement microphone. That worked a little better, but was still not ideal.
After a few years, we made Harriette. A model with artificial ears and two DPA 4061 microphones in stereo. Without question, that gave better results than Harry. But it proved impossible to test on-ear or in-ears: it just doesn’t seal properly. And that distorts the measurement results in the low spectrum.
When Minidsp came out with the EARS set-up, we were very impressed with the specifications. For less than 300 euros, a complete measurement setup? Surely that can’t be any good. Nothing could be further from the truth: this device works very well. And measurements are reproducible!
However, there is one drawback to the EARS: it only has a usb connection to the computer; no analog output to connect to a CLIO or Prism dScope, for example. For this reason, we are now creating a third version of a Dummy-head. We have yet to come up with a name.
HRTF and Harman curves
Anyone who starts measuring headphones may frown for a moment when the first measurements appear on the screen. After all, they are not straight at all. It looks a bit like a “smiley face” with rounded high. Weird… until you consider that we put a pair of speakers right on the ear (or even in it). Up against the ear canal and eardrum. This placement has a huge impact on frequency response. And thus how we should measure it.
If you go searching on how this all works, you are sure to come across the term HRTF. HRTF stands for Head Related Transfer Function.
Simply put, the HRTF is a mathematical representation of the acoustic properties of our ears and head and its influence on the positioning of sound.
When sound reaches us, it does not go directly into the ear canal. It also hits the head and, for example, the auricle. This all affects how we hear as well as how we can locate something. This is all represented by the HRTF. You will understand that an HRTF is unique to each person. After all, we all have a different shape head, ears, nose, et cetera. These in turn cause different reflections and thus different responses!
And you will also understand that the HRTF is different for headphones than for free standing speakers or ceiling speakers, for example.
Harman has done an awful lot of research into acoustics, psychoacoustics and HRTFs. It is also not very surprising that a brand like Revel scores well when it comes to neutrality and radiation behavior in an ordinary room. Headphones like AKG are also widely used in the pro world.
The measurements
These are measurements of the Meze Empyrean. A serious headphone.
The above measurements were made in REW. Now for a long time we used Illusonic’s software (also for speakers), but since we use the EARS, we use REW, because it can easily load the correction files from Minidsp. In these correction files, it both corrects the microphone and applies the HRTF curve for in-ears, on-ears or over-ears. Very convenient.
You can see the influence of these corrections in the first three graphs. The first is a raw measurement without correction. The second and third are corrected for over-ear and on-ear. You can see that the response changes. That has nothing to do with the actual response of the headphones; it doesn’t change. It is purely a correction for how we ultimately hear it.
The fourth is a distortion measurement. That goes without saying: lower is better. Measuring distortion in the bass is very difficult without a anechoic or very quiet room. Even with headphones.
The fifth and sixth measurements show energy distribution (based on frequency response) and resonances via a waterfall graph.
Headphone placement
By the way, it is incredibly important to place the headphones properly on the measurement setup. You can already see above the dashes we drew on Harriette. This is to determine the center when we place headphones. And to be able to do this right time after time.
The seal of the headphone around the ear is very decisive, as is the angle of the driver on the ear. This is what makes measuring headphones so tricky: everything counts and directly affects the measurement.
Concluding
Measuring headphones is complex. And for several reasons. First, it is crucial to learn how the frequency response is affected by the HRTF. Because headphones are so close to the ear – or even ín the ear! – the response is totally different. This is crucial to realize when interpreting the raw, uncorrected graphs.
Second, the placement of the headphones on the dummy is of great influence. If the assembly does not seal properly, it directly affects the response (and the rest of the measurements). Therefore, we always measure a few times to determine which measurement is representative.
All in all, we can say that measuring headphones is a completely different game than measuring speakers!