Our Passive Soundstage Score And Tests

Headphones

In order to measure the amount of interaction between the pinna and headphones, a test was devised that we call PRTF, short for pinna-related transfer function. For this test, we got an extra ear for our dummy head, chopped its pinna off, and measured the frequency response of each headphone twice. Once with the intact ear, and once with the ear with the missing pinna. The difference between the two frequency responses is the PRTF of the headphone.

Headphone's PRTF = (Frequency Response w/ Pinna) - (Frequency Response w/o Pinna)

To come up with a reference to score our PRTF measurements against, a loudspeaker was used. It was positioned 1 meter in front of the HMS (our dummy head) and frequency response measurements were performed with and without pinna every 30 degrees. We picked the loudspeaker PRTF at 30 degrees as the reference, because that’s where loudspeakers are positioned in a stereo setup, but as the graph below shows, there's a clear pattern emerging as the head is rotated.

In the soundstage category in our reviews, PRTF calculations are divided into three: Accuracy, Size, and Distance.

The accuracy value is derived by performing a standard deviation calculation of the PRTF response compared to the speaker reference PRTF, between 2kHz and 7kHz. In other words, it shows how accurate the shape of the response is in that region, compared to the reference PRTF response, and therefore, is mostly responsible for how natural the perception of soundstage will be.

As a rule of thumb, headphones with angled drivers and large and deep enclosures have the best PRTF responses. That's why over-ear headphones tend to do better than on-ears, earbuds, and in-ears in this test. Also, some headphones show unusual peaks and dips in their PRTF responses. One possibility is that these headphones have small ear cups, and they pinch or mangle the pinna in a way that creates an unusual response and the soundstage of these headphones may be perceived as odd and unnatural.

The size value is derived by performing an average amplitude calculation on the PRTF response between 2kHz and 7kHz. In other words, it shows the amount of interaction between the headphone and the pinna, regardless of its accuracy, and therefore, is mostly responsible for how large the soundstage will be perceived.

As a rule of thumb, headphones with angled drivers and large and deep enclosures have the best PRTF responses. That's why over-ear headphones tend to do better than on-ears, earbuds, and in-ears in this test. Also, some headphones show an unusually high amount of pinna interaction in their PRTF responses. One possibility is that these headphones use the pinna to dampen the resonances of the enclosure, and when the pinna is removed for the PRTF test, those resonances are able to ring freely. In this case, the extra activation can be considered as noise in our measurements.

The distance value is derived by calculating the difference between the "average PRTF amplitude of the 2kHz-7kHz range" (i.e. PRTF Size), and the "lowest PRTF amplitude value in the 8kHz-12kHz range". In other words, PRTF distance looks for the depth of the 10kHz notch, which is mostly responsible for cues regarding the angle and elevation of the sound source.

As a rule of thumb, headphones with angled drivers and large and deep enclosures have the best PRTF responses. That's why over-ear headphones tend to do better than on-ears, earbuds, and in-ears in this test.

It should be noted that with some headphones, the center frequency of the "10kHz notch" is slightly shifted upwards or downwards. Based on our tests, an upshift in the center frequency of the notch could actually represent cues for the elevation of the sound source. That is, the auditory system will localize the sound source in-front and slightly above, rather than just in front.

This value is the inverse of the noise isolation score. That is, the less isolating a headphone, the higher their openness value will be. Openness is included in the soundstage category, since open-back headphones, compared to closed-back headphones, tend to give the impression of a more open and spacious soundstage. However, the reputation that open-back headphones have regarding their soundstage hasn't been quite validated in scientific studies yet, and the sense of a more spacious soundstage may actually be related to the acoustic impedance of the headphones, rather than their enclosure properties.

This value is the inverse of the leakage score. That is, the leakier a headphone, the more they acoustically excite the room they are in. Acoustic Space Excitation (ASE) is included in the soundstage category since the soundstage of certain headphones can be subtly affected by the room they are located in. For example, with very open and leaky (loud) headphones, it is possible to hear a small change in soundstage as one moves from a large dead environment to a small and reflective room.

As a rule of thumb, the smaller the driver of a headphone, the less their ASE will be. Also, open-back headphones excite their environment more than their closed-back counterparts. Therefore, closed-back in-ears usually have the least amount of ASE, and open-back planar magnetic headphones tend to have the highest amount of ASE.