Type: circumaural, open-back, electrostatic
Test sample supplied by: Warwick Acoustics Ltd
Reviewed in: Hi-Fi News, November 2019
This ambitious and costly electrostatic headphone builds on the Sonoma Model One – also designed by Warwick Acoustics but manufactured in China rather than the UK – by substituting the Model One's single-sided drivers with balanced, push-pull ones. Like the Model One, it benefits from there being digital signal processing within the DAC/amplifier which accompanies it, which has allowed Warwick Acoustics to equalise the frequency response to its favoured target.
That this target response doesn't quite match either the diffuse-field response or Harman's latest target curve is shown by the corrected frequency responses. As expected, the DF- and Harman 2018-corrected responses differ regarding the Aperio's low frequency balance – the former suggesting that the Aperio has a mildly 'warm' perceived tonal balance below 1500Hz while the latter indicates a gradual decline in perceived output below 200Hz – but both agree that there is a lack of high treble. This largely explains the mildly disappointing Harman PPR figures of 58/58 ≡ 51%/50% (L/R).
Some open-back headphones are largely insensitive to earpad leakage but the Aperio lost a significant chunk of low frequency output in the spectacles and 'hair' tests, with a particularly early shelving-down of response in the left capsule. As a result the PPR scores fell further to 59/43 ≡ 52%/38% and 33/57 ≡ 28%/50% respectively.
As we often find with planar drivers – if more usually in planar magnetic headphones – the Aperio's cumulative spectral decay waterfalls are not as clean as provided by the best moving coil models. A few high-Q midrange resonances are visible, which are less well damped than they appear – particularly the mode at around 350Hz in the left capsule CSD. This also extends the -40dB impulse response decay time in the left capsule to 4.3ms, although the right capsule performs much better at 1.8ms. The pre-responses visible in both IR traces at about 250µs before the main peak are presumed to be an artefact of the DSP.
As HTL's measurements show, 'open-back' headphones vary widely in the degree of isolation they provide from external sound. In the case of the Aperio, the capsules – with their light electrostatic diaphragms – offer, in effect, zero attenuation of environmental noise of whatever spectrum.
An odd feature of the Aperio's frequency response – the kink around 70Hz where the traces for the two capsules virtually overlay one another – is explained by the acoustical crosstalk measurement. A large peak in the crosstalk response at this frequency indicates that a severe headband resonance is responsible.