Sendy Audio Aiva – Magnetostatic Stereo Headphones Review – Lots of wood, but still a fair price and some trouble for the competitors

Measurement setup and basics

Now the moment of truth strikes, once again. The test setup is finally final and the basis remains the well-known measurement microphone that has already proven itself for the in-ears. The suggestions for the realization I found at Oratory and it’s a pity that the site is now down. But I think you should really measure the headphones properly and also underlay the usual hi-fi wall of text with facts and not just mindlessly list some music. Subjective perception and objective measurement in unison should already be standard for reviews.

The complete measurement setup and methodology is described in detail in the article linked below. We can dispense with these redundant details. Nevertheless, it is recommended to have read this article at least once.

Finaler Testaufbau für den Test von Kopfhörern-und Headsets – Messen statt Schätzen und die Hintergründe

Important clue: The Harman curve

The so-called Harman curve is an (optimal) sound signature that most people prefer in their headphones. It is thus an accurate representation of how, for example, high-quality loudspeakers sound in an ideal room, and it shows the target frequency response of perfectly sounding headphones. Thus, it also explains which levels should be boosted and which should be attenuated based on this curve. This also explains the term “bathtub tuning”, which is often quoted, but in which the Harman curve is completely overused and exaggerated.

For this reason, the Harman curve (also called the “Harman target”) is one of the best frequency response standards for enjoying music with headphones, because compared to the flat frequency response (neutral curve), the bass and treble are slightly boosted in the Harman curve. This “curve” was created and published in 2012 by a team of scientists led by sound engineer Sean Olive. At the time, the research also included extensive blind tests with different people testing different headphones. Based on what they then liked (or disliked), the researchers found and defined the most universally popular sound signature.

Headphone tuning can be really problematic because of the human anatomy. Everyone has a slightly different pinna and ear canal, which affects how individuals perceive certain frequencies. In extreme cases, there is a few dB difference from person to person, which then explains the small differences in some measurements with artificial ears. Furthermore, if the sound is not absorbed, it is additionally reflected by other surfaces. Theoretically, a torso could also be included in the test setup, but that would be far too time-consuming.

Measurement of the frequency response

Let’s move on to the measurement, where the Sendy Audio Aiva was driven analog directly to the Creative Sound Blaster AE-9 as a reference (light turquoise curve). The output impedance of the AE-9’s power amplifier is well below one ohm, so that there are no impedance shifts and thus additional measurement errors, especially in the bass range. You can see very nicely the typical drop of the open systems in the low frequency range (acoustic short circuit, see previous page), which cannot be lifted even by more contact pressure. Overall, the response is extremely balanced and pleasantly linear, almost neutral, up to about 1.5 KHz. The peaks in the presence ranges at 3 KHz and later also 7 KHz are ok, while the super high frequencies run out of breath very briefly at around 10 KHz, but then they are pulled through as if there was no stopping them at the top.

If you smooth the whole thing down to the stop, the result is a somewhat rounder curve, which gallantly irons out the small point of criticism in the super high frequencies. The lower midrange is not over-present, but just neatly present and exactly where it really needs to be, while the bass plods rather discreetly in the background, but without really being lacking. At 3 KHz, we again see the hearing-related level increase, which falls a bit more behind the ideal line of the Harman curve, leaving a very silky impression.

In addition, the drivers overemphasize a bit in the super high frequencies at about 7 KHz, which tends to emphasize the sibilants and blow-off noises of instruments. Some of it sounds quite crisp, but some people like it (like me).  Which brings us back to the T1, which is very similar in terms of sound, but really wears on the nerves in the long run and lacks any smoothness that the Sendy Audio Aiva can offer, especially in the presence range.

Cumulative spectra (CSD, SFT, Burst)

The cumulative spectrum refers to various types of graphs showing time-frequency characteristics of the signal. They are generated by sequentially applying the Fourier transform and appropriate windows to overlapping signal blocks. These analyses are based on the frequency response diagram already shown above, but additionally contain the element of time and now show very clearly as a 3D graphic (“waterfall”) how the frequency response develops over time after the input signal has been stopped. Colloquially, such a thing is also called “fading out” or “swinging out”. Normally, the driver should also stop as soon as possible after the input signal is removed. However, some frequencies (or even whole frequency ranges) will always decay slowly(er) and then continue to appear in this diagram as longer lasting frequencies on the time axis. From this, you can easily see where the driver has glaring weaknesses, perhaps even particularly “clangs” or where resonances occur in the worst case and could disturb the overall picture.

Cumulative Spectral Decay (CSD)
Cumulative spectral decay (CSD) uses the FFT and a modified rectangular window to analyze the spectral decay of the impulse response. It is mainly used to analyze the driver response. The CSD typically uses only a small FFT block shift (2-10 samples) to better visualize resonances throughout the frequency range, making it a useful tool for detecting resonances of the transducer.

The picture shows very nicely the transient response and the not quite resonance-free course up to about 1 KHz. You can analyze and dissect the individual sound layers very nicely, and the fact that the peak only really dominates in a very narrow band at 2.5 and 3 KHz is also quite good. The cardboard sound in the lower mids is completely missing and only the slight overemphasis of the sibilants above 6 to 7 KHz is slightly noticeable again. We measure an open, very fast headphone here, but it also “reverberates” about more due to its design. You can like that, too, or not. But I would like to refer you to the following burst diagram, where the whole thing is quickly put into perspective.

Short-time Fourier Transform (STF)
The Short-time Fourier Transform (STF) uses the FFT and Hanning window to analyze the time-varying spectrum of the recorded signals. Here, one generally uses a larger block shift (1/4 to 1/2 of the FFT length) to analyze a larger portion of the time-varying signal spectrum, especially approaching application areas such as speech and music. In the STF spectrum we can now also see very nicely the work of the drivers, which afford various weaknesses in some frequency ranges.

This “dragging” at the lower frequencies below 500 Hz is then repeated several times between about 2.5 and about 6.5 kHz. In this context, I noticed that, for example, the bass components do not, as with other headphones, cause a kind of slight tremolo effect to set in in the mids above them, which somewhat “modulates” the mids and pushes them along. This is certainly also an advantage of planar drivers, which are not at all susceptible to such dependencies of the cone-shaped diaphragms of normal plunger coil drivers (lack of stiffness).

Burst Decay

In CSD, the plot is generated in the time domain (ms), while the burst decay plot used here is represented in periods (cycles). And while both methods have their advantages and disadvantages (or limitations), it is fair to say that plotting in periods may well be more useful in determining the true decay of a driver with a wide bandwidth. And that’s exactly where the Sendy Audio Aiva performs grandiose below 2 KHz, leaving the T1 clearly behind. We only see the (intended) slight resonance oscillations in the high frequencies. This can sound a bit sharp from case to case, but it is less annoying than with the Beyerdynamic T1 when you want to listen to music of very different genres. It is also okay for gaming (and also seems intentional), which is good. Well, the T1 also resolves better in the high frequencies, but I’ll get to that later. There is always something.

Interim summary

Especially the burst decay plot shows the excellent response of the Sendy Audio Aiva. The headphones respond extremely quickly to any level changes and are free of annoying resonances up into the super high frequencies. Only the presence areas are slightly overemphasized. However, you can live with this well, because all this does not annoy you. So you can listen without getting tired, especially when the day is really long.