Sonion’s AcuPass Technology: Making Things Easier for Everyone

All information presented in this article is derived from publicly available documentation.
CYMBACAVUM has not been privy to any confidential industry documents.

In the world of premium earphones, while many manufacturers around the world are able to design and manufacture dynamic, moving coil transducers, the supply of balanced armature technology to earphone manufacturers around the world is essentially a two-horse race between the US-based Knowles Electronics and Netherlands’ Sonion SA.

Out of these two companies, Knowles has always held the market lead. With a long history in microelectronics manufacture, they’ve managed, in recent years, to capture a good portion of the premium audio market with their miniature dual driver design, the TWFK.

This tiny little driver, measuring in at only 5.00 mm × 2.73 mm × 3.86 mm, enabled some absolutely tiny in-ear designs, such as the JAYS q-Jays, the Ultimate Ears UE700, and many more.

The TWFK’s success translated not only well to the mainstream (almost all balanced armature designs originate from applications intended for hearing aids), but the driver’s wide bandwidth and small size also allowed the TWFK to be a preferred driver of many manufacturers of custom in-ear monitors as well.


However, the TWFK, as great as it is in terms of tiny size and wide bandwidth, has had to concede a number of technical tradeoffs. A good number of IEM users lamented that TWFK-equipped IEMs sounded a bit thin and metallic, with unconvincing bass depth, and could even be at times sibilant. Having heard a good number of TWFK-equipped IEMs and their variants, I can agree that these complaints are not unfounded.

Much of this subjective assessment has to do with the small-sized enclosure and the lack of adequate back volume required to provide the bass response necessary for listeners’ satisfaction. A number of companies, such as Westone, EarSonics, and Ultimate Ears, have had to design their flagship products around an assembly of larger-sized BA drivers on top of the TWFK, significantly increasing size and design complexity because of the need to design  around a multi-bore output. For universal-fit IEMs, a manifold design would also be necessary.

In addition, because of the difficulty in creating low-noise/distortion low-pass filters in crossovers for multiple BA-based IEMs, it’s not a surprise that acoustic engineers from all over have been searching for an acoustic-based alternative.

In order to make things simpler for earphone manufacturers and designers, Knowles designed several integrated, dual-driver high/low solutions in not only the TWFK, but also the GP/GD/GQ drivers, and has even custom-designed solutions, such as that used in the Apple Dual Driver In-Ear Headphones. With easy tweaking options, integrated drop-in solutions are what seem to be trending for the rapidly expanding premium earphone market.

Sonion, on the other hand, has not had as much widespread success. They’d been playing second whistle to Knowles, and never managed to release any drop-in solutions until recently, when they introduced a new dual-driver assembly.

In addition, the new driver set utilizes proprietary technology they term ‘AcuPass‘ that promises to make earphone design and implementation easier than ever. Without going into details, AcuPass is essentially acoustic low-pass technology that simplifies crossover circuits and enables multi-driver receivers to be combined into one single output port, making parts easier to design for.

Sonion has a great layman’s explanation in video form:

The whole process works somewhat like the much-ballyhooed Shure SE846’s “true subwoofer”, where the woofer output is sent through a small, diameter-restricted and convoluted output path in order to filter away high frequencies without the need of passive LC/RC circuitry through SMD components. The SE846’s acoustic low-pass filter is a bit more complicated than is AcuPass, but the overarching principles governing both designs are exactly the same.

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