Solo: Planar Magnetic In-Ear Monitors

Many Firsts

Solo constitutes many firsts for us. This universal in-ear monitor (UIEM) model uses a single driver per-side, which compared to most of our other models that range from 6-18 drivers, is completely new for us. Coming onto the scene on the heals of our discontinuation of Duo (2-driver UIEM), the single driver that we deploy is outside of the balanced armature + dynamic driver combos we typically use. The 14.2mm planar magnetic transducer, coupled with some clever acoustic design, is at the heart of Solo. Along with a generally redesigned packaging and accessories, we’re also presenting our first ever pure copper cable; and not just one but two cables included as standard in both 3.5mm and 4.4mm balanced termination.

Planar Tech

The exact inventor of planar magnetic driver technology, sometimes called Orthodynamic, Magneplanar, or Isodynamic, can be hard to pinpoint. James Winey, of Magnepan fame, is generally credited with its discovery and/or popularization back in the early 70s.1

“An array of magnets is placed in front of and behind the diaphragm such that the conductors are immersed in a very even field of magnetic flux (isodynamic magnetic field). When current is passed through the conductors, the magnetic field created by the current flow interacts with the isodynamic field created by permanent magnets, causing the conductors, and therefore the diaphragm, to move.”

Source: innearfidelity.com

The planar magnetic driver features a flat diaphragm with embedded conductors suspended between two sets of magnets, unlike traditional dynamic drivers that utilize a conical diaphragm driven by a voice coil. Because the entire diaphragm moves uniformly due to the planar magnetic field, we can deliver the detail and micro-transient information that would typically require a dedicated high- frequency driver. The diaphragm “is very light, and the electromagnetic force is very strong, so the ability for the signal to accelerate the diaphragm is very, very good.2 Since the diaphragm isn’t driven by a voice coil attached to a cone, which can cause uneven movement, we can reliably extract a true high-resolution full range sound from this single driver.

technical drawing of 64 Audio's planar magnetic driver used in our Solo earphones.

Helmholtz Resonator

While planar magnetic technology has been present in portable hi-fi for many years, we wanted to truly showcase its potential in our products. For this fresh take we needed a combination of a slight but broad reduction in high-frequency amplitude and the attenuation of a few very narrow frequency bands. As the electronic filter acts as a very subtle reductive shelf, we had to think creatively to effectively control some problematic frequencies. For this, we drew on our years of research into Helmholtz Resonators, an acoustic device coined by Hermann von Helmholtz in the mid-19th century, with examples of this device seen in ancient architecture dating back to the 1st century BC.3

The volume of air in and near the open hole vibrates because of the ‘springiness’ of the air inside. A common example is an empty bottle: the air inside vibrates when you blow across the top, as shown in the diagram below.4

Source: The University of New South Wales

While you often see these devices used to amplify certain frequencies we use them to attenuate two regions, a wide shallower band at around 5khz and a narrower deeper band at around 8khz.

Source: bloomaudio.com

While the concept is as simple as blowing into the top of a glass bottle, harnessing this ancient tech is the hard part. We aren’t the first to bring this to portable high-end audio, but just like the other brands we had to create our own repeatable/predictable method to be able to manufacture these with tight tolerances and build similar structures in future projects. The two micro-cavities are 3D printed into Solo’s sound chamber; the precise dimensions of the cavity and neck being pre-determined to target the frequencies previously mentioned.

The two chambers highlighted in green + red.

Additional Patented Technology

To further elevate the planar driver implementation we of course had to integrate our patented pressure relief technology Apex, in the form of apex-core. In Solo it is integrated into the planar drivers’ manifold which vents out of the open copper mesh grill of the faceplate. The pressure relieving channel provides our controlled leak via acoustic filters as opposed to pneumatically interactive foam in our traditional apex implementation. While Solo isn’t technically open back, or semi-open back like Duo, it’s meant to feel very open with about -12db of isolation.

After adding a wide electronic notch filter to gently tame the high-frequency band, we had to account for the change in the impedance curve by adding our patented LID technology to flatten that back out.

Although Solo is our first audiophile UIEM to not include the direct radiating balanced armature driver (tia driver) it is still built using two of the other three parts of this patented technology: the single-bore design and micro-acoustic chambers.

Sound

The agile performance expected of a planar driver achieves maximum realism by extracting subtle details in the music. These transients emerge from the mix, seamlessly keeping pace with your focus on the primary elements of the song. Our acoustic notch filters are strategically placed to eliminate sharpness without compromising driver performance. Apex core assists the high-mid and high frequencies in creating a vast sonic landscape, even in very complex mixes. Alongside this realism comes a tactile immersion in the textures of instruments and even the sonic characteristics of the recording equipment used in your favorite tracks. By intentionally shaping the overall response of Solo, we achieve a bass that has substantial impact and deep sub-bass presence.

Solo Specs

  • Color : Black
  • Frequency Response : 20Hz – 20kHz
  • Isolation : -12 dB
  • Material : T6061 Aluminum Shell + Copper Faceplate Mesh
  • Impedance : 16 Ω
  • 2 x OCC Copper Cables (3.5mm & 4.4mm)
    25 AWG + 23 Wires
    0.26 Ω
  • Driver Configuration
    • 1 14.2 mm Planar Magnetic
  • Sensitivity: 100 dB/mW @ 1 kHz
  • Included Items
    • Carry Case
    • Foam + Silicone Ear Tips
    • Cleaning Cloth
    • 2 x 2-pin cables
    • Cleaning Tool
    • Round Sticker
    • Shirt Clip
  • Price : $1,399

The Reviews Are In

TECH POWER UP: Solo Review

Headfonia Reviews: 64 Audio Solo Review

HEADPHONES . COM: Solo Review

Sources:

1: US Patent US3919499A

2: How Planar Magnetic Headphones Work by Tell Hertsens of Innerfidelity www.innerfidelity.com/content/how-planar-magnetic-headphones-work

3: On the Sensations of Tone as a Physiological Basis for the Theory of Music
by Hermann von Helmholtz

4: UNSW Sydney https://newt.phys.unsw.edu.au/jw/Helmholtz.html