Development Update #9: Audiophilia Part 1

February 24, 2014

The first of our long-awaited audio testing results are out! Now that internal acoustic design has been (mostly) finalized, we can share the current frequency response curve for Arc. Without further ado, here they are:

If you’re unfamiliar with what frequency response means in the context of headphones, or how to read such graphs, here’s a quick primer. Frequency response is the relationship of frequency/pitch to the magnitude and phase of a signal, in this case the sound reaching your ears. In physical terms, this translates to sound pressure (loudness) and time lag, i.e. how quickly different frequencies travel to your ear. Glancing at a frequency response curve will tell you how well different pitches (bass, treble, etc.) come through.

To test audio quality, the audio engineers place headphones around a mechanical dummy head with mics to simulate human ears. For magnitude response, a frequency “sweep” from 20Hz to 20kHz (the lower/upper bounds of human hearing) is played, and the resulting loudness recorded in dB_SPL (a decibel unit specifically used for sound pressure). For phase, a square wave – which theoretically includes all frequencies – is played, and the resulting shape is compared to the original. Today we’ll only talk about magnitude response, shown by the graph above. Our square wave results will arrive soon!

In our graph, the x-axis represents pitch in Hertz and the y-axis represents loudness in decibels. Both are on a logarithmic scale (progressing exponentially instead of linearly) as human ears perceive frequency and loudness differences that way. Channel 1 and 2 are the curves measured for left and right ears, respectively.

Sounds are generally divided into lows/bass (2kHz). The “ideal” response is a totally flat curve, resulting in output that sounds exactly like the input signal. However, a few caveats about human hearing and speaker physics change the definition of “ideal.” For one, low frequencies are picked up not just by your eardrums, but also your chest cavity and skeleton. That’s why you can really feel the oomph from bass on loudspeakers, but not so much on headphones; all the sound is directed straight to your ears, and a little bit to your skull. Hence, bass response is designed to be slightly (but not excessively) louder in most headphones.

Second, once we get into the extreme highs/trebles, all speakers start having some trouble oscillating that quickly, causing a roll-off in amplitude. The later this attenuation starts, and the more gradually it happens, the better your headphones will sound at high frequencies.

Knowing all this, we can evaluate Arc’s frequency response curve.
In the lows we have a fairly even response, which is great! The major problem is the ~3dB difference between left and right, which is about a 20% difference mathematically. To your ears it will be an even smaller discrepancy, but this still needs to be adjusted.

There is still an annoying dip at the beginning of the midrange, but it’s vastly improved from the weak mids mentioned in our first update. This is due to some leakage in the speaker casing, which will be fixed before the next testing round with some internal foam insulation. Otherwise, the midrange is also well-balanced.

Our high roll-off begins at about 8kHz. Between there and 20kHz, the lowest amplitude reached is about 12dBSPL below the standard loudness of 90. Ideally our roll-off wouldn’t happen until after 10kHz, and we’ll try to remedy this before production.

Overall, we’re really happy with the current state of Arc’s sound quality. The shape of the frequency response curve is similar in some ways to well-known audiophile brands like Sennheiser or V-MODA. Here are some reference curves to compare!

We’re excited to continue optimizing Arc’s performance, and get the units to you, our supporters, as soon as possible so you can finally give them a listen :) In the next update, we’ll go over some more sound quality measurements such as isolation and phase response, along with recent manufacturing progress!

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