Elusive Silence: iPhone XLR Mic Interface Noise Floor Comparison [VIDEO]


During my time in the Boston University film production program, we students were given access to some nice mics, and by that I mean the $1000 Sennheiser MKH-416 shotgun mic. It’s called a shotgun mic because it looks a bit like the barrel of a firearm.  Its tubular shape helps it accept sound that’s in front of it and reject sound from all other directions – which is handy when you’re trying to record any specific sound by itself e.g. dialogue and not, say, a nearby bus. In the sound kit film students were lent, this mic was paired with an aluminum Nagra reel-to-reel field recorder the size of a small briefcase of money.  To this day Nagra units are valued at between $1000 and $5000.  Together these two pieces of equipment were capable of capturing sounds richly and beautifully. So much so that during production I was often distracted by the allure of some stumbled upon mundane-turned-magical noise: the piquant clink of a china teacup laid on its saucer, the exquisite baritone creak of a wooden door or the majestic chattering fury of a squirrel on high.

Lately I’d been doing a fair amount of video production with the iPhone and its audio capture had appeared to be ‘good enough’. Either the iPhone’s mic or the louder Brando Mini Mic were used, depending on distance from source, and the audio was not monitored.  Then audio monitoring became an option in Filmic Pro and all the sudden the audio capture did not sound good enough.  These two mics do the job when someone’s talking at the camera, but for recording a quiet ‘found sound’, well, you might as well not bother.  For example, the downstairs freezer would make an intriguing whistling-wind-on-a-mountain-peak sort of sound when it was briefly opened and closed, and I wanted to record it.  However the noise floor with either the built in mic or the Brando was just too high; turn up the volume and you find yourself beneath a roaring waterfall [off system noise].

The urge to capture such sonic curios redoubled itself upon acquiring a Sennheiser ME66 shotgun mic – the venerable MKH-416’s little brother with a nifty option for self-power. Recalling the delightful though cumbersome film school recording setup, I immediately began figuring out how I could use this newfound shotgun mic with the iPhone (which happens to be much lighter than the Nagra).  Since the ME66 uses a balanced, line-noise cancelling three pin connection known as XLR, I knew I needed some kind of XLR-to-iPhone mic adapter affair, and the search commenced.

First I got the Griffin MicConnect; a seemingly good value at $40.  But it’s noisy.  Even when turned to its lowest gain setting, listening to the MicConnect’s audio through headphones at monitoring volume was traumatic. When heard through speakers at normal volume the noisy undergrowth fades into the background, and the signal is so hot that you barely notice it, but if you’re recording something quiet, the result is not what I was expecting.  It is better than the built in mic or the Brando however.   For what it’s worth, I don’t think the MicConnect is necessarily a poor design for what it is; from my research it seems like any external iPhone mic interface that tries to go though the headphone jack gets lots of crosstalk from the iPhone’s headphone preamp.

Thus, to sidestep the headphone jack input entirely, I began looking for dock-connector audio interfaces. For the iPhone 4s, these would be digital, and thus potentially much quieter. I found the iAudioInterface2, which aside from being a little bulky, looks perfectly fantastic.  However, the iAudioInterface2 is also designed to do a variety of things related to pro audio system testing, and at about $500, it was more than I had in mind to spend.  There had to be something simpler and more ‘pro-sumer’ for under $200.

Next I found a few dock connector audio interfaces with 1/8″ miniplug inputs that seemed promising.  To connect an XLR mic, I’d have to use an XLR to stereo-miniplug adapter, which means forgoing the balanced connection for a short 6 inch run of cable, but that’s a short run of cable, so maybe it wouldn’t matter.   The Blue Mikey Digital has one of these inputs, but according to some reviews (1, 2) I could not count on it being ‘dead quiet’.  Same deal with the The Line 6 Mobile In.  I figured if any review wasn’t markedly enthusiastic about the quietness of an interface, I would steer clear.   In my research, the Sonoma Wireworks Guitarjack 2 stood out from the crowd with formal review comments like: “Very quiet with plenty of headroom, and not a hint of RF hum.”  Despite the unbalanced signal path I’d have to take into the Guitarjack2, I figured it was worth a shot.  This nicely done review‘s finding that the Guitarjack2 did not work with Filmic Pro was daunting, but I figured things may have changed by now.  And luckily I was right.   As of now it absolutely does work with Filmic Pro.

The noise floor of the Guitarjack2 at full gain with mic boost enabled was totally acceptable if not totally inaudible.  At this maxxed gain level, the ME66 was quite sensitive and yet fairly resistant to clipping.  Sweet!  For a couple weeks I played with the Guitarjack2 and gradually put together a hand held dead cat iPhone rig based around an L-Bracket.  The dead cat enclosure provides immunity from wind noise while recording outdoors.

Then, while preparing this write-up, I stumbled across a review for the newly-released-last-month iRig Pro.  While the Guitarjack 2 was a slick guitar interface with a mic input that could be made to do what I wanted, the iRig Pro was explicitly designed to do what I wanted: XLR-to-iPhone through the dock connection.  Furthermore, the marketing copy for the iRig Pro was extremely cocky about its quietness: “In iRig PRO, we’ve virtually eliminated noise with our new high-definition preamp design. We can’t tell you how we did it, but you’ll dig it. You’ll have to hear it to believe it. And trust us, you won’t hear it. It’s that transparent.”   “And trust us, you won’t hear it”?  Psssh.  I did not buy that.  But I did order the iRig Pro immediately.

Since the Guitarjack2 cannot not provide  48 volt ‘phantom power’ to a condenser mic, my ME66 had been forced to rely upon its internal 12 volt power supply.  When using this same 12 volt power option with the iRig Pro, the noise floor was more or less identical to that of the Guitarjack2.  Hmmm.  Disappointing.  This suggests that [in light of the next paragraph] the 12 volt ME66 noise floor may be a characteristic of the mic’s internal  power supply itself, and perhaps the Guitarjack2 is actually quieter than my testing suggests.

Anyhow, when running the ME66 phantom powered by the iRig Pro (48 volts), the noise floor practically disappears.  Although the marketing claims exaggerate a little – the noise floor is technically still audible with my setup – it is vanishingly quiet.  In fact, the sound was so clean that I couldn’t help but start singing ad-libbed songs to myself while monitoring with headphones.  For awhile.  I think some of it was actually good.  I wish I was recording.  And not only does the iRig Pro offer excellent performance – its got a physical gain setting dial on the box and a clever level light that tells you when you’re clipping.   This is a terrific little XLR mic interface.  It even comes bundled with cables for iPhone 5, Mac (USB) and MIDI connections.

To help illustrate what I’m talking about with the noise floors of these setups, I devised an apples to apples comparison and put the 1080P video of it up on Youtube.   I found a Canon Elph camera to be a convenient producer of repeatable sound, so I placed it in a quiet environment, enabled the iPhone’s Airplane Mode, and made Filmic Pro video recordings of the Elph as it snapped a countdown timer shot.  Then in OS X iMovie I normalized sections of the recordings based on the peak sound of the shutter click and thereby put each mic setup on equal footing so their relative noise floor levels could be heard side by side.  Note that I level matched the iRig Pro’s gain setting to the Guitarjack2’s maxxed-out gain setting to more easily compare them, so keep in mind that the iRig Pro offers a good bit more volume than you’re hearing here, although that extra volume does come with extra system noise.  The setups I tested were as follows:

1. Griffin MicConnect powering a Sennheiser ME66/K6 with 48 volt phantom power, 15 ft XLR cable
2. Griffin MicConnect with a self powered (12v) Sennheiser ME66/K6, 15 ft XLR cable
3. iRig Pro powering a Sennheiser ME66/K6 with 48 volt phantom power, 15 ft XLR cable
4. iRig Pro with a self powered (12v) Sennheiser ME66/K6, 15 ft XLR cable
5. Sonoma Wireworks Guitarjack 2 with a self powered (12v) Sennheiser ME66/K6 connected via a 6 inch long  XLR to dual mono 1/8″ miniplug adapter

then I figured why not round out the field with:

6. Built-in iPhone mic
7. Brando Mini Mic

You’ll hear the untouched audio of my voice identifying each setup, followed by the normalized audio of the shutter click and several seconds of subsequent silence.  Enjoy!

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