The default answer to “do I need an external microphone for AcousticCheck?” is no. The internal mic on an iPhone 13 or newer is calibrated, has a usable response from about 80 Hz to 20 kHz, and gets you within a few percent of a Class-2 instrument in any reasonable room. I use the built-in mic for maybe 80% of my measurements.

The remaining 20% is what this piece is about. There are three failure modes where the iPhone gives up, and one of them comes for everyone eventually.

Failure mode 1 — the very large room

Anything above roughly 5,000 cubic meters with a reverberation tail past 2.5 seconds starts to push the dynamic range of the internal mic. The decay curve looks good for the first 25 dB, then runs into the noise floor before you’ve seen enough of the tail to fit T30 reliably.

You’ll see this in the app as R² values dropping below 0.95 across the lower bands, and a “truncated decay” flag on the result. It’s the most honest thing the app can do — better than quietly reporting a number that’s ten percent off.

If you only measure small-to-medium rooms, stop reading. The internal mic is fine. You’re here because you’ve seen a truncated-decay flag and you want to know what to do about it.

Failure mode 2 — noisy HVAC

The Schroeder method needs at least 35-40 dB of headroom between the decay’s top and the noise floor. A typical office HVAC system runs 35-45 dB(A) on its own. Add a soft chiller hum at 60 Hz and you can lose the entire bass region of a measurement.

An external mic with directional pickup — a cardioid pattern, ideally — cuts the room ambient by 5-10 dB and brings the bass bands back.

Failure mode 3 — sub-100-Hz interest

The internal mic’s low-end is honest down to about 80 Hz and then rolls off gracefully. If you’re measuring a control room for a mixing studio and you care about modal behavior at 63 Hz, that’s not enough. You need a calibrated measurement mic with response down to at least 20 Hz.

What I actually use

I keep three mics on the road and one in the studio. None of them cost more than a flight to Berlin.

1. Earthworks M30 — the reference

Flat from 9 Hz to 30 kHz, ±0.5 dB. This is what we calibrate against. If you’re going to spend real money once, spend it here. Pair it with a focusrite Scarlett 2i2 and a 30 cm cable and you have a Class-1 measurement chain on your back.

2. iSEMcon EMX-7150 — the working horse

About a fifth of the M30’s price. Flat from 20 Hz to 20 kHz within ±1 dB. Comes with a calibration sheet, which AcousticCheck Plus can import. This is the one I use 95% of the time when I bring external gear.

3. Rode NT-USB Mini — the laptop bag mic

Not a measurement mic. But: USB-C plug-and-play, no interface needed, ±2 dB from 100 Hz to 16 kHz, and you can leave it in a laptop bag and forget about it. For a quick acoustic sanity check in a meeting room when you forgot to bring real gear, it beats the iPhone’s internal mic by maybe 6 dB of SNR.

4. The iPhone, again

Honestly: the internal mic on the latest hardware. If you’re measuring rooms below 1,000 m³ with quiet ambients, the calibration AcousticCheck applies makes the internal mic competitive with a budget external. Don’t spend money you don’t need to spend.

The right tool is the one that gives you a reliable answer in the room you’re standing in. Often that’s the iPhone in your pocket.

How to connect them

  • USB-C iPhones — direct USB-C to whatever the mic uses. The Scarlett 2i2 works via a powered hub; the Rode goes direct.
  • Lightning iPhones — Apple’s Lightning-to-USB Camera Adapter, plus a powered interface. Don’t go through a Lightning-to-3.5mm adapter — it bypasses the digital path that preserves the calibration.
  • Bluetooth — AcousticCheck Plus supports a few specific Bluetooth measurement packs, but in general we don’t recommend Bluetooth for measurement work. Latency is the enemy.

The bottom line

For most work, the iPhone is enough. When it stops being enough, the app will tell you — either by flagging the truncated decay, or by showing low R² values across the bass bands. At that point, the iSEMcon is a one-time purchase that solves the problem for the next decade.

If you need help picking, the support team is fast; if you want to go deeper on the math behind the truncation flag, read how accurate is an iPhone measurement, really?