There are many ways to get an audio signal into an iOS device. We will look at these by model, and then look at other options.

Internal Microphone

All current iOS devices (iPhone 3GS, iPhone 4, iPod touch 4, and all iPads) include built-in microphones. These mics tend to be very consistent from one unit to the next, and are wide-range. However, Apple does include a very steep high-pass filter (which cuts low frequencies), presumably as a wind and pop filter.

Also, in most of the later models, there is a limiter -- making SPL measurements, as well as more sophisticated measures such as Impulse Response difficult to make accurately.

The low-frequency roll-off for the internal mic in these devices is very steep, on the order of 24dB / octave starting at 250Hz.

We have compensated for these mics as much as possible, but given the physics of the situation, the usefulness of the low frequency information is limited. 

Changes in iOS 5

in iOS 5, Apple has given us an actual audio session category called "Measurement". This is a huge step forward, even to have Apple acknowledge that audio measurement is a viable subset of audio users. 

When using this category, (which we have implemented starting in AudioTools 3.7), the limiter is disabled, and we also have for the first time the ability to control the input gain of the built-in mic. This is a huge benefit, and now we can measure dB SPL levels up to (and sometimes over) 120 dB SPL!

AudioTools now automatically senses whether or not input gain control is available on the device, and when available the input gain is adjusted, in two ranges that are present on the Input Select screen for each module. 

Select Low Gain for the best performance in the 30 - 90 dB SPL range, and select High Range for the best performance in the 45 - 120 dB SPL range. This does vary a bit by device, but in most cases works very well.

The one exception in the current Apple product line (based on our in-house testing) is iPad2, which does not support input gain control, at least for now.

Sadly, Measurement mode does NOT change the severe low frequency roll-off, although we know for a fact that for at least one device, the iPod touch 4th gen, the low frequency rolloff can be disabled in software. In iOS 4.1 and earlier, the rolloff was much less, but starting in 4.2, the severe rolloff began, with no change in hardware. We have a bug request in to Apple, but of course we don't know when or if they will address this.

Headset Input

The headset mic input on all current iOS devices also uses the same filter as the built-in mic, and so offers no improvement to performance.

Because of this, we can't recommend any mic that plugs into the headset connector, such as micW. Although micW may be using a higher quality capsule (although it is doubtful) than the iPhone mic capsule, by using the headset mic input, it is subject to the same filters as the built-in mic. 

micW does have lower sensitivity, so it will clip at a higher SPL point, although it's noise floor is also therefore higher, so it can't measure as low of noise levels. The calibration we normally see for this mic is around +13.

Splitting Headset Output Signal

The headset connector is a 4-pin connector on the iOS device. The additional pin is for the mic input. If you want to use the output from the headset jack (for example as a pink noise source) while still using the internal mic, make sure your cable has a 3-conductor plug. This will tell the iOS device to leave the internal mic operational.

There are also splitters available, for example by SoniTalk, that provide a 3-pin headset output and also include a mic similar to the Apple mics.

Dock Input - Analog or Digital?

Apple provides audio input on the 30-pin connector. For iPhone 3GS, iPod touch 3, and older models, this input was analog. 

iPhone 4, iPod touch 4, iPad, and newer models provide a digital audio input on this connector.

Analog Input Devices

The analog line inputs are essentially perfectly flat from 20Hz - 20kHz. Therefore, any dock adapter or microphone that plugs into the dock connector will not be subject to Apple's low frequency filters, and is full-range.

iAudioInterface uses the dock input, in analog mode. So it can only work with the older iOS devices. 

Very few other options exist for the analog dock connector. One is Blue Mikey, although since it is cardioid, it is not recommended for test and measurement.

Digital Audio Input Devices

Starting with iPhone 4, Apple implemented a digital audio link in the 30-pin connector. This provides a way to get high-quality professional audio into it. All current iOS devices use this audio interface method.

Our new iAudioInterface2 connects to the iOS device using digital audio, and provides a phantom-power microphone input with up to 50dB of gain, a second balanced line input, a balanced line output, and a Toslink digital audio output. It includes a built-in li-ion battery system, and can charge the attached iOS devices when the charger is plugged in.

iAudioInterface2 also is tightly connected to our AudioTools app, providing gain control, mode control, and access to advanced features such as impedance measurement.

Over time, more iOS digital audio devices will become available. We will update our Competitive Comparisons page as more devices become available.

Microphone Type -- What makes a Type 1 Mic?

In truth, there is no such thing as a Type 1 (or Class 1) microphone, only a Type 1 Sound Level Meter. According to both the US ANSI S1.43 specification, and the European counterpart, IEC 804, the entire system must be tested against the specification as a single unit.

In our case, all of the algorithms, filters, and features of the SPL Meters and have been written to meet or exceed the standard for Type. And, in the case of the RTA, the filters are written to meet Class 1 of ANSI S1.11. So, if you use our software with a microphone that meets Type 1 or Type 2 specs you will in theory have a meter that meets these specs.

To be really compliant, you would have to submit your microphone and iAudioInterface2 for calibration, to a designated calibration facility, and have a mic that meets all of the Type 1 specifications, which are summarized below.

Here is a summary (not intended to replace the specs) of some of the main features that a mic would have to meet, to be useful as the microphone in a Type 1 Sound Level Meter:

Frequency Response

This is probably what most people think about first. 

20 Hz, +/- 2.5dB

25 Hz, +/- 2.0dB

31.5 and 40 Hz, +/- 1.5 dB

50 through 4000 Hz, +/- 1.0 dB

5000 Hz, +/- 1.5 dB

6300 Hz, +1.5, -2.0dB

8000 Hz, +1.5, -3.0 dB

10000 Hz, +2.0, -4.0 dB

12500 Hz, +3.0, -6.0 dB

16000 Hz, +3.0, -infinity (therefore not required)

20000 Hz, +3.0, -infinity    "

That was the easy one, now for the more challenging ones to meet.

Linear Operating Range

Generally, 60 dB of operating range, with +/- 0.7 dB variability.

Free-Field vs Random Incidence

This is difficult to summarize, but there are strict limits (approx. +/-2.5 dB at 22 degrees) on how omnidirectional the microphone is, in actual use.

Environmental

In most cases, a plastic diaphragm mic cannot meet these specs.

Atmospheric pressure, no more than 0.3 dB change for a +/- 10% change in pressure.

Air temperature, no more than +/- 0.5dB change between 10 and 50 degrees C -- or 50 to 122 degrees F.

Humidity, no more than 0.5 dB change from 30% to 90% humidity at 40 degrees C (104 degrees F).