iPhone Microphone Frequency Response Comparison

With the advent of sound level meter apps for the iPhone OS (of which SoundMeter was the first) people began to ask, “How flat is the frequency response of the iPhone’s microphone?” Early testing indicated that the built-in microphone of the original iPhone was not a good candidate for sound level measurements, but that the iPhone’s headset microphone enjoyed a fairly flat response. Since then, additional iPhone models have arrived on the scene, each with its own set of weaknesses with respect to microphone frequency response. Additional Apple and third party headset microphones have also been introduced.

At long last, some relevant frequency response measurements are presented here for the benefit of those who would really like to “see” how flat a particular microphone is. These results have implications on the use of certain microphones for making sound level measurements, as well as on the use of these microphones for spectral analysis in which relative amplitudes need to be determined with some degree of accuracy.

The following measurements were made relative to a Type 1 precision microphone in a fairly quiet room. These measurements were not made in an anechoic chamber and although the coherence was very good across the audio band, the measurement error is non-negligible at high frequencies, because of diffraction effects.

Built-in Microphones

Built-in iPhone Microphone Frequency Response

Built-in iPhone Microphone Frequency Response Comparison

As I have often said, “The built-in microphone of the original iPhone is not recommended for sound level measurements.” Now, you can really see what I mean. Interestingly, the built-in microphone of the iPhone 3GS isn’t recommended, either, unless you don’t care about frequency content below 200 Hz. This behavior is consistent with the headset input frequency response of the iPhone 3GS (I suspect that the built-in microphone signal goes through the same high-pass filter that gets applied to the headset input). The iPhone 3G microphone’s response is clearly the best of the bunch, but its low end rolls off by 15 dB or more at 20 Hz. Not surprisingly, none of the iPhone models rivals a lab-grade sound level meter with its built-in microphone, but either of the 3G models can potentially give you a decent ball-park estimate of the current sound level, although the low frequencies will be de-emphasized.

Headset Microphones

The goal, here was not to measure every headset microphone on the market, but to take a look at some of the more common options. These measurements were made of each microphone’s electrical output, so they do not include the response of any iPhone input or output circuitry. The microphones included in these measurements are:

iPhone Headset Microphone Frequency Response Comparison

iPhone Headset Microphone Frequency Response Comparison

In the world of headset microphones (at least those that are presented, here), the official iPhone headset microphone and the SwitchEasy ThumbTacks microphone win the day. The USBFever microphone also exhibits a flat response between 20 Hz and 2 kHz, although its response appears to break up more severely by the time it gets up to 10 kHz. In light of recent headset input frequency response measurements, the best case scenario for inexpensive sound level measurement might be to use the ThumbTacks microphone with the original iPhone.

These results are also interesting, in that they strongly suggest that the newer Apple headsets, which are designed primarily for iPods, shouldn’t be used for sound level measurements, either. Their response certainly seems to follow an apparent trend with Apple’s microphone-related circuitry to de-emphasize low frequencies.

It may be important to keep in mind that the goal, here, is to see what makes sense in terms of using iPhone OS devices as inexpensive, portable sound level and spectrum analysis tools. Obviously, there was never an expectation that the iPhone’s inexpensive microphones would perform in a manner consistent with precision measurement mics that are (justafiably) much more expensive. It is possible to connect such high-end microphones to an iPhone, though (via the dock connector)–more on that, later…

iPhone Line Input Frequency Response Comparison

Although I already listed some options for getting line-level audio into an iPhone or iPod touch, that list didn’t include much information that would suggest which option would be best. One important metric that people frequency ask about is frequency response. Well, I finally have some frequency response comparisons available to help answer that question.

These measurements were made of various dock connector devices, attached to an iPhone 3GS. As in other frequency response measurements, the audio was routed through the iPhone, with a little help from SignalScope Pro. This means that each measurement includes the frequency response of the iPhone 3GS headphone output.

The tested devices include:

Line Input Frequency Response Comparison

Line Input Frequency Response Comparison

Based on these measurements, and the unreliable nature of the iTalk Pro, I would recommend the Tunewear Stereo Sound Recorder for iPod as the Line-input device of choice. However, if you need balanced input circuitry, adjustable gain, or phantom power, then the added cost of the Alesis ProTrack may be justified.

As indicated previously, the iVoice Pro may still be the best option for the original iPod touch, depending on your needs.

iPhone Dock and Headset IO Frequency Response

People often ask about the frequency response of iPhone and iPod touch audio inputs. To shed some light on the issue, I made some frequency response measurements of the iPhone, iPhone 3G, iPhone 3GS, and iPod touch 2G with Electroacoustics Toolbox and an Edirol FA-101 audio interface. These measurements are broken into two groups, one for headset input and one for dock connector input.

Since measurements were made by routing audio through each iPhone OS device (by way of the Audio Play Through function built-in to SignalScope/Pro), all measurements include the frequency response of the headphone output in addition to the response of the selected input. The frequency response of the Edirol FA-101 was removed from the measurement, using a not-yet-released version of the Dual FFT analyzer in the Toolbox.

Headset Input

Headset Input Frequency Response

Headset Input Frequency Response

The original iPhone wins, hands down, for the flattest frequency response of the headset input. It’s too bad that the iPhone 3GS drops the low end more severely than any other iPhone OS device.

Dock Connector Input (Line In)

I used a Tunewear Stereo Sound Recorder for iPod for these measurements. It has one of the flattest frequency responses I have seen in a dock connector device (more on that will be published soon).

Dock Input Frequency Response (with Tunewear Stereo Sound Recorder for iPod)

Dock Input Frequency Response (using Tunewear Stereo Sound Recorder for iPod)

There is some ripple evident in the line input frequency response (it’s not yet clear whether the ripple exists on the input, the output, or both), but other than that, the frequency response is quite flat. The low frequency 3 dB cutoff appears to be at around 7 Hz and things stay pretty flat beyond 20 kHz.

iPhone Headset Input Options

One of the most obvious ways to get analog signals into an iPhone or 2nd generation iPod touch is through the headset connector. Several options exist for getting acoustic or electric signals into the headset input, which are discussed below. Any of these options will work with the iPhone, iPhone 3G, or iPod touch 2G. The original iPod touch does not have a headset connector with a mic input channel, so it is left out of this discussion.

When making a decision about what to use the headset input for, or what to connect to it, you may want to take a look at the frequency response measurements of the various iPhone OS devices.

Acoustic Signals

Acquiring acoustic signals requires some type of microphone. Several off-the-shelf options exist for attaching a microphone to the headset jack, as listed here:

Standard iPhone headsets can be used for basic sound level estimates with SignalScope ProSoundMeter or dB, without any further calibration. Using any other microphone (besides the iPhone’s built-in microphone) will require calibration before meaningful sound level measurements can be made. (dB does not support microphone calibration.)

Original iPhone users will be glad to know that the SwitchEasy ThumbTacks microphone will fit into the iPhone’s recessed headset jack. This is not true of the USBFever mic, however, so an adapter cable will be required for that mic. Another benefit that the original iPhone gains from using these mics is that the full audio sample rate will be available rather than being limited to 8 kHz, as it is with the built-in microphone.

Connecting either of the SwitchEasy and USBFever microphones will preclude the use of headphones, unless some special adapter cable is used (I’m not currently aware of an off-the-shelf solution).

Frequency response measurements of these microphones can be found here.

Electric Signals

Acquiring signals from some other source can be a little tricky for the following reasons:

  1. The headset microphone input is very sensitive (it expects a low-level microphone signal).
  2. A bias voltage is present on the headset input to power electret condenser microphone capsules (used by the afore-mentioned microphone accessories).
  3. The headset input expects to see a particular load in order to signal the OS that an external microphone is present.

Of the three issues, the third one is perhaps the most difficult. To be sure the iPhone OS will select your input signal, you can place a suitable resistor in parallel with your input. One user reported that a 3.3 kOhm resistor dropped the bias voltage from 2.7 to 1.9 VDC. When connecting the headphone output directly to the headset input for some basic frequency response measurements, I have had good success with a 670 Ohm resistor. I have also had success connecting external measurement microphones and accelerometers, using a constant-current power supply, without using an additional resistor.

The best adapter cable I have found for connecting to the headset input is a standard A/V cable, which has a four-conductor mini-plug on one end (for connecting to the iPhone) and three RCA plugs on the other end. To work with the original iPhone, the A/V adapter cable needs to have some of the plastic carved off around the mini-plug, or another adapter cable is required to fit the iPhone’s recessed headset jack.

Sometimes, connecting external signals to the iPhone’s headset jack is the most convenient, portable solution. However, working with dock connector input devices allows for up to two input signals without the complicating issues of the headset input.

It should also be noted that the iPhone 3G rolls off the low frequency response of it’s headset input below 100 Hz.

Dock Connector Audio I/O

Several options exist for getting audio signal into and out of iPhone OS devices via the dock connector. However, not all accessories are compatible with all iPhone OS devices. So, we put together this compatibility chart, based on our own tests with SignalScope/Pro and SignalSuite.

Dock Audio Accessory Compatibility

These devices were chosen for their ability to accept stereo audio input from external sources. Some dock connector devices simply feature built-in microphones, which are of limited use for test and measurement applications. It’s also important to remember that the iPhone OS automatically selects the current route for input audio signals (built-in mic, headset, dock connector, etc).

iPhone iPhone 3G iPod touch iPod touch 2G
Alesis ProTrack In/Out(1,2) In/Out(2) Out(3) In/Out(2)
Belkin TuneTalk Stereo In(1) In In(3,4) In
Griffin iTalk Pro (5) In(5) In(5) In(3,4,5) In(5)
MacAlly iVoice Pro In/Out In/Out In/Out In/Out
Tunewear Stereo Sound Recorder In(1) In In(3,4) In
  1. Even when using the dock connector for input, if the receiver or built-in speaker is the current output device on the original iPhone, the sample rate will be limited to 8 kHz (for input and output). Connecting headphones, or an adapter cable, like a stereo mini-plug to RCA adapter, will cause the headphone output to be selected and push the sample rate back up to 48 kHz.
  2. In SignalScope Pro (or SignalScope) the Alesis ProTrack appears as an input only device, so output signals are not routed to the ProTrack’s headphone out connector. The ProTrack’s headphone output does work with SignalSuite.
  3. Using a dock connector input with the original (1st generation) iPod touch appears to require that something be plugged into the headphone jack, unless the dock connector device also supports audio output (like the Macally iVoice Pro, which, ironically, is one of the few devices that is narrow enough to allow you to simultaneously plug your headphones into the bottom of the iPod).
  4. When using the original iPod touch with standard dock connector input devices, like the TuneTalk or the Tunewear device, a dock extender, like the SendStation device, will be required in order to also plug in headphones. You need to be sure your dock extender supports audio (some do not).
  5. The Griffin iTalk Pro that we tested did not work consistently–sometimes it wouldn’t be selected for input by the device. For now, the iTalk Pro is not recommended.

Frequency response measurements of these devices can be found here.

iPhone OS Audio Routes

Getting audio signals into and out of an iPhone OS device can sometimes be a bit tricky. The information presented below outlines the available means for getting audio signals into and out of each iPhone OS device.

Available Input Routes

Built-in mic Headset input Dock input
iPhone Yes(1) Yes Yes
iPhone 3G Yes Yes Yes
iPod touch No No Yes(2)
iPod touch 2G No Yes Yes
  1. The built-in microphone of the original iPhone appears to be routed through a speech-processing codec, which limits the sample rate to 8 kHz and significantly degrades the frequency response.
  2. Using a dock connector input with the original (1st generation) iPod touch appears to require that something be plugged into the headphone jack, unless the dock connector device also supports audio output (like the Macally iVoice Pro, which, ironically, is one of the few devices that is narrow enough to allow you to plug your headphones into the bottom of the iPod).

Available Output Routes

Receiver Speaker Headphones Dock
iPhone Yes(1) Yes(1) Yes Yes
iPhone 3G Yes Yes Yes Yes
iPod touch No No Yes Yes
iPod touch 2G No Yes Yes Yes
  1. Even when using the dock connector for input, if the receiver or built-in speaker is the current output device on the original iPhone, the sample rate will be limited to 8 kHz (for input and output).

Links to additional information:

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