iPhone high-pass filter and tuning curve information for my Ph.D.
I'm working on my Ph.D. studying the bioacoustics of cicada. The recordings I've obtained have been taken from a wide variety of equipment, both professional grade to smartphones. The smartphones are proving to be a bit of an issue though in understanding their capabilities to get a flat frequency response from the microphone.
The main models are iPhones 5 and 6, though 8 and 12s have also been used. The only real information on frequency response I have been able to find was from a study done by Faber on early models.
The cicadas I work with generally call from 3000-14000Hz, with the average being around 8000-11000Hz. I am trying to find whether or not there is current information regarding the later models: 5, 6, etc.
More troubling is the rumor I've heard that iPhones use an internal high-pass filter to pre-process recordings when they are made. I need to know if this is true and if possible what the filter is set at.
Thank you very much,
You can contact me directly too at firstname.lastname@example.org
The microphones built into the iPhone will typically have a fairly flat frequency response. However, there are various issues to consider when understanding what you have actually captured with the various iPhone models you mentioned.
Much depends on the software used to make the recordings. iOS offers a "measurement mode" which apps can enable in order to avoid system-provided signal processing, such as the notorious high-pass filter you have heard about, as well as auto-gain control (fatal for accurate sound level measurements). If the app(s) you used for the recordings don't employ measurement mode, you may need to do some additional work to understand what effects the software (including the operating system) had on the recordings.
To get a better handle on the aforementioned high-pass filter, you can check out this article from our blog:
Finally! iOS 6 kills the filter on headset and mic inputs!
To learn more about using iPhone for acoustical measurement, you may be interested in the article I wrote for Acoustics Today magazine:
Acoustical Measurements with Smartphones: Possibilities and Limitations
Hopefully, in the not-too-distant future, we will publish more measurements on iPhone microphone characteristics.