Part 1: Research
The iPhone XR has three microphones: one in the front at the top of the touch screen, one on the rear side of the phone near the back camera, and a bottom microphone near the edge of the bottom speaker. It primarily uses the bottom microphone when recording.
The maximum sample rate is 48 kHz and bit depth is 32-bit. After researching different voice recording apps for the iPhone, I settled on Voice Record Pro. It has 9.9K reviews, and almost 5 stars, so I was hoping that that meant this is a good app to use. It allows the user to record voice memos and on-site sounds at unlimited length with configurable quality, and we can even export sounds to import from Google Drive. It can also record directly into the .WAV format, which was the requirement of this assignment. Not to mention, it’s free!
Other apps I researched include Voice Memos (of course, since it was already downloaded into our iPhones from the beginning) but I was quick to rule this out because I am unable to record in a .wav format. Another high contender was “Awesome Voice Recorder” which was said to be best for music industry professionals, and also supports .WAV format. However, I think the Voice Record Pro would have been easier to use, and I was able to set its gain control to 0 which is a requirement of the assignment.
Part 2: Data and Recording
I chose to use my MacBook Pro laptop speakers to generate the sine sweep and white noise from Audacity. I chose my independent variable, or the variable that I change, to be the placement of the iPhone XR’s bottom microphone in relation to the laptop speaker. I placed the iPhone next to the laptop speakers, which are located on either side of the keyboard, two ways: with the iPhone bottom microphone next to the laptop speaker or with it flipped around on the other side.
spectrum.txt — the sine sweep generated by Audacity’s .txt file.
For the sine sweep, as I expected, the audio recording was more stable for the iPhone microphone being right next to the speaker in contrast to it facing the opposite direction. Right at around 3000 Hz for the iPhone mic next to the speaker, the dB drops and becomes a lot bumpier than the clean sine sweep of the original audio. However, with the iPhone mic facing away in the opposite direction, the audio grab is rocky from the start and features many bumps. It start being irregular from around 300 Hz itself.
For the white noise, interestingly enough the original generation itself is supposed to be rocky in comparison to the sine sweep. Somehow, the dB in the iPhone mic seemed to be featuring a higher number than the original, but we can see the audio taper down at the end in the higher frequencies for both iPhone grabs. Again, the microphone away from the speaker is bumpier.
In terms of spectral flatness, I think that the iPhone mic, when optimized to be placed right next to a speaker or source of sound, is pretty good at having a flat response with lower frequencies up until 3000 Hz and really high frequencies. For white noise, it has more shaped response.
I will link the Google Document that has the original tables rather than screenshots here: https://docs.google.com/document/d/17ApBtmifboUHYZuteERxK-Ud8an20-QOCtMtIAt63r4/edit?usp=sharing.
The folder containing the four iPhone recorded .WAV files can be found here: https://drive.google.com/drive/folders/1CGA7ifcNeyNqp98-VYT8QKxWwDsMxCAk?usp=sharing.
What I would do differently:
I think I was really apprehensive at first to turn the laptop speaker all the way up since I was in my dorm room with suitemates who, if they could hear me playing the sine sweep, would probably be very angry with me. I tried to play it off like an ambulance, however. I would also try different variables of location, like what it would sound like in a practice room and what the audio recording’s data would look like, since my room has some ambient noise as my window faces the street. The window, however, was closed, and all my fans were off in the making of these recordings.
Citations:
https://discussions.apple.com/content/attachment/8db352cc-b4df-461b-9e14-94d0e5629571
https://apps.apple.com/us/app/voice-record-pro/id546983235
https://www.howtoisolve.com/where-is-the-microphone-on-iphone-xs-max-iphone-xs-iphone-xr-location/
Hey Anjali! I thought it was really cool you tried two different placements of your iPhone. Kudos. It’s helpful to know that the same intuitions about placing your voice/instrument/etc. close and centered over, say, a SM58 probably hold true for most iPhones. I also thought it was interesting that compared to the iPhone SE, which I found had a pretty shaped response, the iPhone XR (a higher-end model) had a flatter response.