I did the same experiment but used test tones played back from my PC and the response seemed to stop at 15 to 16KHz. I suspect the reason was not my test tones or my system, but a limitation of the iPhone microphone.
Frequency response while streaming stops at 15kHz- did I miss something in the fine print?
I am streaming with Qobuz. It sounds great playing both CD quality and hi-rez music. So what's the trouble? While working to get the bass response in my room just right I found test tracks on Qobuz. This was very handy as I played 30-60 Hz test tones over and over. So when I was done with that I streamed a pink noise track and pulled up the FFT analyzer app on my iPhone. Surprisingly, I see the trace drop straight down to 0 dB above 15 kHz. I streamed some test tones and confirmed that above 15 kHz nothing is happening at the speakers. In my old and decrepit state I can no longer hear above 15 kHz so I am relying on the app. In a panic that something is wrong with my speakers I pull out my vinyl test record. I have a nice, pretty flat frequency response from 20-20 kHz showing on the FFT app. I put in a test CD and again show a good 20-20k. Lastly, I ripped the test CD onto my server and again had a good 20-20k response. So why did I not know about the limited bandwidth while streaming? Is it just me? It is an abrupt cutoff above 15 kHz while streaming. What's going on?