Play audio using Audio Queue

I have a question in playing audio.

When I played a 18kHz pure-tone with iphone6, I heard unnatural sound on lower frequency parts(below 12kHz).

But I used iphone6S to played the same 18kHz pure-tone, I heard pure sound on 18kHz. Please refer to the link (*.PNG).

I want to know how to solve this problem on iphone6.

iPhone 6's photo:

https://app.box.com/s/1keuutd653ihqj1jxl35yn0k4krvnt4h

iPhone 6s's photo:

https://app.box.com/s/sw14l2utuucr7xr4buy1wu3jy33ihbla

Thanks!