We took the reading by placing the sound level meter directly under the Lightning port and by placing the meter midway along the side of the device. Getting an accurate sound level on the iPhone 7 and 8 is a little tricky as one speaker is positioned at the bottom of the device and the other is in the earpiece.
Take a look at the video below. A reading 10 dBA higher is about 2x as loud.
Please share if you like.