Calibration

In developing the Decibel app, Gadget Frontier has made sure that the sound level meter is calibrated against professional verified calibration equipment. The microphone of the iPhone 3G was found to be remarkably flat in the response and the dynamic response good. Since the calibration was performed in a quiet room in a normal house, and not in a laboratory, the lower calibrated limit is about 30 dB. The higher calibrated limit is 110 dB, which is also the maximum sound pressure level that the iPhone 3G can read.

The iPhone 2G has the roughly the same quality of the headset microphone, so if the headset is used on the iPhone 2G, the sound level readings are as accurate as with the iPhone 3G.

The Gadget Frontier Decibel App has been carefully calibrated against a fully professional calibration system consisting of

• Brüel & Kjaer 2260 Investigator calibrated by an official external body (SP)

• Calibrated Brüel & Kjaer sound source HP 1001

• Calibrated Brüel & Kjaer sound power source type 4205

Basic rules of sound measurement

When measuring sound levels some basic rules has to be applied in order to get a valid reading.

If the sound is a direct sound, that means, there are no obstacles between the sound source and the receiver, it is very important that the microphone is directed towards the source. Decibel flips the screen when the phone is turned towards the sound source so one always can direct the microphone towards the source and still read the levels conveniently.

When measuring sound pressure levels in dB, the distance between the measurement device and the source is imperative. If the microphone is held too close to the source, for example whistling or screaming into the microphone at 2.5 cm (1 inch), only the nearfield is measured. A nearfield sound measurements and cannot be recorded with a simple sound level meter. One always measures the farfield sound level, at minimum 1 meter (3 feet) distance. This distance is kept to make sure that all nearfield effects have died out before a measurement is started.

Sound source Nearfield values @ 2.5 cm (1 inch) Farfield values @ 1 meter (3 feet)

Whistle 115 dB 70 dB

Scream 125 dB 78 dB