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How good is iPhone as an SLM?

Posted on 24 Sep 2013 in NoiSee | 0 comments

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It might not be the most obvious thing to ponder since the world of measurements and consumer electronics seem to be pretty well separated. On the other hand however, we are forgetting that we use regular desktop consumer computers for professional measurements more times than not. It was a natural transition since consumer computers have gotten increasingly reliable and capable. The aforementioned perspective makes the whole argument and thought more intriguing and interesting. So, we decided to perform necessary measurements to record the frequency and amplitude characteristics.

At first glance it all seems pretty trivial and easy. But only to a degree of estimation measurements. The real measurements are made more difficult since iPhone is not intended for professional measurements and consequently microphone shape does not comply to standardized one. All this leads into a need to use the silent (anechoic) room method of measurements. This method works in a way that the iPhone is placed into the silent room and then sound source is added to produce a special sound field called free-field. It is a field that we would encounter if we were outside where there are no reflections and no walls.

Silent rooms are a standard in the field of measuring sound power and we have them in Slovenia also. A typical silent room costs multiple ten thousands of euros to make and then also expensive to maintain. Luckily the partner company Brüel & Kjær developed a solution that is cheaper and is called silent chamber. The main difference is size – in all aspects. Since the chamber is smaller than the room, it limits the dimensions of the tested object as well. But for our purpose it means a professional laboratory environment that accommodates the iPhone perfectly.

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Measurement accuracy was ensured with a state-of-the-art measurement equipment in the field of electro-acoustics. Analysis was performed with input/output analyzer Brüel & Kjær type 3160 that easily fulfilled all the measurement requirements. Excitation signal was fed from the analyzer to a power amplifier Brüel & Kjær type 2706 from where it went directly to the built-in speaker of the anechoic chamber. Sound field in the chamber was maintained through a backward-loop consisting of a measurement microphone Brüel & Kjær type 4133.

Set-up created and maintained a steady and well defined sound field within the chamber to which two iPhones were exposed – the iPhone 4 and 4s.

The results were interesting and surprising at the same time. The dynamics of the iPhone was pretty impressive, but linearity and repeatability were far beyond our expectations. Good characteristics and all the practical conclusions we were able to draw from these measurements gave us an idea to develop an iPhone application that works just a sound level meter with all the important settings and functions.

Click here to learn more about NoiSee: http://ims.si/noisee/.

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