Friday, 1 May 2015

Visualisation of bat calls on your smart phone by Dr Tom August (twitter: @TomAugust85)


In an age where you can live most of your life via your smartphone it is perhaps surprising that there is little in the way of bat survey tools. Whilst there are a number of applications in the pipe line I found it frustrating that there was no cheap way to visualise sonograms in the field using my smartphone. After all, such a task is well within the computational capacity of even the low end smartphone, and would be a valuable identification and public engagement tool in the field. There are already some products on the market designed to interface a bat detector and smartphone but these come at a significant cost. No doubt it will not be long before cheap apps and appropriate connecting cables are designed and commercialised, but in the meantime I took on the task of connecting my bat detector to my phone for as low cost as possible.
Dr Tom August (twitter: @TomAugust85)

After a couple of days I was able to create a setup that cost around £15 ($22) that connects my iPhone to my BatBox Duet. This allowed me to visualise calls in real-time, record and review calls, and even share them on social media. Here’s how.

This solution has two components, first is a cable that can take the audio out from the detector to the phone, and the second an app that can be used on the smartphone for free, or at a low cost. Whilst powerful, smartphones cannot sample audio at a high rate (only up to ~48kHz), so this solution works best with frequency division and time expansion output. Using a bat box duet as an example the first step is to take the left channel (frequency division) from the audio out, this is done using a stereo to mono splitter [1]. Next, smartphones are setup to accept microphone input through their headphone jacks, but they expect this to come from an unpowered microphone, to attenuate the power that the phone tries to send to the microphone we use an attenuation cable. We can get a cable that at the same time converts our audio connector into an audio and microphone connector suitable for plugging into a smartphone [2]. Finally we need a short connector that connects these two previous cables together [3]

Here is the summary of the cables, remember, there are likely to be many suppliers of these components and some may be better or cheaper than the ones I link to.

[1] Stereo to mono: Maplin - £2.79

[2] Attenuating cable: Ebay - – £8.99

[3] 3.5mm male coupler: Amazon - £3.04

With the audio of our detector feeding into our phone all we now need is an application to view it on. While the cable will work for a variety of phones there is currently no good app that works across platforms. It is worth having a look around to see what apps are out there but at the time of writing I would suggest Spectrum view [4] for iPhones and Spectral Audio Analyzer on Android [5]. Both are free but have a premium versions for about £5 ($7.50).



I would recommend upgrading to the premium version of both apps as the cost is minimal and the additional functionality is well worth it. So there it is, in total my setup cost £25 ($37) including postage and the premium app. Check out the video for a demo of how to put together the set up and use the two apps I mention, I cover using an iPhone, iPad (both using iOS) and a Moto G (running Android).