[Interview] Julian Bunn, California (Audio Engineer, Software Developer, Physics Scientist)

Julian Bunn is a physicist by training with a B.Sc. in Physics and a Ph.D. in Particle Physics. He previously worked at CERN in Geneva, but has been at Caltech for approximately 25 years. At Caltech, he initially worked on data and networks for CERN’s CMS experiment, but later shifted his focus to sensor networks, with a particular interest in the use of sensors for disaster detection and response. Currently, his main area of work is in computational seismology. Julian had an early interest in audio, which started when he built radios and loudspeakers. His AudioTool app was a result of this interest, combining his passions for audio, loudspeakers, and programming. At some point, he contracted with a company called Ivie Technologies and wrote the software for their IE33/35/45 handheld acoustic measurement devices.

Many engineers and scientists have acoustics and music as a hobby. Is dealing with acoustics easier than doing science, and would you get bored being an acoustic engineer only?

I would say acoustics is very much science, as it is based on the properties of sound waves and their interactions with surfaces. As a discipline, I don’t think acoustics is any easier or harder than others – it certainly is far from boring.

Your most known app in Google Play is “AudioTool” which is basically an audio analysis app. But you also have an app called “signal generator”. Are smartphones capable of producing various signals? output is probably routed through smartphones DAC which probably has some limitations?

AudioTool has a signal generator built in, which is essentially a copy of the functionality of the Signal Generator app, but with some added tones like linear and log sweeps. Smartphones are well capable of producing audio signals – that is, after all, how we are able to listen to music on them! The DACs in smartphones can be quite good – it depends on the device. My suspicion is that some are equally as good (based on the same chips) as so-called high end DACs.

What are the input limitations in smartphones, ADC converters are probably Delta Sigma, meaning that low frequency signals cannot be captured. Do you have any idea what the cutoff frequency of a smartphone ADC could be? Also what is the sampling rate and bit depth in smartphones available today?

The performance obviously depends on the device, the hardware it’s using, and the firmware that the manufacturer uses to program the ADC. The Android SDK allows the programmer to specify a “measurement mic”, which is supposed to result in a bypass of any filtering or AGC etc. on the incoming audio signals. Using the measurement mic samples allows acoustic signal processing across the full audible spectrum, at least in principle.

Probably all modern smartphones have MEMS microphones implemented today. Do you see any significant difference from ECM microphones apart from greater sensitivity? Do you envision that microphone engineering will progress so much in the future that it could be used in applications way beyond acoustics?

My understanding is that smartphones are typically using small electret microphone capsules. Some MEMS accelerometers are sensitive at audio frequencies, although they tend to be designed to measure lower frequencies than audio. I am not qualified to predict microphone futures ????

You also have an app called impulse response which can be used as a simple room measurement device interesting to audio fields and live sound engineers and surprisingly it can also be used to analyze audio equipment directly (with some proper impedance matching obviously). But since impulse signals are impossible to recreate in real life physics and especially with the smartphones DACs, how did you actually accomplish that?

Yes, the Impulse Response app uses a special sort of noise signal called MLS to derive the impulse response of a space. The MLS, which sounds like White Noise, has the interesting property that if it is convolved with itself it results in a delta pulse. If convolved with the signal after it’s been recorded in space, you get the impulse response. The impulse response of a room or a piece of equipment tells you how well it handles different frequencies.

Do you think that pink noise audio tuning systems that are commonly used in live music could also be developed on an Android or iOS device?

AudioTool includes White and Pink noise generators. Personally, when I am measuring loudspeakers, I use the Pink noise signal as it is flat when octave binned. It’s easy to see when something is flat 🙂 So AudioTool actually does this already.

You also did a lot of work in seismology. The last earthquake in Turkey one day ago was predicted by a scientist doing planetary tracking. But with advancements of microphones and vibration sensors and especially with AI big data management do you think we would be able to do those predictions better with underground vibration tracking? How is acoustics used today in seismology?

It’s a basic tenet of the field of seismology that earthquakes cannot be predicted. What you can do is to detect an earthquake happening at one place, and then warn people further away that the earthquake waves are traveling their way. This early warning technology is in use in many countries, notably Japan, Mexico and the West Coast of the USA. Various instruments can be used to detect earthquakes, including accelerometers (MEMS and traditional) and other devices that detect movements of the ground, like GPS. One new type of detector uses fibre optic cable – the flexure of the fibre when the ground shakes can be measured by optical pulse reflections. These fibre optic measurements are very sensitive and a rich source of (massive amounts of) real time shaking data.

Where can we follow your work and download your apps, and are you maybe available on some social networks or forums?

The AudioTool and Impulse Response apps are available on the Google Play Store. There is an iOS version of AudioTool for iPhone/iPad, called iAudioTool, in Apple’s App Store, but it is not quite as advanced as the Android version. Feel free to drop me a line on LinkedIn!

play.google.com/store/apps/details?id=com.julian.apps.AudioTool
play.google.com/store/apps/details?id=com.julian.apps.ImpulseResponse