Laser ablation acoustic measurement notes

We did a many tests and measurements on the sound emissions from LIBS measurement. Sound is useful in calibrating the LIBS focus, as in loudest bang means best focus. But it can also be used to identify the sample material. Just by naked ear listening different materials make different sound in LIBS.

Why different materials make different laser ablation sound is by two main effects. First how strongly the material absorbs the laser light determines the intensity or temperature of the plasma spark and as such the volume of the explosion. Secondly when measuring the same spot multiple times, which happened a lot when we were using 7KHz laser, how quick the laser digs into the material to make a deeper crater determines how much sound dampens.

As a rule of thumb, light color minerals make less bang i.e. less intense LIBS and metallic materials make the loudest bang and most intense LIBS.

So the sound carries information about the samples. But if you are already the collecting spectrum information there's not much extra value in recording the sounds as the optical spectrum carries so much more information.

We entertained the idea of an instrument that uses only the laser ablation sounds to do determine materials. It would work in specific circumstances, such as differentiating metallic ore sulphides from other rocks from the loudness of the laser spark. But the information it gives is very poor compared to actual LIBS.

It's been shown by xx that material compactness affects the sound.

Note: this text is mostly old ideas from 2018-2019. There's been new work on this so I'm working on updating this to our current understanding

Note2: we have made one patent Kangas, L. and Leveinen, J. (2022) ‘ESTIMATING THE CONSUMPTION OF CRUSHING ENERGY USED FOR CRUSHING ROCK MATERIAL’, available at https://patents.google.com/patent/FI129558B/en?oq=fi+129558+B somewhat related to this

A novel method presented in this text is the use of sound made by the LIBS spark as another classifier in addition to the light spectrum.

Among with the light spectrum we also measure the sound produced by the LIBS spark.

Research hypotheses dropped from my master's thesis from 2018-2019

• Hypothesis 2: LIBS sound can be used for classifying minerals.

• Hypothesis 2a: LIBS sound intensity can be used for classifying minerals

• Hypothesis 2b: LIBS sound pitch can be used for classifying minerals

• Hypothesis 2c: LIBS sound spectrum(timbre) can be used for classifying minerals

background/methods:

Mineral detection from spark sound

The intense heating of the target causes a plasma spark producing not only light but also a loud sound. The sound is audibly different for different materials. Changes in the intensity of the sound can be heard easily, but more analytical methods are required to find and verify other changes in the sound. In this work we look into how it changes and does other features of the sound depend on the material. We try to prove or disprove the research hypotheses 2,2a,2b and 2c. This chapter gives a terse overview on the possible methods for analysing the sounds and section [sectionresultsound] presents the results obtained.

Here we do not go deep into the theory of sound, but we take a look into the possible useful methods for analysing and differentiating the different sounds caused by different materials. The recorded sound is a variable changing with time which we analyse with signal processing methods, particularly time series analysis.

Table of Spectral analysis methods:

• fourier transform, fft

• periodogram (power spectral density)

• smoothed periodogram

• Thomson’s multitaper power spectral density (MTM multitaper method)

• Bartlett's periodogram

• Welch’s power spectral density estimate (Welch's periodogram)

results:

LIBS sound measurement

The sound was recorded in the microphone setup described in section [microphone_setup]. The recording was started a little time before the laser was turned on and stopped after each measurement series.

Figures [figuresound1] show examples The the sound recorded. Figure [figuresoundsingle] shows a spectrum from a single point. Figure [figuresoundsingleperiodogram] is a periodogram made from the same signal.

The 2 active and 1 passive cycle setup can be seen from the sound signal.

Classifying materials by LIBS sound

Different minerals produce a different sound when hit by a laser beam. The sound intensity changes. How do other properties of the sound change?

Example sound waves Sample 1: Recorded sound fig_sound1

In frequency domain: (FFT) Sample 1: Frequency spectrum of recorded sound fig_soundfft1

Frequency domain by time-window Sample 1: window frequency spectrums (wavelet?) of recorded sound fig_soundfftwindow1

Sound

Along with light, the plasma also produces sound. The sound can be quite loud.

There's harmonics? Check where they come from

"The Fourier theorem states that any periodic waveform can be approximated as closely as desired as the sum of a series of sine waves with frequencies in a harmonic series and at specific phase relationships to each other."

Sound recording

In LIBS-LIDAR:

To record the sound produced by LIBS spark we use a directed microphone(Røde NTG-2) together with a mixer(Behringer Euroback UB802) connected to a computer. The microphone is directed towards the target (from roughly the same distance as the other setup?).

In LASOLIBS:

TODO:update

We could use the acquired spectra and sound to try find out more specifically the minerals in the target and to find out mineral properties like hardness of the rock.