Which sensor to use - basic sonic tomograph for use on wood - Page 1
Forum section: electronics / projects

author: Splitlight
author_rank: Newbie
timestamp: April 06, 2023, 08:57:26 am
content: Hi all,I am a novice programmer and even more novice when it comes to electronics.I'm interested in making a basic sonic tomograph - a device which measures soundwaves going through wood.They work like this:Consumer-grade tomographs I have looked at (for measuring the inside of trees) work by hammering in nails all around the circumference of the tree.On each nail a sensor is placed, which measures sound vibrationsYou then go around the tree, tapping each sensor with a hammer. The tomograph will measure time taken from initial tap, to the time taken for the sound wave to travel to each sensorIt then processes these times, and if the time taken is longer than expected (the sound wave is expected to travel the shortest distance if the wood is healthy), then it means the sound wave has curved around a defect such as a cavity, or rotten woodThis information can be used to create a visual colour representation of healthy wood and rot/cavities.My question is - what type of sensor could be used to detect these sound waves? I have a basic arduino starter kit and ordered some piezo sensors which I'll try out. But anyone with any advice would be appreciated.I've included images of the program I wrote which shows a graphical representation of putting sensors around a tree, and the straight lines connecting each sensor are the shortest path, which you would expect sound to travel through if the wood was healthy.Thanks in advance

author: 741
author_rank: Frequent Contributor
timestamp: April 06, 2023, 09:03:38 pm
content: HiAn interesting topic...This (https://wwv.isa-arbor.com/events/schedule/resources/167/Gocke_Tomography.pdf) also mentions resistance tomography.It would be very satisfying to get good data and be able to create the sort of maps shown.In wood than I imagine the frequencies involved would be quite low, and that might eliminate piezo. I guess that implies low resolution. I can't imagine high-pitched sound getting through. Maybe an electret condensor mic ?Although a 'tap' (for which some manufacturers like 'Picus' supply an 'electronic hammer') could be like a sharp-edged test pulse then presumably only lower frequency components get through.

author: Splitlight
author_rank: Newbie
timestamp: April 08, 2023, 01:43:24 pm
content: Thanks for your reply! I'm getting the code pretty much completely done at the moment, and then I'll try experimenting with different sensors.I didn't consider wood letting certain (low) frequencies through - I'll do research into electret condensor mic'sAnd Picus is the main manufacturer I know of, I wonder if the electronic hammer is necessary to start the timer for each pulse

author: Kleinstein
author_rank: Super Contributor
timestamp: April 08, 2023, 01:52:08 pm
content: Piozos would be the obvious choice. They also work in the audio range. It may just need enough mass on the other side or a flexing construction. Those piezo disks used in beepers are really fun to play with a a sensor and they work even down to the 1 Hz and below range.A classic microphone would be mainly air sound and the transition solid to air is loosing a lot of the power.