Trees as a sound medium - Page 1
Forum section: electronics / projects

author: Whales
author_rank: Super Contributor
timestamp: September 06, 2014, 12:47:29 pm
content: My hackerspace was approached for help to make something that can play sound "through" a tree. Translation: the played sound seems many times louder with your ear against the tree than just from listening in free air.My first thoughts were to pile a speaker, cheap *podlike and amplifier into an existing hole of the tree and expanding-foam the whole unit in-place. A well-disguised copper pair could snake away to a power source. A test with coupling my wooden desk to a speaker suggests frequency response will be poor, but we don't mind as long as the effect is noticeable.Does anyone have any potential tips or advice on how to make a tree sing? All comments are welcome

author: EEVblog
author_rank: Unknown
timestamp: September 06, 2014, 01:16:11 pm
content: My first thought involved one of these old aura interactor bass shakers.

author: jeremy
author_rank: Super Contributor
timestamp: September 06, 2014, 01:19:51 pm
content: I can't really help much, but what sort of frequency range and what sort of wood? All trees are not created equal; both iron bark and balsa are classed as "hard wood" but they are very different. Also, is the tree alive? I presume so.The water content of the wood will make a big difference I think. More water = more elasticity = worse sound conduction

author: Whales
author_rank: Super Contributor
timestamp: September 06, 2014, 01:31:27 pm
content: Quote from: EEVblog on September 06, 2014, 01:16:11 pmHeavier drivers like this are useful -- short of a dimensional anchor, the tree needs to be oscillated 'relative' to some mass.Quote from: jeremy on September 06, 2014, 01:19:51 pmWe are aiming for bird-song at the moment. I expect this higher freq content is going to be absorbed at amuchhigher rate than its lower freq bretherin would be.Yes the target tree is going to be alive (and stubbornly flexible!). I have not yet visited setup location, but I plan to run some trials there to work out if this is going to be practical. It sound likes a good idea to try a few of the trees available of varying size and species to see if any work better than others.

author: Alex30
author_rank: Regular Contributor
timestamp: September 06, 2014, 02:05:51 pm
content: I think you might struggle to get high frequencies (such as bird songs) to be transmitted through the tree. Perhaps you could try something else like sensing the proximity of a person and just play the bird songs from a hidden speaker in the branches?EDITPlus putting a speaker under the tree would be quite difficult, especially without damaging its roots

author: jeremy
author_rank: Super Contributor
timestamp: September 06, 2014, 02:08:49 pm
content: Yes, not only will high frequency likely be absorbed more, it will also likely be much more directional. The reason you can put a subwoofer pointing out the back of you car is not just to show off, it's that it doesn't really matter which way it is.I am interested to see how this turns out. Perhaps this is a question to put to someone more mechanically inclined as well? A lot of mech engineers I know have studied this sort of phenomena to look for resonant frequencies of structures.

author: Whales
author_rank: Super Contributor
timestamp: September 06, 2014, 02:12:29 pm
content: Aye. I think I will have to employ some shady pitch-shifting, or consider playingpowerful owlsmore than cheerful wrens.I'll throw some photographs and results into this thread if it goes ahead.

author: Alex30
author_rank: Regular Contributor
timestamp: September 06, 2014, 02:13:47 pm
content: Quote from: jeremy on September 06, 2014, 02:08:49 pmI am a civil and structural engineering student and I have done some material studies involving wood. Untreated wood, especially if it is still alive, is quite flexible compared to the kind of wood that's used in furniture or construction as the curing process hardens it. Even wood used in construction is flexible compared to other materials, which is why it is used instead of brick to make homes earthquake proof. This is the reason you will struggle to transmit sound through it (especially high frequencies) as it absorbs energy.

author: Whales
author_rank: Super Contributor
timestamp: September 06, 2014, 02:24:12 pm
content: Quote from: Alex30 on September 06, 2014, 02:05:51 pmI was thinking about something like this as a backup plan. A variable attenuation could be done based on the proximity of people, and the setup could be hidden higher in the tree (exposed).I'm not going to be drilling my own holes in the trees or going under the roots, but instead use existing holes and gaps. The trees in question are riddled with natural holes, but opening size is limited to about 30mm diam according to proxied information. Larger will always be better -- the speaker drivers on my desk right now have 50mm as their largest dimension and I hope to be able to squeeze those in without hurting the trees.Exact specifics like this are all in the air. Regardless of whether or not they render my current approach infeasible, I hope to discover some more routes to solving the problem.

author: Alex30
author_rank: Regular Contributor
timestamp: September 06, 2014, 02:33:57 pm
content: Quote from: Whales on September 06, 2014, 02:24:12 pmWill be interested to see how you go and pleasantly surprised if you manage to get an audible sound through the woodSounds like a fun project

author: Yago
author_rank: Frequent Contributor
timestamp: September 06, 2014, 03:22:14 pm
content: How about a bit of smoke and mirrors?Make replica leaves from hard thin metal and bond on a tiny transducer.Scatter these about the tree, and power from a central birdbox.Edit:Could tune each leave to produce one tone, but more efficiently, twelve leaves gives you an octave.

author: G7PSK
author_rank: Super Contributor
timestamp: September 06, 2014, 04:22:10 pm
content: what about using a large piezo driver attached to the tree.

author: Wim_L
author_rank: Regular Contributor
timestamp: September 06, 2014, 06:43:46 pm
content: Quote from: G7PSK on September 06, 2014, 04:22:10 pmThat's one of the best things you can do.There are two problems you'll face:- Attenuation of sound as it travels through the wood. Already mentioned above.- Coupling the sound into the wood.The coupling is an impedance matching problem -- mechanical impedance. A standard speaker, with a thin membrane, is made to couple efficiently to air. You're going to need something a bit more solid to couple vibration into the tree with decent efficiency. Same reason why they don't use standard speakers in underwater sonar.

author: Marco
author_rank: Super Contributor
timestamp: September 06, 2014, 07:12:05 pm
content: Trying to make the sapwood resonate across any appreciable distance seems a dead end ... but maybe the heartwood can propagate sound better? Question is how to couple to it ... boring a hole to put a transducer in is probably not an option? Maybe ultrasonics and heterodyning? (So two offset transducers mixing at the core to create the sound which the heartwood hopefully carries.)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 06, 2014, 07:27:23 pm
content: Yes, impedance matching: you need something with some reaction mass to be able to drive much energy into it. Tree-stuff is about 1000 times denser than air, so you need a speaker that's 1000 times heavier*, with a "cone" (transducer cross-section, really) 1000 times smaller.*Heavier than necessary, that is. A normal speaker (especially when anchored into a box) is way more than heavy enough to push around what little air it does; it probably won't need to be much heavier to transmit the same frequency range into wood.The bigger question is attenuation. I have no idea how to estimate that. Flexibility, by the way, isn't a guarantee: a capacitor is 'flexible' in electric terms (the voltage can be whatever), but that's no guarantee of whether it's lossy (like a resistor: the voltage can be whatever, but the current is always proportional) or not. You can at least guess that it's not going to transmit sound waves much smaller than the average pore size (whether those pores are filled with sap or air). If the pores are 0.2 mm across and the speed of sound is ~3000 m/s (don't know if that's representative of dry or green wood), then it should be okay up to around 15kHz (give or take a factor of 2 for the effect of quarter wave interference) across the grain, and probably more lengthwise.I think your best bet would be a mass driver, something like a regular speaker with the cone cut off, driving a nail staked into the tree. Your main downside is going to be, how to install that inside the tree so it can't be heard (it will couple into the air better than the tree couples into the air, and thus be audible at a distance).Tim

author: eneuro
author_rank: Super Contributor
timestamp: September 06, 2014, 08:11:54 pm
content: Quote from: Wim_L on September 06, 2014, 06:43:46 pmMaybe something like this electromagnet, but high frequency might be a problemJust trying to use this thing withH bridgedriver to "play sound" but unpleasant to hear by this garden mole monster to keep him far away from yardGround is sound medium in my case

author: jeremy
author_rank: Super Contributor
timestamp: September 07, 2014, 04:26:53 am
content: Don't forget that you really have two acoustic matching problems, not one. There is the speaker to the wood and then the wood to the person's ear. For example, if you choose a tree with rough bark it will probably harder to get the sound out of the tree.

author: Whales
author_rank: Super Contributor
timestamp: September 07, 2014, 11:21:25 am
content: Your replies are all amazingFrequency damping in general: I agree that a tree is going to be much like a series inductor, low-pass filtering everything. This is a mostly fixed variable I'm fighting against, as modifying the trees (the urge to iron cast is strong...) is against the aim.Quote from: G7PSK on September 06, 2014, 04:22:10 pmQuote from: T3sl4co1l on September 06, 2014, 07:27:23 pmQuote from: eneuro on September 06, 2014, 08:11:54 pmHmm. Along with most of us here, I have a box overflowing with discarded wall-warts:I'm sure this has a better coil mass than most speakers of the same scale, bar me cutting apart something nice. Options I can see:Run it straight. It probably won't thrust as much as I want, but I could be wrong. It was designed for very LF (50hz) work after all.Remove the secondary side so the primary has more space to move. Might not yield anythingDissect the steel laminations and place the primary coil near some well-fixed harddrive magnets.Quote from: Yago on September 06, 2014, 03:22:14 pmThat idea seems to be branching off in the wrong direction -- perhaps a more back-to-roots approach would be more suitable? I'm going to have to leave it for now. Maybe later I'll twig to it, but until then it's just barking mad.Quote from: jeremy on September 07, 2014, 04:26:53 amIndeed. Rough and loose bark will also reduce the tree <-> air coupling, hopefully increasing the intended effect.Quote from: T3sl4co1l on September 06, 2014, 07:27:23 pmI've tried cutting most of the cone off a speaker already. Unfortunately this made itverydifficult to couple with my test surface as the paper cone held the coil in the optimal position relative to the internal magnet. More than a few mm away from this and its output is heavily diminished.I will try a few more speakers and perhaps modify the existing with a different magnet system.

author: rob77
author_rank: Super Contributor
timestamp: September 08, 2014, 07:31:52 am
content: Quote from: eneuro on September 06, 2014, 08:11:54 pmthat cute mole is not a monster! and he's definitely not after destroying your yard. he's just hunting down some other monsters living in the groundget rid of the ground insects and the mole will migrate to a different place where he can hunt for insects.the insects are chewing the roots of your plants, not the mole.. the mole is just hunting those bastards

author: eneuro
author_rank: Super Contributor
timestamp: September 08, 2014, 03:03:08 pm
content: Quote from: rob77 on September 08, 2014, 07:31:52 amThere is no plans but nice green grass in the garden between trees and... those annoying mole ground mounds each morning in the summerHopefully, I've made research how comercial devices try to keep away mole from yard, but I do not liked idea putting speakers into the ground, so this electromagnet from old low power 230VAC motor teardown should create a lot of noice in the ground, but do not tested it yet. It could be perfect power this thing using small solar panels, but old car starter battery should be fine for tests too

author: VK5RC
author_rank: Unknown
timestamp: September 09, 2014, 02:35:31 am
content: I suspect the Q of the live tree is very low, ie heavily damped, bird calls are generally relatively high in frequency, coupling straight to the air would seem like the way to go but higher frequencies are quite directional, an 'omni' pattern speaker.

author: Marco
author_rank: Super Contributor
timestamp: September 09, 2014, 01:47:48 pm
content: The heartwood isn't alive.