An idea for highly directional and loud loudspeakers Answered
I am currently playing around with vibration experiments.
Mainly in the ultrasonic range though.
But when messing around with some vibration speakers I found a not so well documented misuse for them
Everyone wants a big TV these days but once you have it the sound often turns out to come from a tin can.
Those with a good entertainment or at least stereo system won't mind hooking the to it.
The rest usually opts for a sound bar or how I like to call them shredderbox.
Sooner or later they just fail to produce the sound you had on the first day - or they cost a small fortune.
What is the secret to a powerful speaker?
Secondly the design.
You need the right material to produce a more or less even reaction at all intendet frequencies.
The design makes sure those frequencies that need extra attention get more volume output, like by using a little pipe for the low frequencies.
Some even include a dedicate speaker for these low volumes.
What if you could just build you own soundbar on a budget?
If you have a failing soundbar with the actual speakers as the problem you could salvage the electronics.
In case you can make do with headphone input or RCA connections than any cheap amplifier will do.
Leaves the speakers...
Vibration speakers are still underestimated for their uses...
I tried the the usual approach of using a glue on vibro speaker:
Place it on surface tha gives a re more or less decent sound.
The thing is though that there is no ideal surface for them.
A table can sound like the highs are missing, a hardwood desk might not produce any low frequencies while a window or plaster board wall bring the problem of wiring and vibrations.
To check the reactions of sound on different media I, one day, mounted one speaker under a big tin can.
Right in the center.
The fun hit me when my tests with water were over and tried to play a song with the empty can.
Of course there was some degree of tin can sound but the directional qualities together with the wide frequency spectrum made me experiment.
The key is to find the right material and shape!
You want something that is hard enough to vibrate properly but soft enough to allow for lower frequencies.
I tried pipes, boxes, old plastic containers....
But nothing seemed to provide a broad and even sound spectrum without distortions.
If you vibrate a surface then only at certain, resonant frequencies destinct patterns will form if some dust or similar is place on the surface.
In all other cases there is only chaos.
Preventing the harmonics to form prevents harmonic vibrations to build up to distortion levels.
Like it or not but waste seems to work just fine as a speaker ;)
I made a plug to fit reall tight into a 2 liter juice bottle neck.
The original plastic cap is just too soft.
A vibro speaker glued onto the plug and the bottom of the bottle cut out and the soun was quite impressive.
Proper use is however limited as the speaker would need to be mounted upright.
Next thought was to utilise the bottom of the bottle too.
By cutting a round hole in the side of the bottle I got an even more directional speaker with a better response to low frequencies.
Placement of said hole of course affect how certain frequencies travel and where nodes can form.
The size might also matter as the hoe itself, or better the material around it can get resonant at certain frequencies.
You can cheat at bit though by using sticky tape, duct tape and so on as dampening meterial in badly affected areas.
Especially with a wall mount for the speaker and the bottle hanging down behind the TV the effect is good compared to a standard shredderbox.
The above design is certainly not for everyone although I think it has a wow factor to it if you show a decent sound coming out of a juice bottle ;)
My next step was trying to find out how more fancy designs could work.
So just stop reading here unless you like the idea of creating speakers that should not even work.
Back in the old days we had more than just the speakers with magnets and cones.
Anything that can vibrate can produce sound.
It all depends on how much of it we can actually hear.
My first exotic idea to really misuse a vibro speaker is a vibrating harp.
Does not need to be in the classic shape though.
A string in a resonance box, like a harp, guitar, violine and so on will start to vibrate at the set note.
This is true even if the force for the vibration is external.
You might remember the old school experiment with the two pitch forks on their boxes?
Hit one and the other starts to swing too.
If you make a resonance box for a vibro speaker with internal or external wire strings at different tensions or lengths you can amplify the sound for the notes that correspond to the strings.
Make a relatively large box with some sturdy wires for the frequencies between 60 and 100Hz and you have a really powerful subwoofer from just 20W of input power...
I think you get the idea on how to use amplifying strings now ;)
My second and total misuse is the xmas tree.
If you are a sparky by old trades then you might remember the mechanical frequency meters for generators or other things that required a stable supply.
Well, if not than you should know the little wind up toys that play a melody with a drum and tiny forks.
Imagine you would replace the glue plate with a rod.
Depending on material and length harmonic nodes will form at various places and frequencies.
Between those nodes the swing is maximised while the node itself appear to be stationary and without and vibrations at all.
For example a steel rod of 1.2m would have a single standing wave at about 1kHz while a copper rod of the same lenght will be much lower in the frequency for a single standing wave.
The xmas tree assumes that the rod is of such material that no single standing wave can form below 200Hz and that it won't swing too much at other resonant frequencies.
Since aluminium is easy to work with and available in flat and thin bars already it would be my first choice for the branches of the tree.
The required length is calculate based on the speed of sound in aluminium - you find online calculators for that.
You want the lenght so that you end up with an even fraction of the wavelength you want to "play" with that strip.
This allows for the rod to be placed right in the center of the strip where the standing wave movement is zero in the node.
Make a lot of thin strips to get a broad frequency response.
Placement on the rod can now be crucial.
The best option I found so far is using a threaded rod and tapping the holes in the strips.
The strips swing quite violently if long and at certain frequencies.
And those not in resonance will still transfer their momentum to other strips.
This can cause unwanted harmonics.
Most evident when a single strips swigns violently at a certain frequencies.
Adjusting the angle by turning it is often enough to get out of the overlapping harmonics.
With enough strips it then really looks like a tree with flat branches.
The sound might not be as loud and impressive as a plastic bottle as the virations are going up and down.
But if placed in the right spot it not just looks nice but also makes people wonder where the sound is coming from.
Last but not least my yet to be tested hidden speaker system - due to renting restrictions :(
If you own a house of the standard frame design then you have plasterboard walls and ceilings.
With a large enough surface of the right material, one or two vibro speakers can cover a really wide frequency range at good volume levels.
So far I could only do tests in an old wooden window frame but the priciple works the same way between the wooden frames of a wall or ceiling.
In my experiments a standard plasterboard sheet needs to be 100 x 100cm to get a more or less decent response for the lower frequencies.
Before you rip your walls down use a stud finder and place your vibro speaker on the plasterboard between two studs or beams.
I found that two speakers for the lower frequencies and three or four for the higher ones make a good sound.
That is per channel and if you have the right size plasterboard sheet in the right place.
No point if your left side is further away from the TV than your right.
Ideally you replace the entire sheet with the speaker in the right spot on the new sheet but on the back.
For obvious reasons this is far from being a perfect solution.
But if you plan a full renovation anyway...
A way out for older houses is the wooden floorboards.
They make excellent resonators for low frequencies.
The directional speakers could then still be hidden in picture frames of the right thickness and design.
Anyways, I hope I gave you some ideas here ;)