PDA

View Full Version : Physics of Ukuleles



sleepsinashoe
08-14-2009, 06:17 AM
I suppose this is more a question of acoustically amplified instruments in general, but either way I'm curious about the answer. How exactly does acoustic amplification work? It can't increase the power of the actual sound waves, so presumably it would focus it through the sound hole. However, a ukulele with a body would still be louder from behind (in other words, directly away from the source of focus) than a simple string attached to a wooden stick, right? All I can wrap my head around is that a wooden body would be able to modulate the sound, and maybe even absorb some of it, making the instrument quieter. Any physics minded person want to enlighten me? (I'm going to school for chemistry, how useless)

kissing
08-14-2009, 07:48 AM
actually, i've been quite curious about this myself.

One of my ukes is a Risa electric ukulele (non-acoustic), which basically is strings on a stick with no body. So when you play it without connecting it to an amplifier, you hear just the quiet tone of the strings.

It makes me appreciate at how much a soundboard + body makes so much difference!

I'm no physics expert (my field is chemistry-related too >_<), but I figure that it has a lot to do with vibrations and transfer of that energy to sound energy. The strings cause the soundboard to vibrate, which makes the air vibrate (thus we get sound). The hollow chamber of the uke body allows those vibrating waves to resonate (echo!), making it louder.

Ever wonder why everything sounds louder in an echoey place, like a large hall or a cave?

Rick Turner
08-14-2009, 08:00 AM
Acoustic instruments do not amplify the string vibration. That's a major myth and a misuse of the word "amplify". Acoustic instruments transform string vibration (which is a very poor air mover) into a form of vibration exciting plates and an internal resonator (the air inside the box) which do couple well with air to make sound waves. The electrical analog of this would be a transformer, not an amplifier which uses a small signal from a pickup or mic to control a larger outside force...electricity.

DaveVisi
08-14-2009, 08:00 AM
I think kissing (and Rick, we both posted at the same time) pretty much explained it. The back and sides of the Uke are usually a much harder wood that doesn't resonate (very much, anyway) but instead reflects and bounces the sound waves inside the acoustic chamber of the body and reinforces the vibrations of the top (if the waves are "in phase" which brings in the science of acoustic resonance) as well as directing all the sound out the sound hole instead of simply letting it scatter in all directions.

kissing
08-14-2009, 08:21 AM
The back and sides of the Uke are usually a much harder wood that doesn't resonate (very much, anyway) but instead reflects and bounces the sound waves inside the acoustic chamber of the body and reinforces the vibrations of the top (if the waves are "in phase" which brings in the science of acoustic resonance) as well as directing all the sound out the sound hole instead of simply letting it scatter in all directions.

I find this part very interesting.
Because you have "All-solid" instruments where the backs and sides are (apparently) made from the same solid wood as the top. And these are generally considered to have the best sound. But the sides and back are the same (?) as the top, not harder.

And then you have the "Solid-top" instruments, where the top is solid, but the sides and back are laminate (harder, less resonance). These instruments are generally less expensive than a similar quality "All Solid" instrument.

So according to your reasoning, the "Solid tops" should be better at projecting sound than the generally more expensive "All-Solids"?

Russ Sonny Kemner
08-14-2009, 08:29 AM
Think of it this way. When you throw a stone onto the surface of a calm pond, the water flows out in waves around the spot where the stone hit the surface.

In very simplified terms, the bridge of your instrument is the stone, the sound board or top of your instrument is the surface of the pond, and the sound cavity or inside of the instrument body is the water. When you pluck or strum the strings your are transfering the mechanical engergy of the vibrating string to the soundboard through the bridge and saddle -like throwing the stone. When the sound board responds by vibrating, it creates soundwaves both within the instrument and oustide the instrument, similiar to the surface of the pond. The air inside the body of the instrument responds differently because it is contained , less air to absorb the motion, thus greater air movement and bigger sound.

Sound waves are directed out of the instrument via the sound hole(s) and picked up by the diaphram inside the holes on the side of your head - which transform the moving air (sound wave) to an electrical signal, which in turn is sent to the organic central processing unit inside your head where you recognize it as a note or pitch.

Whoa.... I started to get a little too deep there. Anyway, hope that helps explain it.

DaveVisi
08-14-2009, 08:45 AM
I find this part very interesting.
Because you have "All-solid" instruments where the backs and sides are (apparently) made from the same solid wood as the top. And these are generally considered to have the best sound. But the sides and back are the same (?) as the top, not harder.

And then you have the "Solid-top" instruments, where the top is solid, but the sides and back are laminate (harder, less resonance). These instruments are generally less expensive than a similar quality "All Solid" instrument.

So according to your reasoning, the "Solid tops" should be better at projecting sound than the generally more expensive "All-Solids"?

I think the small size of Ukuleles tends to muddy the works somewhat. Typically in the guitar world, the top is more flexible, usually being Spruce or Cedar with sides and back made from a harder, more "reflective" wood like Rosewood. You don't want the back to vibrate as it would essentially "subtract" from the available sound waves and you'd lose some volume out of the back of the instrument and lost in your body.

Each laminate is different, but I don't think you can generalize that all laminates are harder or less resonant. If it was a rosewood veneer over a softer core wood, the result would be worse, not better than solid wood. Plus you have bracing to contend with. You want a rigid body but still want the top to vibrate as freely as possible. This is easier with small instruments than it is with large spans of wood like you see in a guitar.

Similar to MGM's recent experiment where listeners couldn't really identify solid vs. laminate by sound, the same holds true for guitars. A solid top/laminate back and sides can sound just as good as an all solid wood. Some classical guitar makers do this on purpose to create the sound they're looking for. One builder lines the body (i.e. "laminates" it) with Cypress, to brighten the sound of his instruments, most likely for the tastes of Flamenco players who want a more "lively" sound.

Ukes and Mountain Dulcimers are two instruments where top wood choice seems to defy logic. Perhaps it's simply because the small size and limited vibration of the top doesn't significantly add to the overall "sound" of the instrument, and harder woods sound better in this case. That would explain MGM's results where people typically couldn't guess laminate vs. solid tops, and in some cases their choice for the best sound came from one of the cheapest instruments.

It seems to boil down to a matter of taste, and what sounds good to "you."

DaveVisi
08-14-2009, 08:50 AM
Using Russ's example, all those waves traveling away from you and dissipating off in the distance are waves you'll never hear. If you drop a pebble near the shore and watch the resultant wave "bounce" off the shore and back in the other direction, that's what happens inside the sound box. You want those acoustic "waves" not to get lost, but to get bounced out the front and add to the waves already there so we can hear them better.

RonS
08-14-2009, 09:20 AM
Great post

Its making me rethink some of the things I was planning on doing on my first build.

Thanks

ihavenotea
08-14-2009, 10:13 AM
In very simplified terms, the bridge of your instrument is the stone, the sound board or top of your instrument is the surface of the pond, and the sound cavity or inside of the instrument body is the water. When you pluck or strum the strings your are transfering the mechanical engergy of the vibrating string to the soundboard through the bridge and saddle -like throwing the stone. []


This is why the Dolphin Bridge Makalas are so Magical!

Russ Sonny Kemner
08-14-2009, 12:39 PM
I'm not sure if it makes a difference or not, but I always try to fine sand the inside of the instrument and use a light sealer on the inside surfaces to get better sound wave reflection, plus it makes the inside look a little better and helps preserve the wood.

RonS
08-14-2009, 12:59 PM
Let me ask you this and please tell me if my thinking is correct.

Since the sides and back should reflect the sound and not vibrate, it really shouldn't matter how thick they are.

Can I make the sides 1/8" to 3/16" thick if bending would not be a concern?
For that matter, can I make the sides 1/4" thick"

DaveVisi
08-14-2009, 01:16 PM
Sure, why not? If you can bend it, it should work. Or, do what some hollowbody electric makers do and route out the body from a single piece of wood.

Rick Turner
08-14-2009, 03:54 PM
You guys are headed off into deep misunderstandings here.

It all matters. Every piece of wood matters. The thickness of every piece of wood matters. Sure, you can do whatever you want...and it won't sound like it would have if you did it differently.

Build some ukes this way, build some that way, and decide for yourselves what works. All this talk is just keeping you from making sawdust. Just don't imagine that you can build a uke like nobody else does and hit a magic formula for tone. There is absolutely no substitute for building, building, and then building some more. You've got to make at least a couple of dozen instruments to have a clue as to how to develop your own valid style...your own voice. I've built or participated in building nearly 4,000 instruments now, and I still am learning new things every day.

RonS
08-14-2009, 05:36 PM
Rick you are 100&#37; correct. I agree with everything you say.

If I where to build 1 ukulele per month, in 10 years I would have only completed 120, and my wife would hate me for having to dust them. Even after all that I still would not have your experience.

In building ukes (or anything) there are absolutes that need to be followed. And that is what is what I'm seeking and what this thread is all about.

We learn from our own mistakes by doing. We learn from other people's mistakes by asking. Both are equally important. Otherwise we would still be reinventing the wheel.

I would appreciate any insight that you may have.

Thank you.

DeG
08-14-2009, 05:39 PM
Here is some intresting stuff:

http://ffden-2.phys.uaf.edu/211.web.stuff/billington/main.htm

clayton56
08-14-2009, 05:53 PM
I don't mean to go off on a tangent, but I have been wondering why soprano ukes sound so good with such a small soundboard. One of the things that attracts me to ukes is the sound is cleaner than the same notes on guitar.

I seem to remember that when speaker cones vibrate, only the center vibrates on the high notes and frequencies, and the rest of it only vibrates when there are low notes.

Is that true for guitars, too? If you play notes in the ukulele range on guitar, does only the center of the soundboard vibrate?

Anyway, I've noticed that while ukuleles seem so quiet, when I record something, the mics pick it up at the same recording level as I use for guitars and banjos.

DaveVisi
08-17-2009, 07:48 PM
Read up on "Chladni Patterns"

Our instruments aren't cone shaped, so the vibration patterns are different (much different).

Here's an example of these patterns at work on a guitar shaped soundboard.
http://www.phys.unsw.edu.au/~jw/guitar/patterns_engl.html

This page explains what you're looking at
http://www.phys.unsw.edu.au/~jw/guitar/guitarchladni_engl.html