What is the function of the body?

[billhay4]

I've been mulling this over in the quiet moments before I fall asleep.
It occurred to me that a harp doesn't have a sound board or sound box, yet it still makes beautiful music. So, what exactly are the functions of a sound board and sound box? Do they focus the sound, add certain qualities to it, amplify it, or something else?
That being asked, I'm led to a bigger question: what exactly is sound? I assume it's vibrations that are at a frequency the human ear detects and converts to certain signals to the brain. But is it just vibrations of the air? or of something else (the ether? disturbances in the space/time continuum? what?) I know sound travels through other mediums besides air, so what exactly has to vibrate to create sound? I'm being a bit silly here but I would like a concise definition of sound from someone who understands this better than I do.
For now, however, I am going on the assumption that my definition of sound is more or less correct which leads me back to my original question: how does the mandolin shape, size, construction, composition produce the particular sounds we associate with the instrument? Can they be produced another way?
Papers due at 12:00. No cheating.
Bill

[murrmac]

It occurred to me that a harp doesn't have a sound board or sound box, yet it still makes beautiful music.

All the harps I have ever seen had a soundboard and a soundbox ...

[billhay4]

Concert harps?
Bill

[billhay4]

Seems like they might. So, then is a sound box or sound board necessary for an instrument to produce it's tone?
Bill

[murrmac]

I dig what you are saying ... Alan Carruth would be the man to answer your questions, but I don't know if he posts on here ...

[bassthumper]

best that I can tell the vibration is "formed" inside the sound chamber...

different tone woods influence the sound when used in various components....

instrument construction be it flat/arched tops..varied thickness in tap tuned tops/backs tend toward desired sound in the finished product

I suspect the size of concert harps they bounce the vibrations off surroundings...while I have no experience with harps maybe someone in the know could chime in...I suspect the placement in a room, size of rom etc. causes difference in tone

[Mark Gunter]

I would like a concise definition of sound from someone who understands this better than I do.

Dang, Bill, I wish I knew more about this stuff myself. Unfortunately, I'm not that person who knows more about it than you do. I learned some things in my younger days, some of which may have been dis-proven since. Also, I have made public my own understanding on some related issues that kinda got me into hot water here before. Because of that, I hesitate to air my own thoughts again, but since you brought this up, I may as well have a go.

Sound is processed by humans primarily by vibrations of the eardrums and a biological chain of events that follows. The meaningful vibrations of the eardrum are caused by fluctuations in air pressure, due to waves we might call sound waves which are transmitted by air.

Now, when a man taps a tree, he hears a sound. When a man taps a hollow tree or log he hears a sound of a different quality. When he taps a hollow log with his ear near a hole in the tree he hears a different quality of sound from within the chamber of the hollow log at a clearer, increased volume. There is a resonance to the sound from a hollow log that is not present in the sound from a solid log when tapping. I assume that the origination of drums and of instruments with strings attached to a hollow chamber came about as a result of this type of primitive knowledge, but I can't prove it.

As to the importance of sound holes in an instrument: I got into trouble here over that before. Since that time, I made a quick experiment. I borrowed my bass player's bass fiddle for a few minutes, and plucked the strings placing my ear near different portions of the back, sides ad soundboard. I found that undeniably there was a great volume of sound issuing (presumably) from the sound holes that dwarfed the volume and clarity of sound from any other area! I write "presumably" because as you know, on a bass fiddle, the bridge is situated right there between the sound holes. One might argue that the sound has greater volume and clarity in that area because that is where the string vibration is first transferred to the sound board through the bridge.

Although I'm no expert, I strongly believe that the sound holes serve an important function of directing the resonance from within an instrument outwards. I do not agree with those who post here that no sound comes from the sound hole of an instrument. I have read no scientific papers that convince me that that is the case.

Also, I read a post recently that claimed that the box or body of instrument was not for increasing the volume of projected sound. I also believe that that is a false statement. The volume as well as the quality is enhanced by tapping or vibrating a hollow box, and I believe especially one with a hole in it. Solid body electric guitars have little volume without electronic amplification, while hollow body instruments have great volume which, roughly speaking increases with the size of the chamber (jumbo vs parlor, other things being equal).

[Mark Gunter]

I'm going to add to what I wrote above, that my musings are general observations and when we look at specific frequencies, we might find that very high frequencies are better transmitted with lesser chambers, think xylophone as opposed to guitar or piano.

[Marty Jacobson]

A sound chamber isn't strictly necessary... (muhahaha)

https://vimeo.com/110633932

[Mark Gunter]

A sound chamber isn't strictly necessary... (muhahaha)

https://vimeo.com/110633932

Now that's cool

[pops1]

If you relate the sound to a speaker cabinet a closed back cabinet will take more power to achieve a level of sound than say a ported cabinet. The sound hole, like markscarts I believe, allows air flow so the top is more free to move which in turn creates more volume. I believe if you made a mandolin without a sound hole it would be much quieter as the top would not be able to vibrate as freely due to it pushing and pulling a sealed chamber. That's where I'll stop and let someone more knowledgeable deny or confirm my statements and tell us what really is going on.

[billhay4]

Yeah, many percussion instruments don't have sound chambers as such. Triangles, etc.
I'll take exception to the statement above that vibrations are carried by air. Put your ear on a train track when a train is far away. Or listen underwater.
So, what does have to vibrate?
I can understand that sound boxes affect vibrations but can anyone explain how (in 40 words or less).
Bill

[Bill McCall]

On mandolins I would suggest its to support the strings and allow a greater volume of air to be set in motion than a string on a plank. Cymbals don't have that problem.

[Tom Wright]

Not sure about fitting into 40 words.

Musical instruments need a resonater as a start, and some also have a vibrating radiating surface that is not the resonator. Wind instruments are resonators mainly, the vibration remaining airborne. String instruments have a resonating air volume for some lower frequencies, the top handles mid-range notes, and the higher harmonics mainly radiate from the bridge in the case of violin family. Guitars and mandolins (and banjos) radiate highest frequencies pretty much directly from the string.

This is my understanding of the mechanics, and the important item is the resonator. The reason a trumpet (whose input power is blowing a raspberry with your lips) is generating so much sound is that the input is almost entirely dumped into a narrow range of resonance. The tube keeps all the sound inside, but mainly it transforms a spread of frequencies (buzz) into a note with almost no harmonics. So all that energy is now at one pitch. There is no added energy, unlike electronic amplification. But in that narrow resonant range a trumpet is capable of such power that its wave hits over two atmospheres pressure. This means the bottom of the wave stops at a vacuum, and is why trumpets sounds brassy when loud---the air is being overdriven and is clipping the waveform.

Stringed instruments are less loud because they are less concentrating in the translation from input to resonance. The low notes are supported by the air volume, which is delivered via the ports, whose role is identical to those in speaker cabinets--they add extra resonance at a narrow pitch range at the expense of broader but weaker low response. So you would hear low bass notes there, mainly, and basses and mandolins are indeed miked to pick up some of that. The top is delivering harmonics for those same notes, but more weakly, and distributed across the top, so at any one point it is much weaker and less obvious to the ear. (F-holes also affect the kind of flexibility the top has, being more piston-like than the drum-like movement of an x-braced round-hole top.)

As in the vimeo, much percussion lacks a soundbox---drums may or may not have one. A piano also does not have a soundbox as such, mainly the radiating surface of the soundboard.

All harps do have a soundbox, but they don't have the obvious bass ports. I think this is to spread the response more smoothly across the range in the case of orchestral harps. But they are not quiet---there is lots of area on the soundbox top. I know what it's like to sit next to a strong player. Surface area matters.

[billhay4]

Thanks, Tom,
This clarifies things for me a bit. Could you explain the difference between a resonator and a "vibrating radiating surface" a bit?
Bill

[Tom Wright]

A speaker cone is a radiating surface, so is the top of a guitar. It wiggles and moves air.

A resonator is a system that has a preferred vibration mode, that "wants" to vibrate at a particular pitch. This can be a tuning fork or a bottle you blow over. The string is a resonator, but without being mechanically coupled to something that can move air it is only heard in the tinkly high harmonics.

The soundbox/body is a secondary resonator that has a note, although it is kind of diffuse. The top has resonances also, and they are also a bit spread out, although some are well defined and you can hear one note kind of jump out when you play it. Instruments with "character" tend to have a bunch of sharply defined resonances that make one note sound a little different than the next, making it have a "color". The reason those violin bridge pickups and under-saddle guitar pickups sound "colorless" is this lack of particular resonances adding interest and definition.

This issue involves questions of efficiency and coupling strength. The vibrating string couples weakly to the top---that is why it keeps on ringing. A banjo string couples strongly, so its energy gets taken by the head and the string does not ring long at volume. The difference is the top, of course. Air won't get pushed by a string at lower frequencies, although faster movement does push it a little, so you can hear the twang even on an electric guitar not plugged in.

A trumpet both couples strongly--all the energy is captured---and is efficient in concentrating it into the very narrow pitch range of the basic scale notes.

[Mark Gunter]

I'll take exception to the statement above that vibrations are carried by air. Put your ear on a train track when a train is far away. Or listen underwater.
So, what does have to vibrate?

40 words or less? Dunno 'bout that, but: Vibrations are indeed carried by air, and what has to vibrate is the ear drum. The eardrum can be made to vibrate perhaps by any number of means, but it is specially suited to vibrate to sound waves carried by air. That is not to say that vibrations are not also carried by other means. When you connect two tin cans by a length of string and add tension, sound going into one can vibrates the metal can which sends vibrations in waves through the string, vibrating the second can. With your ear near the second can you will hear the sound due to the second can's disturbing the air in your outer ear canal. Likewise, when you hear a sound from the rails of a railroad track, in order to hear it, the rails cause vibration via the air in your ear canal. That is the primary means of hearing sound.

[sblock]

What is sound? In less than 100 words:

"Sound" is simply a shorthand for describing mechanical vibrations (often vibrations in air, but also vibrations in other fluid media, like water, and in flexible solids, like strings, flat surfaces, and so on) whose frequencies fall into the range where our ears can hear things -- the so-called "acoustic range". Roughly speaking, that's from about 20 Hz up to about 20,000 Hz. Below that range, and we usually call it "infrasound." Elephants can hear infrasound. Above that range, and we call it "ultrasound." Bats can hear ultrasound. But it's all periodic, mechanical vibrations. Nothing more or less.

Why have a soundbox? In about 150 words:

Acoustic stringed instruments have a problem because the mechanical energy in their vibrating strings couples VERY poorly to the surrounding air, resulting in fairly weak sound propagation up to your ears. The solution to this problem is to do something called "impedance matching" (here, acoustic impedance, which is analogous to electrical impedance.) A soundbox, or soundboard, in an acoustic instrument will accept the vibrating string energy as an input, and couple it much more effectively to the surrounding air as an output. It serves not just as a resonator, but also as an acoustic coupler. Another (similar) way to match acoustic impedance is through a horn or speaker, like in the old gramophone horns, which coupled a vibrating phonograph needle to the air, back in the days before electrical amplification. Dobros and banjos act in many ways like a speaker. And nearly all acoustic instruments have some kind of soundboard or soundbox, including harps.

[Mark Gunter]

One of the great things about the cafe is its educational value, and it's great to be able to hear from experts, and to dialog with experts on a wide variety of subjects. This is one that fascinates most of us because of our love for the stringed musical instrument. Obviously, there is more than one way to make an instrument with volume, sound boxes are not necessary for sound to occur, for pleasing sound to occur, or for loud sound to occur. But the question before us here is "What is the function of the body?" - the soundbox - of a mandolin, and by extension of other stringed instruments that have soundboxes. The explanations given are very good and informative, if a bit technical. Would I be correct, in non-technical terms, in answering that one of the functions of the body (soundbox) is to amplify and radiate the sound vibration of the strings? And of course, in the case before us, the sound is transmitted to the eardrum via the air?

[Dave Cohen]

I've answered this question and similar questions on this forum too many times to count. Imo, I haven't done a very good job.. People have seemed appreciative, but after all is said and done, many still don't understand.

When it comes to how musical instruments make sound, everyone thinks they are physicists, except they aren't. However, Tom Rossing is a physicist, and is one who has devoted a lot of his energy to physics education. Tom gave a lecture at the 1988 GAL convention. That lecture became three articles in American Lutherie, and those articles ultimately became two articles in "The Big Red Book of American Lutherie, Volume Two". The first is "An Introduction to Guitar Acoustics", pp 124-134. The second is "Sound Radiation from Guitars", pp 144-152. Questions like "What is the function of the body?" are sufficiently basic, and sufficiently broad, that a forum like this is not the most efficient way to get an answer. Tom Rossing's articles, however, are more efficient. And they are sitting there on paper, waiting for you, so they don't suffer from the limitations of iconic memory or echoic memory.

Al Carruth was mentioned in an earlier post. There are three articles in "The Big Red Book of American Lutherie, Volume One" that Al particularly likes. The first is "Basics of Air Resonances", by W.D. Allen, pp 8-12. The second is "Radiation from Lower Guitar Modes" by Graham Caldersmith, pp 68-72. The third is "Our Great Spherical Friend" by Frederick C. Lyman, Jr., pp 196-201. Spend some time with the five articles I have referenced here, and you will have an idea of what an instrument body does. The articles were all written over 25 yrs ago, and with the exception of perhaps Caldersmith's article, all were written for a lay audience. If you want something mandolin-specific, you have to resort to my papers and to my chapter in the Rossing book, "The Science of String Instruments". But there is really no substitute for going back to the more basic articles in the Big Red Books.

[Mark Gunter]

Dr. Cohen, my understanding then is that my last two questions can't be simply answered. Your best advice is that the OP or I should not entertain such questions on an internet forum, but rather read the articles referenced in $90.00 worth of books. Also, it seems that you feel the need to denigrate everyone here by suggesting that "everyone believes they are physicists when in fact they are not." Misconceptions such as, "I'll take exception to the statement above that vibrations are carried by air" cannot be addressed, because it is more important to belittle the non-physicists that contribute to the discussion.

I would very much love to read the articles you referenced, and perhaps some day I shall. In the meantime, let me make some uneducated observations.

I can skin a calf, make a thong from a piece of the hide, and string it under tension between the ends of a stick, making a crude bow. When I pluck the thong, it makes a sound that I like. I pluck it in time to my brother's blowing on a jug. All is well, but I discover that when I turn a washtub over and firmly plant one end of the bow on the bottom of the tub, the resulting sound is louder and of a different character; I like it. I much prefer the volume and tone. While physics has everything to do with what's going on with sound, I am not required to have a degree in physics to know that my purpose in adding the washtub is to increase the volume and color the sound I'm getting from my bow.

I submit that illustrates the most basic form of most portable stringed instruments - a stick, a string and a resonator, usually hollow: A gourd, a hollow log, a washtub. Why is the resonating chamber there? One reason is to increase the volume. Therefore, I disagree not with any of the experts here, but with folk who post that the soundbox of a mandolin does not serve to amplify the sound, as I read in a recent thread. Likewise, those who post that no sound comes from soundholes are in error, and those who do not seem to understand that sound waves travel through the air are in error. That's no shame to them, but it seems a bit shameful to me that these things can't be addressed except by referring us to scholarly articles.

[Bertram Henze]

It occurred to me that a harp doesn't have a sound board or sound box

Have a look into a harp luthiers shop...
I recommend trying a real harp if you have the opportunity, it's a stirring experience. Not only do they have a soundbox and a top, they have holes on the backside, too (which come in handy for threading the strings through those holes in the top).

[Bertram Henze]

A sound chamber isn't strictly necessary... (muhahaha)

https://vimeo.com/110633932

Strictly speaking, there is kind of a sound chamber (two, in fact), only it's filled with coiled metal instead of air.

Sound chambers serve a double purpose:
1 - make a controlled balance of output between the extremes of big sustain and little volume (electric guitar) and little sustain and big volume (banjo).
2 - concentrate that output into a limited frequency range to acommodate the human ear, i.e. create the output where it can really be heard and not waste energy on inaudible vibrations; this is done by centering volume around a resonance frequency given by shape and size of body and holes.

[s1m0n]

The strings vibrate the soundboard, which vibrates air, which is sound. The back of the soundbox bounces that vibrating air back to the soundboard, making a positive feedback loop that sustains and reinforces the initial note.

It's not essential but it makes the operation more efficient, creating a louder output from a lower energy input (the force you apply to the string).

If you've ever plucked a taut guy wire or steel cable, you know that you can get a note from a vibrating string alone. It's not very loud, tho. If you add a soundboard, you get more noise from the same pluck because there's more vibrating surface to push the air. Add a resonating cavity behind the board and you'll get more noise still.

Incidentally, the laser gun sound effect for the initial Star Wars movie(s) came when a sound tech struck a huge, taut steel cable with a wrench and recorded the noise he made.

[G7MOF]

To me sound boxes just amplify better. For instance, a mandolin with a sound box against a solid bodies mandolin!