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Thread: What is the function of the body?

  1. #26
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    Default Re: What is the function of the body?

    Quote Originally Posted by markscarts View Post
    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.
    I did not say, or even imply, that one should not seek an answer to the question of the thread in the thread itself. I did say that a much better answer could be obtained in a longer format, and one that could be returned to over time. Nor did I say or suggest that non-physicists should bother to seek an answer to such questions. I have gotten tired of people making up non-physical answers to physical questions without even bothering to find out what is already known. Nor did I reference only "scholarly articles". As I said, all of the referenced articles were intended for lay audiences, and are in fact not published in scholarly publications. As for spending $90 for the two volumes of the Big Red Books, (i) they are available in some libraries, and (ii) you spend over $100 for a plumber or other tradesperson to come to your house for an hour's work or less. Yet you don't put a comparable value on the either GAL's publications or the scholarly publications that can answer your questions.

  2. #27
    but that's just me Bertram Henze's Avatar
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    Default Re: What is the function of the body?

    Quote Originally Posted by markscarts View Post
    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.
    There are two phenomena that we physicists often encounter, but which take an enlightened philosopher to deal with:
    1 - the same question asked repeatedly, even after we have answered it elsewhere; but an answer is only understood (or even considered) after one has asked the question - it's hard to keep other answers "in store" in case the right question comes around. So answering to one person passes another person unnoticed. We must have the patience to work the communication field every time, because the question may be the same but the person is not. BUT, of course, the one who asks must equally have patience to hear a lengthy answer - a rebuff ("can't you say it as simple as it is?") turns all of us away.

    2 - talking of "simple"... some folks like to make up their own answers which please them for their simplicity but either wouldn't neccessarily stand experimental test or do not explain anything (my favourite: "why does a stone fall down?" "because it is heavy"). These are frequently found in product commercials and even in popular science shows - and on forums. Both persons, asking and answering, perceive the question as a pain to get rid of quick rather than a chance to learn. I am not advocating patience with these. Nobody can have patience with the impatient.

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    The Amateur Mandolinist Mark Gunter's Avatar
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    Default Re: What is the function of the body?

    In my own case, I am not impatiently requiring a simple answer to anything, nor am I unwilling to learn. I added a couple of questions of my own that should not require lengthy answers. One of the questions is meant to pin down a small part of the OP's original question. I asked, "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?" In that question, I am not seeking understanding of the physics or mechanics of anything, I'm simply asking whether one might be justified in stating that "one of the functions of the body is to amplify and radiate the sound of the strings?" Is it too much to ask for a general consensus on that point?

    The second question is much different, it has to do with an understanding of a physical principle, and while I'm asking for a simple confirmation of the statement, I'm happy to hear any amount of exposition as well as recommendations for reading material, if someone is uncomfortable with providing a simple answer. In the meantime, I'm making no claim to be a physicist, nor do I require a physicist to answer the second question, which is this: "And of course, in the case before us, the sound is transmitted to the eardrum via the air?" Is it possible to come to a consensus that in the normal case of hearing the sound of a musical instrument, the sound is transmitted to the eardrum via the air?
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  5. #29
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    Default Re: What is the function of the body?

    Quote Originally Posted by Marty Jacobson View Post
    A sound chamber isn't strictly necessary... (muhahaha)

    https://vimeo.com/110633932
    Isn't the room the sound chamber?
    Cheers Gary

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  7. #30
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    Default Re: What is the function of the body?

    To my mind, it’s all about energy conversion, the 1st law of thermodynamics. That converting the kinetic energy of the string (or the player’s hand) as efficiently as possible into sound energy.
    A couple of posts mention the word amplify. To amplify there would have to be additional energy input which there isn't.
    In my opinion the soundboard and body facilitate this conversion.

    I wrote this piece on mandolin design a while ago, you might find it of interest.-

    Some thoughts on mandolin design

    For most of the mandolin players, that I come into contact with, the sound that an instrument produces is paramount. As a professional luthier, I always try to build the best constructed and most aesthetically pleasing instrument that I can, but that all counts for very little unless the tone is there too.

    We know from the law of conservation of energy, that energy cannot be created or destroyed but is transformed from one type to another. Essentially a mandolin (or any other acoustic instrument for that matter) has to transform the kinetic energy of the musician’s hand into sound energy. That transformation has to be done as efficiently as possible to get the maximum sound out of the instrument.

    So you pluck the string, it vibrates (kinetic energy) and you want the maximum amount of this energy transferred into the soundboard.

    The job of the bridge is to hold the strings in the correct position relative to the rest of the instrument (for string spacing, action, intonation) and to make the soundboard vibrate.

    A bridge needs to be as light as possible, so that the minimum amount of energy is used in moving the bridge itself. However, it has to be strong enough to support the strings and wide enough at its base to be stable. I make my bridges without any metal height adjusters; I feel that any mechanism that cuts the bridge into two parts must interfere with the bridge’s efficiency. Admittedly, I use a removable bone saddle to aid the adjustment of the action, but that saddle is firmly embedded within a deep slot and the bone saddle helps with acoustic impedance matching- metal to bone to ebony to spruce.

    My bridge actually looks like a bridge! It makes contact with the soundboard only at the two ends- the central portion is removed; this reduces mass and lessens any restriction of the soundboard’s vibrations. However, by reducing the area of contact, you increase the pressure at the points of contact with the soundboard, which in my opinion, allows the transfer of energy to be more efficient. The feet of the bridge are not in some random position- they rest on the bracing which is designed to support the downward force and to help transmit energy across the soundboard.
    The break angle at the bridge is also critical- the greater the angle the greater the downward force. However, if the bridge is pressing down too much on the soundboard, it will choke it- you know yourself, if you press down on the soundboard you will dampen the instrument (a good argument for armrests). The break angle is governed by the angle of the neck relative to the soundboard and is not just a function of the bridge.

    The tailpiece should be a rigid anchor for the strings, so that energy is not absorbed by it; my theory is that any of the available energy will follow the path of least resistance and that path should be into the soundboard via the bridge.
    The tailpiece must be a minimum length. If the length of the string between the bridge and the tailpiece is too great, you get this part of the string vibrating in sympathy and that’s wasting our precious input energy. You’ll notice that on some mandolins that have the anchor point of the strings close to the tail block, they will have harmonic suppressors fitted to the strings, behind the bridge.
    I like to think that the body of the mandolin is like a loud speaker; the soundboard is the equivalent to the paper cone and that the back and sides are the metal chassis. Therefore, the back and sides should be rigid so that they don’t absorb energy from the soundboard. I also like the inner surfaces to be extremely smooth so they act as a reflector.
    Having stated some of my design ideas; you should appreciate why I use French polish as my finish of choice. French polish is a very light surface coating and as such will have a minimal damping effect on the soundboard.

    Well there’s is my two pennies worth!
    Cheers Gary
    Last edited by GarY Nava; Feb-20-2016 at 11:29am. Reason: typo

  8. #31
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    Default Re: What is the function of the body?

    Quote Originally Posted by GarY Nava View Post
    To amplifier there would have to be additional energy input which there isn't.
    It seems that Dr. Cohen may be correct that these subjects become too complex because they are too broad to discuss in a internet forum. If amplification refers to the output which are waves of sound in the air, then we are discussing the amplitude of the wave. Isn't the amplitude of the wave increased when the vibration is transferred to the soundboard, resulting in greater volume in the perception of the listener? It seems to be a matter of efficiency in the mode of transferring vibrations to the air, rather than an increased input of energy. No matter, if we cannot even agree that an increase in volume is one function of the soundbox.
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    Default Re: What is the function of the body?

    Quote Originally Posted by Mark Gunter View Post
    No matter, if we cannot even agree that an increase in volume is one function of the soundbox.
    That's why physicists' explanations are often tedious to read: they try to avoid misconceptions created through easy use of imprecise wording.
    We can all agree that
    - the sound box makes the instrument louder (instrument output)
    - the sound from the instrument reaches the ear by air (ear input)

    However, the instrument does not only have output, it has also input: the pick stroke. The soundbox lifts the instrument output, but not higher than the instrument input - it's rather an optimization than amplification. Amplification only comes into play when instrument output exceeds instrument input, with the help of extra power from elsewhere (the mains, mostly).

    So - every amplification is an increase of output, but not every increase of output is an amplification.
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    Default Re: What is the function of the body?

    Quote Originally Posted by Bertram Henze View Post
    That's why physicists' explanations are often tedious to read: they try to avoid misconceptions created through easy use of imprecise wording.
    We can all agree that
    - the sound box makes the instrument louder (instrument output)
    - the sound from the instrument reaches the ear by air (ear input)

    However, the instrument does not only have output, it has also input: the pick stroke. The soundbox lifts the instrument output, but not higher than the instrument input - it's rather an optimization than amplification. Amplification only comes into play when instrument output exceeds instrument input, with the help of extra power from elsewhere (the mains, mostly).

    So - every amplification is an increase of output, but not every increase of output is an amplification.
    Thank you Bertram! As regards amplification, it seems to me that you have addressed that in reference to amplification of a signal prior to its transfer to the air, as accomplished by means of an electron tube or silicon chip. Am I wrong to use that term in reference to the amplitude of the wave that reaches the eardrums? In one case, with no sound chamber, the waves that reach the ear have lesser amplitude. Once the sound chamber is added, the waves that reach the ear have greater amplitude. There is presumably a loss of some frequencies and sustain that pay for the gain. And I mused that the gain in the amplitude of the output is due to an increased efficiency in transference of the energy to the air rather than an additional input of energy.

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    Default Re: What is the function of the body?

    Quote Originally Posted by Mark Gunter View Post
    Once the sound chamber is added, the waves that reach the ear have greater amplitude. There is presumably a loss of some frequencies and sustain that pay for the gain. And I mused that the gain in the amplitude of the output is due to an increased efficiency in transference of the energy to the air rather than an additional input of energy.
    That's correctly put, as far as I can see. Efficiency is the word I didn't find when I needed it.
    But the word amplification really points to that process of a small signal controlling a big signal, not only in music (other example: laser - Light Amplification by Stimulated Emission of Radiation).
    Last edited by Bertram Henze; Feb-20-2016 at 1:13pm.
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    Default Re: What is the function of the body?

    Quote Originally Posted by Bertram Henze View Post
    That's correctly put, as far as I can see. Efficiency is the word I didn't find when I needed it.
    But the word amplification really points to that process of a small signal controlling a big signal, not only in music (other example: laser - Light Amplification by Stimulated Emission of Radiation).
    I was not aware that the term had such a narrow use. In my youth I studied electron theory, vacuum tube theory and solid state theory, and I'm familiar with the effects of the little signal you mentioned. I only assumed that the word amplification would apply in comparing the amplitude of output waves from a device receiving (roughly) the same input signals when a modification results in significantly increased amplitude of the output. Seems I always get in trouble thinking out loud here, (sigh).

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    Default Re: What is the function of the body?

    Hang in there Mark, this is an interesting thread. There were some things mentioned that caught my attention like the need to remember the law of conservation of energy and the conjecture that the conversion of mechanical energy into sound is associated with some impedance matching element, likely the bridge. I had first heard this suggested in a seminar given by Michael Kasha about 50 years ago and I think it may be true based on how minor variations in the bridge can have significant affect an instrument's performance.

    On the lighter side, though I love physicists and wish I was smart enough to be one, I am reminded of an old story where a physicist was asked to calculate the maximum velocity of a given race horse and said that the first step in that calculation was to assume the horse was a small uniform sphere.
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    Default Re: What is the function of the body?

    Quote Originally Posted by Mark Gunter View Post
    It seems that Dr. Cohen may be correct that these subjects become too complex because they are too broad to discuss in a internet forum. If amplification refers to the output which are waves of sound in the air, then we are discussing the amplitude of the wave. Isn't the amplitude of the wave increased when the vibration is transferred to the soundboard, resulting in greater volume in the perception of the listener? It seems to be a matter of efficiency in the mode of transferring vibrations to the air, rather than an increased input of energy. No matter, if we cannot even agree that an increase in volume is one function of the soundbox.
    The subject is not too broad to be discussed on an internet forum, but it is inefficient to discuss it there. As Bertram said, scientists like to give qualified answers, and those require the long posts, and people tire quickly of the long posts. Also, it usually takes a lot of back-and-forth to finally answer a question satisfactorily. That is where the inefficiency comes in. What is your time worth? If you put a value your time, you will probably find that it is cheaper to buy the Big Red Books than to go back and forth on the internet fora.

    Another detail. Explanation of scientific concepts is often done best with the addition of sketches, diagrams, graphs, etc. The visual aspect of those things is important and simplifying. Posting existing visuals on internet fora takes time; posting visuals that have to be made anew for the particular discussion takes even more time. When those visuals so often exist already in, e.g., GAL articles, that ends up taking you a lot less time. It especially ends up taking me a lot less time.

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    Default Re: What is the function of the body?

    Hey, getting in trouble is the purpose of thinking out loud - that's the way we learn. Embrace it. Another example for that is playing a break in a jam or leading a set in an Irish session: we walk out on a limb and learn from success or failure. Welcome to the rollercoaster called life.
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  21. #39
    Mandolin tragic Graham McDonald's Avatar
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    Default Re: What is the function of the body?

    I would remind folks that Dave Cohen has for many years been politely and patiently explaining the complex acoustic systems that make stringed instruments so fascinating. Put his name into the search box and read back through years of posts where he has described, usually in the most accessible way, what are sometimes very complex ideas. Only occasionally has he snarled at some self-proclaimed expert whose pomposity exceeded his or her understanding and even then he has been kind about it. Anyone interested in these little instruments of ours owes Dave a great debt of gratitude for his generosity in the sharing of his knowledge.

    Cheers

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    Default Re: What is the function of the body?

    I'll give my understanding (as a non-physicist) who has read some of these more scientific works on the subject of sound and musical instruments. I don't pretend to be 100% correct.

    I'm posting just to give my simplifying conclusions after reading these works.

    I'm not going to address how sounds works other than to say you have to have some medium and therefore it can't be in a vacuum.

    Sound comes from the strings vibrating and the cross section of the strings hitting the air. That's not very much surface to move the air so it isn't very loud.

    A soundboard is much larger and when it vibrates it moves much more air and is therefore louder.

    The bridge transfers the vibrations from the strings to the soundboard. You don't want the bridge to be too efficient or the resulting sound would be more like a sudden clap.

    The hole in a guitar (mandolin) is more or less like an air pump. The soundboard vibrates, the air in the body cavity compresses, the back moves a bit as well and the whole thing serves as an air pump.

    Most of the higher frequencies come from close to the top of the soundboard. Most of the lower frequencies come from inside the body cavity. An explanation of what is going on around the sound hole is described as the Helmholtz effect (as I recall) so you can look that you if interested.

    You can cover the soundhole on a guitar (mandolin) and the decrease in the volume is much greater when you are hitting let's say the "a" string on a guitar or the "d" string on a mandolin than when you are hitting the "e" string.

    This just shows that more of the higher frequency sound is coming from on top of the soundboard and the lower frequency sound from inside the body cavity.

    I think this explanation is fairly correct and fairly simply laid out.

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  25. #41
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    Default Re: What is the function of the body?

    I can think of no better purpose of the body than to show off that gorgeous curly maple.


    And that reminds me... What do you call a banjo with the back resonator removed? A good start.

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    Default Re: What is the function of the body?

    And that reminds me... What do you call a banjo with the back resonator removed? A good start.
    A less efficient banjo.

    I am not a physicist so have been trying to stay out of this, but some of the posts have made me groan and cringe. There is NO amplification involved, the amount of energy remains the same. In basic laymans terms, a soundbox makes the conversion of the kinetic energy in a vibrating string into sound energy more efficient. Instead of most of the kinetic energy in the vibrating string going to heat energy, more of it goes to sound energy (which then ultimately ends up as heat energy). How that happens gets complicated and you should read the papers cited by Dave if you want to understand it in more detail. I have read most of them, but it was a long time ago and I have forgotten just about all of it. The end result is it sounds louder, but the efficiency gain is not the same for different frequencies so is non linear. Hence different instruments sound different.

    Hopefully that does not get me into trouble, that is enough physics for me for one day.
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    Default Re: What is the function of the body?

    And what do you call a "less efficient" banjo? A good start.

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    Default Re: What is the function of the body?

    http://youtu.be/MenTEcA4jVk

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    Default Re: What is the function of the body?

    Part of the difficulty in this discussion is that the word "amplify" has a common meaning which is "to increase or augment". That meaning and usage dates back more than 600 years to the French word "amplifier" in the 1300's, some 300 years or more before the beginning of our modern scientific approach to understanding sound, and it is still in common use today. Most everyone knows that a non-powered megaphone increases the loudness of a voice at a distance and in a particular direction. The common description is that the megaphone amplifies the voice, and that would be correct given the long-standing meaning of this word. Our physics and science-minded friends have given the word a special meaning dating back about 100 years to the use of electricity to add energy to the sound source and thereby make it louder by increasing the amplitude of the sound waves. This more limited meaning is a term of art which is understood by those trained in its use, and which appears to others to be consistent with the normal meaning. Confusion arises when folks using the limited meaning tell the rest of us that our use of the word is wrong, that no amplification is taking place because no energy is added. Impedance coupling is a more accurate description from the standpoint of the discipline called Physics. The megaphone and the mandolin body may be impedance coupling devices, but in common parlance and experience they increase or amplify the volume of the initial sound source - the voice or the strings. Choosing to use a more limited definition of a common word does not of itself make wrong the common usage.
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    Default Re: What is the function of the body?

    my Martin B1 "pushes more air" than my mandolins do...great tone but not enough volume to be heard in a BG jam...

    my Englehart "pushes enough air" to easily be heard..AND FELT...even when played gently....

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    Default Re: What is the function of the body?

    Quote Originally Posted by Tommando View Post
    the use of electricity to add energy to the sound source and thereby make it louder by increasing the amplitude of the sound waves.
    Increased amplitude of sound waves = amplified, in the way I used the term. I can tell by what you've written that you understand that. In the interest of clarity, the sentence above needs to be explained: The basic use of electronics for amplification in audio devices uses electricity to strengthen a signal, which is delivered to an output device like a speaker. The moving cone of the speaker then disturbs the air creating sound waves. The distinction is that the sound is represented within such an amplifier (e.g., transistor tube or solid state transistor) by pulses of electronic current (weak signal) that is used to modify a strong current, which is then used to drive the speaker. In the case of such an audio device using electronic amplification, the "sound wave" is what actually goes into the air; the part that is affected by electrical energy is a "signal" that represents the sound wave.

    @peter.coombe, Cringe and groan away at what I write. I'm not impersonating a physicist, claiming infallibility or even claiming to be able to express these ideas as a scholar in the particular field, I'm expressing my opinion here and trying to learn. Sorry if I don't fit in the right box for y'all. I have plenty of respect for the experts here, but can't say I take everything they write as gospel, and really consider myself no more pompous than the establishment is. In fact, I'm a pretty good guy once you get to know me, pomposity and all.
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    Default Re: What is the function of the body?

    Quote Originally Posted by Tommando View Post
    Part of the difficulty in this discussion is that the word "amplify" has a common meaning which is "to increase or augment". That meaning and usage dates back more than 600 years to the French word "amplifier" in the 1300's, some 300 years or more before the beginning of our modern scientific approach to understanding sound, and it is still in common use today. Most everyone knows that a non-powered megaphone increases the loudness of a voice at a distance and in a particular direction. The common description is that the megaphone amplifies the voice, and that would be correct given the long-standing meaning of this word. Our physics and science-minded friends have given the word a special meaning dating back about 100 years to the use of electricity to add energy to the sound source and thereby make it louder by increasing the amplitude of the sound waves. This more limited meaning is a term of art which is understood by those trained in its use, and which appears to others to be consistent with the normal meaning. Confusion arises when folks using the limited meaning tell the rest of us that our use of the word is wrong, that no amplification is taking place because no energy is added. Impedance coupling is a more accurate description from the standpoint of the discipline called Physics. The megaphone and the mandolin body may be impedance coupling devices, but in common parlance and experience they increase or amplify the volume of the initial sound source - the voice or the strings. Choosing to use a more limited definition of a common word does not of itself make wrong the common usage.
    Well stated. Thank you.

  34. #49
    but that's just me Bertram Henze's Avatar
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    Default Re: What is the function of the body?

    Quote Originally Posted by Tommando View Post
    Choosing to use a more limited definition of a common word does not of itself make wrong the common usage.
    Not wrong, but easier to misunderstand (amplification is not the only example for a common word reduced to a meaning by physics - force, energy, momentum, power are others).

    OTOH, if usage of the limited definitions is in danger of being taken for common language, we physicists can always resort to mathematical equations
    the world is better off without bad ideas, good ideas are better off without the world

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    I may be old but I'm ugly billhay4's Avatar
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    Default Re: What is the function of the body?

    Sorry not to contribute, but I've been away from the computer over the weekend.
    I posed this question precisely in hopes of a "messy" discussion like this one. I agree with Dave Cohen that such discussions are inefficient. However, learning (in my case, anyway) is quite inefficient, a lengthy process of misunderstanding and clarification.
    I thank all of you for participating in this process.
    I understand the distinction between amplification and impedance coupling now (whether I will forget this is up in the air). I have a flawed understanding of the function of the sound box and soundboard. I am still not sure that air is required for sound to be made (I think not but some medium is). I do not think anyone has come up with a unified theory of mandolins yet, but they are working on it. As soon as it is completed, others will rush to challenge it.
    Finally, I would like all the scientists among you to recall the each individual learns differently and incompletely (even you). Some learn by reading Red Books (I have read these articles), others by listening, some by making fools of themselves (my way), and some simply by being superior human beings. All of these methods are valuable IF expansion of the mind ensues.
    Bill
    Last edited by billhay4; Feb-22-2016 at 2:44pm.
    IM(NS)HO

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