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how does a soundbox work ?


McCrunchy

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If you make a string that's attached at both ends vibrate, it will emit an almost imperceptible sound. The role of the soundbox is to amplify that sound. My question is : where does the energy required for the amplification come from ? You surely don't add any vibrational energy by attaching a piece of wood. Is it just that the soundbox dissipates the string's energy faster, so that we hear a louder, albeit shorter note ?

 

Thanks for your input,

 

McCrunchy

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Resonance: the sides of the soundbox can reflect the sound, which is hitting all sides from a lot fo angles. Due to the shape of the soundbox, the reflected waves will amplify eachother, and focussing the sound outwards in a single direction.

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But if all you do is "orient" the energy, you'd expect to have regions where the sound is very weak, which obviously isn't the case for a guitar (ie, regardless of direction, it always rings louder than without soundbox) ...

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If you make a string that's attached at both ends vibrate, it will emit an almost imperceptible sound. The role of the soundbox is to amplify that sound. My question is : where does the energy required for the amplification come from ? You surely don't add any vibrational energy by attaching a piece of wood. Is it just that the soundbox dissipates the string's energy faster, so that we hear a louder, albeit shorter note ?

 

Thanks for your input,

 

McCrunchy

 

The energy all comes from the string: it's just that the string is absymally bad at moving air. By coupling it to a resonant sound box, you increase it's air-moving efficiency -- the large flat surfaces of the soundbox move air much more effectively.

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Is it just that the soundbox dissipates the string's energy faster, so that we hear a louder, albeit shorter note ?

 

From conservation of energy this could be the case, but it's also possible that the string more efficiently couples to the primary mode and less into the harmonics.

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In stringed instruments, the energy of the vibrating string is transmitted to the soundboard via the saddle and bridge. All stringed instruments, fretted or otherwise have a saddle and bridge arrangement of some kind.

 

The string is under tension and is raised from the soundboard by the saddle, which is usually made of some hard material such as bone, ivory or hard plastic. The saddle is inserted into the bridge in a tight, interference fit and the bridge is connected directly to the soundboard.

 

When the string is struck, its vibrations are transmitted through the saddle and bridge to the soundboard, which is usually made of some selected tone wood (i.e. a wood that is known to vibrate in a certain way, e.g. in Martin guitars it's Sitka spruce or mahogony, depending on the model).

 

The soundboard vibrates and this moves the entire volume of air in the body of the guitar. The pressure waves (i.e. sound) exit the guitar through the sound hole (the big hole in the front that you drop the pick into every now and again).

 

So, in short, the saddle and bridge transmit the vibrations of a string to the large area of the soundboard which also vibrates and causes the entire volume of air on the body to vibrate (the vibrating soundboard is the source of amplification) and pushes air waves (sound) out of the sound hole. The saddle and bridge are the key to the whole thing.

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  • 5 years later...
  • 1 month later...

Paraphrasing... The musician brings essentially the same power or energy in the string, but the soundbox permits to transmit more of this power to the air instead of being only lost through air viscosity and more causes.

 

The essential reason for inefficient transfer from a bare string to the air is an impedance mismatch. A solid like the string is dense and stiff, which means a high impedance: it moves with big force and small speed. As opposed, air is light and compressible, meaning a small impedance: a high speed means only a small force or pressure. So a solid like the string, which vibrates with little speed due to its high impedance, and only transmits this speed to the air at the contact area, achieves a tiny pressure and sound power in the air: it's inefficient. A trumpet is louder.

 

A part of the trick is to increase the contact area with the air to move more or it. This is what a loudspeaker's membrane does. To a large extend, a piano soundboard relies on its mere area; mechanical waves propagate in it, most are shorter than the board's dimensions, and they escape to the air over their journey when they're faster in the board than in the air. Since the speed of these flexural (=bending) waves increases with the frequency, such a board gets efficient above a limit frequency; to radiate also low frequencies, the board must be stiff, and this is one advantage of thicker wood with additional stiffeners over metal which is denser hence thinner hence more flexible (there are more differences).

 

By the way, a string can radiate into air through this process, but only the highest frequencies, and only if the sound is faster in the string than in the air. This is a difficult conditions for the material, which must be light and strongly pulled to propagate vibrations so fast. It explains why a string suddenly makes a brilliant sound when you tune it high enough. Few string materials achieve it: good metals, nylon, catgut - and among them nylon dampens too much (other plastics like Kevlar even more and are unuseable, I tried), catgut and metal are good but with a different sound.

 

An other part, for instruments but not loudspeakers, is to increase the solid's speed by resonances - many resonances, because by nature they act on a limited frequency band. This is how a violin's soundbox works, at least at the lower frequencies; the higher are radiated by the wave travelling in the soundboard without reflection nor resonance, and the highest go directly from the string to the air.

 

One of the few things we understand of a violin is that the intertwining of the resonances make a part of the instrument's quality. The lowest resonance is the whole box with its openings that make a Helmholtz resonator tuned around the pitch of the A string; it amplifies the second harmonic of the lowest notes to give a strong and warm sound. Then, the top board comes, and at a higher frequency the bottom one, and again the top the bottom... good instruments alternate them, and regularly spread, well over the 10th resonance, to cover nicely the lower part of the violin's range. End of what is understood from violin's physics, more or less.

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Yes I agree with enthalpy, the soundbox acts as an acoustic transformer to better match the acoustic impedence of the input to the air.

 

The energy ultimately comes from the performer's elbow grease and cannot be greater than she inputs.

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