Torco wrote: ↑27 Feb 2021 07:47
omg there was a lot more thread!
the thing about coupling kind of went right over my head
I'll try to clarify, because it's an important concept for anyone interested in instruments (nat- or con-...). I'll go from the beginning, so sorry if this is patronise - feel free to jump on whererever you find appropriate!
A string instrument generates sound through the vibration of a string. However, you can't hear the vibration of the string directly: instead, you hear a vibration in the air, caused by the vibration of the string.
The big question for a string instrument, then, is how to convert the vibration of the string itself into the vibration of air.
A plain string by itself can do this: the string pushes the air as it vibrates, vibrating the air. However, a 1-dimensional string, being thin, does not push a lot of air. The total amount of air moved is very small. As a result, when you pluck a taut string by itself, you only get a very quiet sound, even if you pluck (or strike, or bow) really hard!
Therefore, most string instruments use a 'soundboard'. This is a flat, 2-dimensional surface that is in some way 'activated' by a vibrating string: the vibration of the string is transferred into a flat soundboard attached to the string, making the soundboard vibrate. The soundboard then, in turn, vibrates the surrounding air. Why is this better? Because the 2D soundboard vibrates a much bigger amount of air. The big flat soundboard pushes a big chunk of air, whereas the string by itself almost cuts through the air - so the soundboard is better at moving a larger quantity of air. This makes the sound louder.
Many instruments also go a step further, including a 3-dimensional 'soundbox' (or more generally 'resonating chamber'). This is a volume of air that is either fully enclosed, or at least partially enclosed (an open bowl is an early and common form of resonating chamber). A soundboard by itself can have the problem that when it pushes the air, it doesn't all bounce back, but rather some of it gets pushed away entirely; a soundbox traps a volume of air, so that it can be fully vibrated before it escapes - but if it's too trapped, the sound stays in the instrument, so 'soundholes' let a bit of the trapped vibration out to vibrate the surrounding air.
But of course, none of this increases the total energy in the system, which comes from the original pluck (hit, scrape, etc). Instead, there's a tradeoff: a soundboard lets you move MORE air (louder), but doing so uses up the energy of the pluck more quickly, so the vibration dies away faster. A vibrating string tends to yield a quieter but more sustained sound; a vibrating soundboard gives a louder sound, but one that dies away faster. There's also the issue of inefficiency caused by stiffness of the vibrating objects (creating heat rather than sound) - in general, soundboards are stiffer and less efficient than strings. However, some of the die-off can be counteracted with a resonating soundbox, as the vibration of the trapped air takes some time to escape (and can even continue to drive the soundboard's vibration when that would otherwise have died off).
Meanwhile, however, as soon as you introduce complicated things like soundboards and soundboxes, you introduce irregularities into the sound. A simple string, if it's sufficiently thin and flexible (in practice this only begins to be a problem with heavy metal piano strings), produces an extremely regular harmonic sound: the fundamental frequency, and then the harmonic series of partials above it [recap: a harmonic vibration consists of a basic vibration, but also 'echoes', called 'partials' or 'harmonics', with frequencies exactly 2, 3, 4, 5, etc times the frequency of the basic vibration]. But less flexible objects, like boards and boxes and even volumes of air, have their own inherent 'resonant frequencies', depending on their shape and flexibility. Where these frequencies happen to match a partial of the fundamental being played, that partial is amplified; where the resonant frequencies don't match a partial, that partial is weakened. To give a simplistic example: a large soundbox tends to reinforce low-pitch partials, but not suppot high-pitch ones, while a small soundbox tends to reinforce higher-pitch partials, but not lower ones - hence, as a general rule, the lower the intended pitch of your instrument, the bigger you make the soundbox.
The string itself sounds very similar on most string instruments (though it'll vary a bit depending on weight, flexibility, length, etc). Most of the distinctive sound of the instrument (aside from features of how you pluck/hit/scrape the string) is actually caused by the 'irregularities' introduced by the soundbox and soundboard.
So the sound of each instrument is made up of different elements, coming from different parts of the instrument, which for simplicity we'll divide into two: the sound of the string, and the sound of the body. String sound tends to be quiet, to be sustained (or at least to die away at a regular rate), and to be 'pure' and mathematically simple. Body sound tends to be loud, to be brief (it dies away abruptly - ignoring here of course scraped instruments where you continually re-generate the sound), and to be 'dirty' and mathematically complicated (and hence distinctive).
[we can then probably subdivide body sound: sound coming from the board itself tends to be briefer and dirtier; sound coming from a soundbox (or an unusually resonant soundboard, like a metal sheet) tends to be more sustained and 'richer' - more complicated than a pure string, but more harmonic than a simple board. To put it really simplistically: string sound goes "zhzhwimmmmmmmmmm", soundbox sound goes "dwummb", and soundboard sound goes "twunk"]
Now! Depending on how you convey the energy from the string to the body, and how good the body is at receiving that vibrational energy, you can alter the balance of these two sounds. This is what we mean by "coupling". The string is "coupled" to the body: a lot of, or 'good', coupling means that more of the sound comes from the body; little or bad coupling means that more of the sound comes from the string. So when you pluck a string of your cello the sound is plunkier - more compressed, but potentially louder - than when you pluck a string on a guitar, which gives a more resonant, sustained sound: the cello has a lot more coupling than the guitar. [although it's complicated by the fact that the cello is also a bit better at activating its soundbox, and in particular activates the BACK of the instrument as an additional soundboard, and also by the fact its rigidly arched soundboard is inherently less dirty than the relatively flappy flat soundboard of the guitar. But in simplistic terms...] And in turn, an instrument like a kantele has a soft, singing tone, because it has very little coupling.
What determines how much coupling there is? Well, lots of things, unfortunately. But the biggest thing is simple: how much force is applied to the soundboard by the string. A higher-tension string pushes the bridge into the soundboard, helping vibration move from the string to the board. Similarly, a very high bridge is pushed harder into the soundboard by the same tension (because it bends the string more, and the string wants to unbend). So high-tension strings and high bridges yield high coupling, and low-tension strings and low bridges yield low coupling. Although, as I say, there are also other complications.
An instrument like the guqin or the kantele has no bridge. Therefore, vibration has to travel through the pin, which is much less efficient, hence lower coupling, hence a quieter instrument.
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, but I have a few intuitive thoughts about the english / portuguese guitar.
the portuguese strings sound to me like they're nylon, which imitates gut, which is elastic and soft and loses a lot of the energy of the higher harmonics cause, well, it's soft. the english guittar sounds like the strings are made out of wire, which preserves a lot of the higher harmonics (and thus, ends up more plinky and less dumky, heh, I seem to have answered my own question lmao) also the british one feels like the strings are tighter and thinner, and if my electric guitar is any indication, the thicker the strings the mellower the tone. [/quote]
Both instruments actually use metal strings. However, I think I remember reading the guy who played one of the English instruments saying that he actually used harpsichord wire, which would probably be lighter than modern portuguese guitar wire. Although I don't actually know. This may be an important part of the difference in sound.
The effect of the weight is complicated. Ceteris paribus, thicker strings are more inharmonic - they're 'dirtier' and more 'percussive' (naturally: as you make a string thicker and stiffer, it's less like a string and more like a bar; this is a distinctive feature of modern pianos, which have a very 'percussive', dirty sound (which listeners may interpret as either "ugly" or "characterful"...)). However, thicker strings at the same pitch and length must be higher tension, which makes this 'problem' worse in some ways (higher tension = stiffer), but also counteracts it by removing the inharmonicities of strings flapping around. In general a higher tension string is 'purer' and more filled with harmonics.
the portuguese guitar is bigger, and sounds more like a regular old spanish acoustic guitar <cause, well, it is more like one>. some of this is probably the nylon but I have the feeling it's also about how much body there is _behind_ the bridge? cause the pipa, which is very bright, has the bridge way towards the butt of the thing, whereas a guitar (or the portuguese instrument here) sort of have the bridge towards the middle.
This is, again, related to coupling. If you put the bridge near the edge of the soundboard, it's not as good at making the soundboard wobble, because the edge of the soundboard is fixed to the sides of the instrument. You're effectively "plucking" the soundboard at the end, whereas plucking in the middle of the soundboard, like plucking the middle of a string, gives a purer and louder sound.
I don't really understand the pipa yet, but I think it also has the issue that its soundbox is relatively shallow and narrow, so it doesn't resonate much with the lower harmonics. This is a feature of many non-European instruments, which tend to have much smaller soundboxes than their European equivalents, and tend to have a 'twangier' (if plucked) or 'breathier' (if bowed) sound: they intentionally amplify the upper harmonics rather than the lower ones.
The weird thing about the spanish guitar, meanwhile, isn't the bridge, but the tailpins. Most string instruments have the string pass down to the bottom of the soundboard, but the modern guitar has them glued to a thing right on the soundboard. This actually pulls the soundboard upward, while the bridge pushes the soundboard down; together, this creates a weird twisting motion. This helps you create waves in the soundbox that AREN'T up-and-down like the string and the board, but instead side-to-side - which is why the guitar is relatively shallow. It uses lateral waves instead. Cellos do this to some extent as well, generating their twisting mostly through the asymmetrical bridge. Erhus, on the other hand, just bite the bullet and have a really deep, but not that broad, soundbox - erhu soundbox waves are therefore mostly at 90 degrees to guitar or cello soundbox waves! This is partly why the erhu bow is around the string rather than over it - it lets you play at a sharper angle, putting more vertical waves into the string, and hence into the vertical soundbox. Whereas the cello mechanism is able to make use of the more transverse waves created by bowing over the surface of the string.
I think - though I'm not certain - that instruments like the pipa and the portuguese guitar are less good at creating these transverse waves to optimise their soundbox space, and that this is part of why they have a 'brighter' sound than a spanish guitar, or indeed a lute.
The spanish guitar's real virtue, thought, is its flexibility - literally. Its weird shape, big soundhole and twisting soundboard result in multiple different, very different, vibrational 'modes' for the soundboard: if you play different notes, the soundboard doesn't just vibrate at a different frequency, it vibrates in a different SHAPE. [iirc there's 8 or 9 different vibrational patterns?] And this allows the soundboard to have multiple different resonant frequencies, supporting different ranges of notes. All instruments do this to some degree, but the spanish guitar is unusual in how many distinct vibrational modes it has, which helps to maintain its mellow, resonant sound across a wide range of pitches.
the same is true of the lute, which is brighter than a guitar too.
This may also relate to soundbox shape (bow vs box), but I suspect primarily to strings - lutes had thinner strings than guitars have. Of course, that's partly because the lute is a more inherently resonant shape, so lighter (quieter) strings can make it hum more easily than they could on a (relativey clumsy) guitar.
I feel as if this is something of an extension of the timbral effect of point of contact on a bowed string (which is this: the closer to the bridge you play the more bright, resonant, loud and harsh the thing sounds, whereas if you bow onto the fingerboard you start getting a mellower, darker note. and if you continue up you get a wimpy, flute-like kind of tone. I understand this is the case with any bowed string, and is probably a function of the amount of energy which goes into which harmonic as above).
I don't know, but I think there's probably two factors here:
- playing near the end of a string results in a quieter, less harmonic sound (because some of the energy gets dragged into whatever the string is attached to, and the string is stiffer)
- but playing near the bridge increases the coupling, as more of the energy goes into the soundboard, whereas playing nearer the nut just wastes the energy entirely
Also it could be the case that the construction of the english guittar is... tighter? i.e. the body of the thing is stiffer: this could be due to a number of things, but is most likely due to it just being smaller.
I'm not sure about this. My impression is that despite being slightly smaller (and you can pick pairs of english and portuguese guitars where the size difference is smaller) the construction is pretty much the same.