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How do semi-hollows work?

Discussion in 'Basses [BG]' started by fireglo forever, Dec 3, 2001.


  1. Sorry if this is the wrong thread for this, i wasnt sure where to put it.

    Picups pick the vibrations from the strings through an EMF, and not the actual sound waves, right?

    So how do semi-hollow basses work? They have the same type of pickups, dont they?

    I've played a few at music stores and they generally do sound different.. but then, every bass sounds different. What exactly does the hollow section in the body of the bass do?

    Sorry if this is a " :rolleyes: " question.
     
  2. Brad Johnson

    Brad Johnson Commercial User

    Mar 8, 2000
    Gaithersburg, Md
    Boom Bass Cabinets, DR strings
    It's not a ":rolleyes:" question. It's a very good one.



    Beats me.


    :D


    The composition of an instrument affect the way a string vibrates. For example, a body that is more vibrant may have a fuller sound than one that isn't.
     
  3. CrawlingEye

    CrawlingEye Member

    Mar 20, 2001
    Easton, Pennsylvania
    It's most likely relative to the composition of the bass, as Brad said.

    A rosewood bass will sound different than a maple bass, right?

    So, what makes them different, would be the question. How dense each wood is, would be my guess... Now, it's only obvious that the density of air's much less than that of wood. :)

    I'm sure this could get really far into acoustics, and what-not. :)

    I'll use the same example I've used before...
    Jazz basses...
    There's so many, tons use the same woods, and nearly the same parts, what makes them different?
    Who knows? The way they're put together, probably.

    The Rumblefish surly sounds different than the Fender Jazz. I think that's a good example. :)

    You could even look into the ones with different woods, and it'd still be relative to the context of this thread. :)


    ...$.02...
     
  4. I have no clue either, but with Gretsch... (guitars, not bass) a Duo Jet (solid) sounds much different than a Nashville (hallow) dispite the two having the same pickups, and wood material, ect. One of the only factors is the solid or hallow body.

    Anyone know if F holes do anything either? Are they just for show or what?
     
  5. geshel

    geshel

    Oct 2, 2001
    Seattle
    Guitars are commonly viewed this way:

    1. The strings vibrate
    2a. The pickups sense the strings' vibration
    2b. The vibrations are transmitted into the instrument and do what they may

    But it's really more like this:

    1. The strings start to vibrate
    2. The instrument starts to vibrate
    3. This affects the way the strings vibrate
    4. This affects the way the instrument vibrates
    5. See #3 :)

    Really it's a system. The pickups happen to sense only what the "strings" part of the system is doing at any one time. But this is tied very much to what is going on in the rest of the instrument.
     
  6. geshel

    geshel

    Oct 2, 2001
    Seattle
    F-holes change the instrument from a sealed vibrating cavity into a helmholtz resonator. :) It's similar to the difference between sealed and ported speakers.

    1. The f-holes change the frequencies at which the body of the instrument resonates (when the air inside the body cavity vibrates strongly)

    2. They allow some of this sound to escape, increasing the acoustic output (why most all acoustic string instruments have sound holes of some sort)
     
  7. Christopher

    Christopher

    Apr 28, 2000
    New York, NY
    On an acoustic instrument, the strings are used to drive the top to produce acoustic volume. The combination of modes in which the top vibrates gives the instrument its character. On an electric solidbody, it's reversed: the body ideally is supposed to drive the strings (hence the long sustain), and the frequencies the body absorbs (and hence subtracts from the vibrating strings) gives the instrument its character (and its dead spots).

    I'd imagine a mag-equipped hollowbody responds more like a solidbody electric, since the magnetic pickups are sensing primarily the frequencies produced by the strings; of course, the hollowbody will have an effect on what frequencies the strings retain and what frequencies they give up. With piezo-equipped hollowbody instruments, the pickup is sandwiched in between the top and the strings, so the top probably contributes more to the tone than it does with a mag-only setup.
     
  8. geshel

    geshel

    Oct 2, 2001
    Seattle
    I'm with you on that except for the part about the body "driving" the strings. In the "ideal" solid body, the body doesn't do squat. :) It just provides two fixed endpoints between which the string vibrates. In the real world, any effect the body has must be passive - it can change the frequency content of the string's vibration (including making it larger at some frequencies at the expense of others), but it cannot increase the sustain - the net effect must be that it absorbs some of the string's energy.
     
  9. CrawlingEye

    CrawlingEye Member

    Mar 20, 2001
    Easton, Pennsylvania

    I disagree.
    I think that the body mass has a significant amount to do with the strings vibrations. The more mass you add = more sustain, as I've found. I think it's more the fact that the string vibration transfers it's inertia to the body, which in turn will keep up with the cycle. In doing so, the body vibrates the strings, it's kind of reciprocal. :)
     
  10. Christopher

    Christopher

    Apr 28, 2000
    New York, NY
    Yeah; to be more accurate, the solid body doesn't actually return any more energy to the strings than the mechanical energy that you initially put in when you pluck them. Some of that mechanical energy by necessity is converted to electrical energy (hopefully a miniscule amount); ideally, the body should absorb as little of the remainder as possible. Unfortunately, there's no such thing as a perpetual motion machine, and no such thing as natural infinite sustain. (Standing in front of your amp and feeding back doesn't count--that's an open circuit and draws energy from the mains. But I digress.)
     
  11. geshel

    geshel

    Oct 2, 2001
    Seattle
    You can disagree all you want. . . :) . . . won't change physics. But I think there's just a point of confusion. Adding mass reduces the amount of energy the body takes from the strings, because that mass has inertia and doesn't vibrate as easily. It may sound counter-intuitive, but take away the body completely and have the ends of the string supported magically in mid-air, and the sustain would be greatest.

    Sure there is reciprocal action between the body and strings, see my first post :), but it can never add energy to the overall system. Inless you have a motor inside the bass body that vibrates just right when you play. :) You're putting energy in the system when you pluck the string, and that energy dissipates through a) friction in the string, b) sound, c) same things going on in the body. If the body of the bass somehow added to the equation, it would eventually have to "run out". Don't tell the vintage market. :D
     
  12. CrawlingEye

    CrawlingEye Member

    Mar 20, 2001
    Easton, Pennsylvania

    It's not the fact that I'm telling you that you're wrong as a whole. I just think that the more mass you add, the more there is to vibrate, therefore proving that it'd increase sustain.

    I agree with your overall approach, though.


    I think the contents of the body does play a roll.
    If you think about it, you're even relatively saying it. The body of the bass does receive some of the inertia from the strings vibrations. So, how the body would vibrate (or how much) would play a roll, wouldn't it?

    If you think about it, the harder material, it should prove the more sustain. If you have a metal bass, how long of sustain would you get?
    If your bass is made of jello, how long of sustain would you get?
    (speaking in terms of bodies)

    It's about how well the body wood interacts with the vibrations (softer wood would most likely then cut into the sustain)

    I've thought of an example to prove how body consistency and body size affects sustain.

    Take a metal bowl of jello with, say a grape inside (so you have a marker), now say you drop an 8oz weight inside the metal bowl of jello, from say 2 feet. Watch the marker, and time how long it keeps in motion.

    Now, repeat the same proccess with a bowl of 3/4 the size, or 1/2 the size.

    Which will stay in motion longer?
    Will they stay in motion the same amount of time?
    (That's essnetially what you're telling me)

    If you're wondering how this is relative, this is how:
    The jello bowl would be your body.
    The 8 oz weight would implement your initial vibration (as in plucking your string).


    You can also try this with other things, like water. This is a silly type of example, but I think it should prove sufficient for substantiating my point. :)
     
  13. geshel

    geshel

    Oct 2, 2001
    Seattle
    CrawlingEye, if you read my post carefully you'll realize that I never said the body doesn't play a role (nor a roll) in sustain. Try it, you'll see.
     
  14. Great way to put it. Thanx geshel. I dont know about you guys, but i think this is a very interesting topic.

    About the Helmholtz resonators, that's the way ported speaker cabs work too. Manufacturers tune them to a desired resonant frequency according to the diameter and depth of the port(s) they use. The frequency at which the air inside the cab vibrates is different than the one of the air suspended at the port, and the combination of both gives the resulting resonant frequency. So, i guess its the same with acoustic instruments and ported semi-hollow basses, like geshel said.

    To what CrawlingEye said about mass: I dont know if it really has more to do with mass or with the density of the material. I guess they are both connected since in order to increase the mass while keeping the average size that most basses have, you have to increase the density of the material. Does this explain why my 9-pound rickenbacker has such a kickass sound? :D
     
  15. CrawlingEye

    CrawlingEye Member

    Mar 20, 2001
    Easton, Pennsylvania
    Explain this:

    :confused:
    You said it did previous to this, then later you claimed otherwise.
    :confused:
     
  16. geshel

    geshel

    Oct 2, 2001
    Seattle
    "The frequency at which the air inside the cab vibrates is different than the one of the air suspended at the port, and the combination of both gives the resulting resonant frequency. "

    I've always looked at it this way:

    - air in the box: a spring
    - speaker: driver attached to one end of the spring, that has some intrinsic mass, resistance and springiness of its own

    sealed box: the other end of the spring is attached to a fixed point. The mass, spring, and resistance all added up determine the resonance frequency.

    vented box: the other end of the spring is attached to another mass, determined by the port (basically the mass of air in the port at rest). The funny part is that the "mass" (air in the port) moves faster for a skinny port, so it's like there's a lever between the end of the spring and the mass. The diameter of the port determines the lever ratio. So, a larger-diameter port needs to be longer to have the same effect because, even though the mass is higher (which lowers the resonance of the system), it doesn't move as far, which effectively changes the spring constant as far as it is concerned (raises it, which increases the resonance of the system). The second effect outweighs the first, so you have to make a larger diameter port longer to get the same result.

    The "output" of the system is the motion of the driver plus the motion of the mass at the other end. With each adjusted for their area to get the proper acoustic result.

    This will only help if you have an intuitive grasp of spring-mass systems, or you could build some and try it out :) But the cool thing is that this is how most speaker design programs work, at a basic level (the good ones have the more complicated effects taken into account). For EEs, if you replace "mass" with "inductance", "spring" with "inverse capacitance", and resistance with, well resistance, with the appropriate conversion factors thrown in, you get an electrical circuit analog. The solution to the system is the same either way! Pretty cool in my book. Although don't forget to add the output from the port to that of the speaker. I can't remember if the acoustic output corresponds to the voltage across or the current through the port/speaker however.
     
  17. john turner

    john turner You don't want to do that. Trust me. Staff Member Administrator

    Mar 14, 2000
    atlanta ga
    in an ideal solid body, the body does nothing to the tone. hence the term "ideal" in geshell's quote that you used. do you know what "ideal" means?

    it means "in a perfect world" or "not taking into account losses due to dampening, friction or other lossy criteria."

    by using the term "ideal", it is implied that the object being discussed doesn't perform up to that standard in the real world.

    oh, and lastly, mass is not the primary factor in determining sustain. an aluminium bass would sustain substantially better than one made of lead.
     
  18. geshel

    geshel

    Oct 2, 2001
    Seattle
    Sorry if I wasn't clear.

    In the ideal case, the body has no effect. The sustain is maximized. That is the best you can do.

    In the real world, the body has some finite mass and vibration absoption, and reduces the sustain and affects the tone.

    Generally, more mass equals more sustain. This is because the inertia of the mass offsets the losses in the material itself - so stiff (not hard) material helps, like your jello analogy. :) But, theoretically, if the instrument was just a very thin, infinitely stiff and strong, but lightweight rod, the sustain would be excellent. Like graphite necks that weigh less than wood ones but have more sustain.

    I guess my main point is still: the instrument body can never add overall sustain, it can only subtract.
     
  19. john turner

    john turner You don't want to do that. Trust me. Staff Member Administrator

    Mar 14, 2000
    atlanta ga
    this is assuming similar stiffnesses in the materials
     
  20. geshel

    geshel

    Oct 2, 2001
    Seattle
    Yeh, instead of "generally" insert "all other things being equal". :D

    "It was a time of many overlapping posts..."