Audio Theory: What frequencies make drivers travel the furthest

Discussion in 'Archived Threads 2001-2004' started by Dave Schofield, Mar 29, 2002.

  1. Dave Schofield

    Dave Schofield Second Unit

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    I have a CD from the Propellerheads with a track of "scratching" and a guy acting as a "human beat box". I had the grilles of my speakers off once when listening to this track and noticed that the drivers of my old Monitor 3's traveled a ridiculous distance. My question is, do smaller speakers (the Monitor 3's had an 8" driver) travel a ridiculous distance in order to try to produce the lowest of frequencies? Secondly, I've been afraid to listen to this track loud for fear that it will literally send my drivers shooting out of the cabinet, is this an understandable fear? Will turning it up exacerbate this problem even further?
    You can listen to a sample of this track (the "problem note" is in the first 3 seconds of the exerpt of track 9 - Bigger?) here: http://www.amazon.com/exec/obidos/AS...618344-7034350
     
  2. Dave Poehlman

    Dave Poehlman Producer

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    I assume by "travel" you are referring to the actual motion of the driver's cone (also known as excursion).

    To reproduce low frequencies at a decent volume, a driver has to move more air. This is why you see it jumping in and out on bass notes.

    As far as drivers shooting out of the cabinet... well... that depends on the power the driver is capable of handling and how much power you are sending it. I'm not familiar with the Monitor 3 specs, you may want to consult your literature or it may be marked on the box itself.

    If you are concerned about frying 'em, you might want to invest in some fuses for them. It's always better to buy a $.75 fuse than a new speaker.
     
  3. MichaelGomez

    MichaelGomez Stunt Coordinator

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    Drivers have an extension that is a result of the spider and vent pole. The better drivers usually have a larger extension coefficient because they have to move farther to reproduce lower base. That is why the surrounds on a sub are so big and the ones on a tweeter are non existant. I think that it is the xmas on a spec sheet.
     
  4. Dustin B

    Dustin B Producer

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    As frequency goes down the amount of air that has to be moved to produce the requency at a reasonable volume goes up. Drivers have a spec called Sd. This is the effective surface area of the driver that can move air. The amount of air a driver can displace is called it's Vd. To get the Vd of a driver you take its Sd and multiply it by it's peak to peak Xmax. One way Xmax is the the distance the cone can travel linearly from it's resting position either out or in. Two times the one way Xmax is the entire distance the driver can sweep linearly. Once a driver passes Xmax it starts to distort heavily as the motion is no longer linear. When it starts to sound bad you know it's time to turn it down (your passing Xmax). There is a little distance after Xmax to Xmech. Xmech is the mechanical limit of the drivers motion. When you hit Xmech you bottom the driver. You'll know this happens as you'll hear a loud clank as metal hits metal. The driver won't come shooting out at you, it will just make a god aweful sound as it bottoms.

    And yes smaller drivers have to move rediculous distances to produce low bass. It varies quite a bit from driver to driver based on surround and dustcap size. But for comparison sake lets take the area of the different sized circles.

    " - "^2

    dia - area

    6 - 28

    8 - 50

    10 - 78

    12 - 113

    15 - 176

    18 - 254

    So an 8" driver would have to travel twice as far as a 12" and 3.5 times as far as a 15" driver to produce a given frequency at the same volume.

    There is one other hiccup to this excursion thing, sealed versus ported enclosures. In a sealed system as frequency goes down excursion goes up at a constant scale.

    In a ported system this isn't the case. Above the tuning point of the port excursion goes up while frequency goes down at the same rate as a sealed system. A little above the tuning point though, excursion starts to go down again as the port starts to take over the output. Excursion continues to go down until you get to the tuning point of the port. Then as you go below the tuning point of the port excursion starts to increase at a much more rapid pace than the sealed system.

    I'm guessing your Mon3 is tuned somewhere between 45hz and 50hz. So frequencies just above 45-50hz won't cause the driver to move that much, but frequency below 40hz will have it flapping.
     
  5. Dave Schofield

    Dave Schofield Second Unit

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  6. Dustin B

    Dustin B Producer

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    Nope (Paradigm defines that as the -3dB point, and in my opinion what they post for that spec is often pretty optimistic). You can email Paradigm, but I doubt they will tell you. The other option is to post the box and port measurements of your Mon3 and I'll calculate it for you with LspCAD.

    I'll need the outside box dimensions (W, H, D). I'll need the diameter of the port and how long it is.

    This won't be exact, but I should be able to get pretty close with that info.
     
  7. Dave Schofield

    Dave Schofield Second Unit

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    For the record, I haven't got the Monitor 3's anymore. I moved up to the Monitor 7's about a year ago. And I don't have the Monitor 7's with me right now (they were in the first "move out" shipment, as I have far too much stuff in my appt to move out in one trip)

    Thanks for clearing that up for me.
     

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