Home Forums DIY Speakers and Subwoofers Anyone familiar with the C-note bookshelf speaker kit? Reply To: RE: Anyone familiar with the C-note bookshelf speaker kit?

  • tvor-ceasar

    September 17, 2020 at 2:12 am

    Are you ready for some math? This is from the QBASIC routines I wrote going on 30 years ago.

    Ported enclosure – enclosure size given (existing box)
    Minimum required info:
    Internal volume of box in Cu.Ft. (Box vol)
    Vas of Driver
    Qts of Driver
    Fs (or Fo) of Driver
    Diameter of port in inches in decimal form (PD)

    F3 = ((Vas/Box Vol)^0.5)*Fs
    Fb = ((Vas/Box Vol)^0.32)*Fs
    Port length = (1.463*(10^7)*((0.5*PD) ^ 2))/(Fb^2*Box vol*1728)-(1.463*0.5*PD)

    This next section has to do with the peak response due to non-optimal box size. The peak may be positive or negative, indicating the level of the peak around the Fb in dB. There are 2 steps: the first is to calculate a temporary value (V) that will then be used to find the dB level. I will give you 2 ways to do the second part since QBASIC uses a different LOG function than standard calculators such as the TI35 and similar. (one is LOG and the other is Natural LOG, I forget which is which)

    V = ((Vas/Box vol)^0.35)*Qts*2.6
    Peak Response in dB = (LOG(V))*0.43429445*20 [this is the QBASIC version]
    Peak Response in dB = (LOG(V))*20 [this is the Calculator version]

    A helpful hint: most software calculators have conversion sections built into them. You just have to expand them and look under their menus. Often times they also include a Scientific layout that gives you access to the extra functions in the formulae.
    Btw, in case you (the reader of this post) wonder what the caret symbol (^) means, it is to raise the preceding number to the power of the following number. So 2^3 is 2 to the 3rd power, or 2^3=8

    Now, using all this info we start with the required info:

    Box Vol 0.388 CuFt (11L)
    Vas 0.28 CuFt. (7.93L)
    Qts 0.38
    Fs 51.81 Hz
    PD 1.375″ (1-3/8″)

    Plugging all that info into the formula gives you:
    For a box size of 0.388 Cu.Ft.
    Your target driver will have a
    F3 of 44.01257
    Fb of 46.67432
    a 1.375″ diameter port at 3.728515″ long
    A Peak response of -1.096577 dB above baseline

    Obviously, since the Peak response is negative, it is below baseline.

    Now running the box dimensions in the C-Note kit on PE, you end up with an internal volume of ~0.32 Cu.Ft. This calculates out to:
    F3 of 48.46382
    Fb of 49.64279
    a 1.375″ diameter port at 4.068575″ long
    A Peak response of -0.5108053 dB above baseline

    In your case, a slightly larger box results in slightly extended bass response that is just a tad stronger than originally designed, even though it may seem opposite. Say the baseline is 87 dB. The C-Note would be 86.5 dB at 49-1/2 Hz and 83.5 dB at 48-1/2 Hz, while your box would be 86 dB at 46-1/2 Hz and 83 dB at 44 Hz. So very close, but at lower frequencies.

    I hope I haven’t confused you too much. It can be a daunting task in the beginning. Once you dive in, it can be addicting. Take care, and ask away. I’ll share what I know.

    Reply To: RE: Anyone familiar with the C-note bookshelf speaker kit?