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| A vented loudspeaker is analogous to a 24dB/octave cutoff high-pass filter, characterized by an enclosure having an open tunnel or port which allows the passage of air in and out of the box. |
| At low frequencies, the vent contributes substantially to the sound output of the system. |
| It does so however, by increasing the acoustc load at the rear of the cone, reducing cone motion, and the output of the driver. |
| As such, a vent only adds as much as it subtracts. |
| Compared to closed-box systems, vented enclosure posses several unique characteristics: |
| A: Lower cone excursion near the box resonance frequency. |
| B: Lower cutoff using the same driver. |
| C: In theory, a +3dB higher efficiency for the same volume closed-box system. |
| On the downside, vented enclosurs are much more sensitive to misaligned parameters.This factor makes the vented box loudspeaker somewhat more difficult for the inexperienced homebuilder. |
| Just like at closed boxes nearly everyone Qts-waarde can be used, although generally only values between 0.2 and 0.5 will lead to a good reproduction. |
| f-3 |
-3dB half-power frequency. |
| fs |
resonance frequency of driver |
| fc |
resonance frequency of the closed box system |
| Q |
ratio of reactance to resistance (series circuit) or resistance to reactance (parallel circuit) |
| Qts |
total Q of driver (woofer) at fs, considering all driver resistance. |
| Qtc |
total Q of speaker system at fc, including all system resistance. |
| Vas |
volume of air having the same acoustic compliance as the driver suspension. |
| Vab |
volume of air having the same acoustic compliance as the enclosure. |
| Xmax |
peak linear displacement of driver cone. |
| Sd |
effective surface area of driver cone. |
| Vd |
peak displacement volume of driver cone. |
| Vb |
net internal volume of enclosure. |
| Parameters: |
Vas = 57,2 Liter
fs = 42,5 Hz
Qts = 0,32 |
| Calculation Vb: |
Vb = 15 * Vas * Qts2,87 = 32,6 Liter |
| Calculation f-3: |
f-3 = 0,26 * fs / Qts1,4 = 54,5 Hz |
| Calculation fb: |
fb = 0,42 * fs / Qts0,9 = 49,8 Hz |
| Port : Sv = 72cm² |
Lve = ((10*3432*Sv)/(4*3,142*fs2*Vb))-0,825*wortel(Sv) |
| Lve = ((84707280)/(2322280,88))-7 = 29,5 cm |
| Parameters: |
Vas = 57,2 Liter
fs = 42,5 Hz
Qts = 0,32 |
| Calculation Vb: |
Vb = 17,6 * Vas * Qts3,15 = 27,8 Liter |
| Calculation f-3: |
f-3 = 0,3 * fs / Qts1,33 = 58 Hz |
| Calculation fb: |
fb = 0,42 * fs / Qts0,95 = 52,7 Hz |
| Port : Sv = 72cm² |
Lve = ((10*3432*Sv)/(4*3,142*fs2*Vb))-0,825*wortel(Sv) |
| Lve = ((84707280)/(1980350))-7 = 35,8 cm |
Firstly, calculate the Efficiency Bandwidth Product: EBP = fs/Qe.
If EBP is around 100, this indicates that a vented box would be a better choice.
If you have Qe and Qm, you can take external resistance into account when calculating Qts.
Qts = 1/((1/Qm) + Rs/((R+Rs)Qe)) where R is the resistance of the wiring, typically 0.5 ohm.
Alternatively, use Qts as specified in the TS parameters. |
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Source: The Loudspeaker Design Cookbook, Vance Dickason
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Defination Vented-Box