Resource to calculate, building and measuring Hi Fi Loudspeakers and more...


 
zaterdag 25 november 2017
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    Defination Vented-Box



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.



Woofer Selection

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.



Definition of terms

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.


Formula Reflex Systems (Hoge)


Hooge



Calculation Reflex Systeem (Hoge)

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

Calculation Reflex Systeem (Bullock)

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


Check EBP



Firstly, calculate the Efficiency Bandwidth Product: EBP = fs/Qe.

Fs [Hz]
Qe []
EBP[]
 
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.


Bass Reflex Cabinet

Basis for the recommended housing size Vb and tuning frequency Fb is the Qb3-B4-C4-Tabelle for Ql=7 ("normal" losses).


Fs [Hz] : Qts [] : (0.1 - 0.8) Vas [l] :  
 
  Vb [l] : Fb [Hz] : F3 [Hz] : Ripple [dB] :
 
  Dv [cm] : Lv [cm] : Pa [cm2] :  
 
If you change the pipe diameter [Dv], click on 'Port' to recalculate Port length.



Vb [l] - Net volume of the box in liter (without the volume for the bass reflex pipe)
Fb [Hz] - Tuning frequency of the Port
F3 [Hz] - Frequency, where the level at the lower end of transmission dropped around 3dB
Ripple [dB] - Ripples at the lower end of the transmission range (-0 to +Ripple dB)
Dv [cm] - Diameter of the bass reflex pipe
Lv [cm] - Length of the bass reflex pipe
Pa [cm2] - Area of the bass reflex pipe



Source: The Loudspeaker Design Cookbook, Vance Dickason