Enter your driver parameters to calculate sealed & ported chamber volumes, port dimensions, and tuning frequency
⚡Quick Presets
Unit System:
📏Driver & Box Parameters
⚠️ Please fill in all required fields (Fs, Vas, Qts) with valid values.
🔊 Your Bandpass Box Results
📊Tuning Style Reference
0.9×Fs
Tight / SQL Fb
1.1×Fs
Balanced Fb
1.35×Fs
Extended Fb
1.6×Fs
Boom/SPL Fb
+3 dB
Typical Bandpass Gain
15–40 Hz
Typical Bandwidth
≤17 m/s
Max Port Air Velocity
0.7×Vas
Typical Sealed Vol
🎵Fb Tuning by Fs & Style
Fs (Hz)
Tight Fb (Hz)
Balanced Fb (Hz)
Extended Fb (Hz)
Boom Fb (Hz)
25
22.5
27.5
33.8
40.0
30
27.0
33.0
40.5
48.0
35
31.5
38.5
47.3
56.0
40
36.0
44.0
54.0
64.0
45
40.5
49.5
60.8
72.0
50
45.0
55.0
67.5
80.0
60
54.0
66.0
81.0
96.0
📦Typical Sealed Chamber Volumes by Vas
Vas (L)
Vc1 at 0.60× (L)
Vc1 at 0.70× (L)
Vc1 at 0.80× (L)
Vc1 at 0.60× (ft³)
Vc1 at 0.70× (ft³)
20
12.0
14.0
16.0
0.42
0.49
30
18.0
21.0
24.0
0.64
0.74
40
24.0
28.0
32.0
0.85
0.99
60
36.0
42.0
48.0
1.27
1.48
80
48.0
56.0
64.0
1.70
1.98
100
60.0
70.0
80.0
2.12
2.47
120
72.0
84.0
96.0
2.54
2.97
🚧Port Length vs Tuning Frequency (4" Round Port, Vc2 = 40L)
Fb (Hz)
Port Length (in)
Port Length (cm)
Notes
25
~42.0
~106.7
Very long — use slot port
30
~28.5
~72.4
Long — consider 2 ports
35
~20.3
~51.6
Moderate length
40
~15.0
~38.1
Standard length
45
~11.3
~28.7
Compact
50
~8.7
~22.1
Short port
60
~5.3
~13.5
Very short — check velocity
💡 Sealed Chamber Volume: The sealed rear chamber (Vc1) controls high-frequency rolloff and upper bandpass limit. Smaller Vc1 = higher upper –3dB point. Use 0.6–0.8× Vas as your starting point for most builds.
💡 Port Velocity Check: Divide peak port air velocity by max cone excursion. Keep port air velocity under 17 m/s at full power to avoid chuffing noise. Use larger diameter or multiple ports if velocity is too high.
A 4th order bandpass enclosure is an type of speaker box that includes two separate chambers. The subwoofer is placed in between a sealed chamber and a ported chamber. The sealed chamber act as a high-pass filter so that the mid and high frequencies is blocked from the system.
The ported chamber are used to allow the system cut out the low frequencies and allow the system to reach its tuning frequency. 4th order bandpass enclosures does not focus on providing all of the frequencies that is required of a speaker box. Instead, it focus on providing the frequencies in a narrow band to create an increase in the efficiency of the speaker box.
How a 4th Order Bandpass Speaker Box Works
To build a 4th order bandpass enclosure, you must use Thiele-Small parameters. The first parameter that you must use is Fs, or the free-air resonance of the subwoofer. This measure the frequency at which the subwoofer cone will naturaly resonate.
The second parameter that you should use is Vas, which is the equivalent volume of air of the subwoofer. This parameter will help to indicate the size of the chambers of the enclosure. The third parameter are Qts, which indicates the damping and losses of the subwoofer.
Based off the value of Qts, the bass of the speakers can be punchy or boomy. Fs, Vas, and Qts are all parameters that will help calculate the volumes of each of the chambers of the enclosure. The tuning of a 4th order bandpass enclosure will determine the sound of the subwoofer box.
If 0.9 times the Fs is used for the tuning frequency, it will help to create a narrow bandwidth. If the passband is tuned to 1.1 times the Fs, there will be a wider passband, which allow for more types of music to be played through the speaker. If the passband is tuned to 1.6 times the Fs, the output of the speaker will be higher but the bass will not be as tight.
The sealed chamber size should be between 0.6 and 0.8 times the Vas. Smaller sealed chambers will provide a tighter rolloff of the frequencies. Additionally, the size of the ported chamber should be 1.2 times the size of the sealed chamber.
However, the user can increase the size of the ported chamber if more low-end extension is desired from the speakers. The bandpass gain of a 4th order bandpass enclosure will provide a gain of 3 to 6 dB. Ports is required for the bandpass enclosure to allow for the low frequencies to exit the speaker box.
However, it is also important to choose the correct design of the port for the enclosure. For round ports, the diameter of the port should be large enough to prevent the air velocity in the port to increase to 17 meters per second. If the air velocity is too high, chuffing noises will come from the enclosure that will interfere with the bass sounds that are played.
Another option include slot ports that allow for the ports to fit into tight spaces. However, you should monitor the aspect ratio of the slot port to prevent the formation of turbulent airflow. Additionally, if multiple ports are used, the air velocity within the enclosure will decrease.
However, using multiple ports will take up more space within the enclosure. The length of the port will change based on the tuning frequency of the subwoofer; low tuning frequencies will require long ports for the speaker. In building the 4th order bandpass enclosure, you must use specific materials.
You should use 3/4-inch MDF to build the enclosure. 3/4-inch MDF will reduce the amount of resonance within the enclosure. Bracing should also be used in between the chambers.
This is because the pressure that builds up within the enclosure can damage the wall of the enclosure. If the walls of the enclosure flex, the enclosure will not perform as it is supposed to. Additionally, another consideration is the placement of the subwoofer.
One option is to place the subwoofer in a way that it face the ported chamber. Finally, another consideration is the excursion limit of the subwoofer, or the Xmax of the subwoofer. Because the sealed chamber will place a heavy load on the cone of the subwoofer.
Many people make mistakes when they build a 4th order bandpass enclosure. One of the mistakes that many people make is building the enclosure to be too small. 4th order bandpass enclosures require a great deal of volume for the two chambers.
For instance, a 12-inch subwoofer will require between 1.5 and 2.5 cubic feet of volume for the enclosure. Another mistake is ignoring the Efficiency Bandwidth Product, which is the Fs of the subwoofer divided by the Qts of the subwoofer. If the Efficiency Bandwidth Product is over 100, then a ported speaker enclosure may be the best option.
However, if the Efficiency Bandwidth Product is under 50, then a sealed speaker enclosure might be a better design for the subwoofer. Finally, you can use polyfill within the sealed chamber to even out the peaks in the sound that emanate from the speaker. Additionally, bracing should also be used within the enclosure.
