Vented enclosures
Vented enclosures, also referred to as Bass Reflex and ported enclosures differ greatly from the previous enclosure discussed. In vented enclosures, the rear output of the woofer is filtered though a tuned vent or port. This filtered output augments the front output of the woofer. Only the enclosure and the vent determine vent-tuning frequency. Choosing the correct frequency will depend of the woofer, enclosure volume available and the desired performance.
Woofers in a vented enclosure reach a minimum excursion point at the tuned frequency of the enclosure. This is caused by the interaction of the port and the enclosure. At the tuned frequency, the port generates the majority of the output. Mechanical power handling of the woofer is very high at the tuned frequency due to the reduced excursion at that frequency.
There are several advantages to a vented design. For similar sized enclosures, up to a 3dB increase in output in the low frequency range is possible. Figure 5 The increased efficiency comes from the output generated by the port.
12
inch woofer is a 2.5 cubic foot sealed and vented enclosure.
Tuned
frequency of the vented enclosure is 35 Hz.
At and above the tuned frequency of the port, there is a great improvement in mechanical power handling of the woofer when compared to an equivalent sealed counterpart. Figure 6
12
inch woofer is a 2.5 cubic foot sealed and vented enclosure.
Tuned
frequency of the vented enclosure is 35 Hz.
Power
applied to both enclosure designs is 1000 watts.
The disadvantages of this design are sharper frequency roll-off below the tuned frequency (24 dB per octave versus 12 dB for a sealed enclosure) and the reduced mechanical power handling below the tuned frequency of the enclosure.
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topDesigning Vented Enclosures
For the most accurate results, ORION recommends a good computer aided design program. Basic alignments can be determined with the use of the formulas in Table 5. These formulas can be fudged for lower tuned frequencies, but negative ripple, a dip in the frequency response before the tuned frequency will occur. Still, it is possible to design an excellent performing enclosure that has a negative ripple response output.
How size affects performance
Size of the enclosure relative to the woofer will greatly affect the performance of the system. If the enclosure is to small, the enclosure will be very difficult to tune due to the extra long vent requirements. In addition, no additional output will result due to the interaction of the enclosure, port and the woofer. If the enclosure is too large, the frequency response will have a dip before the tuned frequency of the enclosure. Also, the mechanical power handling of the woofer will suffer before above the tuned frequency response of the enclosure. This will result in a peaky frequency output with little mechanical power handling. Choosing the optimum enclosure volume will depend on the woofer, available power and desired frequency response performance.
How Tuning frequency affects performance
Tuning frequency will have a significant affect on the performance and reliability of the system. If the tuned frequency is too high, the woofer will have a very peaky output and very poor power handling in the lower frequency range. If the tuning frequency is too low, the enclosure will be very difficult to tune due the long length of the port and a potential dip if the frequency response above the tuned frequency of the enclosure. Choosing the correct tuning frequency will depend on the enclosure size, woofer and designed performance of the overall low-frequency system.
Vented Enclosure Formulas
Where:
- Vas =The total volume of air equal to the compliance of the woofer
- Qts =Total "Q" of the woofer
- Lv =Length of port in inches
- Fb =Desired tuning frequency of the enclosure
- F3 =Frequency where the output of the enclosure is reduced by 3 dB. This is the frequency where the output of the woofer begins to roll-off
- Vb =Needed enclosure volume of air in cubic feet
- r =Radius of the port in inches. Note: The radius is half of the internal diameter of a round port material
Choosing Woofers
Some woofers are better suited for sealed alignments, some woofers are better suited for vented alignments and some woofers have no particular preference. Calculating the E.B.P. (efficiency bandwidth product) of the woofer will help determine the woofers overall preference. This does not mean that the woofer will not work well in other enclosures, just that it is better suited for a particular enclosure.
Calculating EBP
- Fs=Resonant frequency of the woofer in Hertz
- Qes=The total electrical "Q" of the woofer
Any good company provides the needed parameter information in the specification sheets.
Maximizing Performance
Maximizing the performance of a vented enclosure is easy if you follow the suggestions listed below.
Seal the enclosure: Any loss in the enclosure is a reduction in output. This is both true for both the woofer mounting and the hole for the speaker wire.
Mount the woofer to a solid baffle: The less the enclosure flexes, the greater transfer of mechanical energy.
Multiple woofers: When using multiple woofers, mount all woofers on the same plane (side) of the enclosure. Failure to due this can cause a reduction in output efficiency.
Port Mounting: When possible, mount the port on the same plane (side) as the woofer/s. Although alternative placement can work, this will ensure trouble-free performance.
Port Air flow: Do not block the flow of air to either side of the port. Leave a minimum distance of the diameter of the port away from any obstructions that can block the airflow.
Shape matters: Do not build a perfect cube enclosure. This will alter the frequency response of the woofer due to standing waves created inside the enclosure. For rectangular shaped enclosures, the Golden Ratio between the dimensions is 0.8 to 1.0 to 1.2.
When ever possible, put an angle in the enclosure. This will reduce the possibility of destructive interference in the enclosure.
Brace the enclosure: Bracing the enclosure increases the mechanical transfer of energy and reduces the flexing of the enclosure with can reduce the efficiency of the system.
