Sealed enclosures
Sealed enclosures, also referred to as acoustic suspension enclosures, share some operating characteristics with the infinite baffle and aperiodic designs. All these designs separate the front wave of the woofer from the rear wave. Where the sealed enclosure differs in operation is how the air in the enclosure assists the mechanical performance of the woofer.
The air in the enclosure acts like a spring and helps control the woofer by limiting the cone motion below the resonant frequency of the enclosure Fc. The benefits of a sealed enclosure when designed properly is excellent mechanical power handling, low frequency roll-off rate at a rate of 12 dB/Octave, and ease of construction. The disadvantage of the design is the trade-off between power handling, maximum output, and transient response and low frequency extension.
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topDetermining Performance
The relationship between the size and volume of the enclosure and the Theile-Small parameters of the woofer determine the overall performance in free-field. The parameter that describes the relationship between the woofer and the sealed enclosure is the Qtc. Qtc represents several performance parameters: transient response, power handling and low frequency extension. With the Qtc values that are relatively low, the transient response and low frequency response is excellent but the power handling is greatly diminished. When the Qtc is high, transient response is poor, power handling is excellent, the low frequency response is poor and the frequency response is peaky above the resonant frequency, Fc of the enclosure. The range of Qtc used for car audio applications should be between 0.5 to 1.5 for acceptable results. Values outside this range will sacrifice one or more of the performance characteristics too greatly for acceptable use. Depending on your performance goals, different Qtc values should be chosen.
Qtc = 0.5
Good
Transient response
Poor
power handling
Good
Low Frequency
Qtc = 0.707
Best
compromise between power handling and frequency response
Qtc = 1.1
Best
compromise between power handling and frequency response
Slightly
degraded transient response
Qtc = 1.5
Reduced
low frequency response
Power
handling good
Poor
Transient response
About the enclosure
Woofer
is a 12 SVC (single voice coil)
Power
applied to the enclosure is 1000 watts.
Enclosure
volumes are 13 cu.ft., 2.75 cu.ft., 0.88 cu.ft., 0.5 cu.ft. respectively.
The
12 SVC can handle so much power that depending on your amplifier
power, this woofer can maximize the output by increasing the
size.
It is important to understand how the woofer reacts in different enclosure volumes. The changes in Qtc and frequency response are only part of the overall picture.
When an enclosure is too large
When an enclosure is too large as in a Qtc near or below 0.5, the suspension of air in the enclosure no longer assists the mechanical control of the woofer. This can greatly reduce the mechanical power handling of the woofer. The mechanical power handling of the woofer is reduced due to increased excursion.
Suspension Limits
In addition, potential damage can be caused by the woofer exceeded the linear suspension limits of the excursion. There are two different types of suspension limits: hard limits and soft limits. A hard suspension limit occurs when the very quickly limited, like a car hitting a brick wall. This type of limit can usually cause permanent woofer damage because the limit is caused by one surface striking another. A good example of this is when the voice coil former of a speaker reached the back plate. This is noticeable by a loud cracking noise. A soft suspension limit is when a woofer’s travel is slowed to a stop without making loud noises that cause woofer damage.
What happens when an enclosure is too small
When an enclosure is too small. There is also a mechanical power-limiting problem. In addition to the changes in frequency response and Transient response, The excursion limits become harder to reach as the enclosure size decreases. However, excursion limits are not the only factors in determining mechanical power handling. When the enclosure becomes too small, the spring effect of the air in the enclosure becomes so great that the woofer cone’s motion is greatly limited. The problem is the voice coil is still trying to move and now the movement of the speaker is really determined by how much the cone flexes. When the cone flexes, the joint between the cone and the voice coil formers can weaken and break. The potential for damage is increased because all of the energy of the motor assembly of the speaker is directed on the cone/voice coil former joint.
When looking at what the enclosure is trying to accomplish, a good analogy is comparing the spring effect of the air volume in the enclosure to the springs on your car. The goal of your car’s suspension system is to keep the best contact between the tires and the road or, in other terms, control the motion. When your springs are too soft, the enclosure is too big, the car cannot maintain control because the car leans too much. If the springs in the car are too stiff, the car cannot accommodate for the irregularities in the road and the stiffness generates great stress on the rest of the suspension system and the car itself. As with automotive suspensions, the enclosure must be balanced for the woofer and the performance goals.
Designing Sealed Enclosures
For the purest audiophile, an enclosure of 0.5 may operate best. A good overall performer is an enclosure with a Qts of 0.707. For a person who desires to maximize both mechanical power handling and frequency response a Qtc of 1.1 will work well. If power handling is your only goal, a Qtc nearing 1.5 should operate best. After determining which "Q" enclosure will best suit your performance needs, we need to determine the size of enclosure needed to achieve the Qtc when your chosen driver is selected. Table 4 gives formulas for determining the performance of your sealed enclosure.
Sealed Enclosure Formulas
Coming soon
Where:
- Qtc =The total "Q" of the woofer and the enclosure
- Qts =The total "Q" of the woofer parameters
- Vas =The total volume of air equal to the compliance of the woofer
- Vb =Needed enclosure volume of air in cubic feet
- Fs =Resonant frequency of the woofer in Hertz
- Fc =Resonant frequency of the woofer and the enclosure in Hertz
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
Where:
- Fs=Resonant frequency of the woofer in Hertz
- Qes=The total electrical "Q" of the woofer
Maximizing Performance
Sealed enclosures are the easiest to maximize the performance. Listed below are some helpful hints to make the most of your sealed low frequency system.
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. 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.
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.
