Design Theories: Sealed - 2nd Order Acoustic Suspension

Sealed - 2nd Order Acoustic Suspension

Basic Theory
Widely popularized by Acoustic Research back in the 50’s and continues to make its way into several popular commercial and DIY loudspeaker designs today. The driver is mounted in a sealed, airtight enclosure, generally with the front of the driver facing outward but is not restricted to this method only. The volume of the enclosure is chosen to achieve a specific desired system Q which defines the response characteristics of the driver and enclosure. Q values range between the 0.5 and 1.5 - with 0.5 being overdamped, 1.5 being underdamped, and 0.7 being critically flat. The total system Q (also known as Qtc) is dependent on 3 things: the volume of enclosure, the T/S parameters of driver and internal treatment compounds. A sealed enclosure is best suited for drivers with an EBP (Efficiency Bandwidth Product) of 50.0 or lower and drivers with Qts values above 0.40 but is not restricted to these exact alignments. EBP is calculated by taking the the fs of the driver and dividing it by the Qes - therefore EBP = fs/Qes. The cutoff rate is typically 12 dB/octave below f3, however higher system Q's result is a somewhat sharper roll-off (~14 dB/octave) while lower system Q's result in a slightly more shallow roll-off (~10 dB/octave). Better damping and better transients is achieved by shooting for a lower system Q which can be accomplished by either making the enclosure larger or by adding stuffing/damping material. Suitable damping materials include polyfill, Dacron, fiberglass, and acoustic foam. Box stuffing will also affect f3 by either raising it or lowering it depending upon the type and amount of stuffing used. Stuffing makes the box "appear" to be acoustically larger than it really is.
Advantages
2nd order sealed enclosures are the simplest to design yet still offer outstanding performance and can therefore be the most rewarding. They are easy to model with software and it is easy to achieve predicted results. Box size and shape are generally the least complex. Great for both beginning and advanced DIY’ers. The exact desired response characteristics can be achieved by simply designing for a particular Qtc (or system Q). Modeled performance is easily altered by varying the size of the enclosure and the amount of stuffing material used. They exhibit a very shallow cutoff rate of 12 db/octave below fB. This results in much better group delay response that may range from 1ms – 10 ms. Fast, quick, natural, smooth, tight, accurate, controlled and warm are some common subjective terms one might use to describe sealed enclosures. Transient response is the best of all enclosure types. The excursion of the driver increases as the frequency applied decreases until fB is reached after which the driver excursion begins to decrease. There is no need for subsonic filtering due to the enclosure’s natural tendency to inhibit extremely low frequencies. This results in less bottoming out of drivers at subsonic frequencies. However, this only applies for smaller enclosures. As the enclosure size gets larger, more Xmax is required in order to prevent overexertion for the same amount of input power. Sealed enclosures have more extended low frequency response than vented enclosures given the same f3 for both due to the shallower rate of roll-off. Phase shift is minimal within its normal operating frequency range.
Disadvantages
Very low frequency output is difficult to achieve without active filtering. The f3 (also know as 3dB down point) is usually fairly high, above 30 Hz in most applications and by simply increasing Vb, one cannot lower f3 for any given driver. Low f3's in a sealed enclosure can be achieved by using drivers with a very low free air resonance or Fs. Less power efficiency by about -3 dB as compared with vented enclosures. Lower over SPL capabilities. There's a strong need for drivers with a very large Xmax in order to ensure safe operation at least down to fB, especially if the box is designed for Qtc values < 0.7 Any enclosure volume that is modeled with the system Q larger than 0.707 results in higher f3. Lowest f3 is achievable only under an ideal Q = .707 alignment which may require unusually large and sometimes unacceptable enclosure volumes.
Best Applications
Best suited where a completely uncompromised sound quality is desired. Best for classical music and most rock and pop type music. Most widely used in car stereo systems where cabin gain can make up for its lack of low end <30Hz bass. Where size is an issue. Sealed boxes can be half the size of vented boxes yet can be made even smaller if a higher Q is allowed. May also be use for small to moderately sized Home Theaters. Usually is the easiest box to pass SAF (spouse acceptance factor). You should also go with sealed when the driver's T/S parameters dictate that the driver should be housed in a sealed enclosure due to a high Qts (above 0.4) or an EPB of 50 or lower - though this just a guideline and not a rule.

Source: www.danmarx.org