Showing posts with label Subwoofer Design. Show all posts
Showing posts with label Subwoofer Design. Show all posts

5/24/2014

Subwoofer Design - Bandpass Dual Chamber

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Bandpass Dual Chamber Basic Theory:

The front and rear of the driver are mounted in separate enclosures and tuned to specific calculated values. Resultant output is suppose to be better than any of the other designs mentioned previously. Bose owns the rights to the exact details behind this design. They explain the theory like this, "The low-frequency speaker drivers are located between separate acoustic compression chambers inside a patented Bose Acoustimass module. As each speaker cone moves, it excites air in the chambers. Trapped in the chamber, this air acts as an acoustic spring, which interacts with the air in the port to produce more low-frequency sound with less power. The system is more efficient and requires less cone motion, which in turn produces less distortion. In the event that any otherwise audible distortion is produced, the patented design traps it inside the acoustic chambers -- so it never enters the room. The result is an Acoustimass module with no audible distortion that can be located anywhere in the listening area.

Bandpass Dual Chamber Advandages:
More efficient system within its bandpass. More control over cone movement. Less audible distortion. This doesn't necessarily mean that there is a true reduction in distortion from the driver, but that any distortion that is present form the driver can't be heard as well due to the chambers acting as filters on any unwanted noise. My opinion only.

Bandpass Dual Chamber Disadvandages:
Combined volume of both chambers may result in large overall enclosures. Very difficult to design properly. You may have to experiment a great deal before getting this design to sound acceptable. Results may vary substantially due to misalignment of both front and rear chambers as well as tuning frequency of each chamber. Drivers can be easily blown due to high compression factors because of lowered cone motion and thereby exceeding the thermal limits of the driver before exceeding its mechanical limits. The driver may in fact tear itself to pieces. There are no exact parameters or calculations for designing 6th order bandpass enclosures due to the patent owned by Bose. So if you build one, you're basically on your own. Original article sourse is danmarx.org.
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Subwoofer Design - Order Vented

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Order Vented Basic Theory:
Also known as bass-reflex, ported or vented. The driver is mounted into an enclosure which houses a large opening, port, vent or slot that extends into the cabinet a specified length. The length and area of this vent are extremely critical to the proper function of a 4th order enclosure. The port and driver contribute together to provide the desired response characteristic. The driver is generally mounted with the front facing outwards, but is not restricted to this method only. The vent which extends into the cabinet tunes the enclosure to a specific frequency (known as fB) thereby acting as a high pass filter on the driver. Driver excursion is at its minimum at fB where the vent then takes over and provides most of the output. Cut-off rate below fB is 24 dB/octave but can be varied up or down 5-6 dB depending upon the exact tuning frequency and volume of the enclosure. There are various types of alignments that all fit into the ported 4th
order category. Some common types are QB3, EBS, SBB4 and SC4. By varying the enclosure size and the tuning frequency, it is possible to achieve a variety of distinct low- frequency performances from a single driver. The vent acts by damping the load produced by the driver above fB causing it to behave somewhat as if it were in a sealed enclosure. Best suited for drivers with an EBP near 100.0 or higher and Qts < 4.0 but is not restricted to these numbers only.

Order Vented Advandages:
Extended low frequency response. 3 dB down points (f3) are capable of being near or even below 20 Hz. Increased power handling above fB due to reduced driver excursion at and while nearing fB. More efficient system. Generally +3dB increased output over sealed enclosures due to the combined output of driver and port. More overall SPL capabilities. Deep, powerful, full, loud, inspiring, incredible, and earth shattering are common subjective terms associated with vented enclosures.

Order Vented Disadvandages:
Larger enclosure size. More difficult to accurately achieve predicted results. Misaligned enclosures can result in very poor bass quality. Very accurate T/S parameters of actual driver is required. Although sometimes you can get away with using manufacture’s specifications. Driver unloading or bottoming out below fB is very common. Xmax is reached easily below fB and may cause sever damage the the driver's suspension, voice coil or cone. This usually requires the need to install additional high pass filtering below fB. But is not a always a necessity as long as power levels and frequency content are kept within reason. Transient response is degraded, yielding typical group delay curves as high as 50 ms. Muggy, boomy, sluggish, one-note, slow, and inaccurate are common subjective terms associated with vented enclosures. Port diameter must be large to avoid unwanted port noise, which in turn requires the port to be long for any given Fb, which then drives up the volume of the enclosure, sometimes to undesirably large proportions. Port chuffing if port area is not kept in check.
Order Vented Best Applications:

Where the deepest and loudest bass is necessary. Where size is not a huge issue but may still be a definite factor. For Home Theater and music. May be best suited for sound reinforcement, theater, live performances, DJ and other situations where lots of loud deep bass is needed and transient response is less critical.
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Subwoofer Design - Order Acoustic Suspension

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Order Acoustic Suspension Basic Theory:

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 desired system Q which defines the response characteristics of the driver and enclosure. Q values may range between the 0.5 and 1.5 - with 0.5 being overdamped, 1.5 being underdamped, and 0.7 being critically damped or 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 a general rule only, sealed enclosures may be 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 values. 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 are 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.

Order Acoustic Suspension Advandages:

2nd order sealed enclosures are simple to design and offer outstanding performance in a wide variety of applications. They are easy to model with software and get 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/or 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. 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 typically 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.

Order Acoustic Suspension Disadvandages:
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 efficient 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 undesireable enclosure volumes.

Order Acoustic Suspension 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. Original article sourse is danmarx.org.
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