Custom Enclosures

MDF VS BIRCH ENCLOSURES

It is a long time debate among DIY box builders but below is a great comparison video of MDF versus Birch wood for building enclosures.

Video Description: In this episode of Amplified, Doug begins the design process of the subwoofer enclosure for the VW Mule. Today he will discuss the pros and cons of MDF vs Birch woods. We will also be taking a look at a Windows applications called Win ISD Pro.

Medium-density fiberboard (MDF) is an engineered wood product made by breaking down hardwood or softwood residuals into wood fibers, often in a defibrator, combining it with wax and a resin binder, and forming it into panels by applying high temperature and pressure. MDF is generally more dense than plywood. It is made up of separated fibers, but can be used as a building material similar in application to plywood. It is stronger, and more dense, than particle board.

A birch is a thin-leaved deciduous hardwood tree of the genus Betula in the family Betulaceae, which also includes alders, hazels, and hornbeams. It is closely related to the beech-oak family Fagaceae. The genus Betula contains 30 to 60 known taxa of which 11 are on the IUCN 2011 Red List of Threatened Species. They are a typically rather short-lived pioneer species widespread in the Northern Hemisphere, particularly in northern areas of temperate climates and in boreal climates.

Birch plywood was specified by the BBC as the only wood that can be used in making the cabinets of the long-lived LS3/5A loudspeaker.

SEALED VERSUS PORTED ENCLOSURES

Car Audio enclosures come in a variety of designs each with their advantages and disadvantages. Depending on your goals, the type of enclosure should be based on the desired applications.

The loudspeaker driver’s moving mass and compliance (slackness or reciprocal stiffness of the suspension) determines the driver’s resonant frequency (Fs). In combination with the damping properties of the system (both mechanical and electrical) all these factors affect the low-frequency response of sealed-box systems. Output falls below the system’s resonant frequency (Fc), defined as the frequency of peak impedance.

In a closed-box, the air inside the box acts as a spring, returning the cone to the ‘zero’ position in the absence of a signal. A significant increase in the effective volume of a sealed-box loudspeaker can be achieved by a filling of fibrous material, typically fiberglass, bonded acetate fiber (BAF) or long-fiber wool. The effective volume increase can be as much as 40% and is due primarily to a reduction in the speed of sound propagation through the filler material as compared to air. The enclosure or driver must have a small leak so internal and external pressures can equalise over time, to compensate for barometric pressure or altitude; the porous nature of paper cones, or an imperfectly sealed enclosure, is normally sufficient to provide this slow pressure equalisation.

PORTED/VENTED ENCLOSURES

Also known as vented (or ported) systems, these enclosures have a vent or hole cut into the cabinet and a port tube affixed to the hole, to improve low-frequency output, increase efficiency, or reduce the size of an enclosure. Bass reflex designs are used in home stereo speakers (including both low- to mid-priced speaker cabinets and expensive hi-fi cabinets), bass amplifier speaker cabinets, keyboard amplifier cabinets, subwoofer cabinets and PA system speaker cabinets. Vented or ported cabinets use cabinet openings or transform and transmit low-frequency energy from the rear of the speaker to the listener. They deliberately and successfully exploit Helmholtz resonance. As with sealed enclosures, they may be empty, lined, filled or (rarely) stuffed with damping materials. Port tuning frequency is a function of cross-section and length. This enclosure type is very common, and provides more sound pressure level near the tuning frequency than a sealed enclosure of the same volume, though it actually has less low frequency extension since the “rolloff” is steeper (24db/oct vs. 12db/oct for a sealed enclosure). Malcolm Hill pioneered the use of these designs in a live event context in the early 1970s.

Vented system design using computer modeling has been practiced since about 1985, when researchers Thiele and Small first systematically applied electrical filter theory to the acoustic behavior of loudspeakers in enclosures. While ported loudspeakers had been produced for many years before computer modeling, achieving optimum performance was challenging, as it is a complex sum of the properties of the specific driver, the enclosure and port, because of imperfect understanding of the assorted interactions. These enclosures are sensitive to small variations in driver characteristics and require special quality control concern for uniform performance across a production run. Bass ports are widely used in subwoofers for PA systems and sound reinforcement systems, in bass amp speaker cabinets and in keyboard amp speaker cabinets.

BANDPASS ENCLOSURES

Bandpass enclosures come in several configurations such as a 4th, 6th, or 8th order bandpass.

A 4th order electrical bandpass filter can be simulated by a vented box in which the contribution from the rear face of the driver cone is trapped in a sealed box, and the radiation from the front surface of the cone is directed into a ported chamber. This modifies the resonance of the driver. In its simplest form a compound enclosure has two chambers. The dividing wall between the chambers holds the driver; typically only one chamber is ported.

If the enclosure on each side of the woofer has a port in it then the enclosure yields a 6th order band-pass response. These are considerably harder to design and tend to be very sensitive to driver characteristics. As in other reflex enclosures, the ports may generally be replaced by passive radiators if desired. An eighth order bandpass box is another variation which also has a narrow frequency range. They are often used to achieve sound pressure levels in which case a bass tone of a specific frequency would be used versus anything musical. They are complicated to build and must be done quite precisely in order to perform nearly as intended.

TRANSMISSION LINE ENCLOSURES

Transmission line or T-line enclosures have several interior walls that are used like a wave guide for sound waves leaving the enclosure.

A perfect transmission line loudspeaker enclosure has an infinitely long line, stuffed with absorbent material such that all the rear radiation of the driver is fully absorbed, down to the lowest frequencies. Theoretically, the vent at the far end could be closed or open with no difference in performance. The density of and material used for the stuffing is critical, as too much stuffing will cause reflections due to back-pressure,[dubious – discuss] whilst insufficient stuffing will allow sound to pass through to the vent. Stuffing often is of different materials and densities, changing as one gets further from the back of the driver’s diaphragm.

Consequent to the above, practical Transmission Line loudspeakers are not true Transmission Lines, as there is generally output from the vent at the lowest frequencies. They can be thought of as a waveguide in which the structure shifts the phase of the driver’s rear output by at least 90°[dubious – discuss], thereby reinforcing the frequencies near the driver’s Fs. Transmission lines tend to be larger than ported enclosures of approximately comparable performance, due to the size and length of the guide required (typically 1/4 the longest wavelength of interest).

The design is often described as non-resonant, and some designs are sufficiently stuffed with absorbent material that there is indeed not much output from the line’s port. But it is the inherent resonance (typically at 1/4 wavelength) that can enhance the bass response in this type of enclosure, albeit with less absorbent stuffing. Among the first examples of this enclosure design approach were the projects published in Wireless World by Bailey in the early 1970s, and the commercial designs of the now defunct IMF Electronics which received critical acclaim at about the same time.

A variation on the transmission line enclosure uses a tapered tube, with the terminus (opening/port) having a smaller area than the throat. The tapering tube can be coiled for lower frequency driver enclosures to reduce the dimensions of the speaker system, resulting in a seashell like appearance. Bose uses similar patented technology on their Wave and Acoustic Waveguide music systems.

Numerical simulations by George L. Augspurger and Martin J. King have helped refine the theory and practical design of these systems.

BOX DESIGN RESOURCES

Free Online Volume Calculator by Subbox.com: https://subbox.pro/en/volume-calculator/

This article includes material from “Medium-density fibreboard.” Wikipedia.https://en.wikipedia.org/wiki/Medium-density_fibreboard Licensed under Attribution-ShareAlike 3.0 United States (CC BY-SA 3.0 US) https://creativecommons.org/licenses/by-sa/3.0/us/ Authors:https://en.wikipedia.org/w/index.php?title=Medium-density_fibreboard&action=historyThis article also includes material from “Birch.” Wikipedia.https://en.wikipedia.org/wiki/Birch Licensed under Attribution-ShareAlike 3.0 United States (CC BY-SA 3.0 US) https://creativecommons.org/licenses/by-sa/3.0/us/ Authors:https://en.wikipedia.org/w/index.php?title=Birch&action=history

This article includes material from “Loudspeaker enclosure.” Wikipedia.https://en.wikipedia.org/wiki/Loudspeaker_enclosure Licensed under Attribution-ShareAlike 3.0 United States (CC BY-SA 3.0 US) https://creativecommons.org/licenses/by-sa/3.0/us/ Authors:https://en.wikipedia.org/w/index.php?title=Loudspeaker_enclosure&action=history