• LA-208

LA-208 is a compact three-way, three - element full range loudspeaker system.

 

The Enclosure

The theoretical continuous line source with essentially no space at all between individual devices (loudspeaker enclosures). In practice, we can only make the devices put together as close as possible.

 

Because the physical dimensions of an array’s transducers effect the interaction between the sound waves they produce. It is most important when the wavelengths become comparable to or shorter than the space between elements. Since the wavelength of a 16KHZ is 21.3mm which is very small relative to the spacing of the elements that produce them. It is therefore the enclosure must be made flat to reduce the distance between elements.

 

Besides, the enclosures that make up the line array must be put together tightly to avoid gaps between them as these gaps are non-radiating. These non-radiating areas must be minimized. (See Figure 6) The front portion of LA-208 enclosure is made into rectangular shape and the rear portion trapezoidal to minimize the non-radiating areas. (See Figure 7)

Acoustically, the enclosure is of phase inverted (or vented) design. The advantage of this design is that the sound wave radiated from the rear of the speaker be in phase with the sound wave radiated from the speaker front by proper phase adjustment after passing through the vent. The vent itself functions as an additional radiator in this case, and its radiation can add constructively to that of the speaker over certain frequencies. It requires less speaker excursion and therefore reduced amplitude distortion for a given acoustic output in the lowest portion of its usable bandwidth. This effect is most noticeable for frequencies in the neighborhood of the system lower cutoff frequency.

 

For the same lower cutoff frequency, the enclosure can be made smaller and due to this reason, the air within the enclosure acting as an air cushion at very strong transient signals. (See Figure 1 )

Figure 1

High frequency compression driver is coupled to a device called Compression Transfer Ribbon (CTR) which is developed by iBO specifically for the Line Array Series. This device transfer the sound wave radiated from the compression driver to plane wave. The CTR is then coupled to a horn which form part of the enclosure. The specific opening of this horn and its geometry is designed to control the directivity of the high frequencies. (See Figure 8)

The enclosure is made of 18mm (11-ply) birch plywood. All joints are tenon and mortise of Chinese tradition plus screws and adhesive to ensure the rigidity and durability.

Many domestic and imported speaker enclosures using medium density fibre (MDF) board to reduce cost, but MDF has a fatal weakness. It soften when moisten and loss its rigidity. iBO rather uses expensive birch plywood to build the enclosures to ensure its strength and rigidity no matter used indoor or outdoor, rain or fine.

The enclosure is heavily braced internally and a black powder-coated perforated steel mesh grille protects the drivers from damage. The finishing is in black semi-matt texture paint. A rear connector plate fitted with two Neutrik Speakon NL4MP connectors for amplifiers and for loop-in and loop-out to additional enclosures. (See Figure3)

The Components

Low/mid-frequency sections of LA-208 use two L8/50/08PS 8-inch drivers which is developed by iBO specifically for LA-208.

  • Frame is die cast alloy
  • The cone is a special formula of carbon fiber, fiberglass and wool
  • The cone is light and thin but very rigid
  • Magnet is of high flux y30 strontium ferrite weights 3.45kgs
  • The diameter of the voice coil is 2-inch edge wound with flat aluminum wire. The coil former is of Kapton. (See Figure 9 and 10)

Both L8/50/08PS operate on 50HZ to 500HZ band , on 500HZ to 1.4KHZ, only one operates to avoid an array effect on the horizontal plane and cause lobbing error.

 

High frequencies band use one H3/75/03Z compression driver coupled to a CTR-3A plane wave generator.

The H3/75/03Z is developed by iBO featuring:

  • The diaphragm is made of titanium and its surround is Mylar. The voice coil is edge wound with flat aluminum wire, (See Figure 11 and 12) the diameter is 3-inch (75mm)

Figure 12

  • The diameter of the throat is 1.4-inch (35mm)
  • The slits pattern of the phase plug is of circumferential type
  • y30 high flux strontium ferrite is employed and the magnetic structure weights 6.05kgs
  • CTR-3A is made from die-cast resin with several equal acoustic passages and the sound wave radiated from the compression driver transfer to plane wave after passing thru these passages. (See Figure 17)

Crossover Network

The sensitivity of LA-208’s high frequency component (106.5 dB w/m) is much higher than the low frequency component (96 dB w/m). The difference is more than 10 dB. If a build-in passive crossover network is employed, the sensitivity of LA-208 can only be 96 dB w/m. It does not make sense to sacrifice the 10 dB of the high frequency component. Besides, any passive crossover network have an insertion loss. The actual sensitivity of LA-208 would be less than 96 dB w/m. We decided not to employ passive crossover network for LA-208 and recommend using external speaker processors that incorporate delay and equalization functions such as iBO D026 (2 in, 6 out) digital speaker processors.

 

Applications and Technical Specifications

LA-208 can be used in conjunction with any of the LA-215B and LA-218B subwoofers to form a four-way system.

 

Major applications are front of the house speaker of mid-scale sound reinforcement system such as touring, arenas, stadiums and theaters. It works well both indoor and outdoor.

 

Technical Specifications and diagrams (See below)

Rigging

 

 

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