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HOW SPEAKERS ARE BUILT: A Look Inside the Eminence Factory

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  • HOW SPEAKERS ARE BUILT: A Look Inside the Eminence Factory

    By Cobi Stein

    The first electronic loudspeaker designs were introduced in the 1920s, and while there have been many improvements in component materials over the decades, not much has changed in the basic functionality of a loudspeaker: a permanent magnet interacts with an electromagnet (voice coil) to move a cone back and forth, which produces sound waves. Founded in 1966, Eminence has been a leading
    supplier of loudspeakers for professional audio, musical instrument, car audio, and home hi-fi applications to many of the world’s most recognized brands. Here’s how we do it.

    Voice Coils
    At the heart of every loudspeaker is the electromagnet, more commonly known as the voice coil, which uses an electric current to produce its magnetic field. The first step in making a voice coil is the winding process. A Kapton, Nomex, paper, or fiberglass former material is wrapped around a steel mandrel, which is used to keep it round during this process. These former materials are chosen for their thermal power handling and sonic contributions. Next, copper or copperclad aluminum wire is wound to specific lengths according to the design. The coil is then baked to cure the adhesive used for coating the wire. A “backup” bead of glue is added at the top and bottom of the winding. As an added quality assurance measure, we doublebake our coils at this point. A paper collar is wrapped around the coil to protect the
    wire that goes from the winding to the tensile leads. These leads are spliced onto each end of the winding. We then check the DC resistance and overall quality of each coil.

    Fun fact: Eminence makes 95% of our voice coils in-house

    Permanent Magnets
    When the voice coil receives an electric current and produces a magnetic field, it is repelled by the permanent magnet fixed to the loudspeaker basket. These magnets come in various sizes and materials, but interestingly enough, they aren’t magnets at all until the loudspeaker assembly reaches the end of our production line and passes through a magnetizer. Eminence uses ferrite, lightweight neodymium, and alnico (a combination of aluminum, nickel, and cobalt) magnets.

    Fun fact: Eminence uses over 360,000 magnets each year

    Metal Parts
    There are a few metal parts used in the construction of a loudspeaker: the basket, top plate, back plate, and core (pole piece). The top and back plates are stamped from rolls of steel in our Press Shop. The top plate is either welded or staked to the basket, and the back plate will have a round core welded or staked to the center of it. These two plates will eventually sandwich the permanent magnet.

    At Eminence we use a variety of loudspeaker basket types and sizes. Our cast-aluminum chassis range from 5" to 18" in diameter, and our stamped steel models range from 2" to 15". We alsohave a variety of powdered metal and steel cores, in both vented and solid construction, and range in size from 1" to 6".

    Once our metal parts are assembled, they are then sent through our in-house E-coat paint process. A uniform coating of cationic epoxy paint is applied over the entire surface of the metal parts to a controlled thickness of less than 0.001".

    Fun fact: Eminence purchases over 1,160 tons of American-made steel each year, the majority of it from Steel Technologies right across the street from our factory

    Soft Parts
    The soft parts of a loudspeaker are the cone, dust cap, spider, and surround. Our cones come in many different sizes and the bodies are typically made of paper. A surround can be constructed from paper,
    cloth, Santoprene, rubber, or foam. The spider and surround make up the mechanical suspension, which brings the cone back to its original resting position.

    Fun fact: Eminence purchases over 310,000 cones each year from suppliers in the USA, UK, and Malaysia

    Final Assembly
    The first step of the final assembly process is to attach terminals to the painted basket/top plate assembly. Next a rear gasket is applied to the basket to create a seal with the enclosure if the speaker is to be front-mounted. The magnet and painted back plate/core assembly are glued to the top plate, using a centering gauge to ensure a uniform magnetic gap—the narrow space in which the voice coil sits between the permanent magnet and the metal core.

    Glue is applied to the flange of the basket to attach the cone surround. A vacuum is used to remove any debris from the magnetic gap, and a Mylar gauge is inserted through the voice coil to help the assembly technician set the exact coil height for the given loudspeaker design. A bead of glue is then added around the coil to adhere it to the spider.

    The next step is to apply glue to the bottom of the spider, place it over the voice coil, and attach it to the basket. At this point the voice coil is glued to the spider. The cone is then inserted into the basket and pressed into the bead of glue that was applied earlier. Another bead of glue is applied from above to attach the cone’s neck to the voice coil. Voice coil lead wires are fed through two small pre-drilled holes in the cone and then on
    to the terminals.

    The Mylar gauge is removed from the voice coil and the exposed lead wires are covered, or “dressed,” with a rubber cement for added strength. An edge treatment is used to seal porous cloth surrounds, as well as for sonic and performance enhancements. A final bead of glue is applied to the cone to attach the dust cap, which is used to keep debris from getting inside the magnetic gap.

    Once the dust cap is in place, the speaker is sent through our C-core magnetizer, which uses a field strength of 20 tesla meters to create the magnet. A front gasket is then added to the basket, to create an enclosure seal if the speaker is to be rear mounted. The tensile leads are crimped to the terminals and any excess wire is trimmed. A small bead of glue is applied to each terminal to help fix the lead wires in place.

    The speaker is fully assembled at this point. A conveyer belt slowly moves the speakers through an oven to cure the glue, and a technician at the end of the line “sweeps” the speaker with an audio signal. This is to ensure the final product is working properly and has no visible or audible defects.

    Fun fact: 100% of the speakers manufactured in the Eminence factory are listened to by a person

    Cosmetic enhancements are applied last, including a back plate label and a rubber magnet boot to provide a clean and professional look. If the product is going to an OEM customer, it is typically stacked
    on a pallet, shrink wrapped, and staged in our shipping department. Otherwise, the speaker is sent to our packing line to be boxed and stored in our warehouse for distribution to virtually any destination in
    the world.

    In addition to pro sound, car audio, signaling and musical instrument applications, Eminence products can be found in cinemas, arenas, aircraft carriers, Carnegie Hall, and even the Smithsonian Museum as part of a sound art exhibit.

    About the Author
    Cobi Stein is formerly the Marketing Director / Artist Relations Manager for Eminence. He is also a drummer for the disco band, 100% Poly.

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