Stereo Speaker

Background

A loudspeaker or speaker is a device that converts electrical energy waves into mechanical energy waves or audible sounds. Sound is produced by the vibration of an object. This vibration sets up a series of ripples or waves much like what is seen when a stone is thrown into a pond. Speakers reproduce sound waves (or audio) at various frequencies. The frequency is the rate at which the particles in the air vibrate. Sound that the human ear can hear is from about 20 hertz (Hz) to 20,000 Hz or 20 kilohertz (kHz). Speakers are used in all types of communications and entertainment equipment such as radio and television receivers, tape recorders, telephone answering machines, baby monitors, and stereo home entertainment systems

History

The basic principle of a dynamic speaker has changed little since it was patented by Ernst Siemens in 1874. Siemens described his invention as a means for obtaining mechanical movement of an electrical coil from electric currents that flowed through it. The original intent of his invention was to move a telegraph arm. Alexander Graham Bell applied the principles of Siemens device to the telephone two years later. Thomas Edison is credited with inventing the loudspeaker as it is known today. It consisted of a flexible diaphragm (cone) attached to the throat of an acoustical horn. The cones of early loudspeakers used various materials such as thin metal sheets, leather, and paper. Paper was (and still is) used in the construction of speaker cones because it is cheap and readily available.

Raw Materials

The dynamic speaker has not changed in decades. The frame is made from stamped iron or aluminum. The permanent magnet is a ceramic ferrite material consisting of iron oxide, strontium, and a ceramic binder. The cone, surround, and spider are made of treated paper coated with an adhesive glue. The voice coil consists of a plastic bobbin with fine gauge insulated copper wire wound around it.

Design

The most common speaker is the dynamic speaker. It consists of a frame, permanent magnet, soft iron core, voice coil, and cone. The frame supports the cone and permanent magnet assembly. The voice coil consists of an insulated wire wound around a plastic bobbin. One end of the bobbin is attached to the cone and the body of the bobbin slides over the soft iron core. The wires from the voice coil are connected to an audio amplifier. When electrical audio signals from the amplifier are applied to the voice coil, an electromagnetic field is produced around the voice coil. This causes the voice coil to move back and forth along the soft iron core which aids or opposes the magnetic field produced by the permanent magnet. The movement of the voice coil causes the attached cone to vibrate and produce sound. There are four major types of speakers: full-range, tweeter, midrange, and woofer. The full-range speaker can reproduce most of the audio sound spectrum. However, a single speaker cannot accurately reproduce the entire audio frequency range of the human ear. Therefore, other speakers were designed to overcome the limitations of the full-range speaker. The tweeter is designed for the higher audio frequencies or treble sounds in the 4-20 kHz range. They are very small, usually around 2 in (5.1 cm) in diameter or under. The midrange speaker reproduces sounds in the 1,000 Hz to 10 kHz frequency range. Their size ranges from 2 in to 8 in (5.1—20.3 cm) in diameter. The woofer reproduces the bass or very low sounds in the 20-1,000 Hz frequency range. A subwoofer can extend this to as low as 3 Hz. Woofers range from 4 in to 15 in (10.2-38.1 cm) in diameter with 10-12 in (25.4-30.2 cm) in diameter being the most common.