How Do Speakers Work? [video]

Abstract

How Do Speakers Work? Hello. I’m Dr. Bruce Denardo here in the Physics Department of the Naval Postgraduate School in Monterey, California. The purpose of a loudspeaker is to convert electrical energy into acoustic energy. How does a conventional or dynamic loudspeaker work? In this video, we will present demonstrations and explanations of the effect. It would almost certainly help if you first view two other videos: One on the fundamental magnetic force on a moving charged particle, and the other on the jumping wire and reverse demonstration. LOUDSPEAKER Here is a cut-away of an actual loudspeaker. A circular coil of wire called the voice coil is attached to a cone. The voice coil sits in the gap of a circular magnet. The magnetic field in the gap is radial, pointing directly outward or inward. The brown flexible spider keeps the voice coil centered in the gap so that it does not rub on the magnet. The black flexible surround attaches the open end of the cone to the frame of the loudspeaker. Here is a diagram of a loudspeaker. A current through the voice coil in the magnetic field causes a force on the coil and thus the cone. The action here is essentially the same as the jumping wire demonstration! Here is a damaged voice coil and cone that was removed when a 15-inch diameter 1000-watt loudspeaker was being “reconed.” The voice coil is wrapped around a lightweight cylinder for rigidity and for attachment to the cone. The current normally oscillates in a loudspeaker. For outgoing current on the right and thus ingoing current on the left, there is an upward force on the cone, by the right-hand rule. When the current is reversed, the force is downward. The cone thus oscillates, generating compressions and expansions of air which travel outward. This is sound! The loudspeaker transduces electrical energy into acoustic energy. We use a function generator and amplifier to drive a loudspeaker. The frequency is 200 Hz (which is 200 cycles per second). Note that the loudspeaker here is a raw loudspeaker; it is not enclosed in a cabinet. This substantially reduces the loudness because the sound waves produced on either side of a raw loudspeaker bend (or diffract) around the loudspeaker and tend to cancel each other. We prefer to use a raw loudspeaker due to its simplicity and also because it is convenient for the next demonstration. LOUDSPEAKER AS A MICROPHONE Is a conventional loudspeaker reversible? That is, can the loudspeaker be used as a microphone which detects sound? In the video of the jumping wire demonstration, we moved a wire in the magnetic field and observed a voltage on an oscilloscope. The jumping wire is reversible. Essentially the same effect should occur for a loudspeaker! Incoming sound causes motion of the cone and thus the voice coil. The motion of the voice coil in the magnetic field should produce a voltage across the coil, but the effect may be too small to observe. Let’s do an experiment. The loudspeaker is now directly connected to an oscilloscope. [Speak, clap, whistle] As you can see, sound produces a varying voltage on the oscilloscope. Touching the cone also produces a voltage. Young children love this demonstration because they can “see” their screams on an oscilloscope. For more mature audiences, we connect the amplifier to another loudspeaker, which is used to produce sound that is detected by the original loudspeaker, as you can see. It is interesting that a loudspeaker acting as a microphone can actually be useful. This was done during World War II for the first detonation of a nuclear weapon. A normal microphone could have been overloaded or damaged by the blast wave. A conventional loudspeaker has a circular coil of wire in the gap of a circular magnet. The coil is attached to a cone. When an oscillatory electric current is passed through the coil, there is an oscillatory magnetic force on the coil, which moves the coil and cone, thus causing sound to be emitted. The loudspeaker can act in reverse, as a microphone, although it is not very sensitive and is subject to noise. A conventional loudspeaker is not the only kind of loudspeaker! An unusual and interesting type is an electrostatic loudspeaker. This works in a fundamentally different way than a conventional loudspeaker, and has its own advantages and disadvantages. We will demonstrate and explain an electrostatic loudspeaker in another video. Physics lecture demonstrations are always fascinating, and the quest for them never ends. This is the Physics Department of the Naval Postgraduate School, and I’m Dr. Bruce Denardo. Thank you.