A sound system consists of a microphone for input, the amplifier circuitry, any volume and tone controls, and a loudspeaker for output.
Every component of the sound system is equally important, therefore engineers have coined the term "High Fidelity", or HiFi, which refers to the ability of a sound reproduction or recording system to accurately reproduce original sound in a wide range of frequencies.
This article is an overview of the design and operation of microphone and loudspeaker acoustics, and is intended to highlight the essentail workings in terms of sound physics.
Microphones
Microphones are transducers that convert sound energy into electrical energy. A vast variety of microphones have been developed for specific purposes. They range sustantially in quality:
- From the very simple carbon microphone that needs to respond to a limited frequency range of the human voice
- To the moving coil or ribbon microphone (Fig 1) that provides the broad frequency response required for recording music
Note that the moving coil microphone has a diaphragm that causes a coil to move between the poles of a magnet, creating an alternating induced voltage. The ribbon microphone has a metallic ribbon that moves in response to a pressure fluctuation, and in the same manner an electrical current is produced.
Loudspeakers
Loudspeakers are also transducers but they work in the opposite sense to microphones. That is, they convert electrical energy into sound energy. Many specialist speakers have been developed applicable to particular frequency ranges.
The loudspeaker cone system shown schematic in Figure 2 is a dynamic loudspeaker that has a cone made to vibrate by the movements of a coil, which undergoes an alternating current while sitting in a strong magnetic field. The shape of the cone directly influences the directivity and frequency response of the loudspeaker. When the cone is attached to the voice coil, a large gap above the voice coil is left exposed. A dust cap is placed on the cone to cover this air gap.
Limiting Sound Distortion
Speaker enclosures or baffles (Fig 3) limit the diffraction effects that can occur for low frequencies, where sound waves coming round the back of the speaker can cancel out waves from the front. A baffle limits these resonance effects inside the speaker but its use is not always practicable, whereas an enclosure is often more appropriate. With a better quality loudspeaker, the sound pressure level becomes more uniform in the room.
Summary
A sound system is made up of many parts, each needing to be of high quality. Microphones are transducers that convert mechanical energy (pressure waves) into electrical energy. Loudspeakers work in reverse, converting the electrical signal back into mechanical pressure waves, which our ears can decode. Baffles and enclosures around speakers improves the speaker quality, making sound more uniform and enjoyable in the room.
The reader may be interested in more detail about this topic, or the measurement of sound.
Harry P. Schlanger - Hello, I started out as a physicist working for research organisations. Mostly in the area of heat transfer in solids and porous media. ...
