CABINET DESIGN
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HISTORY OF SPEAKER ENCLOSURE DESIGN
How a horn loudspeaker works. (A) compression driver (B) horn
THE HORN
The earliest speakers were horns, made up of a small compression driver composed of an electromagnet and a metal diaphragm in a small sealed compartment, connected to a large horn, which spreads the sound out to the air, turning the small, high-pressure waved produced by the diaphragm into larger, lower-pressure waves. Since the horn is essentially it's own cabinet, the speaker enclosure was not necessary, and early radios were often just an amplifier and a separate, standalone horn. This style of speaker is still widely used for high-frequency production, and in public-address systems, due to it's efficiency and directivity, which means that it sends sound only where it is pointed, reducing echoes and reflections. However, it did not work as well for lower-frequency sound due to the size of horns required, leading to the development of the conventional loudspeaker in the 1920's.
The earliest speakers were horns, made up of a small compression driver composed of an electromagnet and a metal diaphragm in a small sealed compartment, connected to a large horn, which spreads the sound out to the air, turning the small, high-pressure waved produced by the diaphragm into larger, lower-pressure waves. Since the horn is essentially it's own cabinet, the speaker enclosure was not necessary, and early radios were often just an amplifier and a separate, standalone horn. This style of speaker is still widely used for high-frequency production, and in public-address systems, due to it's efficiency and directivity, which means that it sends sound only where it is pointed, reducing echoes and reflections. However, it did not work as well for lower-frequency sound due to the size of horns required, leading to the development of the conventional loudspeaker in the 1920's.
THE LOUDSPEAKER
When the first modern, paper-cone loudspeakers were introduced, little thought was given to the design of the cabinets, which were designed for aesthetic and practical reasons, with no thought given to their acoustic consequences. However, it was quickly noted that the design of the cabinet has a significant effect on the bass response of the speaker. Early cabinets often had open backs, which reduced weight and materials costs, made the unit easier to service, and allowed the electronics to cool from convection (Early amplifiers used vacuum tubes, which are essentially rudimentary transistors that operate similarly to an incandescent light bulb. They are inefficient, and produce a considerable amount of heat.). Since the waves from the back of the speaker are out of phase with those of the front, and that phase difference is controlled by the design of the enclosure (and in the case of an open-back cabinet, the position of the cabinet and what is behind it), the waves can add together either constructively or destructively, with this effect occurring differently at different frequencies. This is called Comb Filtering, because it produces a sound spectrum that looks like a comb, with some frequencies boosted upwards and some cut downwards. In addition, certain cabinet dimensions could produce standing waves, causing the entire cabinet to oscillate, boosting certain frequencies and causing undesirable mechanical vibration to the amplifier. This is clearly undesirable. However, engineers quickly realized that it might be possible to use this interference boost the response of certain desirable frequencies, increasing the efficiency of the speaker, while reducing the response of undesirable frequencies that lie outside of the intended frequency range of the system.
When the first modern, paper-cone loudspeakers were introduced, little thought was given to the design of the cabinets, which were designed for aesthetic and practical reasons, with no thought given to their acoustic consequences. However, it was quickly noted that the design of the cabinet has a significant effect on the bass response of the speaker. Early cabinets often had open backs, which reduced weight and materials costs, made the unit easier to service, and allowed the electronics to cool from convection (Early amplifiers used vacuum tubes, which are essentially rudimentary transistors that operate similarly to an incandescent light bulb. They are inefficient, and produce a considerable amount of heat.). Since the waves from the back of the speaker are out of phase with those of the front, and that phase difference is controlled by the design of the enclosure (and in the case of an open-back cabinet, the position of the cabinet and what is behind it), the waves can add together either constructively or destructively, with this effect occurring differently at different frequencies. This is called Comb Filtering, because it produces a sound spectrum that looks like a comb, with some frequencies boosted upwards and some cut downwards. In addition, certain cabinet dimensions could produce standing waves, causing the entire cabinet to oscillate, boosting certain frequencies and causing undesirable mechanical vibration to the amplifier. This is clearly undesirable. However, engineers quickly realized that it might be possible to use this interference boost the response of certain desirable frequencies, increasing the efficiency of the speaker, while reducing the response of undesirable frequencies that lie outside of the intended frequency range of the system.
THIELE SMALL PARAMETERS: SPEAKERS AND CAB DESIGN
In order to design a proper enclosure for a given speaker, we need to know something about how that speaker will react in a given setting. This requires some set of standardized measurements by which loudspeakers from different manufacturers can be compared and selected, and enclosures can be correctly tuned for them. This set of standardized measurements is known as a loudspeaker's Thiele Small Parameters, named after Neville Thiele of the Australian Broadcasting Commission and Richard Small of the University of Sydney, who together developed the modern system of loudspeaker analysis. While the tables developed by Thiele were found to be inaccurate, because he neglected to consider energy lost through the vibration of the enclosure, his basic system of breaking down the performance of a speaker into a set of fundamental parameters lives on. These fundamental parameters are
In addition, the Thiele Small Parameters include a number of qualitative descriptions, which serve to characterize the performance of the driver and help engineers design ports and cabinets.
- Sd - Projected area of the driver diaphragm, in square meters.
- Mms - Mass of the diaphragm/coil, including acoustic load, in kilograms.
- Cms - Compliance of the driver's suspension, in meters per newton.
- Rms - The mechanical resistance of a driver's suspension in N·s/m
- Le - Voice coil inductance measured in millihenries (mH) (measured at 1 kHz).
- Re - DC resistance of the voice coil, measured in ohms.
- Bl - The product of magnet field strength in the voice coil gap and the length of wire in the magnetic field, in tesla-meters (T·m).
- Vas - The volume of air which creates the same amount of compliance as the loudspeakers physical suspension.
In addition, the Thiele Small Parameters include a number of qualitative descriptions, which serve to characterize the performance of the driver and help engineers design ports and cabinets.