The Isobaric or isobarik loudspeaker
construction technique was originally introduced by Harry F. Olson
in the early 1950s. It is derived from the term Isobaric
, which is of Greek origin: "Iso" meaning "constant" and "Baric" meaning "pressure". It defines the operational characteristics of the use of at least two woofers
(bass drivers) in a loudspeaker unit.
The use of isobaric loading in a loudspeaker in practical terms is to lower bass frequency response.
The two bass drivers are coupled to work together as one bass unit: they are mounted one behind the other in a casing to define a chamber of air of constant pressure in between. The two drivers are placed either "cone to magnet" and wired in phase with one another or "cone to cone" or "magnet to magnet" and wired out of phase with one another so that cones move together when driven with an audio signal, keeping the pressure of air in the chamber substantially constant (the "isobaric" condition). The loudspeaker which is mounted on the front wall of the cabinet thus operates under substantially "ideal" conditions, mimicking its potential performance in free space. This holds true only in the region above the resonant frequency.
The two drivers operating in tandem exhibit:
- A resonant frequency that is lower than that of the single driver by a factor of .707 (i.e. multiplied by the square root of 2)
- Twice the moving mass compared to the single driver (this is essentially the same statement as the previous one), without the concomitant rise in mechanical Q factor an ordinary doubling of mass would entail
- Lower even-order distortion above the resonant frequency, due to the lack of air compression in the front chamber
- Halving of the resistance if the drivers are connected in parallel (the most common arrangement), which in turn doubles the electrical current requirements for the amplifier
- Obviously, doubling the driver cost
The result is that the coupled driver pair (iso-group) can now produce the same frequency response in half the box volume that a single driver of the same type would require. For example, if a speaker is optimized for performance in a 40 litre enclosure, one iso-group of the same speakers can achieve the same low frequency extension and overall response characteristics in a 20 litre enclosure. Stated alternatively, an isobaric array mounted in a box of the same volume as a single driver will produce half an octave more of bass extension.
Any non-linear behavior of the speakers affect sound pressure within the chamber, and could give rise to distortion components. These may occur if the speaker driven to high levels, or if the speaker coils in the internal unit progressively overheat. These distortions may be attenuated by absorptive material inside the cabinet.
List of isobaric speakers