This speaker design was first proposed by Olson in the 1950s. This composite system is also called an isobarik (constant pressure) system, which has some surprising advantages compared with other dual low-frequency speaker unit structures. Its physical structure and electrical structure type are shown in the following figure:
![Compound woofer system]()
The composite low-frequency speaker system has the following characteristics:
(1) Qts will be the same as a single speaker unit.
(2) Fs will be the same as a single speaker unit.
(3) Vas (and the corresponding cabinet volume Vb) will be half of a single speaker unit.
(4) The impedance will be half of a single speaker unit (assuming it is connected in parallel).
(5) When forming a push-pull structure, it will have all the advantages of a push-pull speaker structure.
(6) The sensitivity of the complex will be the same as that of a single speaker unit (the sensitivity will be increased by 3dB due to the 4Ω load, but it will be reduced by 3dB due to the doubled quality of the cone diaphragm).
The main advantage of the constant pressure method is that the cabinet volume can be halved compared to a single speaker unit, making it an ideal choice for ultra-low unit applications.
The following manufacturing details should be noted:
(1) The cabinet size must be calculated according to the closed cabinet system, using Qts and Vas/2 of a single speaker unit.
(2) Make the short pipe shown in the figure above. You can use wood to make a square shape, or use Sonotube (the cardboard used to make cement pillars) into a cylindrical shape. The length is not critical, but it is necessary to ensure that the cone-shaped diaphragm of the rear low-frequency speaker unit does not touch the magnet structure of the front low-frequency loudspeaker unit. The pipe must be sealed, and no damping material should be put in except for some sound absorber (such as felt) pasted on the pipe wall.
Wooden Speakers:
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