This article is copyright © 2006 by Audio Amateur Corporation. PO Box 876, Peterborough, NH 03458, USA. All rights reserved.

The system that I present is unusual, it's the Double Chamber Reflex (DCR) proposed in December 1961 on Electronics World by G.L. Augspurger and later on resumed first by R. N. Marsh (Speaker Builder 03/80) and then by David B. Weems in his articles on Speaker Builder (04/1985 and 01/1992) and in several of his books. In order not to create confusion, look at the following image that shows the Bass Reflex and four of its variations, between which the one of Augspurger-Weems:

Let's try to understand how the DCR works: the speaker is made of two chambers, one the double of the other, with the woofer placed in the bigger chamber. In the first chamber, V₁, there are two ports, the first, P₁, tunes V₁ with the outside while the second port, P₂, tunes the two chambers, V₁ - V₂ ; finally the third port, P₃, tunes V₂ with the outside. Therefore three different resonance frequencies will be created: F₁ related to V₁-P₁; F₂ related to V₁/V₂ - P₂ ; F₃ related to V₂- P₃ . Keeping V₁ = 2 x V₂ and the three ports equal to each other, in length and diameter, we will have that F₂ = 1.126 x F₁ while F₃ = 2.17 x F₁ = 1.93 x F₂ .

Looking at the impedance response of a DCR system, there will be three peaks instead of the two typical of the bass reflex; it will be possible to see also two F_B: the lower one (F₂) where the woofer sees just one chamber (V₁+V₂), as if there isn't any partition between the two chambers; the other one (F₃) is present at a little bit less than one octave of F₂ being F₃ = 1.93 x F₂ . In the next chart we have that F₂=46.1 Hz and F₃=89 Hz, measured looking at the two dips where the impedance module assumes the relative minimum; the found values of F₂ and F₃ are confirmed by applying the just said formulas.

To confirm that the DCR F₂ is equal to the F_B of the corresponding reflex system, thus taking off the separation between the two chambers, I compared the two impedances; both the systems weren't using any absorbing mat inside and the Reflex total volume was 0.8 liter bigger than the DCR one, cause of the absence of the chambers separation wood. The result confirms that at F₂ the DCR sees just one chamber (V₁+V₂), as if there isn't any partition between the two chambers:

A typical frequency response of a DCR speaker has a dip above F₃ that, according to Augspurger and Weems, is not to worry about since it's of modest amplitude and also because dips are less audible than peaks. In case the dip is very deep, Weems suggests to place some absorbing mat inside P₂; however doing so will increase the port losses altering the known parameters in a not easily predictable way. The following chart is the near field response of just the woofer, thus without having measured and added the port responses; F₂ is recognized by the big notch, since in the reflex systems the woofer less energetic emission happens at its F_b. In this chart you can notice that F₂ is equal to 44.5 Hz while in the impedance chart it is equal to 46.1 Hz: this difference is normal to happen cause , usually, acoustic data and impedance data do not agree exactly; however I suggest to use the woofer near field notch as the more reliable measure of the reflex tuning frequency. You can notice the typical DCR dip near F₃.

To understand what causes the typical DCR dip, we have to study the two external ports acoustic emissions, P₁-P₃, looking at the near field phase chart of P₁-P₃ and of the woofer: below F₂ (46.1Hz) the ports are in phase with each other but out of phase with the woofer; between F₂-F₃ the ports are in phase with each other and with the woofer as well; over F₃ (89Hz) the ports are out of phase with each other, and this is what causes the dip we were talking about.

The advantages of the DCR compared to a conventional reflex system, as reported by D. B. Keele, are a reduction of the cone excursion around F₃, with a consequent reduction of the distortion; we know that in reflex speakers the cone excursion is minimal at F_B (while in closed box the cone excursion is maximal near the F_C ), therefore the diaphragm nonlinearity distortion as well as the inter modulation one are less than in a closed box at the system tuned frequency. In a DCR the reduced cone excursion happens at two frequencies, F₂ ed F₃, thus in a wider range compared to a reflex system. Moreover the use of two chambers in a tower speaker, thus where it is easier to establish standing waves, helps to reduce the phenomenon of the resonances. To verify this assumption, I compared the impedance of the DCR with a reflex box obtained by eliminating the division panel; both systems didn't have any absorbing mat inside. The result is encouraging: the reflex peak at 180 Hz disappeared and there aren't any new dangerous peaks except the one at 89 Hz; notice how similar are the dips between the peaks, just to confirm that the F₂ of the DCR is equal to the reflex F_B.

Now let's see how to design a DCR.