This is not mine, I found it somewhere, but it could explain what you were seeing with regards to isobaric:

By electrically and acoustically coupling two woofers (coupled-chamber or compound principle), the required cabinet volume can be reduced by about half of what would be required if only one chassis were used.

Compound systems, however, are basically no different in construction than single-chassis systems. The calculation of cabinet sizes, tuning tunnels etc. is completely identical, only the (modified) TSP of the compound chassis has to be used. The acoustic (airtight) coupling and electrical interconnection of two (usually identical) chassis creates a new chassis with new TSP. If the coupling volume is practically 0 ltr. the TSP of the new compound chassis are also practically identical with the exception of the Vas, Re, Le and the efficiency.

The Vas is halved, since per chassis only "one side" of the diaphragm contributes to the acoustic power respectively the diaphragm mass is at least doubled. Therefore, the efficiency also decreases by half, SPL decreases by 3 dB. Re and Le change in dependence on the electrical connection, with parallel connection Re and Le are halved with series connection doubled.

In practice, the coupling volume is neither 0 ltr. nor 100% tight. Therefore, all TSP change more or less strongly compared to the single chassis. The air of the coupling volume has a mass of approx. 1.18 g/l which is added to the moving mass of the two single chassis. Due to the more than double mass compared to the single chassis, the resonant frequency fo decreases and Qm, Qe and thus Qt change. Since the decrease of the resonant frequency is square root proportional to the increase of the mass, the Vas of the compound chassis decreases even with a coupling volume of more than 0 lt. also only by half of the Vas of a single chassis. However, due to the increased mass, the efficiency drops by more than 3dB compared to the single chassis.

K ~ correction factor due to chamber volume in Ltr.

Vas'=Vas/2

K = (2* mass + coupling volume* 1.18 g) / 2 / mass

fo' = fo / (K ^ 0.5)

Qm' = Qm / (K ^ 0.5)

Qe' = Qe * (K ^ 0.5)

Qt' = (Qm' * Qe') / (Qm' + Qe')

N%' = 0.000000000964 * fo' ^ 3 * Vas' / Qe'

SPL' = 112 + 10 * (Log(N%') / Log(10))

e: Added some more explanation.

By electrically and acoustically coupling two woofers (coupled-chamber or compound principle), the required cabinet volume can be reduced by about half of what would be required if only one chassis were used.

Compound systems, however, are basically no different in construction than single-chassis systems. The calculation of cabinet sizes, tuning tunnels etc. is completely identical, only the (modified) TSP of the compound chassis has to be used. The acoustic (airtight) coupling and electrical interconnection of two (usually identical) chassis creates a new chassis with new TSP. If the coupling volume is practically 0 ltr. the TSP of the new compound chassis are also practically identical with the exception of the Vas, Re, Le and the efficiency.

The Vas is halved, since per chassis only "one side" of the diaphragm contributes to the acoustic power respectively the diaphragm mass is at least doubled. Therefore, the efficiency also decreases by half, SPL decreases by 3 dB. Re and Le change in dependence on the electrical connection, with parallel connection Re and Le are halved with series connection doubled.

In practice, the coupling volume is neither 0 ltr. nor 100% tight. Therefore, all TSP change more or less strongly compared to the single chassis. The air of the coupling volume has a mass of approx. 1.18 g/l which is added to the moving mass of the two single chassis. Due to the more than double mass compared to the single chassis, the resonant frequency fo decreases and Qm, Qe and thus Qt change. Since the decrease of the resonant frequency is square root proportional to the increase of the mass, the Vas of the compound chassis decreases even with a coupling volume of more than 0 lt. also only by half of the Vas of a single chassis. However, due to the increased mass, the efficiency drops by more than 3dB compared to the single chassis.

K ~ correction factor due to chamber volume in Ltr.

Vas'=Vas/2

K = (2* mass + coupling volume* 1.18 g) / 2 / mass

fo' = fo / (K ^ 0.5)

Qm' = Qm / (K ^ 0.5)

Qe' = Qe * (K ^ 0.5)

Qt' = (Qm' * Qe') / (Qm' + Qe')

N%' = 0.000000000964 * fo' ^ 3 * Vas' / Qe'

SPL' = 112 + 10 * (Log(N%') / Log(10))

e: Added some more explanation.

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