To smooth out the response !
An to possibly add a bit of lowest end notes.
Going from isothermal to adiabatic
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How exactly does the filling "effectively" increase the box volume? I
can imagine the filling increase the box's absorptive losses, thereby
increasing the damping. But what exactly causes the lower resonance
frequency of filled vs. unfilled? Does the filling cause an
increased air load on the driver diaphragm and/or add moving mass
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For the sealed system, we measured an 18� driver in a large sealed cabinet. Without any stuffing or other materials in the cabinet, the impedance showed a peak of 36 ohms centered at 25 Hz and a resonance or response issue in the impedance trace at about 170 Hz. After adding about 9 poly fill pillows to the enclosure, the impedance measurement showed that the peak had dropped to about 21.5 ohms in magnitude and about 23 Hz in frequency. The issues which showed up as wiggles in the impedance are smoothed out. Resistance goes up just a hair outside of the impedance peak area.
Looking at the before and after frequency response, it is easy to see that there was a large resonance issue related to the enclosure dimensions near 170 Hz. This was completely absorbed and smoothed out with the addition of the stuffing. We can also see that the overall response shape has changed a little with the very lowest and highest bass frequencies boosted just slightly (red curve). However between 16 Hz - 55 hz, the response is flatter but lower in volume. This in addition to the greatly diminished magnitude of the impedance peak, indicates that efficiency has been decreased somewhat over this range.
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The adiabatic compression of a gas causes a rise in temperature of the gas. Adiabatic expansion against pressure, or a spring, causes a drop in temperature. In contrast, free expansion is an isothermal process for an ideal gas.
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An adiabatic process occurs without transfer of heat or mass of substances between a thermodynamic system and its surroundings. In an adiabatic process, energy is transferred to the surroundings only as work.
