A speaker in a sealed enclosure could be used. However a small vent is required unless you also want to measure barometric pressure!
In any conventional microphone, if both sides of the diaphragm are open, low frequency signals are lost due to the fact that the place of the signal arriving behind the diaphragm is almost the same as that arriving at the front.
A similar feature also adds the directionality of a microphone where the size of the diaphragm becomes a significant fraction of the wavelength of the sound (in this case where both high and low pressure regions exist on the same side of the diaphragm). In your case, with low frequencies, you'll lose directionality - but it lets you get away with a larger diaphragm.
The other thing you need to worry about is that any microphone which produces an output proportional to the rate at which it moves (and a speaker is one of these) will produce a smaller output at lower frequencies for the same excursion distance.