According to examples, an apparatus may include a chamber having a hole and a microphone unit. The microphone unit may include a first substrate having a first opening aligned with the hole of the chamber, a second substrate positioned with respect to the first substrate to form a gap between the second substrate and the first substrate, the second substrate having a second opening, and a diaphragm housed within the gap formed between the first substrate and the second substrate, in which the first opening is positioned on a first side of the diaphragm and the second opening is positioned on a second side of the diaphragm.