A droplet generation device comprising: a rigid reservoir split into at least two regions (42, 43; 47, 48) by a substantially liquid-impermeable barrier, the at least two regions comprising a liquid-containing region (43; 47), and a non-liquid containing region (42; 48); a perforate membrane connecting the liquid-containing region (43; 47), containing, in use, a liquid to be dispensed, to the atmosphere, such that vibration of the perforate membrane causes the liquid to be ejected through the perforate membrane into the atmosphere; and a pressure control system consisting of one or more valves connected only to the non-liquid containing region (43; 47) of the reservoir , in which: at least one valve is configured to allow gas to enter the non-liquid containing region (43; 47) of the reservoir in response to a pressure difference (ΔP) across the at least one valve of a certain amount, ΔP in , wherein -0.1 kPa > ΔP in > -10 kPa; and at least one valve is configured to allow gas to leave the non-liquid containing region (43; 47) of the reservoir in response to a pressure difference (ΔP) across the at least one valve of a certain amount ΔP out , wherein 0 kPa < ΔP out < +0.1 kPa; wherein the pressure difference (ΔP) is the absolute pressure of gas in the non-liquid containing region (43; 47) of the reservoir minus the absolute atmospheric pressure, with ΔP in < AP out , characterised in that , the barrier consists, at least in part, of a gas-permeable membrane (411; 45; 46; 58). a rigid reservoir split into at least two regions (42, 43; 47, 48) by a substantially liquid-impermeable barrier, the at least two regions comprising a liquid-containing region (43; 47), and a non-liquid containing region (42; 48); a perforate membrane connecting the liquid-containing region (43; 47), containing, in use, a liquid to be dispensed, to the atmosphere, such that vibration of the perforate membrane causes the liquid to be ejected through the perforate membrane into the atmosphere; and a pressure control system consisting of one or more valves connected only to the non-liquid containing region (43; 47) of the reservoir , in which: at least one valve is configured to allow gas to enter the non-liquid containing region (43; 47) of the reservoir in response to a pressure difference (ΔP) across the at least one valve of a certain amount, ΔP in , wherein -0.1 kPa > ΔP in > -10 kPa; and at least one valve is configured to allow gas to leave the non-liquid containing region (43; 47) of the reservoir in response to a pressure difference (ΔP) across the at least one valve of a certain amount ΔP out , wherein 0 kPa < ΔP out < +0.1 kPa; at least one valve is configured to allow gas to enter the non-liquid containing region (43; 47) of the reservoir in response to a pressure difference (ΔP) across the at least one valve of a certain amount, ΔP in , wherein -0.1 kPa > ΔP in > -10 kPa; and at least one valve is configured to allow gas to leave the non-liquid containing region (43; 47) of the reservoir in response to a pressure difference (ΔP) across the at least one valve of a certain amount ΔP out , wherein 0 kPa < ΔP out < +0.1 kPa; wherein the pressure difference (ΔP) is the absolute pressure of gas in the non-liquid containing region (43; 47) of the reservoir minus the absolute atmospheric pressure, with ΔP in < AP out , characterised in that , the barrier consists, at least in part, of a gas-permeable membrane (411; 45; 46; 58). <EOS>