A gas delivery system adapted to deliver a gas to a subject may have a gases flow passage with a modulation section where the velocity of gases passing through the modulation section is modulated. The modulation section can have a first region at which the modulation of the velocity of the gases is greatest. The gases flow passage can be configured in such a way that the pressure measured at or near the first region is substantially the same as the static pressure of gases at or near a second region along the gases flow passage downstream of the first region.
