However they can be very noisy, with increases in noise in the axial (forward and aft) direction of up to 30 dB, and tangentially 10 dB. Most of this extra noise can be found in the higher frequencies. These substantial noise problems limit commercial applications. One possibility is to enclose the contra-rotating propellers in a shroud. It is also helpful if the tip speed or the loading of the blades is reduced, if the aft propeller has fewer blades or a smaller diameter than the fore propeller, or if the spacing between the aft and fore propellers is increased.