Device to reduce strength of pressure pulses in oilwells resulting from jarring

A downhole device 10, has an upper sub 12, a hollow housing 14, a lower sub 16, and a hollow inner tube 18. Drilling fluid enters the tool at port 20 at the upper end of the upper sub 12. Drilling fluid exits the tool at port 30 at the lower end of the lower sub 16. The upper end of the inner tube 18 is attached to the lower end 22 of the upper sub 12, and has its lower end 34 free within the hollow housing. The inner tube creates an annular cavity 54. When the device 10 is lowered into the wellbore, the air inside the cavity 54 is trapped. Pulses in the wellbore fluid propagate into the device 10. The pressure wave is absorbed by the slight compression of the trapped air pocket in the cavity 54.