Sound absorber

1. A sound absorber for an engine of an aircraft, the sound absorber comprising:

a base body having a bottom plate, a wall enclosing the bottom plate and an opening enabling entry of air into the base body; and

a tongue having a fixed end connected to the wall and a free end;

wherein the tongue is porous, and wherein the tongue has a normalised specific acoustic resistance not less than 0.5 ρc and not more than 10.0 ρc, where ρ is the density of the air and c is the speed of sound through the air.

2. The sound absorber of claim 1 , wherein the tongue has a normalised specific acoustic resistance not less than 1.0 ρc, where ρ is the density of the air and c is the speed of sound through the air.

3. The sound absorber of claim 1 , wherein the tongue has a normalised specific acoustic resistance not less than 2.0 ρc, where ρ is the density of the air and c is the speed of sound through the air.

4. The sound absorber of claim 1 , wherein the tongue has a normalised specific acoustic resistance not less than 3.0 ρc, where ρ is the density of the air and c is the speed of sound through the air.

5. The sound absorber of claim 1 , wherein the tongue has a normalised specific acoustic resistance not more than 5.0 ρc, where ρ is the density of the air and c is the speed of sound through the air.

6. The sound absorber of claim 1 , wherein the tongue has a central portion disposed between the fixed end and the free end, the tongue further having a longitudinal direction (L) extending from the fixed end towards the free end, a width direction (Br) extending substantially perpendicularly to the longitudinal direction (L), and a transverse direction (Q) extending substantially perpendicularly to the longitudinal direction (L) and substantially perpendicularly to the width direction (Br), the central portion having a first free edge extending in the longitudinal direction (L) from the fixed end towards the free end, and a second free edge opposite to the first free edge and extending in the longitudinal direction (L) from the fixed end towards the free end.

7. The sound absorber of claim 6 , wherein the tongue divides a space of the base body into a partial space adjacent to the bottom plate and a partial space adjacent to the opening, the two partial spaces being connected to each other via a gap, the free end being disposed at a first distance from the wall in the transverse direction (Q) and at a second distance from the bottom plate in the longitudinal direction (L), such that the first and the second distances determine a size of the gap.

8. The sound absorber of claim 6 , wherein the central portion is configured to extend at least partially straight along the width direction.

9. The sound absorber of claim 1 , wherein at least the central portion has two lateral portions extending in the longitudinal direction, with the lateral portions being connected with the wall.

10. A sound absorber assembly for an engine of an aircraft, the sound absorber assembly comprising a plurality of sound absorbers of claim 1 .

11. A gas turbine engine for an aircraft comprising the sound absorber of claim 1 .

12. A gas turbine engine for an aircraft comprising the sound absorber assembly of claim 10 .

13. The gas turbine engine of claim 11 , comprising:

an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor;

a fan located upstream of the engine core, the fan comprising a plurality of fan blades; and

a gearbox that receives an input from the core shaft and outputs drive to the fan so as to drive the fan at a lower rotational speed than the core shaft.

14. The gas turbine engine according to claim 13 , wherein:

the turbine is a first turbine, the compressor is a first compressor, and the core shaft is a first core shaft;

the engine core further comprises a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor; and

the second turbine, second compressor, and second core shaft are arranged to rotate at a higher rotational speed than the first core shaft.

15. A sound absorber for an engine of an aircraft, the sound absorber comprising:

a base body having a bottom plate, a wall enclosing the bottom plate and an opening enabling entry of air into the base body; and

a porous tongue having a fixed end connected to the wall and a free end, wherein the tongue is bent along a longitudinal direction in an arcuate shape and wherein the tongue has a normalised specific acoustic resistance not less than 0.5 ρc and not more than 10.0 ρc, where ρ is the density of the air and c is the speed of sound through the air.

16. The sound absorber of claim 15 , wherein the tongue has the shape of a branch of a parabola in the longitudinal direction, and wherein the tongue is straight in a width direction substantially perpendicular to the longitudinal direction.

17. The sound absorber of claim 16 , wherein the tongue comprises a foam or a fiber filling.