Question

what is the speed of sound in vulcanised rubber?

Answer 1

since sound can't travel in vacuum so speed is zero

Answer 2

Speed of sound in thin long solid bars is given by 

v=\sqrt{\frac{Y}{\rho}}v=ρY​​ 

  Young's modulus for rubber Y = 4 * 10⁶ N/m² and  density ρ = 1.2 or so.

   Speed of sound in rubber using the above formula =  57.7 m/sec. This is much smaller than the speed of sound in air, which is 340 m/sec at atmospheric pressure/Temperature.

   For vulcanized rubber the Young's modulus Y = 2.8 * 10⁶ N/m² and the density = around  920 kg/m². So speed of sound = 55.1 m/sec.

    Vulcanized rubber is even softer than rubber.  So the pressure changes in it, due to sound waves are not much. The particles in vulcanized rubber do not vibrate or oscillate as fast as in other solids or in air. 

     Sound propagates only if the particles of the media oscillate. Then there are pressure differences that propagate.  Vulcanized Rubber particles are very soft and they compress and expand side ways. They absorb some pressure from the sound waves. The vibrations of the particles is not perfect and elastic. It is reduced as compared to hard solids

   Hard metallic objects, if you hit it, it vibrates for a long time.  Soft objects like rubber regain their shape back quickly and stop vibrating quickly.  That property is the one that makes velocity of sound more in hard metals and less in rubber.