1. A voice activity detector comprising a first primary voice detector; a feature extractor; a background estimator, said voice activity detector being configured to output a speech decision (vad_flag) indicative of the presence of speech in an input signal based on at least a primary speech decision (vad_prim_A) produced by said first primary voice detector, the input signal being divided into frames and fed to the feature extractor, said primary speech decision being based on a comparison of a feature extracted in the feature extractor for a current frame of the input signal and a background feature estimated from previous frames of the input signal in the background estimator; said first primary voice detector having a memory in which previous primary speech decisions are stored, said voice activity detector further comprises a short term activity detector, said voice activity detector is further configured to produce a music decision (vad_music) indicative of the presence of music in the input signal based on a short term primary activity signal (αvad_act_prim_A) produced by said short term activity detector based on the primary speech decision produced by the first primary voice detector, said short term primary activity signal is proportional to the presence of music in the input signal, said short term activity detector is provided with a calculating device configured to calculate the short term primary activity signal based on the relationship: vad_act ⁢ _prim ⁢ _A = m memory + current k + 1 where vad_act_prim_A is the short term primary activity signal, m memory+current is the number of active decisions in the memory and current primary speech decision, and k is the number of previous primary speech decisions stored in the memory.
3. The voice activity detector according to claim 1 , wherein said short term activity detector is further provided with a filter to smooth the short term primary activity signal and produce a lowpass filtered short term primary activity signal (vad_act_prim_A_lp).