A method of estimating a sound source location for a sound source by particle filtering, the method comprising: generating an initial set of particles, the set of particles representing a probability density function for a state variable comprising the sound source location; iterating the steps of, for each particle of the set of particles: generating a state value for the particle at the current time instant in response to a state value of the particle at a previous time instant, and generating a weight for the particle at the current time instant in response to a weight of the particle at the previous time instant and a measurement at the current time instant; the step of generating the weight for the particle at the current time instant comprising: determining a first acoustic transfer function from the sound source to a first recording position at the current time instant, determining a second acoustic transfer function from the sound source to a second recording position at the current time instant, determining the weight in response to a correlation between the first and the second acoustic transfer functions; the weight for the particle being determined in response to a value of the correlation for a relative delay offset corresponding to the state value of the particle, the method further comprising the step of generating a state variable estimate comprising a sound source location estimate for a current time instant in response to a combination of state values for the set of particles at the current time instant, the contribution from each particle of the set of particles depending on the weight of the particle. generating an initial set of particles, the set of particles representing a probability density function for a state variable comprising the sound source location; iterating the steps of, for each particle of the set of particles: generating a state value for the particle at the current time instant in response to a state value of the particle at a previous time instant, and generating a weight for the particle at the current time instant in response to a weight of the particle at the previous time instant and a measurement at the current time instant; the step of generating the weight for the particle at the current time instant comprising: determining a first acoustic transfer function from the sound source to a first recording position at the current time instant, determining a second acoustic transfer function from the sound source to a second recording position at the current time instant, determining the weight in response to a correlation between the first and the second acoustic transfer functions; the weight for the particle being determined in response to a value of the correlation for a relative delay offset corresponding to the state value of the particle, the method further comprising the step of generating a state variable estimate comprising a sound source location estimate for a current time instant in response to a combination of state values for the set of particles at the current time instant, the contribution from each particle of the set of particles depending on the weight of the particle. generating a state value for the particle at the current time instant in response to a state value of the particle at a previous time instant, and generating a weight for the particle at the current time instant in response to a weight of the particle at the previous time instant and a measurement at the current time instant; the step of generating the weight for the particle at the current time instant comprising: determining a first acoustic transfer function from the sound source to a first recording position at the current time instant, determining a second acoustic transfer function from the sound source to a second recording position at the current time instant, determining the weight in response to a correlation between the first and the second acoustic transfer functions; the weight for the particle being determined in response to a value of the correlation for a relative delay offset corresponding to the state value of the particle, determining a first acoustic transfer function from the sound source to a first recording position at the current time instant, determining a second acoustic transfer function from the sound source to a second recording position at the current time instant, determining the weight in response to a correlation between the first and the second acoustic transfer functions; the weight for the particle being determined in response to a value of the correlation for a relative delay offset corresponding to the state value of the particle, the method further comprising the step of generating a state variable estimate comprising a sound source location estimate for a current time instant in response to a combination of state values for the set of particles at the current time instant, the contribution from each particle of the set of particles depending on the weight of the particle.