There are many qualities required to be a skilled sound engineer: the theoretical knowledge of sound and audio, the technical mastery of studio hardware and software, and diplomatic people skills, for example. But perhaps the most obvious (and often overlooked) quality required of all sound engineers is the ability to listen. Sound engineers must constantly evaluate the sound quality of individual sound sources in a mix and evaluate how these sound sources interrelate with one another. The ability to listen critically and make these judgements is a skill that must be learnt through trial and error and repeated practice. This free online course aims to provide you with tools for developing and refining the critical listening skills relevant to contemporary sound engineering and will be of interest to musicians, sound enthusiasts and experienced sound engineers alike. The course begins by examining the nature of sound and how properties of sound (frequency, amplitude and sound intensity, for example) are expressed. The relationship between these physical properties and the more subjective characteristics of music such as pitch, loudness, dynamic range and timbre is then examined. The remainder of the course focuses on the signal processing techniques commonly used by sound engineers when mixing multitrack recordings - for example, equalisation, compression and reverberation. On completion of the course you will have a thorough understanding of the workflow involved when mixing a multitrack recording and will have developed the critical listening skills necessary to make informed judgements regarding individual and overall sound quality.