The book "Content-Based Audio Search" by Pedro Cano is aimed at professionals in the field of audio information retrieval. It addresses the challenges and opportunities of content-based audio search and explores ways to close the existing semantic gap that currently limits the widespread use of content-based audio search engines. While traditional music search engines heavily rely on human-generated metadata, which is often time-consuming and error-prone, this work aims to enhance the relevance of automated methods. The dissertation is divided into two parts: the first part analyzes the strengths and weaknesses of audio fingerprinting as a purely low-level audio description technique. The second part discusses the necessity of a knowledge base that encodes general knowledge to bridge the semantic gap. It presents a system for retrieving sound effects that utilizes both low-level and semantic technologies.