The idea of the horn loudspeaker was first conceived by Ernst Siemens in the late 19th century, but the true development of such devices occurred in the 1920s and 1930s, as visionaries like Sir Charles Parsons (renowned for his work in steam turbine technology) refined the concept, utilising conical horns to amplify sound waves effectively. The natural amplification and 'loading' effect meant that quite large SPLs could be achieved with the relatively modest power of valve amplifiers of the day.
Then renown engineers such as Paul Voigt and Edwin Jensen played important roles in the proliferation of horn loudspeakers during this era, particularly in theatres, radio broadcasting, and cinemas. The unique conical design of horns catered to the demand for powerful sound projection in large public spaces.
By the mid-20th-century, advancements included exponential and tractrix horns, addressing issues related to distortion and frequency response, thereby enhancing audio fidelity. Today, contemporary horn loudspeakers incorporate state-of-the-art technologies, offering precise control over sound dispersion and frequency response but with high efficiency too – thus allowing us to enjoy low-powered valve amplifier designs that would struggle to drive non-horn-loaded designs.