INVESTIGATION INTO THE EFFECTS OF THE GEOMETRY IN CONE DRIVEN MIDRANGE HORNS AND PHASE PLUGS

In professional audio, compression drivers and horns are almost always used for high frequency sound reproduction to increase sensitivity and control directivity. Horns are essentially acoustic transformers, and work by helping to match the impedance between driver and the air, leading to increased efficiency. This increase can be quite dramatic; 0.5% to 5% is common for direct radiators, whereas a well-designed horn loudspeaker may reach 20% to 40% efficiency1 . The same principal can be applied to achieve efficiency gains with paper cone drivers. But while traditional lumped parameter theory models horns accurately at low frequencies, in the mid to high frequency region the behaviour becomes more complex and the theory is much less accurate. Consequently, many of complexities of the design after the horn expansion are still considered by many to be a ‘Black Art’. This is especially the case in midrange horns where phase plugs are used which vary tremendously in size and shape, sometimes up the length of the horn. These phase plugs could potentially be used to prevent beaming and also to control directivity, since they allow the expansion of the horn to be maintained whilst controlling the flare of the horn walls.