Loudspeaker design tips

Often DIY loudspeaker designers/builders will spend many hours playing with a crossover to obtain a flat on-axis response but find that the resulting sound is very disappointing. In our experience there is a key reason for this...

It would seem intuitive that the flatter you can make the on-axis response of a loudspeaker the better it will sound, this is not always the case.

Why flat on-axis response can sound poor

The off-axis response is equally important because we are usually listening to loudspeakers in a 'small room' (any room in your house is a considered to be small room) and early reflections from room boundaries will smear the sound if the off-axis response is different to the on-axis (direct) response - early reflections are those occurring within about 10 milliseconds following the direct sound.

So in the design of a conventional box speaker it important to take a series (increments of 5 degrees) of off-axis measurement out to 90 degrees, the closer this response is to the on-axis response the better sounding the speaker will be in your room. You may find that there is a compromise angle and I have found that setting up the speaker at around 22.5 degrees for optimum flatness can give the optimum results... you will find that a toe in of the speakers to this flat angle will give a more acceptable sound as the on-axis sound may well have a rather bright response after optimisation.

You may find that 2-way speakers are more prone to larger differences between on and off-axis responses due to the directional performance of the mid-bass driver - you may see quite a dip in the response as you go further off axis. If the tweeter can handle a reduction in the crossover frequency then that may improve the off-axis response, however with a second order filter slope 1.5kHz is probably the limit to avoid excessive tweeter diaphragm excursion which can lead to increased distortion.

Another, potentially better solution is to adopt a more directional speaker design. Controlled directivity can be achieved using a suitable wave-guide in the critical frequency band of 500 Hz to 5000 Hz... this is the decade where the ear is most sensitive and the flatter and more directionally uniform your response the better the sound will be (everything else being equal).

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