Professor Vanderkooy presented empirical measurements showing that below some frequency (around 200 Hz) we can model the loudspeaker as a point sound source, a monopole, placed at a point in front of the loudspeaker. He called this point the acoustic centre. Professor Vanderkooy then provided the necessary equations from the acoustic theory to prove that such a point exists. Typically it is positioned at a distance of one half of the baffle diameter.

Later, his talk was illustrated with simulations of various loudspeaker cabinet configurations (cylindrical and box) and discussion on the aspects of the acoustic centre in the design and modeling of loudspeakers. The position of the acoustic centre as a function of the cabinet depth and the presence of a port (aka vent) was richly illustrated with simulation results.
The talk sparked an interesting discussion around the temporal effects of the acoustic centre. It was shown that the acoustic centre is also the temporal acoustic centre, i.e. the loudspeaker actually can be modeled as a monopole below some frequency.
The lecture continued with practical effects of placing subwoofers (the acoustic centre is a low frequency phenomenon), its applications on more complex loudspeaker cabinet designs, and how this theoretical approach is applicable to microphone and microphone enclosure design. For microphones the acoustic centre mathematical apparatus is applicable for frequencies up to 5000 Hz.

The lecture finished with a series of slides containing analysis of the human head as the enclosure of two microphones. The acoustic centre effect in low frequencies actually increases the dimension of the base (the distance between the two microphones - our ears), which helps the localization of low frequency sound sources. This effect is observable up to 400 Hz.
The last part of the talk was about the impact of the acoustic centre theory on dipolar speakers (a loudspeaker on a finite baffle) and loudspeakers with a cardioid directivity pattern. In these cases the acoustic centre theory is less applicable and the best practice is to use the geometric center of the loudspeaker.

The evening concluded with an array of questions from the audience. The AES PNW Section would like to thank Professor Vanderkooy for his interesting talk and detailed answers. We will welcome him back any time he chooses to visit us.