One of the questions that has come up with regards to the specifications of the new BeoLab 90 is about the size of the woofers. The specifications state that it has one 13″ front woofer and three 10″ woofers for the sides and rear. However, if you look in the technical specifications in the Technical Sound Guide, you’ll see that the “effective diameter” of the front woofer is 260 mm (about 10″) and the remaining woofers is 212 mm (about 8″). Why is there a discrepancy?

The difference is in how a woofer – or any loudspeaker driver – is measured. When you say 13″ woofer, the measurement is the external diameter of the circular metal frame around the front of the driver. If you look on the first page of the datasheet shows that this diameter is 320 mm for the BeoLab 90’s front woofer – so it’s a 13″ driver. (I’ve copied the technical drawing from the datasheet below – see the two dimensions given on the left side of the drawing.) However, this diameter includes non-moving parts (at least they should not move – they’re screwed to the enclosure).

If you measure the moving parts of the woofer, then we have to decide on where to measure – what is the actual diameter of the diaphragm? Normally, the way to measure this is from the high points on the surround that encircles the diaphragm and connects it to the loudspeaker frame. As you can see in that same drawing, this diameter is 258.8 mm, which, in the “official” datasheet is rounded to 260 mm.