Graham Audio writes: “There’s been quite a lot of interest and intrigue surrounding the design of our new LS5/5s, especially regarding the unconventional slots. To give clarity and clear up any confusion, here’s a message from the designer, Derek Hughes, to give you an insight into how the LS5/5 slots work”…

“All drive units are omnidirectional at low frequencies, and have an increasingly narrow dispersion (beam width) as the frequency increases.

This means that at the crossover frequency, there will often be areduction in the response to the sides of the speaker, and then an increase above the crossover frequency, as the smaller driver has a wider dispersion.
To reduce this effect, the LS5/5 uses slots in front of the LF & MR drivers.
The slot makes the drives perform as if it was a narrower source, giving it a wider dispersion, thus leading to a more uniform response off axis.
This is particularly advantageous where there are several listeners as it widens area where the response is smooth.
There is a limit to how narrow the slot can be in order to avoid resonant cavities behind it, giving dips in the response, so the slot width is such that these resonances occur above the crossover frequency.”
This effect is explained in great detail in the BBC Research report into the design of the LS5/5. This can be downloaded from link.