What the Helmholtz is a Helmholtz resonator?
If you want to know quickly, just click here. You will find all the search engine links you could possibly need.
But the point that is really important is why you should know about the Helmholtz resonator.
The main acoustic problem of any untreated room is standing waves. Most sound waves, indoors or out, are traveling waves. This means that the pattern of low-pressure and high-pressure regions is constantly in motion. In a room, a traveling wave will reflect back and forth and around the room, but its energy will be distributed along its path.
A standing wave occurs easily between parallel surfaces, although it can still occur in rooms with non-parallel walls. It happens when the wavelength of the wave fits exactly between the walls. So walls that are three meters apart will produce a standing wave at around 114 Hz. Standing waves also occur where half a wavelength fits exactly (in fact this will be the strongest standing wave), and at multiples of the lowest frequency. So in this case we would get standing waves at 57Hz, 114 Hz, 171 Hz, 228 Hz etc.
Standing waves can be absorbed by a Helmholtz resonator, perhaps more than one for best effect.
A Helmholtz resonator consists of an enclosed air space, which is connected to the outside by a narrow neck. A beer bottle works well as a Helmholtz resonator in reverse - blow across the opening to sound a note. A Helmholtz resonator used to absorb standing waves would be bigger, and partially filled with absorbing material such as mineral fiber. The neck of a Hemholtz resonator doesn't have to be external; it can just as easily extend inside the air space.
You can find a spreadsheet to calculate the resonant frequency of a Helmholtz resonator here (right click and 'save target as'). This includes a 'port length correction factor', but it would be wise to make the port length adjustable for fine tuning. Sorry about the feet and inches - I'll make a metric one when I have time.
This is pretty heavy duty acoustics, but it is not impossible to understand. Standing waves are a problem, the Helmholtz resonator is a possible solution. Acoustic consultants know all about this and might try to blind you with science, but now you have a starting point to talk to them in their own language.