One of those elusive concepts about acoustics that many people involved in high-end loudspeaker design might not be fully familiar with is that of acoustic lobing.
While doing some research on this, I found that there is a very limited amount of information about acoustic lobing, so I decided to make the internet a better place by providing a brief write-up.
Acoustic lobing is an effect that occurs when two neighboring loudspeakers emit the same frequency while being a distance apart that is larger than a quarter of a wavelength of that sound.
In reality, any system that involves a mid range driver and a tweeter will crossover at frequencies that are high enough for lobing to occur unless we are looking at coaxial drivers or drivers that are nestled within each other.
So what is lobing? As frequencies increase, a loudspeaker will gradually stop projecting sound into a spherical pattern but become more directional. More energy is projected forwards than backwards and higher frequencies are therefore heard louder than lower frequencies,. This is what is known as baffle step diffraction.
Now if a nearby driver is playing the same frequency, and it is more than a quarter wavelength apart as is usually the case with normal sized drivers, the sound pattern will change from one directive lobe, to several lobes expanding out sideways.
This will lead to narrow bands of cancellations and superimpositions leading to a change of coloration of the sound at different listening angles and a dreadful task called sweet spot hunting. The effect is known as a “comb filter” since the ranges of louder and softer tones are distributed like the spikes of a comb along the frequency axis. This can be quite tiring to listen to.
Common ways to avoid acoustic lobing are to use of coaxial drivers or full range drivers. All multi-way driver systems will exhibit lobing to some degree and in those cases the effects of lobing can be mitigated, but not eliminated
Forms of mitigation include: careful setup and speaker toe in, to where the sound coloration becomes neutral and pleasing to any particular listener, and maintaining a narrow listening position.
There are cases such as line arrays where lobing is actually a desirable effect by placing many identical drivers in a column which deliberately provokes lobing. This is done to convert a spherical waveform of a single driver into a cylindrical one, with a narrow, vertical direction. The main reason people started building such line arrays was to be able to intelligibly transmit human speech in highly reverberant spaces like churches by avoiding sound being projected onto ceilings and floors.
This only works in a narrow frequency band and line arrays tend to be highly collared when listened to off axis.
Arved Deecke is founder of the Danish / Mexican Loudspeaker company KVART & BØLGE that makes audiophile quarter wave loudspeakers and sound systems at a price anyone can afford. In his free time he blogs about all things related to sound, music and audio.
