Lobing

Uneven loud and quiet patches in the air that appear when two speakers or drivers cover the same frequency.

Two speakers playing one frequency: paths that match add to +6 dB lobes, half-wavelength differences cancel to dead nulls.

What it is

Uneven loud and quiet patches in the air when two sources play the same frequency and their waves add or cancel.

Key facts

Speed of sound = 343 m/s in air at 20 degrees C (about 1235 km/h)
Wavelength: lambda = c / f. Lambda is wavelength (m), c is speed of sound (343 m/s), f is frequency (Hz)
Examples: 100 Hz = 3.43 m, 1 kHz = 0.343 m (34 cm), 10 kHz = 3.4 cm
Two coherent sources IN PHASE add to +6 dB (double voltage = quadruple power)
Two coherent sources OUT OF PHASE cancel to a deep null (near minus infinity dB)
Constructive peak: path difference = whole wavelengths (0, lambda, 2 lambda...). Null: odd half-wavelengths (lambda/2, 3 lambda/2...)
Comb filtering: first null at f = 1 / (2 x delay); e.g. 1 ms offset = null at 500 Hz, notches every 1000 Hz
Keep coupled sources spaced under lambda/4 of the highest shared frequency to stay coupled
Free-field distance doubling loses 6 dB (inverse-square law)
Two uncorrelated sources only sum to +3 dB (power add), not +6 dB

How it works

Two sources radiate the same frequency at once
Their sound paths to a given seat differ in length
Where paths match (in phase) waves stack and get LOUDER (+6 dB)
Where paths differ by half a wavelength they cancel to a NULL
This repeats across angles, making fingers of loud and quiet (lobes)
Move, re-space, or re-aim a box and the whole pattern shifts

Real examples

Two subs spaced wide apart: bass strong dead-centre, weak off to the sides
Stacking two tops on a pole: high frequencies comb-filter into hot and dead spots
Left and right mains overlapping at the mix position: vocal level dips as you step sideways
A speaker plus its reflection off a hard wall: classic comb-filter notches

How it helps in live sound

Space subs under lambda/4 of crossover freq (~0.85 m at 100 Hz) to keep them coupled
Or use cardioid/end-fire sub arrays to steer the lobe and kill rearward energy
Aim overlapping tops so coverage edges meet at minus 6 dB, not bullseye-overlap
Use a measurement rig (Smaart, REW) to find comb nulls before doors open
Mono the subs and centre-cluster, or use one stack, to avoid wide-spacing power-alley nulls
Time-align (delay) stacked or distributed boxes so arrivals match at the crowd

Everyday analogy

Like two stones dropped in a pond, the ripples pile up tall where crests meet and flatten dead where a crest meets a trough.

Watch out

Myth: more speakers always = louder everywhere. Truth: overlapping identical signals create nulls (dead zones) as well as +6 dB peaks; spacing and aim decide who gets which.

Fun fact

Two perfectly matched speakers fed the same signal can sum to +6 dB in one seat and a near-silent null one step away, even though each box alone plays full level.

Key takeaways

Lobing = interference between two sources covering the same frequency
In phase adds +6 dB, half-wavelength out of phase cancels to a null
Wider spacing (in wavelengths) = more, narrower lobes and nulls
Higher frequencies have shorter wavelengths, so they lobe much more easily
Fix it with tight spacing, smart aiming, delay/time-alignment, and measurement