A critical factor in the bass response of a monitor in a room is the distance of nearby walls (or boundaries) from the monitor. If a monitor is positioned freely standing in the room, the wall behind the monitor may have a strong effect on the output of low frequencies. The wall will reflect the low frequency energy radiated by the monitor. This energy will be reflected and sums with the sound radiated by the monitor. The low frequency energy is reflected and sums with the sound radiated by the monitor.

Destructive interference:

When the reflected sound is out of phase with the original sound, it destructively interferes with the direct sound, causing the sound level to go down, and causing a notch in the bass response of the monitor. This notch can cause a significant reduction in the bass output. For example, if a monitor is placed so that the front of it is 86 cm (34") from the wall behind it, the first cancellation frequency will be at approximately 100 Hz:

The reflection off the wall behind the monitor causes the notch at 100 Hz. The comb filtering ripple between 1 and 2 kHz is caused by the acoustic reflection from the mixing desk surface. The tolerance of this monitor's anechoic frequency response is ± 2.5 dB. Any deviation larger than that range is an effect of placing the monitor into the room!

Solutions:

The first and best cure for the ‘wall behind the monitor’ cancellation dips is to flush-mount the monitors in a hard wall – also called ‘infinite baffle mounting’ or ‘flush-mounting’ - totally eliminating backwall reflection.

The second best cure is to placing the monitor very close to the wall. A distance below 20 cm (8") from the wall keeps the monitor bass response unaltered (the cancellation dips are at or above 430 Hz). The resulting low frequency boost can be compensated with room compensation adjustments.

Finally, the third cure is to move the monitor very far away from the wall. Then the cancellation frequency goes down. But the direct-to-reflected level ratio also becomes very favourable, and the reflections from walls loose significance as the direct audio from the monitor to the listener dominates.

Placing a free-standing monitor in the room:

To avoid cancellation of audio because of the sound reflecting back from the wall behind the monitor, follow the placement guideline below. This reflection happens at relative low woofer frequencies only. Avoiding the cancellation is important because the reflected sound can reduce the woofer output, causing the monitor low frequency output to appear to be too low. To avoid the cancellation, push the monitor close enough to the wall. Typically the distance of the monitor front to the wall should be less than 0.6 meters. This ensures that the low frequency output is not reduced. The monitor needs a minimum clearance of 0.05 m to the wall to ensure full output from the rear bass reflex port.

Distances recommendation: from a single wall to the front baffle of free-standing monitors:

Frequency domain notches and distances from the single wall behind a free-standing monitor and its front baffle:

Different ways to solve other reflection problems:

- Select a room shape that will direct the reflections away from the listening position.

- Ensure that the back wall behind the listening position (the rear wall) is more than 3 m (9.8 ft) away from the listening position to avoid low frequency cancellation at the listening position. This problem often exists in rooms less than 5 m (16.4 ft) in length.

- Add absorbing to reduce the level of the reflected sound.

Formula for calculating cancellation frequencies:

Quarter wavelength cancellation frequency:

ƒc = c / 4dx

ƒc is the cancellation notch centre frequency

c is the speed of sound in air at 20°C at sea level = 344m/s

dx is the distance from the front of monitor to the wall behind it.

Minimum distance of the monitor to the wall

dmin =1.4 c / 4 ƒ-3dB

dmin is the minimum distance from the front of the monitor to the wall behind it

c is the speed of sound in air at 20°C at sea level = 344 m/s

ƒ-3dB is the -3 dB low cut-off frequency of the monitor

Half wavelength cancellation frequency

ƒc = c / 2(dreflect-ddirect)

ƒc is the cancellation notch centre frequency

c is the speed of sound in air at 20°C at sea level = 344 m/s

dreflect is the distance of the monitor to the listening position via the reflecting surface

ddirect is the direct path distance from the monitor to the listening position

Please also visit our Monitor Setup Guides for additional information:

https://www.genelec.com/monitor-setup