The wall behind the monitor
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 ‘free-standing’ in the room, the wall behind the monitor will usually have a very strong effect on low frequencies, as it will reflect part of the omni-directional low frequency energy radiated.
This energy will be reflected back towards the listening position and the initial sound point source - see diagram below.
When the total distance traveled by this reflected sound is half a wavelength of the original sound, it destructively interferes with the direct sound causing a notch to appear in the bass region of the monitor. This first notch will be typically about 2/3rd octave wide and cause a significant decrease of the total bass energy in the room. You must check that the monitor is not at a distance from the wall behind it that will cause this cancellation notch to appear in the frequency range that the monitor can reproduce. For example, if a monitor is placed so that the front of it is 86 cm (34") from the wall behind it the cancellation frequency will be approximately 100 Hz. There is a formula to calculate this cancellation frequency. Below is a measurement of a 1038A placed 86 cm (34") from the wall behind it.
The reflection off the wall behind the monitor causes the notch at 100 Hz as the front of the monitor was positioned 86 cm (34") away from the wall. The comb filtering type ripple between 1 kHz and 2 kHz is caused by the reflection from the mixing desk. The tolerance of this monitor's anechoic frequency response is ± 2.5 dB in the pass-band. Everything outside this tolerance range is an effect of placing the monitor into the room!
There are various ways to solve this problem. It will depend on the room as to which one is most suitable and can be used:
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’ or ‘flush-mounting’ - which totally eliminates this wall reflection and cancellation.
Second best is placing the monitor very close to the wall, which raises the cancellation frequency higher. This works well when the monitor is not too small. The risk is, with small monitors, which inherently are less directional in mid frequencies that the dip just moves to the low mid-band and causes even worse coloration. The distances between 0 and 20 cm (8") from the wall let the monitor response to be, in most cases, unaltered (first cancellation dip down to 430 Hz only); i.e. the directivity of the monitor is high enough so that the rear radiation cannot cause a severe cancellation. Additionally, the low frequency boost should be compensated for when the monitor is mounted close to the wall (maximum of +6 dB).
Alternatively, the third cure is to move the monitor considerably away from the wall: the cancellation frequency goes down so far that it is below the low frequency cut-off of the monitor. Thus, the minimum distance ‘monitor/wall behind’ depends on the monitor low frequency performance. However, at low frequencies and for large monitor, the minimum distance becomes very long and impractical.
Placing the free-standing monitor in the room
GENELEC bi-amplified systems should be placed so that a minimum gap of 5 cm (2”) is left behind the monitor for amplifier cooling and rear opening reflex port sound radiation. In general, when positioning the monitor’s front baffle further than 30 cm (12") from the wall, a reflection can cause a cancellation in the low frequency response and hence a loss of bass sound quality. For two-way monitors, LF cancellations in the range 40…80 Hz should definitely be avoided (see figure below). Low frequency cancellations in the 80…200 Hz range should also be avoided where possible. If this is not possible the sound quality can still be perceived to be good. Translating these frequency ranges into distance recommendations shows that an acceptable response can be achieved at distances from the wall up to 1 m (3.3 ft). Beyond, the 1 m…2.2 m (3.3...7.2 ft) range should definitely be avoided. Large monitors placed at a distance greater than 2.2 m (7.2 ft), may suffer from a cancellation in the very low frequency region around the LF cut-off thereby compromising the monitor’s LF extension.
Distances recommendation: from a single wall to the front baffle of free-standing monitors. Correct (green), acceptable (orange) and avoid (red).
Frequency domain notches and distances from the single wall behind a free-standing monitor and its front baffle
Two observations are also immediately apparent:
For large monitors, with very low cut-off frequency (Three Way monitors and Main monitor systems), placed away from the wall the necessary distance is far too long for any practical rooms.
At the same time, the distances to other boundaries in the room become similar to the desired distance to the wall behind the monitor, and the reflections from these other surfaces start to dominate the response.
The conclusion is that large monitors should not be placed free-standing but flush-mounted to perform in the best possible way.
Reflections off other boundaries
Other sources of bass cancellation are from reflections off the floor, ceiling, side walls and studio rear wall. The cancellation is still based on half wavelength cancellation of the path difference but, as the sound does not travel back on itself, a slightly different formula is required. In fact this is the general equation for cancellation frequencies and can also be used for desk reflections that occur in the mid frequency range.
There are some different ways to solve these reflection problems
- Change the room shape so that the reflections are not directed towards the listening position. This is not always possible for physical and financial reasons.
- Ensure that the wall behind the listening position (rear wall) is more that 3 m (9.8 ft)away from the listening position to avoid low frequency cancellation problems. This is often a problem in rooms that are less than 5 m (16.4 ft) in length.
- Add absorbing materials to reduce the level of the reflected sound. Although generally a cheaper solution this has the disadvantage that the reverberation time, T60, can be adversely affected.
Formulae for calculating cancellation frequencies
Quarter wavelength cancellation frequencyfc = c / 4dx
where:fc is the cancellation notch centre frequency c is the speed of sound in air at 20°C at sea level = 344m/s
dxis the distance from the front of monitor to the wall behind it
Minimum distance of the monitor to the walldmin =1.4 c / 4 f-3dB
where: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
f-3dB is the -3 dB low cut-off frequency of the monitor
Half wavelength cancellation frequency
fc = c / 2(dreflect-ddirect)
where:fc 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 monitor to the listening position via the reflecting surface
ddirectis the direct path distance from the monitor to the listening position