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Bass Loudspeaker Position in PA Systems
Bass speakers have their own particular issues.
Because the wavelengths involved are relatively large (about 2.8 metres at 125Hz), the horn sizes needed to provide adequate pattern control are impractical for many touring (or, indeed, permanently installed) systems. Most speakers dealing with frequencies below about 250Hz (i.e. with wavelengths greater than about 1.4 metres) can be treated as omnidirectional for most practical purposes.
Also, our ears use higher frequencies (with shorter wavelengths) to work out where sounds are coming from: we are not very good at identifying the position of low-frequency sounds.
From these factors, it is often (mistakenly) assumed that it doesn't much matter where the bass speakers go.
In fact, bass speaker position affects three main aspects of sound:
In a multi-enclosure PA system the ideal position for mid and HF enclosures is above the audience head-height. Conversely, bass speakers benefit from being placed on the floor (see Level, below).
Where the bass and mid/HF speakers are in different places, one may be further than the other from any given audience position. This may cause phase-related problems around the crossover frequency, and can also make the rhythm less focussed.
Most active crossovers and controllers allow some correction - by delaying the feed to respective amplifiers - for where enclosures are placed. If the mid/HF enclosures are on stage and the bass enclosures are on the floor in front of the stage (a couple of feet further forward, as well as closer in height to the audience), delaying the feed to the bass speakers by a couple of milliseconds will improve the time alignment of bass and mid/HF components. It isn't practically possible to locate speakers so that time-alignment is exactly correct for all audience positions. While sophisticated measurement and phase-checking devices are available, any final alignment is inevitably approximate for much of the audience, and a ‘guesstimate’ can be almost as effective as precise measurement. Again, because of the wavelengths involved, a couple of feet (or milliseconds) are unlikely to make much difference, but getting it even approximately right will offer some improvement over getting it completely wrong.
Percussive elements of the sound are usually more coherent if the ‘attack’ (containing higher frequencies) arrives before - rather than after - the lower-frequency fundamentals. If in doubt, position the bass speakers slightly further than the mid/HF units from an average (mid-audience) position (or - if you have the option - set any delay on your crossover or controller so that the bass signal arrives slightly later rather than earlier). ‘Slightly further’ (or ‘slightly later’) means no more than a couple of feet (or milliseconds).
Placing bass speakers together - so that their output is acoustically coupled - will yield an increase in level. This can be as much as 3dB (the equivalent of a doubling in power) when compared with output from the same speakers placed further apart.
Similar increases in level can be achieved by placing bass speakers against solid boundaries (floor and walls): bass speakers should almost always be placed on the floor. As this is their usual position, placing them elsewhere - on stage, for example - will usually result in a perceived reduction in level.
Placing speakers against walls - whilst yielding further increases in level - may compromise other aspects of the sound. The mid/HF units always need to be forward of the performers, so placing bass speakers against the back wall adversely affects time alignment. Depending on the size and layout of the venue, placing the speakers against side walls may also compromise time alignment. Placing them against walls also has effects on room resonance (see below).
Because sound at lower frequencies is relatively non-directional, sensitivity to low-frequency feedback is more dependent on distance than on direction. Better feedback immunity can be achieved if bass-sensitive microphones and pickups are placed as far as possible from bass speakers.
Positioning a bass speaker against a wall maximises its effect on room resonance along the dimension at right angles to the wall (placing a speaker against the back wall has effect along the length of the room, and placing it against a side wall has effect across the width of the room).
The space under the stage is also a resonant enclosure, and placing bass speakers on or under the the stage will increase the effect of that resonance.
Room resonance always has adverse (and sometimes ghastly) effects on amplified sound. For this reason it is generally better if you don't place the bass speakers against the walls or on or under the stage. They are better placed in front of or (depending on room dimensions) on either side of the stage. If the mid/HF speakers are on stage, it may be necessary to delay the signal to the bass or mid/HF amplifier(s) to correct any time alignment issues that result from this.
If low-frequency ‘boom’ is a problem, moving the bass speakers may yield some improvement.
Moving them across the width of the room will usually have more useful effect than moving them along its length or height: except in very small auditoriums, the frequencies involved along the length of the room are very low indeed (in a 20-metre room the first-order mode is at only 9Hz, and even the fourth mode is below 35Hz). Although height-related modes can in theory present problems, raising bass speakers off the floor sacrifices more output level than many systems can afford, and placing them at exactly the right height is not usually easy, or even practical.
As a brief rule-of-thumb, try positioning the bass speakers symmetrically at either side of the stage (as in the first diagram, below) in one of two positions: 1/4 of the room width from each side wall, or 1/8 of the room width from each side wall. Generally, try 1/4 in smaller venues, or 1/8 where the total width is more than about 7 or 8 metres. This should minimise the effect of at least one or two of the most noticeable width-related resonances.
If neither position seems to work - or if the problem seems to get worse - try positioning them centrally (as in the third diagram, below) and do your best with the EQ. The show must go on.
*Although the detail and underlying mathematics are beyond the scope of this article, anyone who would like to look into it further can find some useful technical discussions (as well as an Excel spreadsheet for calculating modes from room dimensions) in the White Paper section of the Harman International website.
Overall, you are aiming for a bass sound that is tight and well-controlled, rather than ‘boomy’ and unfocussed. Although moving bass speakers will often make a big difference, the ‘best’ position for them will always depend on the room (as well as the speakers and the type of music involved). However, if you have time, even trial-and-error can sometimes offer worthwhile improvements.
Some effects of bass speaker position are shown below.
In the first diagram, the bass speakers have been placed (as they often are) on the floor at either side of the stage:
The way the sound radiates means it is louder when a listener is either near one of the speakers, or in a position where the sound from both speakers combines in-phase (i.e. up the centre of the room). The resulting pattern has been known by a variety of names. Perhaps your imagination can help you guess at these.
In practice, reflections cause low frequencies to predominate near walls (as well as other dense solid objects: generally it is best not to place the mixer in such positions, as this will make it difficult for the sound-engineer to judge bass levels). For this reason, the perceived pattern indoors is more like the one below:
Note that a listener at the position marked by the red dot is 2m further from one speaker than from the other. At 80Hz this is roughly half the wavelength, and the perceived result would be a loss of level at that frequency in that position. Peaks and notches in level at various frequencies would be found in most positions in the listening area. While such a situation is obviously best avoided, the effect indoors is somewhat offset by reflected sound (from walls, floor and ceiling). Depending on the exact position, this layout may also limit the effect of room resonance (as discussed above). Outdoors (or in enclosures - such as marquees - that are acoustically transparent at low frequencies), the layout below will generally offer some improvement:
In this example, the bass speakers have been placed together in a central position, on the floor in front of the stage. As discussed above, placing them together - coupling their output - produces a measurable increase in level. Placing them on the floor also gives an increase in level.
Because the sound radiates from what is now effectively a single source, phase cancellation between the bass speakers themselves is eliminated, and although the effects of distance and boundary walls may still be noticeable, the spread of sound is much more even.
This layout is therefore a good way to maximise output, minimise phase cancellation, and provide relatively even audience cover. Indoors, however, the these advantages may be outweighed by effect of room dimensions - see above - which EQ is relatively powerless to correct. Where room dimensions do not noticeably affect the sound (rooms in which the walls are not parallel often fall into this category), this may still be a good way to position the speakers.
However, where more bass cabinets are available, a layout based on line-array design (see the section on phase) may be the most effective way to maximise output and simultaneously maintain pattern control.
Here, the cabinets are arrayed so that their centres are half a wavelength apart (λ/2) at the crossover frequency (at 120 Hz this would be 1.4 metres). This minimises output to the sides of the array (reducing reflections from the side walls) whilst allowing effective coupling between the cabinets. Adding to the number of cabinets further reduces side-spill. Note, however, that using this layout with most bass cabinet designs (which are relatively non-directional), bass levels on stage will also increase.
Where bass speakers have to be placed on stage, so that spill compromises the sound for nearby musicians, we can use this array effect to our advantage.
Here, the driver centres of paired speakers at either side of the stage are separated by 170 cm (λ/2 at 100 Hz). The result is coupling in forward (audience) positions, while high cancellation to the sides means negligible spill to the nearby musicians. Although there will still be some frequency-dependent unevenness in level over the audience area, this is reduced when compared with clustered cabinets at either side of the stage.