The performance of any loudspeaker will be influenced by the acoustics of the space in which they operate. Difficult room acoustics, combined with improper loudspeaker placement, can interfere with achieving the fidelity of which your loudspeakers are capable. This article will guide you around some of the pitfalls when configuring a PA system.
Recognizing Problem Rooms
In most live environments, the room is rarely designed to maximize the listening experience. More often then not, money is spent on aesthetic appeal rather than acoustic treatment. For large-scale tour productions, venues are often sports arenas that have been designed to maximize crowd noise. Smaller music venues are often chosen for location or architectural aesthetics, rather than music reproduction. While an empty warehouse or old wine cellar might make a great environment to hang out in, it’s necessary to recognize and correct what that space does to the sound system in order to optimize the PA’s performance in the venue.
In general, the following physical features of a room can affect a sound system’s performance:
- Room size
The size of the room directly impacts how well certain frequencies will be reproduced. For example, if you measure a room diagonally, you will discover how well that room will be able to sustain low frequencies. This may seem odd until you think about the physical size of audio waves at various frequencies. For example, a 50 Hz wave is about 22.6 feet long. (To calculate how big an audio wave is, divide the speed of sound—1,130 ft./second—by the frequency. For a 50 Hz wave, 1,130/50 = 22.6 ft.) So a room that is 45 feet on the diagonal is going to regenerate low frequencies more effectively than a room that is 15 feet on the diagonal.
When a room’s width or length correlates directly to the size of a waveform at a specific frequency, a standing wave can occur where the initial sound and the reflected sound begin to reinforce each other. Let’s say we have a long, narrow room where the distance from one side to the other is 22.6 feet. When a 50 Hz wave bounces off the wall, the reflective wave travels right back along the same path and bounces off the other wall and cycle repeats. In a room such as this, 50 Hz reproduces very well—maybe too well. So any mix will have a heavier low end.
In addition, low-frequency waves are powerful enough to cause the walls, ceiling, and even the floor to flex and move. This is called “diaphragmatic action,” and it dissipates energy and strips away the low-end definition. So if you’re in an old cotton mill, and the walls and floor are made of thick concrete that don’t vibrate much, the bass response is going to be much more powerful than if you’ve set up a show in an old warehouse where the walls are made of barge board and tin.
Maintain a High Direct-to-Reverberant Ratio
Another way a room interacts with sound waves is through reflectivity. Like most room anomalies, reflections can be good and bad. Consider the effect of a cathedral’s reflections on a choir or a piano. This type of reverberation (reverb) is quite desirable. But not all reverb is good reverb. Reflections can also cause comb filtering. For example, if a speaker is placed near a reflective surface (such as a concrete wall), the direct sound coming from the speaker and the reflected sound coming from the wall can arrive at the listener’s ears out of phase with each other, causing cancellation and reinforcement. If they’re 180 degrees out of phase with respect to each other, they will cancel each other out.
If you are using your loudspeakers in a reverberant environment, position them so that as much sound as possible is focused on the congregation area and steered away from reflective surfaces. When you do your placement and positioning, it’s a good idea take some time to do a “walk around” of your loudspeakers, playing either pink noise or program material, so you get a feel for how the sounds are translating into the room.
It is important to keep in mind that vertical coverage is just as important as horizontal coverage. If you are using a ground-stack approach with pole mounts, make sure your coverage matches the listening plane. Suspension of speakers will provide even further control.
Important note: Always use a licensed and insured system integrator when suspending loudspeakers. These individuals are not only trained to know how to safely suspend heavy loudspeakers, they also know how to hang them so they sound their best.
Different types of loudspeakers offer different coverage patterns. PreSonus® StudioLive™ AI-series full-range loudspeakers employ coaxial mid/high-frequency drivers that have a nominal 90-degree horizontal by 60-degree vertical coverage pattern.
Unlike conventional loudspeakers, coaxial systems have the mid- and high-frequency drivers on the same axis, providing a single point source for a consistent acoustic center. This results in symmetry of response on both the horizontal and vertical axis, at any given angle. (By “symmetry,” we mean that whatever response is observed at a given angle with respect to the axis, the same response will be observed at that angle in the opposite direction. The loudspeaker’s behavior is “mirrored” about its axis.)
If you are using conventional (non-coaxial) loudspeakers, you won’t get this even, symmetrical coverage but if you know what your speakers’ coverage pattern is, the vast majority of the information presented here will still apply.
Some loudspeakers, like the StudioLive 312AI and 328AI full-range loudspeakers, feature dual-position pole mounts that allow you to mount the speaker atop a stand at 90 degrees or at a 7 to 10-degree downward tilt. (The StudioLive 315AI does not offer a downward tilt.) Using the downward-tilt mount will focus the loudspeaker’s energy onto the audience and avoid destructive reflections. This is ideal for situations where the loudspeaker is mounted atop a tripod stand and placed on a stage or where the pole-mounted loudspeaker is on the floor and the coverage area is relatively shallow (conference, coffee house, etc.)
Wall and Corner Loading
Very low frequencies are not directional, so they radiate out of the sides and back of the loudspeaker, as well as out of the front. If you place a loudspeaker against a wall, the rear sound propagates back into the room. This can increase output of bass frequencies as much as 6 dB, and as much as 12 dB if you put the loudspeaker in a corner.
In order to have the most control over your sound, it’s best to always start with the flattest response, so you normally should avoid wall and corner placement. On the other hand, if you need some extra bass boost, this technique may be worth a try. It is important to be aware of what’s happening and be prepared to take advantage of it or compensate for it.
In most situations, a PA system relies on two main speaker systems positioned at the front of the room to reproduce audio for the entire performance space. As a result, the level of the system is considerably louder at the front line then it is at mix position.
With a point-source, horn-loaded loudspeaker (such as a single, powered loudspeaker), sound intensity is lost at a rate of -6 dB per doubling of distance. This is true regardless of tuning, amplification, power rating, or any other speaker specifications. So if your signal level is 118 dB SPL at 1 foot, at 8 feet away it’s down by 18 dB!
Here’s a simple chart that illustrates the math:
In situations where sound must be reproduced outside of the main system’s optimum range, well-placed delay systems offer support by extending the intelligible range of the PA. Rather than relying on a pair of front-of-house speakers to fill the entire room, you can create listening zones throughout the room so that your front-of-house system only needs to be loud enough to cover the front of the room. This allows you to lower the level, giving the front-row listeners’ ears a break and getting better fidelity from your speakers.
However, it’s not as easy as just bringing an extra pair of speakers. Since electricity travels much faster than sound, listeners in the rear of the room are likely to hear the sound coming from the nearest set of speakers before they hear the sound from stage, which can dampen the attack and intelligibility of the sound and create an unpleasant phasing effect.
To compensate, you need to delay the signal going to the additional speakers. For example, it takes about 55 ms for sound to travel 50 feet. So if you put your speakers 50 feet back, you need to delay the signal by that much.
Fortunately, with StudioLive AI-series loudspeakers and SL Room Control, you can delay each speaker by up to 300 ms. All you need is a reasonably accurate measurement of the distance between your main speakers and delays.
Also note that if you are using conventional loudspeakers with a StudioLive-series digital mixer, you can use the mixer’s subgroup outputs, each of which has a variable delay of up to 300 ms, to feed your satellite (delay) speakers.
If neither your mixer nor your loudspeaker has a built-in delay, use a separate delay processor to achieve the same results.
Delay speakers allow you to run the main speakers at a lower volume, as they relieve the mains of handling high- and mid-frequency content for part of the space. As a speaker is pushed harder, the edges of its frequency response begin to distort, so by easing the demands on the mains, delay systems increase fidelity sonically, as well as mechanically. This also means that the front row doesn’t need to be blasted just so the people at the back can hear the show.
The goal of distributed sound is to extend the intelligible range of the system, without killing the front of the crowd with excessive level. As noted earlier, sound travels much slower than electricity, so the audio coming out of the delay system will arrive to the listeners before the audio coming out of the main system. Without proper alignment, the multiple arrival times create confusion to the listener and sonic definition is lost. Speech and beat transients become less intelligible. In large venues, this can actually create a flam or echo effect. By delaying the audio going to side and rear fills, you can create a cohesive listening environment for the entire audience.
It should be noted that frequencies in the sub-bass range of a delay system do not require distribution. In fact, a delay system’s highpass filter should be rolled up as high as 300 to 400 Hz to avoid sound going back toward the stage as low frequencies become omnidirectional.
When placing delay systems, the main goal is to maintain intelligibility of the PA, especially in the vocal consonant range (2 to 4 kHz). However, this goal is achieved by overcoming different obstacles depending on whether you are indoors or outdoors. In both situations, the delay system should be set where the main system’s intelligibility falls apart. As with the main system, the placement of the delay systems will determine how successfully you are able to achieve these goals.
Inside. Indoors, you are trying to overcome the direct-to-reverberant reflections. The location of the delay system is dependent on the critical listening area (typically just behind front-of-house). Your goal is to find where the direct signal-to-reverberation ratio has reached about 50/50. At this point, the reflections in the room are at an equal level to the direct sound of the PA, and vocal intelligibility is lost. Listen for a lack of intelligibility in the vocals and find the point at which the drums and rhythm section don’t feel tight.
A great way to find the best position for your delay speakers is to set up and tune your main system and play audio through it. Play something similar to what you will be mixing later. Set the level so that it is comfortable from the front row. Walk backward away from the main system until you notice a lack of clarity. This is the beginning of the space that will need delay-system coverage.
Outside. Outdoors, you are trying to maintain level as the noise floor of the crowd begins to be at equal level to the PA in the intelligibility range. When working outside, the delay system is used to overcome outdoor noise, including (but not limited to) crowd murmur, concessions, generators, tractors, babies, etc. At this point, the main system needs more support in order to deliver the same perceived loudness as you get further from the source.
Once you have positioned and delayed your satellite system, use an SPL meter to match the output of the main and delay systems at the measurement point. If you are standing 20 feet from the left side of the main system and 30 feet from the left side of the delay system, and if the output of the main system is 85 dB, the output of the delay system should also be 85 dB.
Delaying subwoofers relative to their full-range counterparts compensates for the cancellation or reinforcement of low frequencies that occurs when the same frequencies are reproduced by two sound sources set some distance apart. Low frequencies in the crossover region between full-range and subwoofer have wavelengths that are several feet long—the wavelength of a 150 Hz wave is about 7.5 feet—which means that reinforcement and cancellation will occur as the waves interact in the room.
Delaying a subwoofer will compensate for this effect when the loudspeaker is about the same distance away from, or in front of, the subwoofer, as specified in the setting. As room acoustics will influence effectiveness, we recommend listening tests using different delay settings, in conjunction with alternate polarity settings, to determine the best results.
If you are aligning for a custom installation you will need to do some calculating:
Find the spot in the room where coverage from the main speakers and the subwoofers overlap. Measure the distance from the overlap area to each speaker location. Subtract the smaller distance from the larger. Divide that number by 1,100 and apply that delay value to the speaker that is closest. Keep in mind that the overlap area may be behind front-of-house.
The PreSonus StudioLive 18sAI subwoofer has been designed and customized for use with the full-range AI-series loudspeakers but also can be used effectively with other full-range loudspeakers. It features three delay presets to guarantee that your 4-way system stays in alignment:
0M. Select this setting when the full-range system is directly over the subwoofer (i.e., mounted atop using the SP1BK sub-pole accessory). No delay is added to the subwoofer amplifier output.
1M. Select this setting when the subwoofer is about one meter (39.37 inches) from the full-range loudspeaker—a typical distance when the loudspeaker is on an adjacent tripod stand. A delay of about 2.9 ms is introduced to the signal that is output through the subwoofer amplifier.
2M. Select this setting when the subwoofer is about two meters (78.74 inches) from the full-range loudspeaker—typical when the full-range system is on the stage and the subwoofer is on the floor. A delay of about 5.9 ms is introduced to the signal that is output through the subwoofer amplifier.
Real-World Delay System Example
The goal in a complicated system with loudspeakers distributed throughout the venue is to delay each satellite system to its counterpart in the main system (e.g., the left front fill to the left front-of-house loudspeaker).
- Delay the main system relative to the source on stage. On small stages where the guitar amp and drum kit can be clearly heard above the front-of-house (FOH) loudspeaker system, delaying the main system can “move up” the backline so that it aligns with these instruments and decreases blurring in the mix. This will tighten the overall mix and give it more punch.
- Delay the front fills relative to the main system by delaying each side of the system independently (e.g., delay the left front fill to the left FOH loudspeaker).
- Delay subwoofers relative to the main system. How you do this will depend on how your subwoofer system is positioned and configured. In general, you will want to delay each subwoofer relative to the full-range loudspeaker closest to it.
- Delay down-fill speakers (upper and under balcony) relative to the main system, again delaying each side of the system independently.
System Configuration Suggestions
The following discussion and graphics will demonstrate some system configurations for common rooms. The size and shape of your room and the application for which it will be used determine, to a large extent, how many speakers you will need and where they should be placed. In every situation, keep in mind two important design factors: your loudspeaker’s coverage pattern and half-space loading.
Every full-range StudioLive AI loudspeaker offers a 90˚ horizontal x 60˚ vertical coverage pattern. If you are using StudioLive AI-series speakers, be sure to pay close attention to these angles when using your speakers. Rotating the cabinet changes the horizontal and vertical coverage. If you are using conventional (non-coaxial) loudspeakers, find out what their coverage pattern is and figure accordingly.
When configured for stereo use, make sure the cabinets are not placed too wide for the room or too far back into the corners. Too wide of a placement will direct too much energy onto the walls and can potentially add destructive interference to the room. Adjust the left and right speakers, as well as the toe-in angle, to produce the best stereo image. If a room is very narrow, a mono cluster might be a better choice than stereo.
Wherever you place your loudspeakers, you should be aware of half-space loading. Half-space loading occurs when a speaker comes in close contact with, or touches, a hard surface like a floor or wall. As its name indicates, this type of summation happens when the circular radiation of the speaker is blocked by a hard surface and forced to radiate in a crescent shape. Depending on the proximity and position, there may be a boost in low-frequency energy. Testing your speaker placement and doing some critical-listening tests will help determine the best final location for your loudspeaker system.
If your speakers are sitting on the floor, you can expect a certain amount of half-space loading. If you are using your speakers as floor wedges, you might want to experiment with using a highpass filter (built into all full-range StudioLive AI loudspeakers) to reduce low energy. In some cases, this might improve intelligibility. Using the Floor Monitor DSP contour will also help you get the best use out of StudioLive AI loudspeakers in this position.
Note: Because a floor monitor placed on the stage is unavoidably subject to half-space loading, the Monitor DSP contour in a StudioLive AI-series full-range loudspeaker is specifically designed to compensate for bass buildup and maintain a tight mid-bass response.
A stereo system allows panning and adds depth to the acoustic image. This is good for speech reinforcement and greatly enhances live or pre-recorded music. Locate speakers to give the best horizontal coverage. Ensure that the listeners are well covered by the pattern.
Mono Cluster with Down Fill
Center or mono systems can provide a simple, economical solution for venues where speech intelligibility is the priority, rather than music. As with a stereo system, make sure the coverage pattern of the speaker focuses the energy on the audience.
This graphic actually shows two speakers. The upper speaker is for throwing to the back of the room, and the lower speaker covers the space in the front of the room, closest to the stage.
An LCR system is a stereo system with a center speaker added. This configuration allows panning and adds depth to the acoustic image. This type of system will provide more control than a basic stereo system and is ideal in situations where music and speech intelligibility are equally important.