Understanding Speaker Placement

In Uncategorized, Web Articles by tfwm

by Bryan Cole

Speaker placement has to some always been a black art – sound being a technology that you can’t physically “see” it’s not always a simple self-explanatory thing. If you are installing a new sound system in your facility, one of the biggest influences on sound quality that you can have is to have an industry professional “measure” the room response using proper test equipment. This will allow you to understand more about what sound does to the room – or maybe better put – what your room does to the sound. So much of the end sonic product of a room is the result the room’s shape and structure, not the choice of loudspeaker as many would think. Unfortunately, churches all too often reach for the “best sounding” (aka best marketed) loudspeaker, and the result is less than hoped. A less costly loudspeaker, properly placed in a well-treated room, will allow your loudspeaker to do what it does best – deliver sound to the listener – while having the room do less of what it is good at: adding many more “out of time” arrivals at the listener’s ear.

In an ideal world, the listener would hear only the sound from one loudspeaker. Unfortunately, this requires an anechoic chamber to accomplish, and even if your church was close to being anechoic, there would be many undesirable side effects to that. An Anechoic room is the sonic equivalent of a room that is 100% black (no light.) In an anechoic chamber, the walls are treated in such a way that no reflections occur at all. Also in an anechoic room – which is most often used for loudspeaker testing, there is usually only one loudspeaker present at a time. Our church sanctuaries generally fail on both counts, the first because we must use multiple loudspeakers to cover the given area, and the second because we rarely have a 100% acoustically treated room (most of the rooms that I am writing about likely have no acoustic treatments at all.)

Minimize Arrivals at the Listener

In our anechoic chamber example, the single loudspeaker in the room is putting out sound, and the listener in the room (most often a microphone) is hearing only that sound. That is the ideal world that we are looking for. Our reality is that sound is leaving our loudspeakers, and arriving at the listener first, then followed by reflected sound from hitting walls, poles, and people. Each listening position receives many reflected sounds, and they all arrive at the listener at different times. The time lag is because each has its own path, and the lengths vary. The path to listener from loudspeaker is often the short, direct route. The path of sound reflecting off of walls and ceilings are varied distances, taking different lengths of time (at the speed of sound) to reach the listener. This undesirable effect has another name in audio circles, “reverb.” Reverberation is made in digital reverb processors by creating multiple variable-length paths for the audio to flow to the output of the processor. You have adjustments in the processor for early and late reflections, which are adjustments in the amount of sound taking a shorter or longer path. You also have adjustments like “bright” and “dull” which are digital approximations of the type of reflective surface that these virtual reflections are bouncing off of. The problem with naturally created room reverb is that unlike our digital reverb unit, it has no “off” button.

So reflections will be what they will be in our room – with the only exception being floor reflections. I’m fond of telling worship leaders complaining of the sound during practice in an empty sanctuary, that “I’ve ordered acoustic treatments – and they should be here about 9am.” Turns out the bag of mostly water that is our human body is pretty good at absorbing sound – and in most churches, the floor gets covered to a pretty high degree with humans during the service – and the varying height and shape of these humans makes for a very random dispersal pattern for that sound that still gets reflected.

So our only option is to aim our loudspeakers in such a way as to minimize the number of arrivals at the listener. Every loudspeaker has a specification for the coverage in degrees. Keep in mind that this coverage is at a certain frequency, and it usually (in a 2 way driver) is applicable to the sound emanating from the High Frequency driver – the one most often “sculpted” in the shape of a horn. Line arrays are required to control the directivity of lower frequencies. Let’s say that your loudspeaker has a “90 degree” coverage pattern. Take a 2 dimensional “top down” drawing of your facility. You can even “cut out” triangular “cones” out of paper that mimic the sound coming out of the speaker, and place them on your top-down drawing – with the goal being to cover as much of the room as possible, minimizing sound that points at places (like walls) where there are no listeners. Experiment with placement on your drawing, finding the best location for coverage.

Minimize Overlaps
Often loudspeakers are placed in “clusters” to “tight packed” loudspeakers. This can be either vertically or horizontally oriented, and there’s a significant amount of math that goes into the design. The ultimate goal of an array is to make all of the elements of the array act as one loudspeaker. If you are going to be placing arrays in this fashion, it is recommended that you work with someone qualified in array design (the manufacturer or a system integrator.) Most of our systems will be of the type we call an “Exploded array” – that’s where we place a loudspeaker to cover a particular seating area, and we minimize any overlaps where more than one loudspeaker is covering a single seating area. Remember that multiple-arrival issue we dealt with above? If a listener hears audio from more than one loudspeaker, it is highly likely that the time arrivals at the listener are different – just like reflections off of floors, walls, and ceilings, this can drastically alter the frequency response at the listener’s position. If you must have overlap, try to design your overlap over an aisle way where there generally are no listeners.

One sound source per channel
This is not a popular discussion, because it is most often misunderstood, and misapplied (with passion, I might add.) A live sound system has very different goals than a home playback system. A home playback system is most always designed for playback of 2 channels (stereo) or more – and this is why our home audio systems have at least two loudspeakers – and the intent is that the listener hears sound from both channels (because it was mixed intentionally for that type of listening.) In live sound, our goals are much more coverage oriented – we choose the quantity of loudspeakers based on the coverage needed – and we do this for – wait for it – ONE channel. Yes, most live sound systems are intended to be mono – after all, that is the way it is mixed in most cases. What about that amazing stereo piano patch your keyboard has? Unfortunately, in a properly designed mono live sound system, if you send stereo content to the left and right channels, the listener will only hear 1/2 of the content – they will hear the channel that is covering the seating area that they are in.

Can you have stereo coverage? Absolutely – your live sound system can be designed in such a way that two channels cover each seating area. Please know that this is a rare configuration, and it requires diligent panning of every channel on your mixing console to fill the stereo space properly. The part to understand is that to receive stereo content at the listener’s ears, the system has to have been designed with that intent from the start. Yes, your system may be 2-channel – one channel per loudspeaker – but that does not make it appropriate to deliver stereo content to the listener – remember most of your audience will only be able to hear one channel.

Vertical Aiming
We have spoken mostly about aiming in the vertical space (what you would see via a “top down” drawing of your space.) There is also the horizontal to consider – and while similar rules apply, the front to back coverage becomes the most important one. Remember the coverage pattern of your loudspeaker (90 degrees in our example?) That coverage pattern decreases in decibels as you move left or right from 0 degrees (dead center.) Note that your loudspeaker may have two specifications – a 90 x 60 loudspeaker is 90 degrees in the horizontal, 60 degrees in the vertical. The general rule for vertical aiming of a single loudspeaker is to the center of the loudspeaker’s coverage at a seating position 2/3 of the way back in the area you intend to cover. The logic is this: The listeners nearest the loudspeaker will be receiving “off axis” sound, which is at a reduced volume – so even though they are much closer than the listener 2/3 of the way back, the volume is only slightly louder than if we were to aim the center of the speaker directly at them. This simple rule will balance the coverage from front to back – however if your room is exceptionally deep, you may require “delay fill” loudspeakers to even out front-to-back coverage – if so, it is recommended that you seek help from those that know how to properly configure a delay fill speaker in your system’s loudspeaker processor (DSP.)

What about subwoofers?
Bass wavelengths are physically very large and mostly omni-directional, and while the same rules of physics apply, due to the wavelength size, the results in a room can be very different than a high frequency (smaller) wavelength. If you have more than one subwoofer in your room, without modeling the configuration in software, you have two options – either place the subwoofers directly next to each other, or at least 8 feet apart. This general guideline will help prevent the subwoofers from working together to create “lobes” or areas where the sub energy is much louder than the adjacent area.

Now that you have a general set of guidelines to follow, you can set about to design your speaker placement. The first real world obstacle you will undoubtedly discover is that there is almost always some kind of obstruction right where you want to place your loudspeaker. Unless you were able to design your facility with audio as a component from the beginning (rare) then you will have to compromise. Every sound system is a compromise, either for budgetary reasons, or physical placement limitations. The bottom line is to place your loudspeakers as close to “ideal” as possible, while still living within the physical restrictions of your space.

What do you gain by proper placement?
Placing loudspeakers in this manner will insure that your loudspeakers and your room are not fighting against each other. Your goal of a single loudspeaker “covering” a single area will help reduce the number of unwanted reflections (there will always be some.) One of the side benefits to reduced reflections is higher gain-before-feedback.
Bryan Cole is the founder of Axia Concepts, bringing high end creative audio, video, lighting and IT solutions to the local church using a unique, people-centric approach to technology.

Reprinted from Technologies for Worship Magazine May 2015 issue