Before you invest in your next sound system upgrade, it’s important to understand exactly how sound waves propagate in an enclosed space, and how the complex interrelationships of direct and reflected sounds are perceived by listeners in the context of music and speech program material. In order to wrap your head around that process, you first must master four fundamental terms that define how sound interacts with any architectural space.
- Direct sound
- Early reflections
For each of these terms, we’ll look at a working definition, consider how this characteristic affects speech and music in a church environment, and evaluate how it can add to or distract from the worship experience.
Defined – This one is relatively straightforward. Direct sound refers to those acoustical waves that emanate directly from the sound source. The source could be a person speaking or singing, a musical instrument, or a loudspeaker. With pure direct sound, no reflected sound waves at all are reaching your ears.
However, it is rare that you will hear nothing but direct sound. Indoors, you’ll experience it only in an anechoic chamber — a room made for acoustical measurements that absorbs all sounds at all frequencies. Even outdoors it’s uncommon. You’d need to be away from any large objects since buildings, pavement, water, or even large tree trunks will reflect some sounds.
Benefits – Direct sound is always good, for two reasons.
First, direct sound is the foundation of both musical clarity and speech intelligibility. That’s because all of the sounds you hear, at all frequencies, are phase coherent when they reach your ear — at least relative to the immediate source. (If these direct sounds are from a loudspeaker, the phase relationships may be altered relative to the original source, but that’s another story.) Direct sounds will be free of any smearing of consonants in speech, so similar words are easily distinguished. Also, staccato music sounds with rapid transient changes are perceived individually with full clarity.
Direct sound is also important for localization and imaging. Direct sound tells our ears where the sound is coming from, and it allows us to distinguish the location of sounds in space with amazing accuracy. If you’ve ever sat in the front row of a symphony concert, you know that you can close your eyes and localize instruments in three-dimensional space from their direct sounds.
Downsides – There are no [downsides] to direct sounds unless they are distorted or too loud.
Definition – Early reflections are sounds that arrive at your ear less than about 1/10 second after the direct sound. They are generated by acoustically reflective surfaces near the sound source and are perceived as an integral part of the direct sound.
Benefits – Even though we don’t perceive these reflections separately, we know when they are absent. Early reflections give an acoustical space a feeling of “life” and “air.” Without them, the space sounds “stuffy” or “dead.”
Early reflections actually contribute to speech intelligibility by increasing the total sound level while maintaining [the] coherence of consonant sounds. Early reflections also enhance musical clarity and allow members of ensembles to hear each other. Stage orchestra shells are basically made to create early reflections. Traditional church architecture often produces ample early reflections — usually mixed with mild reverberation — that encourage congregational singing.
Downsides – Again, no real downsides, unless the perceived boost in level from strong early reflections makes the overall sound too loud.
Definition – Reverberation is complex reflected sound energy that is perceived as separate from the direct sound. Reverberation “hangs in the air” after the direct sound stops. The nature of reverberation is defined by three characteristics:
Strength – How strong (loud) is the reverberant sound in comparison to the direct sound?
Decay – How long does it take for the reverberation to die away after the direct sound stops? This is usually given as a mid-band RT60, or the time it takes mid-band frequencies (those centered at 500 Hz and 1 kHz) to decay 60 dB from the initial level of direct sound. A very dead room, like a good movie theater, will be less than one second. A large cathedral can be 10 seconds or more, whereas a good symphony hall will be around two seconds.
Frequency balance – Depending on the size, architectural layout, and building materials, some frequencies in the reverberation envelope may be stronger than others. A persistence of low frequencies gives a “warmer” reverberation while higher frequencies will make the reverberation tail “brighter.”
Benefits – The right balance of reverberation can make a room “come alive,” for orchestral and choral music in particular. Reverberation contributes to a sense of depth and spaciousness, adding a dimensionality and enfoldment not realized with only direct sound and early reflections.
Downsides – Too much reverberation can be very damaging to speech intelligibility, as phase relationships crucial to recognition of consonant sounds become jumbled. Excessive reverberation also can be problematic with amplified music, with direct and reverberant sounds clashing with each other. Also, an imbalance in the reverberation envelope can make a room “boomy” (excessive low frequencies) or “edgy” (excessive high frequencies).
Cathedrals like this one in France are highly reverberant, so multiple small loudspeakers are needed to deliver enough direct sound to maintain speech intelligibility.
Definition – Echoes are discrete sound reflections that are perceived as separately identifiable sonic events. A slap echo is perceived as a single sound return, where flutter echoes are a rapid series of decaying echoes.
Benefits – Echoes can be great fun in a desert canyon but are universally problematic in church acoustics.
Downsides – Slap echoes can be generated in larger churches by a large flat or concave reflective surface at some distance from the stage. Sharp sounds, like a snare drum hit, will bounce back a discernible fraction of a second later and cause much consternation among the musicians.
Flutter echoes are caused by hard parallel surfaces, such as in a mirrored dance studio, and can be extremely annoying. Fortunately, flutter echoes are rarely generated in most church architecture, either traditional or contemporary.
Armed with knowledge …
With a clear grasp of these terms, you should be able to understand and evaluate suggestions put forth to deal with acoustical problems in your church. Possible solutions could be as simple — and cost-effective — as re-aiming your current PA arrays to better avoid reflective surfaces and concentrate more power on the seating areas.
It’s more likely, however, that you’ll want to look at various acoustical treatments that reflect, diffuse or absorb sounds at specific frequencies to achieve the desired effects, whether increasing early reflections or either enhancing or deadening reverberations. We will look at these options in a future article.
At the very least, now you’ll be able to tell when a supposed “acoustics expert” might be lacking proper credentials. If a consultant comes in, claps his hand and says “sounds pretty echoey in here” when there is only a reverberation tail with no discrete echoes — be suspicious.
This article originally appeared here, and is used by permission.