Sound in Reverberant Spaces, Part 1: Why It’s Hard

Zac HicksSound and Audio, Worship Leading Tips, Worship Technology1 Comment

I’ve had the privilege over the last year to work with Steve Bailey, a musician and sound engineer who I believe has “conquered” our hard-surfaced, reverberant, classically-oriented worship space at Cherry Creek Presbyterian Church here in Denver.  As a worship leader, my vision for a good mix is not about making our musicians sound awesome as much as it is that the music we make sounds inspiring enough such that we eliminate distractions toward the people of God singing boldly on Sunday mornings.  A lot of my friends and readership work in older church buildings which are highly reverberant and therefore difficult to mix for amplified music.  Because Steve so excels at mixing our difficult room for our diverse musical sounds (everything from big rock, to Dixie, to bluegrass, to folk-classical), I invited him to post here.  Hopefully we’ll hear more from him!

The Problem

Amplified sound in highly reverberant spaces is a very common problem, particularly in churches. It’s common enough that the vast majority of the work that I have done as a live sound engineer has been for churches with overly reverberant rooms. The problem comes intrinsically from oppositional forces being at work within the church. The forces of the past demanded large spaces that are designed for acoustic worship music. The forces of the present demand contemporary worship music, which is almost always amplified. So we run into the issue that suddenly a room that was designed for a “classical” musical aesthetic is now being used to play “rock” music.

When building rooms that are designed for acoustic music (or that don’t take amplified music into consideration) architects seem to favor lots of hard, reflective surfaces and funny angles. Honestly, I don’t know if architects take into consideration what their space will sound like or if they just build with hard surfaces and angles because it looks good. It probably goes both ways, but the end result is the same: you have a room that sounds really good for unamplified music or speech because it is highly reverberant.

The Science

As a general rule, I’m going to try not to get overly technical in these posts, but some science talk is demanded here. Sound, amplified or not, is essentially a vibration that disturbs the particles of nearby air and causes them to bump into each other. So, the soundboard on a piano, or the cone of a speaker, or someone’s vocal cord, vibrates and pushes air particle 1. Particle 1 bumps into particle 2, which bumps into particle 3 and on and on until particle 13,732,085 bumps into a tiny hair in your ear. This hair is connected to a nerve, which sends an electrical impulse to your brain. So everything is really just pushing air around.

But our ears are really tiny. When you think about the amount of air, and therefore sound, that’s actually traveling into your ear versus the amount that’s being pushed around by a piano, or a cluster of speakers, there’s a lot of air that’s going to places apart from our ears. This is where those hard surfaces come into play.

With hard surfaces in a room, the sound literally bounces around more. So, the chain of air particles that didn’t go into your ear hits a hard wall and bounces off of it, and might then also bounce off of another wall and maybe another before it reaches your ear. All that bouncing ends up taking a lot of time. So there’s one set of particles that come straight from the sound source to your ear, and a whole other set that bounce around the room for a while and then hit your ear later. Realistically speaking, we hear dozens of reflections coming from various points in the room all reaching our ears at different times. This creates what we in the ‘biz’ call “reflections.”

Reflections aren’t bad. In fact, a lot of musical effects are designed to emulate reflections that don’t exist. Things like echo, delay, and reverb create a little bit of time between the original, unprocessed signal and what’s called the ‘wet’ or effected signal. These effects create a sense of space in music that otherwise wouldn’t have it which is great and, honestly, totally necessary to keep things sounding natural.

So rooms designed with hard surfaces create a natural reverberation, which sounds great at lower volumes. But once things get amplified everything falls apart. Here’s why: imagine bouncing a super-ball. When you bounce it softly, it bounces for less time before it dribbles out and eventually comes to rest. If you jump in the air and throw it as hard as you can, it bounces for much longer before it comes to rest. This is what’s happening to the air particles in your room.

Think of amplitude (another, more precise word for “volume”) as being the force that you impart to the super-ball. When acoustic music is performed it happens at a low enough amplitude that only the sound reflecting off of one surface reaches your ear. These reflections are called “early reflections” and are the good kind of reflections. The other, bad reflections that come from bouncing off of two or three walls (called “late reflections”) lose all of their energy through bouncing around so that they come to rest before they can actually reach your ear. This would be like when you bounce the ball softly.

When you amplify sound, you are imparting more energy to it and thus more amplitude and volume. So those late reflections have a lot more energy when they start out and never come to rest before they reach your ear. Think of when you jump in the air to bounce the super-ball. All these late reflections are competing in your ear against the direct sound and the good early reflections, making the whole thing sound messy and unintelligible. 

Toward a Solution

So that’s the problem in a nutshell. The solution to the problem is complicated and comes from a lot of different approaches. In later posts, I’ll start addressing what these approaches are and the philosophy that will help you to get the most out of the fixes that you decide to use.

If this was hugely interesting to you I would first call you a nerd, and then suggest that you do some reading on the physics of sound. The first few chapters of almost any sound reinforcement textbook will have a wealth of information for you. I particularly like Audio in Media, by Stanley Alten. For a more comprehensive approach to these concepts, and really for everything audio, pick up the The Sound Reinforcement Handbook, by Gary Davis and Ralph Jones. It’s a hefty read and can get extremely technical, but I consider it to be the most complete source of information on the subject.

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