Last month, we examined the concept of reverb time — A.K.A., RT60. This month we’ll look at two more important parameters found in most reverbs: early reflections and diffusion.
Early Reflections
Imagine you’re seated in the front row of an auditorium and someone is on stage smacking a snare drum. The direct sound is the sound that reaches your ears straight from the snare without reflecting from any boundaries. Sound reaching your ears from any other path is either reverberation or early reflections. The shortest reflected paths from the snare to your ears are the early reflections. The path from the snare to the floor to your ears would be an example of an early reflection. The path from the snare to the rear walls to your ears is a reverberant path — it’s much longer than an early reflection. In large rooms, sound usually travels farther before it’s reflected, so the timing between individual early reflections is longer than that of a small room.
Early reflections are important because they give our brain the first hints regarding room size, and often they can be heard as discrete, short delays. When those early reflections are blurred or diffused, your brain thinks that sound is being bounced from irregular surfaces (more on that below).
Reflecting on Your ‘Verb
When adding reverb to a mix, you should be using it in an aux send/effect return arrangement as opposed to inserting it on individual tracks. There are several reasons for this, one of which is to conserve processing resources in software-based mixing systems (see “Serial and Parallel Processing” from the June 2011 issue of FRONT of HOUSE for a detailed discussion).
Waves’ TrueVerb is an example of a reverb that provides output level controls for direct, early reflections, and reverb (see Fig. 1). Adding reverb using an aux send means you already have a channel with the dry signal, so the direct output should be all the way off (or 100 percent wet if your ‘verb has a wet/dry mix control). In the case of TrueVerb, you then have the option of mixing the amounts of early reflections and reverb. If you turn the early reflections down all the way and leave the reverb up full, you’ll get the impression that you’re listening from a more distant position. If you crank up the early reflections and slowly bring down the reverb, the reverb tail will be attenuated, making it sound like you are getting closer to the source. You’ll notice that adding early reflections makes the effect sound brighter. That’s because early reflections haven’t yet experienced the energy loss (more noticeable in the high frequencies) as have the reverberation components of the sound.
You can use a higher mix of early reflections to reverb as a way of adding ambience without adding length to a sound. Favoring the early reflections over the reverb on an electric guitar can add a sense of space without messing with the articulation of the guitar parts. Doing so on a lead vocal will make the vocalist appear to be in a locker room or a garage; on the other hand, removing the ERs and cranking the reverb removes the immediacy and gives the vocal an ethereal sound. You may notice with low diffusion that you hear separate delays or “chatter,” especially on percussive sounds; increasing the diffusion parameter can reduce this phenomenon — which leads us to…
Diffusion
The diffusion parameter on a reverb can be difficult to understand and hear. In fact, I’ve used some reverbs (hardware and software) where I swear that changing the diffusion parameter makes absolutely no sonic difference.
If you look up the meaning of the word diffusion you’ll likely find “the act of being diffuse.” Fabulous. Didn’t Mrs. DeMarco tell me in the sixth grade that I’m not allowed to define a word by using that word in the definition? I’m pretty sure she did. In the video industry, diffusion refers to soft-focus of an image. Diffusion in acoustics refers to scattering of reflected sound. Understanding diffusion in acoustic terms can help your understanding of diffusion as a reverb parameter.
When a sound bounces off of a hard surface such as glass or concrete, a lot of energy is reflected. If that energy is reflected back toward an opposite, parallel wall, a standing wave can be created at a certain frequency or frequencies. This results in inconsistencies in the frequency response as you listen from different areas of a room. We’ve all experienced this when mixing: you move five feet away from the mix position, and the kick drum disappears. That’s because the sound field in the room is not diffuse — it’s not consistent throughout the room.
Acoustic diffusors are used to break up standing waves (and their high-frequency counterparts, flutter echoes) in studios and live venues. A diffusor scatters reflected sound to help make the sound energy more uniform across the frequency range throughout a room. This would help the problem of the disappearing kick drum. Diffusion can have a profound effect on the sound of a room, increasing sonic consistency throughout the room, and reducing the sense of localization.
An increase in a reverb’s diffusion parameter blurs your perception of the reflections so that you don’t hear them as individual echoes but more as a cohesive decay. Here’s an analogy. If you drop one marble onto a wood floor you will clearly hear the sound of that marble hitting the floor and bouncing. As it loses energy the bounces get smaller and closer together, until they stop. That would be like one reflected sound wave. If you dropped a hundred marbles into the floor at the same time, that would be like hitting a snare drum in a reflective room. Sound bounces off of the surfaces of the room, creating hundreds of small echoes. Increasing the diffusion decreases your ability to hear those individual bounces or echoes and breaks up the standing waves.
Generally, a higher diffusion value blends the individual reflections, and lower diffusion numbers result in more discrete echoes. Diffusion can be difficult to hear on certain instruments. On a snare drum, low diffusion adds graininess to the reverb tail and in some cases can add a chattering sound on the decay. Adding diffusion to a vocal reverb reduces transients in the reverb and in some cases can add a sort of ringing or resonance. Fig. 2 shows Waves’ Renaissance Reverb, which provides control over diffusion amount.
It’s worth noting that some reverbs have a density parameter and not a diffusion parameter. Density usually refers to the thickness of the reflections in a reverb tail, and there may be a separate density parameter for the early reflections. Higher density results in a smoother tail, and lowering the density may produce an effect much like reducing the diffraction (graininess and chatter on percussive sounds). You may find it easier to hear changes in diffusion or density if you temporarily plug the reverb into a channel and listen only to the “wet” signal.
The audibility of diffusion and density is affected by damping, which we’ll discuss next month in On the Digital Edge.
