What is reflection?
Reverberation or Reverb happens when a sound is reflected. Reverb time is the time it takes for an impulse to decay by a certain dB level in a space. In an acoustic space, there tends to be many reflections that return to the receiver each time they hit a surface – until the absorption of the surfaces causes them to decay. Some enclosed spaces are more reflective than others. The reflectivity (opposite of absorbency) of a room depends on the absorbency of the materials/surfaces in it and the surface area of those materials.
For example, imagine a large hall with hard, smooth walls. When the hall space is empty, there will be a more reverb than when it is full of objects like people and furniture. When something is in the way of sound, some frequencies will go around it and some will get absorbed – this phenomenon is called diffraction. This along with the absorption provided by material typically used in clothes and furniture diffuses reflections and decreases reverb time.
Reflection and Reverb
When producers and mixing engineers hear “reverb”, they typically don’t think of reflections in a space. Rather, the wide array of digital hardware and software units that exist today comes to mind. Reverb effects are evident throughout history – we could say they are a subtle but crucial part of any mix. Using them tastefully on the right sources in your mix can enrich your song as a whole. They can add warmth, depth and space to even the simplest of ideas. On the other hand, choosing the wrong type of reverb or too much reverb makes a mess.
BASIC REVERB PARAMETERS
Most software and hardware reverbs feature these basic parameters for determining the following aspects of the effect. You will find these parameters in just about every unit out there.
Type – Determines the emulated type of space (Hall, Room), device (Plate, Spring), or digital process (Reverse, Gate). Different settings emulate different reflectivity. I’ll go over this later in detail.
Room size – Adjusts the size of the virtual room determined by the Type parameter.
Pre-delay – Determines/simulates the time it takes for the first reflection to return to the source after its been propagated. The longer the pre-delay, the larger the sense of space. Keep in mind, at longer pre-delay settings, the effect becomes more of an echo than a reverb tail.
Reverb Time (Tail) – Determines the time it takes for an impulse to decay (lose energy) by a certain amount (determined by the publisher) in the virtual space. It is a function of the absorbency and volume of the room. Big, empty, reflective spaces have longer tails while smaller, absorbent spaces have shorter tails.
Damping – Since softer materials (like clothes and curtains) are better at absorbing higher frequencies, reflected sound is often low-pass filtered naturally. Damping determines how significant the reverb’s emulation of this phenomenon will be.
Mix (dry/wet) – Determines the balance between processed and dry signal. In doing so, it also determines whether the effect is functioning in serial or parallel (See Blog: Powerful Parallel Processing).
SOME REVERB TYPES AND EXAMPLES
Stereo Pair in a Room
This is an early method of incorporating reverb into a mix used long before the invention of emulators. It is achieved by mixing the output of a stereo pair (2 mics) in the room with the mic on the performance such that the balance can be blended to the engineer’s taste. This method of capturing reverb is most faithful to the complexity of naturally radiated sound. No emulator has the processing means to so accurately recreate the subtleties in the acoustics of a real room. It is however a limiting technique if the room recording is not sympathetic to the overall mix
Spring reverb simulates long telephone line delays from the 1930’s. It is an electromechanical device that uses a system of transducers and steel springs to create clones of the input. When vibrations hit the output, that signal is transduced as well as the parts of the vibration that are bouncing back and forth along the springs. The science behind spring reverb is more complex than this, but spring reverbs are perhaps the cheapest setup to manufacture.
This advanced reverb unit analyses the response of a room based on a recording that was made in it using DSP technology. Many of these reverb units provide impulse responses recorded in famous spaces like the Sydney Opera House, event stadiums, cathedrals, and natural environments – but you can record and import your own samples. The analysis captures a number of different qualities in both the time and frequency domain of the response. This has been a revolutionary tool for the film and TV industry (since they can impose the ambiance of the filming location onto the audio post-production).
A reverb chamber represents an enclosed space with several microphones. They can be of any size and shape. It can be build with any reflective type of material: concrete, wood, glass, tile…etc.
Digital chamber reverbs usually emulate a smaller space compared to hall. Reverb tail is also typically shorter. Perhaps the biggest difference between hall and chamber is clarity. Lets apply chamber reverb to this drum loop and hear how it sounds.
Like a spring reverb, a plate reverb is a man-made device. The difference is that it operates on one dimension and has more creative qualities. It uses an electromechanical transducer, similar to the driver in loudspeakers, to induce vibration along a large plate (surface) of sheet metal and usually has a dampening mechanism to adjust decay time. It sounds nice on a vocal. For example…
Digital Emulator (Algorithmic)
Digital reverbs emulate the natural and mechanical aspects of the reverbs we have discussed so far and are perhaps the most commonly used reverbs in studios today. They have little physical limitations and provide a lot of control. There are many different internal designs for algorithmic reverbs and many different pre-set virtual spaces. For example….
Hall – Emulates the space of a concert hall who’s size and construction is variable. This type is often applied on string sections and orchestral arrangements. In these audio examples, you can sense how the hall reverb tail decays for a while after the string performance ends.
Room – Considerably smaller in size than the previous virtual room, resulting in a stronger initial reflection followed by a fast series of echoes that decay quickly. Again, the sound of the reflections depends on the materials and construction of the room. We will apply it on the same drum loop from chamber example here.