More general reverb tips

As a followup to my Eos tips and tricks post, I thought I’d share some more reverb tips. All of these have been tested with Eos, but should also work with a wide variety of hardware and software reverbs.

• Set the high cut filters to a fairly low frequency. Older hardware reverbs, such as the EMT250 and Lexicon 224, had a hard cutoff at 8 KHz to 10 KHz, due to the low sampling rate of the machines. The high cut filters in many reverbs have a much more gentle slope than the high order filters used at the inputs of these old boxes. To emulate these old boxes, try setting your high cut filters to a fairly low frequency, such as 2 to 4 KHz. This also corresponds more closely to the absorbtion of high frequencies by air in a large space, such as a concert hall.
• Use the low cut controls to make the reverb sound less “tubby.” Many concert halls actually have a much longer decay for low frequencies than mid range frequencies. This is useful for classical music, but for most popular music forms, the amount of bass energy that is present will sound flubby when reverberated. Set the low cut controls at 200 to 400 Hz, or even higher, for a clearer reverb sound.
• If you don’t have low or high cut controls, put the reverb in a send bus, and put the EQ of your choice before or after the reverb.
• Try compressing the input or output of the reverb, for some neat sounds. A limiter before the reverb will keep spikey transients from dominating the response, and will better emulate the transformer-coupled inputs of the old high-end hardware units. Compressing the output of the reverb will change the exponential decay response to something much weirder, depending on your settings.
• Plate reverbs have a lot of high frequencies in the decay, so make sure that the high frequency decay multiplier (or the high frequency decay filter cutoff) is set fairly high. These controls are usually separate from the high cut controls, that shape the sound at the input (or output) of the reverb.
• Adjust your modulation depth based on the decay. For long decays, you may wish to back off on the modulation depth, as the sound will travel through the modulators many more times compared to a short decay. Each pass through the modulators causes more detuning. A modulation depth that works for short decays may sound seasick for long decays. Of course, if that’s your thing, then go for it.
• Use the Size control for the desired echo density, but be mindful of how it affects the modal density. For example, if you want a small drum room, then set Size to a smaller setting, as it will make the echos closer together. However, a smaller Size setting will sound more metallic for longer decays, as the modal density goes down as the Size decreases. Longer delay lengths = higher modal density = less metallic = lower echo density. For short decays, the low modal density may not matter.
• The Size control is often given in meters. This has nothing to do with any real physical world metrics, in most cases. A real acoustic space with a 30 meter maximum dimension will have a few orders of magnitude higher modal density than your typical digital reverb with a 30 meter Size setting. Just tune it by ear to where you like it.
• Shorter Size settings may also result in deeper modulation for the same decay setting, so be sure to retune this for your tastes.

Hope these are useful to people. If you have any more tips, feel free to add them in the comments.

Modulation in reverbs: reality and unreality

The use of modulation in digital reverbs dates back to the first commercial digital reverberators. The EMT250 used an enormous amount of modulation, to the point where it sounded like a chorus unit. Lexicon’s 224 reverberator incorporated what they called “chorus” into the algorithms, working along principles not dissimilar to the string ensembles in use at the time. The Ursa Major Space Station was based around an unstable feedback arrangement, that relied upon randomization to achieve longer decay times without self-oscillating.

Recently, Barry Blesser has written about randomization in his book, “Spaces Speak: Are You Listening?” Blesser argues that thermal variations in most real-world acoustic spaces results in small variations of the speed of sound within those spaces. Multiply this by several orders of reflections, and the result is an acoustic space that is naturally time varying. Blesser goes on to argue that random time variation in algorithmic reverbs emulates the realities of an acoustic space more accurately than time-invariant convolution reverbs.

Blesser makes a convincing argument, but I am not convinced that the heavy amounts of delay modulation used in the older reverbs makes for a more “realistic” space. The randomization in the older algorithms does a nice job in masking the periodic artifacts that can be found when using a small amount of delay memory. However, the depth of modulation used in the old units goes far beyond what can be heard in any “real world” acoustic space. The thermal currents in a symphony hall will result in a slight spread of frequencies as the sound decays, but will not create the extreme chorusing and detuning found in the EMT250, or in the Lexicon algorithms with high levels of Chorus.

Having said that, I would argue that the strengths of algorithmic reverbs is not in emulating “real” acoustic spaces, but in creating new acoustic spaces that never existed before. Blesser recently said that the marketing angle of the EMT250 was to reproduce the sound of a concert hall, but later describes the EMT250 in terms of a “pure effect world.” The early digital reverbs, in the hands of sonic innovators such as Brian Eno and Daniel Lanois, were quickly put towards the goal of generating an unreal ambience, where sounds hang in space, slowly evolving and modulating. Listen to Brian Eno’s work with Harold Budd, on “The Plateaux of Mirror,” to hear the long ambiences and heavy chorusing of the EMT250 in action. A later generation of ambient artists made heavy use of the modulated reverb algorithms in such boxes as the Alesis Quadraverb to create sheets of sound, that bear little resemblance to any acoustic space found on earth.

Creating these washy, chorused, “spacey” reverbs has been a pursuit of mine since 1999. My early Csound work explored relatively simple feedback delay networks, with randomly modulated delay lengths, in order to achieve huge reverb decays that turn any input signal into “spectral plasma” (a term lifted from Christopher Moore, the Ursa Major reverb designer). With my more recent work, I have tried to strike a balance between realistic reverberation, and the unrealistic sounds of the early digital units. The plate algorithms in Eos are an attempt to emulate the natural exponential decay of a metal plate, but were also inspired by my understanding of the EMT250. The Superhall algorithm in Eos was not attempting to emulate any “natural” space, but rather the classic early digital hall algorithms, with heavy randomization, nonlinear build of the initial reverberation decay, and the possibility of obtaining near infinite decays. The “real” world continues to be a source of inspriation for my algorithms, but I find myself more attracted to the unreal side.