Choosing the right reverb

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  Choosing The Right Reverb How best to use different reverbs Recording engineers have been adding artificial reverberation to recordings formany years, and have developed a variety of different ways of doing this. Homerecordists now have access to modern versions of all these tools, so let's look athow they compare to each other, and how best to use each.  Artificial reverb is an integral part of music production, as it puts back the sense of space and place that'sremoved by close-miking voices and instruments in an acoustically dead studio. In the real world, reverb iscreated by sounds reflecting and re-reflecting from surfaces in an enclosed or partially enclosed space, andthe resulting pattern of sound is infinitely complex. The geometry of the space and the materials from whichit is made affect both the pattern and intensity of the reflections, and the rate at which different frequenciesdecay. Our brains derive information from these audio characteristics, enabling us to learn something aboutthe nature of the space without necessarily seeing it. In music production, this means that the reverb typeand its settings need to be chosen carefully if the human hearing system is to accept it as natural — or atleast believable.In the early days of recording, there was no artificial reverb, so the effect was created by placingmicrophones and loudspeakers in a reflective room such as a tiled basement. Rockfield Studio in Waleseven had a room made with suspended glass plates to create a variable reverberant environment. Thedifferent rooms built by different studios were often instrumental in how popular the studio turned out to be,but most of these rooms became obsolete when artificial reverb was invented. Probably the first successfulartificial reverb device was the spring, though all kinds of weird and wonderful devices were created,including coiled pipes with a speaker at one end and a mic at the other. Analogue Reverb: Springs, Plates & Delays  Apparently the first spring reverb was developed by accident after a couple of sound engineersexperimented by dangling a coiled spring from a gramophone pickup cartridge, shaking it to create thunder effects for radio. By fitting a transducer to the other end of the spring, they found that sounds passed alongthe spring were picked up with a kind of reverb added, and so the spring reverb was born. Springs are stillused in guitar amplifiers, and they have such a distinctive sound that modern digital reverb units ofteninclude spring emulations.While the spring worked well enough on vocals, guitars, and electronic organs, it wasn't so good onpercussive sounds, as it had a tendency to 'twang' — the sound fluttered and pulsed as the reflectionsbounced back and forth along the length of the spring. You can get an idea of what spring reverb soundedlike on vocals by trying one of those toy microphones with a spring and a diaphragm inside. (In fact somepeople have used these for recording in various ways!) All kinds of tricks were tried to improve the quality of    spring reverb — immersing the spring in oil, using multiple springs with different characteristics, using heavylimiting on the input signal and extensive EQ — but it wasn't until the reverb plate was developed thatartificial reverb really got serious .  Before the reverb plate came on the scene, the most common artificial reverb option was based around steel springs, like these ones from Accutronics which can be found within some Fender guitar amps.  As its name suggests, the plate reverb utilised a thin metal plate, which could be as large as one metresquare or more, suspended in a soundproof box and driven into vibration by means of a transducer similar to the voice-coil assembly of a loudspeaker. This arrangement was fed from a mixer aux send, via a suitableamplifier to drive the plate. Pickups mounted on the surface of the plate picked up the vibrations and fedthem to a preamplifier, and the signal was then fed back to the mixer's aux returns. Normally two pickups,mounted at different positions on the plate's surface, were used to give a pseudo-stereo output from a monoinput.The way the plate works is that sound energy from the input transducer moves very quickly from thetransducer near the centre of the plate to the edges of the plate, where it bounces back. These reflections inturn encounter other plate edges and continue to re-reflect, losing energy but gaining complexity. Becausethe plate is rectangular and the sound waves propagate in a circular manner, even the first wave of reflections is very complex, as the expanding wavefront will encounter different parts of the edge of the plateat different times.The early part of real-life reverberation in larger spaces comprises a number of discrete reflections that areclearly audible before the dense reverb tail builds up. However, a reverb plate produces a very dense soundvery quickly, and has no discernable early reflections. Because early reflections are fundamental inconveying a sense of room type to the listener, the plate is somewhat devoid of spatial character insomuchas it doesn't suggest any particular type of acoustic space, but in a musical context this can be anadvantage, as we often use reverb as a musical effect, and not to create the illusion of a specific type of room or hall . While the plate has no real spatial character, it certainly has a recognisable tonal character and can tend tosound rather metallic, so EQ often needs to be used to improve the end result. The decay time of a plate isalso fixed, unless some form of mechanical damping is applied, so the better plates were fitted with remote-control fabric pads that could be brought into contact with the plate to reduce the decay time. Undamped,the reverb time could be as long as five seconds or more, which is too long for most routine applications.Early spring or plate reverbs were also often used in conjunction with tape delay to help create a sense of space and size. As springs and plates generate no early reflections in the true sense of the word, addingone strong psychoacoustic cue using delay helps create a false sense of spatial identity. In a real room, thiswould equate to how long the sound takes to reach the first surface and then bounce back to the listener.The longer the pre-delay, the larger the space feels. Modern digital reverbs do the same by offeringadjustable pre-delay times. For example, a typical vocal reverb treatment might comprise a plate reverbemulation with a decay time of 1.2 seconds or so and a pre-delay of 60-80ms.While plates are no longer commonplace, there are some extremely good digital simulations of the plate,such as the Universal Audio Plate 140  plug-in for the UAD1 card, and special algorithms need to be createdto reproduce the rapid build-up of density that occurs in a real plate. These plate effects work particularlywell on vocals, and would have been used on virtually all the classic records in the '60s and '70s. Multi-head-tape or magnetic-disc echo devices were also often used to create pseudo reverb, sometimes inconjunction with spring reverbs, but used alone these devices were unable to create the necessary densityof repeats to emulate the real thing. Instead, the echo machine became an effect in its own right. Digital Reverb  Digital reverb became a commercial reality with the EMT 250 in 1976, and early digital reverbs attempted toapproximate what goes on in a real room by first using a multitap digital delay line to recreate those tightlyspaced early reflections. The delay times would often be adjusted empirically to produce a pleasing result,rather than being a direct emulation of any particular space, and the amplitude of each tap was alsoadjusted to produce a natural result. Just 20 or 30 delay taps might be enough to approximate the earlyreflections, and by choosing different patterns and wider or narrower spacings the impression of varioussizes of room or hall could be created. Photos: Mark Ewing.Two classic early digital reverb units which are still in use today: the Klark Teknik DN780 (above) and the Quantec Room Simulator (below).  As a rule, a wider early-reflection spacing is interpreted by the brain as a larger space. In order to reproducethe build-up in complexity in the decaying reverb tail, multiple re-circulating filters (usually a mixture of combfilters and all-pass filters) were used, sometimes fed from the srcinal input, sometimes from the outputs of    the tapped delay line, depending on the designer. In either case, these re-circulating filters had to becarefully tuned so that the reverb didn't ring excessively at specific frequencies, and they also included EQ-like elements to damp the high end in a way that replicated the behaviour of a real space.Everyone had their own method of designing reverb algorithms, which is why the products from differentmanufacturers had, and still have, very different characters. Some of the most sought-after reverb devicesdidn't sound particularly natural, but happened to be musically flattering. While digital synthetic reverbs of this type rarely sound exactly like the rooms they purport to emulate, their sound has become part of popular musical culture. The most famous of the digital reverb manufacturers is probably Lexicon, who havedefined the reverb sound of pop music over the past two decades, though there were numerous other earlydigital reverb devices such as Quantec's Room Simulator, the Ursa Major Space Station, the classic AMSRMX16, and Klark Teknik's DN780.Digital reverbs are often brighter and more sparkly than anything you come across in real life, but they workperfectly in a musical context. In fact, they work so well that, when sampling or convolution reverb made itpossible to 'copy' reverb from actual spaces, many people found that it didn't sound exciting enough for popmusic, so turned the process to sampling pieces of reverb hardware. Perhaps the most important aspect of a good synthetic reverb is the way that it seems to become part of the srcinal sound, rather than seeming tobe an effect layered on top — which of course in reality it is! With the better units, adding more reverbincreases the sense of space, but doesn't swamp the srcinal sound and doesn't make your mix soundcongested. Cheaper reverbs may sound OK in isolation, but can end up making your mix sound messy andcluttered when you try to use them on a real project. This has little to do with technical specifications — it isall down to how the reverb algorithms are designed. Convolution Back in the '80s when I first asked manufacturers whether it would ever be possible to sample the reverbcharacter of a real space, the response I always got was that there would never be enough computingpower available. However, computing power has continued to follow Moore's Law and today we havecomputers that would have been inconceivable back then. In the mid-'70s I was working for a company thathot-rodded Commodore PET computers by expanding the stock 1K memory to 8K, but now 8GB of memoryis a practical option. In other words, in only 30 years or so the amount of memory you can fit into a typical  desktop computer has expanded by around one million times, and clock speeds are now so high you couldcook frozen chicken with them! The Atari ST, on which so many of us started, had a clock speed of just8MHz, whereas some of today's machines run at over 3GHz.  Another successful early digital reverb unit was the Ursa Major SST282 Space Station. Although the srcinal hardware unit (top) is still in use insome studios, most home-studio owners are most likely to come into contact with the Space Station's distinctive sound via the smaller SevenWoods Audio SST206 Space Station (centre) or Princeton Digital's TDM plug-in recreation (bottom). Now, of course, convolution has become a reality, and can run on just about any well-specified Mac or PC,   as well as in dedicated hardware effects units. Convolution is essentially a brute-force, number-crunchingmeans of producing reverb (or other linear delay-based treatments) which is brilliant in its simplicity. It starts   with the premise that you can measure the reverb character of a room by putting up a stereo mic at thelistening position, then recording the reverb created from a single spike of sound one sample in length — astarting pistol or a balloon burst gets close to this. The resulting sound picked up by each microphone iscalled an 'impulse response' and it can be recorded conventionally. If you then use a computer to multiplythis impulse response by every sample of new audio signal, the net result is that you add the srcinal room'sreverb to that audio signal.Nowadays, instead of trying to create a loud sound one sample in length (which isn't very easy), impulseresponses tend to be recorded using a sine wave swept over the whole audio frequency range over a periodof a few seconds. A mathematical process is then used to time-compress the resulting data so that itequates to what you would have got had you used a single-sample pulse. The advantage is that it's easier to reproduce a suitable signal level for measurement, and because the measurement is integrated over afew seconds it tends to be less susceptible to corruption by low levels of background noise in the roombeing sampled. Integrating multiple impulse-response recordings from the same source can further reducethe noise floor. Of course, to get a good measurement you need very accurate microphones and speakers,and to do a comprehensive job you may need to take different measurements at different places in the roomso as to give the user a choice of reverb characteristics. Fortunately, commercial convolution reverbs comewith a wide range of presets based on specific venues, and many third-party impulse responses areavailable on the Internet, some for sale and some free.To capture the sonic signature of an electronic reverb unit, the same test signal can be fed through it andthen mathematically processed to produce an impulse response, enabling that particular hardware setting to
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