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Reverberation

One of the most important sound treatments is discussed in this month’s Advanced Music Synthesis.


It's probably true that the quality of ambience heard is a fairly important factor in determining whether or not a sound is liked or disliked, and it certainly adds depth, dimension, and some 'live' perspective to the potential artificiality of many a multitracked piece of music. One of the many trials and tribulations of modern recording techniques is the small problem of meeting the rather exacting demands of the human ear for the right amount of ambience in the right place at the right time. As a result, a good deal of time and effort is taken up ensuring that recorded signals are 'dry', so that when ambience is added it's of a uniform variety and not likely to give rise to the sort of mushy, acoustically-anarchic mess that would result from recording and mixing together a load of signals each with their own idea of what ambience is all about.

Au Naturel



When a sound is made inside a room, the listener initially hears the sound as it comes directly from the source (Figure 1a). A short time later, after a delay of somewhere between 5 and 50 milliseconds, the first echo reaches the listener, followed by several more individual echoes (Figure 1b). Many more echoes follow on the body of the first group (Figure 1c), and, as the quantity increases, so the brief time span between them becomes shorter and shorter until they join together in a healthy reverberant melee as a seemingly continuous sound. The original sound is now diffused throughout the room due to the multiple echoes and random paths taken by the original sound waves. As the degree of diffusion increases, the amplitude decreases, and it's this fading of the reverberant sound that constitutes the reverb decay.

Figure 1. Phases of ambience.


The apparent rate of reverb decay during this initial period depends both on the size and shape of the hall or room and on the position of the listener, and, according to those that study the complexities of auditory perception, the ear uses the first 200 milliseconds or so of reverb to perceive the size and shape of the ongoing ambient situation. The overall reverb time is usually measured by making a loud noise and measuring the time the sound takes to decay through 60dB. The value measured will obviously be dependent on the factors that we've already mentioned, but, at later stages of the reverb decay, the absorption coefficient of materials used in the room's construction is an important consideration.

Thus, values for ∝, the absorption coefficient, range from 0.03 for plastered wall (i.e., it's relatively reflective) to 0.6 for curtains hung in folds (i.e., they're like acoustic golden syrup). Ploughing these figures into Sabine's formula, namely, Tr=0.164V/S∝ where V is room volume (m3) and S is room surface area (m2), gives the value for Tr, the reverberation time, in seconds.

The difficulties of designing either an acoustically 'dead' recording studio or an ideally reverberant concert hall shouldn't be underestimated, and two of London's finest concert halls only achieved their current acoustic status after a lot of nail-biting to iron out various glaring design problems that manifested themselves after the edifices had been erected. The Royal Festival Hall may have been remarkably innovatory in design for a post-War building, but, for various reasons stemming mainly from the enthusiasm of the architects, the hall also functioned as a curious sort of comb filter as soon as an audience was in position. Peaks and troughs may be fine for flanging and the like but they aren't everyone's idea of the best treatment for Beethoven or the other Top Ten composers on London's South Bank, and the nonlinear reverb characteristics were rapidly remedied by means of judicious reinforcement of certain critical frequencies at key spots in the hall. And, whilst Queen Victoria might not be that amused by the sudden intrusion of UFOs into the upper reaches of her beloved Albert's very royal hall, the addition of the infamous 'flying dishes' certainly rescued the RAH from a fate worse than death.

Mechanical Aids



In the sort of recording situation facing the average electro-musician, ambience only comes about as a result of the injection of some delaying tactics into the process of mixdown. Mechanical reverb devices are certainly the most cost-effective way of inducing some semblance of natural ambience into the otherwise clinical clarity of synthesiser tracks. However, the range of modae operandum and variation in cost of units, for such an apparently straightforward procedure, can make choosing the right sort of reverb device a rather painful business. Four methods prevail:

1. Live Acoustic Chamber


Generally, high quality live acoustic chambers are constructed with non-parallel and hard, sound reflective surfaces so that the sounds will echo (or, more correctly, reverberate) within the chamber. Though live chambers are difficult to design and correspondingly difficult to build, some smaller studios have successfully found suitable 'lively' environments close to hand — bathrooms and toilets, for example. In operation, a loudspeaker is placed at one end of the room and a microphone at the other, with baffles added in between to maximise the path length across the room. Sounds to be treated can then be replayed through the speaker, and the resultant direct plus reverberant soundfield picked up by the microphone.

2. Reverb plate


One of the prized possessions of most large recording studios is the omnipresent EMT reverb plate (also erroneously called an 'echo' plate). This consists of a steel sheet approximately 2.5m x 1.5m, sometimes plated with gold or silver, and suspended vertically on springs. A moving coil transducer is mounted at a critical position to 'energize' the plate, and two or more piezo-ceramic contact microphones mounted at strategic points around the plate pick up the reverberant vibrations as they radiate to and from the edges of the plate. Plates tend to give a rapid response to transients and good high frequency resolution, two factors which account for their popularity in recording rock music — particularly for simulating a live 'cage' of ambience around a drum kit.

3. Reverb springs


Bandhive Great British Spring.

The simplest and cheapest reverb device consists of two or more sets of springs, often of different lengths, joined together at the ends. Sound waves are induced into one end of the springs in a torsional manner and recovered at the other end. The energy reflects back and forward between the two ends of the spring until it dies away giving a more or less reverberant effect. The length of springs used in reverb units varies enormously from a 2" twin unit in the very cheap Accessit reverb unit to a 16" triple unit in the Furman Sound RV-1. It's difficult to know what type and arrangement of springs makes for a good sound — doubtless spring tension and diameter play as important a part as anything else. 'Floppy' springs often account for nasty jangles as the spring decides it can't cope with a loud transient — hence the logic of limiters, pre-spring EQ, and LED overload indication. In theory, by using various amounts of bass or treble boost, different types of 'rooms' (or reverb patterns) can be synthesised, ranging from one containing a lot of absorbent material (an auditorium with well-padded walls, for example) in which the bass frequencies are more prominent, to very 'bright' or unabsorbent environments where the high frequencies contribute more to the reverberation effect. Most spring-line reverb units have a decay time that is characteristic of a particular spring set up. The AKG units, on the other hand, make use of what they term their 'torsional transmission line principle', whereby frequency-dependent decay time variation is effected via motional feedback. Damping elements are placed along the spring-line so that feedback can effectively shorten the spring length and vary the overall decay time from 2 to 4.5 seconds.

The most frequent descriptions applied to spring-line reverb are 'boing', 'twang', and 'flutter', and there's no doubt that the best evaluation technique for reverb units is the use of a drum track or any other sharp, percussive signal which will show up the presence of these less than desirable fellows. Signal processing tricks can be employed to overcome or conceal reverb deficiencies, but the usual result is a loss of the natural reverb qualities possible with a good spring unit. The four reverb units compared in Figure 2 all produce high quality reverberation, but with more or less subtly different characteristics. As modern mixing desks enable both the 'echo send' and 'echo return' to be separately equalised, pre-spring and post-spring EQ in the reverb unit itself is hardly an essential feature, and 'no frills' designs like the Bandive Great British Spring make a lot of sense.

Figure 2. Comparison of spring-line reverb units.
Master Room XL-121 Furman Sound RV-1 Bandive Great British Spring AKG BX5 Orban 111B
post-spring EQ Yes Yes No Yes Yes
pre-spring EQ No Yes No No Yes
limiting No Yes No Yes Yes
overload indication Yes Yes No Yes Yes
rack-mounting Yes Yes No Yes Yes
twin-channel (stereo) Yes No Yes Yes Yes
variable decay time No No No Yes No
price (£s/VAT exclusive) 302 257 174 375 631
springs/channel 3 3 3 ? 6


4. Tape echo


Tape recorders can be used to create a 'flutter' echo effect; by feeding back the delayed, off-tape signal to the input, sounds will be subjected to multiple repeats before dying away. However, such an effect is only like prolonging the initial echo phase of a sound, and, because of its repetitive and unrandom nature, becomes somewhat tedious. Where flutter echo is useful, though, is in conjunction with a spring or plate reverb, the straight tape delay enhancing the reverb, and a signal that has been delayed by a short interval before being connected to a reverb device often sounds more realistic when added to the final stereo mix.

Ursa Major Space Station SST-282


Digital Doobries



As we've seen, true reverb is a rich, random pattern of sound reflections whose echo density increases as the sound decays. The strength of each reflection, its time delay, and its frequency response determine the sound of the reverberation. Simulating realistic reverb effects electronically is a challenging problem and can't be done easily by tapping off increments of delay along a delay line and then summing the outputs together (these tend to create flutter echo with a low density of reflections). In theory, a digital delay line with a very large number of taps (something of the order of 5,000 per second of delay) could be constructed and used to duplicate natural reverb exactly. A high-speed processor could then be used to access numbers from the memory, multiply them by constants representing the required amplitude for that portion of the decay curve, add them together, and send the sum to the output. This operation needs to be controlled by a program that precisely describes the time delay and amplitude for each reflection — the reverberation algorithm. However, if the sampling rate is 20kHz (giving a bandwidth of something less than 10kHz), then the processor has only about 50μs in which to perform about 5,000 multiplications and additions — some 1,000 times faster than available 8-bit technology is capable of at present!

Digital reverberation systems solve this problem of inadequate processing time by using many short delays, with each tap being fed back into memory, and the magical ingredient of the reverberation algorithm randomizing the tapping to a greater or lesser extent. This combination of a smaller number of taps and feedback is the principle of all current digital reverb units. The principal factors to be considered in designing a digital reverberator are the time delay of each segment, the amount of feedback, the digital filtering applied to the feedback around each loop, and how all the taps are summed to make the final reverberant stew.

Figure 3. Inside reverb.


Lexicon 224 Digital Reverb controller.

This reverberation algorithm may go a long way to providing a usable type of reverb, but, as compromises have to be made, it isn't possible to exactly duplicate the natural reverberation of a typical concert hall, or whatever. Even with the best of intentions, such algorithms can often produce very metallic-sounding, fluttery reverb, and it's only after some very ingenious program-writing that Schroeder came up with the all-pass reverberation algorithms commonly used by units such as the Lexicon 224 and Ursa Major Space Station. Schroeder's work enabled these manufacturers to produce units that are capable of a quality of reverb sound that approaches the 'naturalness' of top-flight plate reverberation units. Just as small rooms and echo chambers produce high initial diffusion, the reverberation tending to spread and colour the sound it is added to (e.g. the bathroom 'solo'), modern algorithms tend to prescribe high diffusion after the reverberation has built up, but with different degrees of initial diffusion.

The real beauty of digital reverb is the adaptability of the system to all manner of reverb types, both natural sounding and outrageous, as well as the production of special effects via notch filtering and so on. A short, rapid diffusion reverb, such as that from a fully damped plate, may be used on drum tracks, a live chamber type on vocals, and a medium-damped plate with some initial delay on brass.

The Lexicon 224 is probably the most sophisticated digital reverb system available at present, and current software enables six reverb types to be punched up at will: small concert hall, vocal plate, large concert hall, acoustic chamber, percussive plate, and alternative small concert hall.

Orban Dual Reverberation Model 115.


Conclusions



Digital reverberation is so utterly open-ended in terms of the variety of ambient environments that can be synthesised that its relevance to the limitless sound-world of electronic music is crystal clear. However, the technology is very expensive (Ursa Major Space Station — £1,475; Lexicon — £4,800) and likely to stay that way until large capacity RAM drops significantly in price.

Whilst it is true that spring-line reverb units have a habit of jangling to their hearts' content, and to the musician's discontent, if used carelessly, the various signal-processing features included in some units can reduce colouration, jangle tendency and so on, but the user would be well advised to make sure that this isn't because the manufacturer is trying to make a silk purse out of a sow's ear. Spring-line reverb certainly offers the most cost-effective solution to quality reverb for the less well-off studio (unless, of course, you happen to have a convenient bathroom to hand — and one that's not being used for the usual purposes!). A number of spring-line units selling for around £200 are perfectly capable of deceiving the brain into thinking it's perceiving real ambience, and the addition of some pre-reverb tape delay adds the finishing touch to the story.


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MF1 Sync Unit

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Multireverb


Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.

 

Electronics & Music Maker - Apr 1982

Scanned by: Stewart Lawler

Feature by David Ellis

Previous article in this issue:

> MF1 Sync Unit

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> Multireverb


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