ARTIFICIAL REVERBERATION Ainnol Lilisuliani Ahmad Rasidi (SID : 430566949) Digital Audio Systems, DESC9115, Semester 1 2014 Graduate Program in Audio and Acoustics Faculty of Architecture, Design and Planning, The University of Sydney -------------------------------------------------------------------------------------------------------------------------------------- 1.0 Abstract Digital reverberation is an audio effect that is very common in musical production. It can be used to enhance recorded sounds that often sounds “dry” and “flat”. The principal idea of artificial reverberation was initiated by Manfred Schroeder in the 1960’s. Since then, many artificial reverb algorithms have been created. This review will look into two types of reverberation, convolution and algorithm based reverberation, focusing on Schroeder’s delay network algorithm and the applications of artificial reverberation in many areas. 2.0 Introduction Sound is a mechanical energy that travels through air at the speed of about 344 m/s. The speed varies upon the properties of air it travels, mostly due to the change of temperature and sometimes due to the humidity. In an enclosed space, this longitudinal waves of sound would reduce its amplitude the further it travels from the source until it reaches a surface. Depending upon the characteristic of the surface, some of the energy of the sound will be absorbed while some shall be reflected back into space. The reflected sound will bounce again as it meets other surface or obstacles, hence creating a complex pattern of reflection. Reverberation is the term we use for the collection of reflected sounds from the surfaces in an enclosed space. It is measured by reverberation time, which is perceived as the time for the sound to die away 60 decibels after the sound sources ceases (Sabine, 1972) . It is dependent on the intensity of the sound. The intensity of the sound would drop as the density of the reflection pattern escalates. This is due to distance and absorption by the surface materials. The need for reverberation effect in musical production is due to the need for the output to sound ‘natural’. Due to the acoustic conditions or a recording studio, sonic reflections are needed so that the produced sound will not be perceived as ‘fake’ or ‘dead’. Instead it should give listeners a sense or space and reality. In essence, artificial reverb is needed to restore a sense of reality. 3.0 Artificial Reverberation There are generally two types of artificial digital reverb; convolution and algorithmic. Convolution Reverb Convolution reverb takes recordings of real acoustic spaces and turn them into impulse responses. This is through the assumption that the acoustic space is approximately linear and time invariant (Välimäki et al, 2012). When sounds recorded in studios are applied (convolved) through this impulse response, the output will produce a reverberant sound, with the acoustic characteristics of that particular room. Convolution in its mathematical definition is the result of the multiplication of two functions, one function is a feed-forward finite impulse response, and the other a reverse function of the impulse, which would create a third new function. In simple form, it ‘blends’ one function with another. Convolution reverbs are so-called because the sequence of signal "events" is reversed: A space is excited by a signal and recorded, and the resulting sound or ambience of that space is then processed and used to treat and react to an entirely different signal (Hamberg, 2014). Among the common types of convolution reverbs are hall, room, plate, and spring reverbs. A hall reverb is normally used to add an ambience of a large space into a sound mix. It normally has a longer reverberation time, a contrast to a room reverb. A room reverb is normally used to enhance the output of instruments which have been recorded with a close mic arrangement. This will make the instruments sound as though they were not recorded in an acoustically absorbent studio. Plate reverbs were created by using an electromechanical transducer to create a vibration from a large metal plate. The sound it produced is then picked up and turned into impulse responses. It is mostly used to make a vocal sound rich and adding a bit of strength to a weak recorded snare drum. Spring reverbs create dense echoes due to repeated reflections of an input signal. Applying this to instruments like guitar will create a back and forth ringing effect, which would sound great in playback. Algorithmic Reverb Algorithmic reverbs uses varying algorithms to model the specific type of reverb. This type of reverb has many parameters that can be manipulated, making it common and favourable among music producers because it provides a more customizable mix. The principal behind this approach is feedback delay networks. This idea was initiated by Manfred Schroeder in the 1960’s. In 1979, Moorer enhanced Schroeder’s theory by inserting one-pole filters into delay loops to control reverberation time as a function of frequency (Välimäki et al, 2012). Gerzon introduced the concept of feedback delay network (FDN), which is essentially an orthogonal matrix of feedback comb filter around a parallel bank of delay lines. He stated that cross- coupled filters would result in a good stereo spreading. Jot and Chaigne then established a systematic FDN design methodology which allows largely independent setting of reverberation time in different frequency bands (Smith, 2010). A retrospect of all these approaches is that they all use the common underlying principle of sound delays over time. Ultimately, a smooth decaying sound delay is what is desired, with the target of creating a reverberating effect that is felt but not noticed. 4.0 Schroeder’s Reverberation Algorithm Schroeder’s approach to mimic a room reverberation effect was by feeding signals into a parallel arrangement of feedback comb filter (IIR) and a series connection of allpass filters (Schroeder 1961, Schroeder 1962). The conceptual design of this audio effect is based on the following diagram: Figure 1 : Conceptual Design of Schroeder's Reverberation Design The parallel bank of four infinite impulse response filters creates a feedback comb filter that is implemented for their resulting exponential decay. That is then fed to a series connection of three allpass filters, which would create the density of reflections needed. In essence, the feedback comb filters provide the length (how long) of the reverberation while the allpass filters provide the intensity of the reverb effect. The delay values for the filters should consist of prime numbers to avoid amplifying the delayed signal, which would lead to a more dense and uniform decay (Moorer, 1979). Comb Filters Comb filters were proposed for the design of Schroeder’s artificial reverberation. Feedback comb filter work by adding a delayed version of the original signal to itself. When the filter is stable, the signal is seen as a series of identical impulses decreasing in amplitude. Thus, the response sounds like the original signal decaying over time. The following is the transfer function of a feedback comb filter. Figure 2: Feedback Comb Filter Y (z) z –M H(z) = = X (z) 1 - g z –M When M is sufficiently large, the impulse response of the feedback comb filter is heard as discrete “echoes” of the input impulse (Välimäki et al, 2012). Allpass Filter Schroeder aIso suggested a series of allpass filter in the design of his artificial reverberation. In theory, the characteristic of an allpass filter is it allows all frequencies to pass through a system at equal amplitude. Figure 3: Allpass filter g + z –M H(z) = 1 + g z –M However, it also produces frequency-dependent time shifts, a feature which helps disperse or diffuse the sound. For this reason, Schroeder’s allpass sections are sometimes referred to as impulse expanders or impulse diffusers (Strube, 1982). The all pass filters increase the echo density and do not introduce coloration (timbre) unless the delay time is greater than the integration time of the ear, i.e. about 50m (Schroeder, 1962). The allpass filters provide “colorless” high-density echoes in the late impulse response of the reverberator. While allpass filters are “colorless” in theory, perceptually, their impulse responses are only colorless when they are extremely short (less than 10 ms). Longer allpass impulse responses sound similar to feedback comb-filters. For steady-state tones, however, such as sinusoids, the allpass property gives the same gain at every frequency, unlike comb filters (Smith, 2010). 5.0 Applications Artififial reverb is an absolute necessity in musical productions that it is almost like what water is to plants. Without it, the result would turn out ‘dry’ and ‘dead’. It is now almost a trend to produce a song as though they are being recorded in famous places such as The Vienna Opera House, or the famous Abbey Road studio. Artificial reverb also finds itself useful in other areas as well. The need to add reverb in a film production has also long been acknowledged in the industry. It is what makes a film sounds real. Nowadays, with the advancement of loudspeaker technology, what with surround sound and atmospheric format such as the ones introduced by Dolby, artificial reverberation plays a more major role in creating a sense of envelopment to audience. It helps sound designers to create a ‘spatial’ effect, as though audiences are ‘in’ the movie. This need of creating a sense of envelopment also makes its way to ‘Virtual Reality’ applications, such as Video Games. Artificial Reverberation is essential to introduce the realism of the virtual sound scene as well as in perceiving distance and directivity. The use of artificial reverbs in binaural gives the perception that the sound image is externalized. This is otherwise perceived inside the listener’s head. Therefore, adding reverb helps create a three dimensional perception of sound.