Research: Physical Modelling Synthesis

Research: Physical Modelling Synthesis

I was recently involved in the sound design, along with Chris Ware, for the game Hearth (Matt Allen, Alex Hockley) and required a dramatic cello as to set the mood the game scene. For this I used Sculpture, which is a physical modelling synth within Logic Pro. Below I will describe physical modelling synthesis and the process I took to create the desired sound.

Physical Modelling

"In music, physical modelling refers to a sound synthesis technique, which is based on models of the sound production mechanisms involved in musical instruments. The idea is to generate sound by reproducing how real musical instruments actually function and produce sound," (Verge).

This differs from most other synthesis techniques in that most synthesisers use oscillators or prerecorded samples to produce signals. Physical modelling on the other hand uses mathematical models that mimic how instruments behave in the real world. Taking a guitar for an example, a signal based synth can produce a real guitar sounding note. However, if that note is played twice in a row, the second note will reset to be exactly that same as the first. This is not how a guitar string behaves. When a string is plucked twice in a row, the second pluck will differ due the string still vibrating from the first strike. This is what physical modelling is designed to emulate. Physical modelling also takes into account where along the string has been plucked, the pickup placement and tension of the string. 

Sculpture

                              Sculpture

Sculpture is a physical modelling synth within Logic Pro. It features three object models labelled simply as Objects, which emulate that way in which the string is struck (pluck, strike, bow, blow). It also emulates how the string is sustained and released. It features a visual representation of a string on which to place each object and pickup. In the centre of the interface is where the tension of the string is controlled. This is labelled as Material and represents the mix between the sounds glass, steel, nylon and wood. These are the features specific to physical modelling. It has the general features of many synths, such as EQ, LFO and Delay.

Original Cello Patch

                        Object

To create my desired cello sound I began by setting Object 1 as a normal bow. This didn't have the resonance that I was looking for to make a dramatic sound so I left this as the key on response and added a Bow Wide as a key off in Object 2. This created a longer release time. 

            String Visualisation

I positioned the two pickups side by side just off centre of the string. This was to give a stereo image of the instrument without spreading the image too far. The further the pickups are positioned from each other, the more of a delay that occurs between the left and right speakers. The three objects were placed within a close proximity to each other and the pickups as this is consistent with the way a cello is bowed. A player won't stray too far from the one position when playing.

                Material Scale

I decided on a mix between steel and nylon for the material with a low amount of stiffness. This synth plugin also gives the possibility of controlling the tension, resolution and media loss between different keys on a keyboard. I set these to give the string more tension and media loss when higher notes are played. Similar to the way a string instrument is played.

In order to emulate the vibrato of the cello, I added an LFO affecting the amplitude and pitch of the signal. This was set to a 5.80Hz sine wave with a 1600ms delay. The purpose of the delay was that a player will never do vibrato at the very beginning of the note. They will instead gradually fade in the vibrato on each note.

 

The final patch can be heard below.

References

Verge, M. Physical Modelling. Retrieved August 12th, from https://www.applied-acoustics.com/techtalk/physicalmodeling/

Sculpture [Image] (2009). Logic Pro 9. Retrieved August 10th.