Julius Smith

• Professor at Stanford University

Modal testing is a commonly used method to measure the transfer function or frequency response function of musical instruments or their components. Various excitation and measurement tools are used, and the recorded signals are analyzed to estimate the modal frequencies, damping ratios, and mode shapes of the object. These modal parameters are used...

No PDF available ABSTRACT Laser Doppler vibrometers (LDV) are used for non-contact vibration measurements of various structures and are frequently used for stringed instrument measurements. Single-point LDVs can be used with the roving hammer or LDV method for mode shape measurements, but this is time-consuming and requires constant attention. Scan...

Ce texte présente mon utilisation du langage Faust pour l'enseignement. J’utilise Faust pour des démonstrations de traitement du signal audio. Je trouve que le moyen le plus efficace d’apprendre la programmation est de « coder en direct » en classe avec l'aide des étudiants. Un langage de haut niveau comme Faust est excellent pour cela, en raison d...

The Reflections series takes a look back on historical articles from The Journal of the Acoustical Society of America that have had a significant impact on the science and practice of acoustics.

A signal processing method to impart the response of an acoustic string instrument to an electric instrument that includes frequency-dependent string decay alterations is proposed. This type of processing is relevant when trying to make a less resonant instrument, such as an electric guitar, sound similar to a more resonant instrument, such as acou...

While recording an ensemble of musicians, it is often desired to isolate the instruments to avoid interference from other sources. Close-miking and acoustic isolation booths are some techniques for mitigating microphone cross-talk (or "bleed"). In this paper, we propose an algorithm for canceling microphone bleed in ensemble recordings in the post-...

A method is presented for simulating the free-field, frequency-dependent directivity of linear sound sources for use in real-time within geometric acoustic environments. The method, which is applied to modeling the directivity of a violin body and a clarinet air column from experimental acoustic data in this study, is based on using minimum-phase m...

No PDF available ABSTRACT A method is proposed for simulating the frequency-dependent directivity of sound sources for use in interactive geometric acoustic rendering applications. The method is based on using measurements to design a state-space filter allowing the interactive simulation of a time-varying number of radiated sound wavefronts, each...

No PDF available ABSTRACT A real-time method of string instrument acoustic transfer which includes damping is proposed. Acoustic transfer of string instruments is relevant when trying to make a non-resonant instrument, such as an electric guitar, sound more similar to an acoustic guitar. Unlike previous acoustic transfer methods which only perform...

This paper proposes a real-time, sample-by-sample pitch tracker for monophonic audio signals using the Extended Kalman Filter in the complex domain (Extended Complex Kalman Filter). It improves upon the algorithm proposed by the same authors in a previous paper [1] by fixing the issue of slow tracking of rapid note changes. It does so by detecting...

No PDF available ABSTRACT Modal synthesis is a common technique to model linear musical systems efficiently using a parallel bank of second-order digital filters to represent the modes of a resonating instrument. However, many instruments exhibit nonlinear behavior when driven at high amplitudes, often resulting in amplitude dependent mode center f...

Resonator guitars are acoustic instruments which have one or more spun metal cones embedded in the top plate, with strings driving the cone directly through a bridge. They were originally designed to be louder than traditional acoustic guitars and are often played with a metal slide. The vibrational characteristics of resonator guitars having a sin...

It is well known that a digital filter transfer function can be converted between the direct form and parallel connections of elementary sections, typically second-order ("biquad") sections. The conversion from direct to parallel form is performed using a partial fraction expansion, which usually requires long division of polynomials when expanding...

Noise subspace methods are popular for estimating the parameters of complex sinusoids in the presence of uncorrelated noise and have applications in musical instrument modeling and microphone array processing. One such algorithm, MUSIC (Multiple Signal Classification) has been popular for its ability to resolve closely spaced sinusoids. However, th...

This paper proposes a method to filter the output of instrument contact sensors to approximate the response of a well placed microphone. A modal approach is proposed in which mode frequencies and damping ratios are fit to the frequency response of the contact sensor, and the mode gains are then determined for both the contact sensor and the microph...

In the context of digital waveguide synthesis of string instruments, strings are modeled as pairs of digital waveguides which encounter a scattering junction at each boundary. The string scattering junction at the bridge can be modeled to match the measured driving point admittance at the bridge of a physical instrument, and is used in calculating...

We present a digital filter design technique for the modeling of impedance and radiation transfer functions as measured from an alto saxophone. For each fingering position, both the input impedance and sound radiation are jointly modeled via a recursive parallel filter structure akin to a modal expansion, with the filter coefficients estimated in t...

A hybrid method for sound scattering calculations is presented in this paper. The boundary element method (BEM) is combined with a recently developed edge source integral equation (ESIE) [J. Acoust. Soc. Am. 133, 3681–3691 (2013)]. Although the ESIE provides accurate results for convex, rigid polyhedra, it has several numerical challenges, one of w...

There has been fascinating work on creating artistic transformations of images by Gatys. This was revolutionary in how we can in some sense alter the 'style' of an image while generally preserving its 'content'. In our work, we present a method for creating new sounds using a similar approach, treating it as a style-transfer problem, starting from...

String instrument synthesis models have typically ignored the influence of the string stopping position when modeling the string boundary conditions at the neck. This simplification neglects the differences between frequency-dependent wave reflection phenomena taking place at the string boundary when it is stopped at different positions along the n...

Developments in immersive audio technologies have been evolving in two directions: physically motivated systems and perceptually motivated systems. Physically motivated techniques aim to reproduce a physically accurate approximation of desired sound fields by employing a very high equipment load and sophisticated, computationally intensive algorith...

Physical modeling of musical instruments is of great interest, both due to the interpretation of the parameters that make the model as similar to reality as possible, and because of the faithful sound synthesis. In this paper we will focus on the modeling of percussion instruments and more concretely drums consisting of a vibrating membrane and a r...

The sound radiation from unbaffled vibrating plates is a classical problem that is often solved numerically by coupling a vibration model to the surrounding air with the boundary element method, or the finite differences in the time domain (FDTD). Here, the surrounding air is instead represented with Green's functions computed with the edge source...

We present a method to simulate the late reverberation of a non-convex rectilinear space using digital waveguide networks (DWNs). In many delay-line-based reverberators, diffraction effects and even occlusion are often neglected due to the need for hand-tuned, non-physical mechanisms that complicate the extreme computational economy typical of such...

We present a technique to iteratively optimize poles of a recursive digital filter in parallel form. Only exposing the poles as the variables to optimize, we employ a linearly constrained gradient descent routine in which the numerical estimation of the error gradient involves first obtaining the zeros by projecting the target response over a basis...

We extend the Wave Digital Filter (WDF) approach to simulate reference circuits that involve operational amplifiers (op-amps). We handle both nullor-based ideal op-amp models and controlled-source-based linear op-amp macromodels in circuits with arbitrary topologies using recent derivations for complicated scattering matrices. The presented methods...

A large class of transcendental equations involving exponentials can be made explicit using the Lambert W function. In the last fifteen years, this powerful mathematical tool has been extensively used to find closed-form expressions for currents or voltages in circuits containing diodes. Until now almost all the studies about the W function in circ...

We derive closed-form expressions for the poles and zeros of approximate fractional integrator/differentiator filters, which correspond to spectral roll-off filters having any desired log-log slope to a controllable degree of accuracy over any bandwidth. The filters can be described as a uniform exponential distribution of poles along the negative-...

A hybrid method for acoustic scattering problems is studied in this paper. The boundary element method is combined with a recently developed edge diffraction based method [J. Acoust. Soc. Am. 133, pp. 3681-3691, 2013]. Although the edge diffraction method has been shown to provide accurate results for convex, rigid objects at a very attractive comp...

A hybrid method for acoustic scattering problems is studied in this paper. The boundary element method is combined with a recently developed edge diffraction based method [J. Acoust. Soc. Am. 133, pp. 3681–3691, 2013]. Although the edge diffraction method has been shown to provide accurate results for convex, rigid objects at a very attractive comp...

Recent research related to artificial reverberation is reviewed. The focus is on research published during the past few years after the writing of an overview article on the same topic by the same authors. Advances in delay networks, convolution-based techniques, physical room models, and virtual analog reverberation models are described. Many new...

Wave Digital Structures (WDS) are particularly interesting for applications of interactive modeling of nonlinear (NL) elements in the context of Virtual Analog modeling. NL circuits, however, often include multiple nonlinearities or multi-port nonlinearities, which cannot readily be accommodated by traditional WDS. In this work we present a novel m...

We report on the playability of a bowed-string physical model that combines digital waveguide and finite-difference time-domain frameworks. We extend previous approaches by combining a finite-width bow-string interaction model with a dynamic friction model based on simulating heat diffusion along the width of the bow. Bow hair dynamics are incorpor...

Wave Digital Structures (WDS), with their inherent stability and robustness, would be particularly suitable for non-linear (NL) circuit modeling in Virtual Analog applications, if it were possible for them to accommodate multi-port nonlinearities. In this work we present a method for modeling a rather general class of multi-port NL elements in the...

In this paper, we study error in wave digital filter nonlinearities from an energetic perspective. By ensuring that power dissipation corresponding to this error is non-negative, we respect the basic wave digital filter premise that errors should not correspond to an increase in system energy. In particular, this has implications for the formation...

A selection of enabling and significant recent developments in music/audio signal processing will be highlighted. Developments considered fall into three categories: (1) new hardware such as tablet computers having powerful processors and expressive touch-screens, (2) domain-specific software such as the Faust language for audio signal processing,...

An acoustic reverberator consisting of a network of delay lines connected via scattering junctions is proposed. All parameters of the reverberator are derived from physical properties of the enclosure it simulates. It allows for simulation of unequal and frequency-dependent wall absorption, as well as directional sources and microphones. The reverb...

Alleyways bounded by flat, reflective, parallel walls and smooth concrete floors can produce impulse responses that are surprisingly rich in texture, featuring a long-lasting modulated tone and a changing timbre, much like the sound of a didgeridoo. This work explores alleyway acoustics with acoustic measurements and presents a computational model...

We present an analysis of the cowbell voice circuit from the Roland TR-808 Rhythm Composer. A digital model based on this analysis accurately emulates the original. Through the use of physical and behavioral models of each sub-circuit, this model supports accurate emulation of circuit-bent extensions to the voice's original behavior (including arch...

We designed models of three TR-808 voice circuits which are physically-informed and derived as closed-form expressions in terms of electrical component values where possible. An important part of the performance practice of analog drum machines involves modifying their voices circuits. These techniques would be impossible to recreate in a digital m...

We present an analysis of the cymbal voice circuit from a classic analog drum machine, the Roland TR-808 Rhythm Composer. A digital model based on this analysis (implemented in Cycling 74's Gen~ retains the salient features of the original. Developing physical models of the device's many sub-circuits allows for accurate emulation of circuit-bent mo...

We present an analysis of the bass drum circuit from the classic Roland TR-808 Rhythm Composer, based on physical models of the device’s many sub-circuits. A digital model based on this analysis (implemented in Cycling 74’s Gen~) retains the salient features of the original and allows accurate emulation of circuit-bent modifications—complicated beh...

A method for recognizing an audio sample locates an audio file that closely matches the audio sample from a database indexing a large set of original recordings. Each indexed audio file is represented in the database index by a set of landmark timepoints and associated fingerprints. Landmarks occur at reproducible locations within the file, while f...

Alleyways bounded by flat, reflective, parallel walls and smooth concrete floors can produce impulse responses that are surprisingly rich in texture, featuring a long-lasting modulated tone and a changing timbre, much like the sound of a didgeridoo. This work explores alleyway acoustics with acoustic measurements and presents a computational model...

Handheld mobile computing devices are now ubiquitous. These devices are powerful, connected and equipped with a variety of sensors. Their pervasiveness has created an opportunity to realize parametrically controlled, physically modeled, virtual musical instruments. We will present a brief history of physically modeled musical instruments and the pl...

The worldwide presence of Brazilian culture grows every day. However, some of the musical instruments used in its principal cultural activities lack of a formal acoustic analysis which would make them more understandable for the rest of the world. One of them is the berimbau-de-barriga (berimbau), which consists of a string (wire) attached to an ar...

(Invited paper for an Interdisciplinary session) New activities in any field are often precipitated by new enabling technologies. In musical acoustics applied to real-time sound synthesis, one exciting new development is smart-phones and tablets having high audio quality, multicore processing power, and display screens with multitouch controls. For...

In light of the promising results obtained by driving a low-complexity digital waveguide (DW) violin model with synthetic bowing gestures, we are currently exploring the possibilities of combining DW and finite-difference time-domain (FDTD) frameworks to construct refined physical models of string quartet instruments. Departing from state-of-the-ar...

A harmonic exciter is an audio effects signal processor applied to enhance the brightness and clarity of a sound, particularly used for vocals. This is achieved by inducing a measured amount of high-frequency distortion. In this paper, an exciter is digitally modeled and implemented as a standalone application (or plugin) using the FAUST (Functiona...

Haptic technology, providing force cues and creating a programmable interface, can assist users in more accurately using an interface. This paper investigates haptic assistance in combination with auditory feedback instead of visual feedback. A user test is carried out in which participants select fundamental frequencies from a continuous range to...

A nonparametric allpass filter design method is presented for mat- ching a desired group delay as a function of frequency. The tech- nique is useful in physical modeling synthesis of musical instru- ments and emulation of audio effects devices exhibiting dispersive wave propagation. While current group delay filter design meth- ods suffer from nume...

The first artificial reverberation algorithms were proposed in the early 1960s, and new, improved algorithms are published regularly. These algorithms have been widely used in music production since the 1970s, and now find applications in new fields, such as game audio. This overview article provides a unified review of the various approaches to di...

Classical geometric waveforms used in virtual analog synthesis suffer from aliasing distortion when simple sampling is used. An efficient antialiasing technique is based on expressing the waveforms as a filtered sum of time-shifted approximately bandlimited polynomial-spline basis functions. It is shown that by optimizing the coefficients of the ba...

We provide a single channel speech enhancement method leveraging the harmonic structure of voiced speech. A sinusoidal model, based on the pitch of the speaker, is used to filter noisy speech and remove any noise components that lie between the harmonics. To remove noise that lie on each harmonic frequency, we use a noise estimation procedure that...

Traditionally, the average professional musician has owned numerous acoustic musical instruments, many of them having distinctive acoustic qualities. However, a modern musician could prefer to have a single musical instrument whose acoustics are programmable by feedback control, where acoustic variables are estimated from sensor measurements in rea...

We present a data-processing pipeline based on sparse feature learning and describe its applications to music an-notation and retrieval. Content-based music annotation and retrieval systems process audio starting with features. While commonly used features, such as MFCC, are hand-crafted to extract characteristics of the audio in a succinct way, th...

The harpsichord is a plucked string keyboard instrument that was popular during the Renaissance and Baroque music eras. Although it was later replaced by the more expressive piano, it has mounted a comeback due to the early music movement today. A physical model of the harpsichord's plucking mechanism is presented, detailing the plectrum-string int...

When approaching violin sound synthesis, the theoretical advantage of body modeling by finite difference or finite element paradigms comes from parameterizing the model by geometry and material properties. However, difficulties in representing the complexity of physical phenomena taking place have kept such approaches from raising more success, due...

The harpsichord is a plucked string keyboard instrument with a distinct sound, and previous work has been done to synthesize it [V. Valimaki et al., EURASIP J. Appl. Signal. 7, 934-948 (2004)]. However, these excitation signals are extracted through recorded tones and are not physical models in the true sense. A physical model of the harpsichord pl...

In this paper, we present a thorough treatment of a harpsichord plectrum-string interaction which allows for large end deflections and both transverse motions of the string. We start from the gen-eral equations of motion of a bent beam, and an accurate shape of the plectrum is calculated, agreeing with existing known can-tilever beam models when en...

Virtual analog effects are a consequence of the on-going digitization of all equipment used in music production. Various digital methods to imitate the warm or lo-fi sound qualities that remind listeners of analog times are covered in this chapter. In particular, many algorithms presented in this chapter are physical models of audio effects boxes t...

Current graphics processing units (GPUs) are massively parallel computing environments offering remarkable performance boosts in parallelizable tasks. Audio signal processing is a potential application area. Three different cases for GPU implementation are studied: additive synthesis, fast Fourier transforms (FFT), and time-domain convolution. For...

Passive nonlinear filters provide a rich source of evolving spectra for sound synthesis. This paper describes a nonlinear allpass fil-ter of arbitrary order based on the normalized ladder filter. It is expressed in FAUST recursively in only two statements. Toward the synthesis of cymbals and gongs, it was used to make nonlinear waveguide meshes and...

In this chapter, we discuss methods for real-time synthesis of stringed instruments. Interest in this topic is wide and varying, as both studio and performance uses for realistic virtual stringed instruments are becoming increasingly possible with gains in computing power.

In quasi-bandlimited classical waveform oscillators, the aliasing distortion present in a trivially sampled waveform can be reduced in the digital domain by applying a tabulated correction function. This paper presents an approach that applies the correction func-tion in the differentiated domain by synthesizing a bandlimited im-pulse train (BLIT)...

The harpsichord, in a broad sense, refers to the family of plucked keyboard instruments. The sound of the harpsichord tone is unique and is not mistaken for other plucked string instruments, such as the guitar. The synthesis of harpsichord sound has been done previously [Valimaki et al., \ EURASIP J. Appl. Signal Process. 7, 934-948 (2004)], where...

The mass-spring chain allows accurate modeling of vibrating strings in all three spatial dimensions. [Rowland and Pask, Am. J. Phys. (1999).] However, such a model is for non-stiff strings. A stiff mass-spring string model is proposed, consisting of three or more cross-coupled parallel mass-spring chains.

The mass-spring chain allows accurate modeling of vibrating strings in all three spatial dimensions. [Rowland and Pask, Am. J. Phys. (1999)]. This presentation describes striking one or more adjacent masses of such a string model with a forcing function corresponding to the piano hammer [http://www.ioc.ee/stulov/PUB.htm]. An advantage of striking a...

Inspired by on-site observations and measurements, a computational acoustic model of the interior architecture of the 3000-year-old ceremonial center at Chavín de Huántar, Perú is presented. The model addresses the foundational study by Lumbreras et al. (1976) which posited an acoustic system integral to Chavín architecture involving "a network of...

In previous waveguide synthesis models for piano, longitudinal waves have been neglected, although it is known that there is audible coupling from transverse to longitudinal vibration in piano strings [e.g., Conklin lectures, http:∕∕www.speech.kth.se∕music∕5∕_lectures∕]. A general method for accurate 3-D string simulation is the mass-spring chain [...

Trivially sampled geometric waveforms such as the rectangular pulse wave used in subtractive sound synthesis suffer from aliasing caused by the discontinuities in the waveform or its derivative. Several algorithms for the reduction of aliasing distortion have been suggested, providing either complete removal or great suppression of aliasing. Some a...

The 16 papers in this special issue focus on virtual audio effects and musical instruments.

An efficient approach to the generation of classical synthesizer waveforms with reduced aliasing is proposed. This paper introduces two new classes of polynomial waveforms that can be differentiated one or more times to obtain an improved version of the sampled sawtooth and triangular signals. The differentiated polynomial waveforms (DPW) extend th...

This paper presents a procedural approach to derive nonlinear filters from schematics of audio circuits for the purpose of digitally emulating analog musical effects circuits in real time. This work, the first in a two-part series, extends a well-known efficient nonlinear continuous-time state-space formulation for physical modeling of musical acou...

The linear coupling of two strings or of a single string vibrating in two orthogonal polarizations leads to two observable phenomena: two-stage decay and beating. In this paper, we present methods for accurately measuring and modeling the lower partials of a recorded guitar tone, where coupling effects are most audible. These estimated parameters a...

One of the challenges in virtual analog synthesis is avoiding aliasing when generating classic waveforms such as sawtooth and square wave which have theoretically infinite bandwidth in their ideal forms. The human auditory system renders a certain amount of aliasing inaudible, which allows room for finding cost-effective algorithms. This paper sugg...

An efficient allpass filter design method is introduced to match the dispersion characteristics of vibrating stiff strings. The proposed method designs an allpass filter in cascaded biquad form directly from the target group delay, placing the poles at frequencies at which the group delay area function achieves odd integer multiples of ??, and fixi...

This paper collects together various facts about digital waveguide networks (DWN) used in acoustic modeling, particularly results pertaining to lossless scattering at the junction of N in- tersecting digital waveguides. Applications discussed include music synthesis based on physical models and delay effects such as artificial reverberation. Connec...

A haptic musical instrument is an electromechanical musical instrument that provides a musician not only with audio feedback but also with force feedback. Many commercial force feedback devices consist of a tool, such as a thimble or pen, attached to the end of a robotic arm. The musician can grab onto the tool and use it to manipulate a virtual mu...

Active electroacoustic systems are commonly employed to transmit sound from one location to another. For example, consider the following configuration: a person talks into a microphone, which produces an electrically amplified signal for driving a loudspeaker, and the loudspeaker induces an output acoustic signal allowing someone else at a more dis...

Prior active room reverberation enhancement systems have typically employed microphones, artificial reverberator filters, and loudspeakers to change the reverberant properties of a room. However, acoustic feedback from the loudspeakers to the microphones has had the potential to drive such systems unstable. In contrast, techniques from feedback con...

Homemade musical instruments are wonderful tools for getting students interested in learning about acoustics. Homemade digital musical instruments can similarly engage students, provoking them to learn more about the modern world surrounding them. In addition, the tools for developing homemade digital musical instruments are now becoming so easy to...

Active electroacoustic systems are commonly employed to transmit sound from one location to another. For example, consider the following configuration: a person talks into a microphone, which produces an electrically amplified signal for driving a loudspeaker, and the loudspeaker induces an output acoustic signal allowing someone else at a more dis...

Additive synthesis is one of the fundamental sound synthesis techniques. It is based on the principle that each sound can be represented as a superposition of sine waves of different frequencies. That task can be done fully parallel and thus it is suitable for GPU (graphics processing unit) implementation. In this study we show that it is possible...

In Mechanical Sound Synthesis, real mechanical devices are em-ployed to create sound. Users can interact directly with the vari-ables of the sound synthesis, making interactions more intuitive to both users and audience. We focus on real-time feedback con-trol for Mechanical Sound Synthesis and provide a classification scheme using the reality-virt...

In this paper we propose new models for the plucking interac-tion of the player with the string for use with digital waveguide simulation of guitar. Unlike the previously presented models, the new models are based on structurally passive scattering junctions, which have the main advantage of being properly scaled for use in fixed-point waveguide im...

Arecentlyintroducedstructuretoimplementacontinuouslysmooth spectral delay, based on a cascade of first-order allpass filters and an equalizing filter, is described and the properties of this spectral delay filter are reviewed. A new amplitude envelope equalizing filter for the spectral delay filter is proposed and the properties of structures utili...

A new model for transverse piano string vibration, second-order in time, which models frequency-dependent loss and dispersion effects is presented here. It is then shown how a digital waveguide structure may be related di-rectly to this model. Finally, the model parameters are fit to experimental data from a grand piano.

Aliasing, usually considered an artifact of discrete time systems to be avoided, is found to be an aesthetic fea- ture of the E-MU SP-12 sampler/drum machine. This pa- per presents the steps in modeling the SP-12 as a signal processing system. Measurements of the characteristics of the SP-12 are presented. The signal path is analyzed to produce a p...

The implementation of spectral delay using filters comprising a cascade of many loworder all-pass filters and an equalizing filter is discussed. The spectral delay filters have chirplike impulse responses causing a large, frequency-dependent delay that is useful in audio effects processing. An equalizing filter design and a multirate technique that...

In this research, a method previously applied to improve a dig- ital simulation of the avian syrinx is adapted to the geometry of the clarinet reed. The clarinet model is studied with particular atten- tion to the case when the reed beats again the lay of the mouth- piece, closing off air flow to the bore once each period. In place of the standard...

Digital sound synthesis has become a standard staple of modern music studios, videogames, personal computers, and hand-held devices. As processing power has increased over the years, sound synthesis implementations have evolved from dedicated chip sets, to single-chip solutions, and ultimately to software implementations within processors used prim...