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Logarithmic decrement

The logarithmic decrement is a dimensionless parameter in the dynamics of underdamped oscillatory systems, quantifying the damping-induced exponential decay of amplitude in free vibrations. It is defined as the natural logarithm of the ratio of two successive peak amplitudes in the system's displacement response, providing a direct measure of energy dissipation per oscillation cycle.^[1] In practice, the logarithmic decrement, denoted as δ, for a single cycle is calculated using the formula δ = ln(x_n / x_n+1), where x_n and x_n+1 represent consecutive peak displacements. For improved precision in experimental data with noise, it is often determined over multiple cycles as δ = (1/N) ln(x₁ / x_N+1), where N is the number of cycles between the measured peaks. This method assumes a single-degree-of-freedom (SDOF) system undergoing underdamped free vibration, with the decay following the form x(t) = A e^-ζω_n*t cos(ω_dt + φ), where ζ is the damping ratio, ω_n is the natural frequency, and ω_d is the damped natural frequency.^[1]^[2] The logarithmic decrement relates directly to the system's damping ratio through δ = 2πζ / √(1 - ζ²), enabling the extraction of ζ from observed decay rates; for lightly damped systems where ζ ≪ 1, this simplifies to the approximation δ ≈ 2πζ. This connection makes it a fundamental tool for identifying damping properties without requiring direct measurement of the damping coefficient c, which can instead be derived as c = 2ζ√(km), with k as stiffness and m as mass. The approach is particularly effective for ζ < 0.1 but becomes less accurate as damping increases beyond approximately 0.5, where alternative methods like curve fitting are preferred.^[3]^[1] Logarithmic decrement finds wide application in mechanical and structural engineering for analyzing vibrations in systems such as vehicles, bridges, and machinery, where it helps assess stability and energy loss from free decay tests or impulse responses. In aerospace and acoustics, it is used to evaluate modal damping in structural dynamics and combustion stability, respectively, often applied to time-domain data from accelerometers or pressure sensors. Additionally, it supports material characterization in nonintrusive evaluations, such as determining viscoelastic properties through oscillation decay in resonant tests.^[2]

Fundamentals

Definition

The logarithmic decrement, denoted as δ, is defined as the natural logarithm of the ratio of two successive amplitudes in a decaying oscillation, expressed as δ = \ln\left(\frac{x_n}{x_{n+1}}\right), where x_n and x_{n+1} are the amplitudes of the nth and (n+1)th peaks, respectively.^[1] This measure quantifies the rate of energy dissipation per oscillation cycle in underdamped systems, where the amplitude decreases exponentially over time due to damping forces. It originates from the free vibration response of linear damped systems, such as the damped harmonic oscillator.^[4] For example, in a simple pendulum subject to air resistance, the logarithmic decrement indicates the rapidity with which the swing amplitudes diminish from one cycle to the next.^[5]

Physical Interpretation

The logarithmic decrement represents the fractional rate at which the amplitude of an underdamped oscillator diminishes over each successive cycle, quantifying the dissipative effects of damping forces that convert mechanical energy into heat or other forms.^[1] This measure captures the incremental energy loss per oscillation, providing a dimensionless indicator of how rapidly vibrations decay in systems like suspended masses in viscous fluids or structural beams under friction.^[6] In underdamped motion, the oscillation amplitude follows an exponential decay envelope, where the peak displacements decrease as e^{-\zeta \omega_n t}, with \zeta as the damping ratio and \omega_n the natural frequency; the logarithmic decrement \delta assesses this decay on a per-cycle basis, enabling straightforward evaluation of the damping's impact without tracking absolute time.^[7] By taking the natural logarithm of the amplitude ratio between successive peaks, \delta transforms the inherently nonlinear exponential process into a linear scale, facilitating constant values for analysis in linear systems.^[1] This logarithmic approach is particularly advantageous because, for linear viscous damping—where resistance is proportional to velocity—\delta remains constant irrespective of the initial amplitude, unlike nonlinear damping mechanisms (such as dry friction) where the decrement varies with oscillation size.^[6] Such constancy simplifies the characterization of damping in engineering contexts, allowing \delta to serve as a reliable metric for predicting long-term vibration behavior under proportional energy dissipation.^[7]

Mathematical Formulation

Derivation from Damped Oscillations

The equation of motion for a damped harmonic oscillator is given by m \ddot{x} + c \dot{x} + k x = 0, where m is the mass, c is the damping coefficient, and k is the stiffness.^[8] This second-order linear differential equation describes the underdamped case where the system oscillates with decaying amplitude.^[2] The general solution for the displacement x(t) in the underdamped regime (\zeta < 1, where \zeta = c / (2 \sqrt{m k}) is the damping ratio) is

x(t) = A e^{-\zeta \omega_n t} \cos(\omega_d t + \phi),

where A is the initial amplitude, \phi is the phase angle, \omega_n = \sqrt{k/m} is the undamped natural frequency, and \omega_d = \omega_n \sqrt{1 - \zeta^2} is the damped natural frequency.^[1] The exponential term e^{-\zeta \omega_n t} represents the envelope of the decaying oscillations.^[2] To derive the logarithmic decrement, consider the successive peak amplitudes, which occur approximately at times separated by the damped period T_d = 2\pi / \omega_d. The amplitude at the n-th peak is x_n \approx A e^{-\zeta \omega_n t_n}, and at the next peak, x_{n+1} \approx A e^{-\zeta \omega_n t_{n+1}}, with t_{n+1} - t_n \approx T_d.^[1] The ratio of these amplitudes is thus x_n / x_{n+1} = e^{\zeta \omega_n T_d}.^[8] The logarithmic decrement \delta is defined as the natural logarithm of this ratio:

\delta = \ln \left( \frac{x_n}{x_{n+1}} \right) = \zeta \omega_n T_d.

Substituting T_d = 2\pi / \omega_d yields \delta = \zeta \omega_n \cdot (2\pi / \omega_d) = 2\pi \zeta / \sqrt{1 - \zeta^2}. For light damping (\zeta \ll 1), the term \sqrt{1 - \zeta^2} \approx 1, simplifying to \delta \approx 2\pi \zeta, where higher-order terms are negligible.^[1] This derivation assumes an underdamped system ($0 < \zeta < 1) and focuses on the envelope approximation, ignoring small phase shifts at peaks.^[2]

Relation to Damping Parameters

The logarithmic decrement \delta is precisely related to the damping ratio \zeta by the formula

\delta = \frac{2\pi \zeta}{\sqrt{1 - \zeta^2}},

which arises from the solution to the damped harmonic oscillator equation, where the amplitude decay over one oscillatory period is analyzed.^[4] This exact expression allows for the determination of \zeta from measured \delta via the inverse relation \zeta = \frac{\delta}{\sqrt{4\pi^2 + \delta^2}}.^[4] For lightly damped systems where \zeta \ll 1 (typically \zeta < 0.2), the exact formula simplifies to \delta \approx 2\pi \zeta, enabling a direct estimation of the damping ratio as \zeta \approx \frac{\delta}{2\pi}.^[4] This approximation holds because the term \sqrt{1 - \zeta^2} \approx 1 under light damping conditions, focusing analysis on systems exhibiting many cycles before significant decay. The logarithmic decrement also connects to the quality factor Q, a measure of energy storage relative to dissipation per cycle, through Q \approx \frac{1}{2\zeta} \approx \frac{\pi}{\delta} for small \zeta.^[9] This relation implies that after approximately Q cycles, the system's energy decays to a factor of e^{-2\pi}, corresponding to the amplitude decaying to e^{-\pi}. These connections facilitate non-dimensional comparisons of damping properties across diverse oscillatory systems, independent of their natural frequencies, by normalizing decay rates via \delta and \zeta.^[4] However, the approximation's accuracy diminishes for \zeta > 0.2, where higher-order terms in the exact formula become significant, necessitating the full expression for reliable parameter extraction.^[4]

Measurement Techniques

Standard Method Using Successive Amplitudes

The standard method for measuring the logarithmic decrement relies on analyzing the amplitudes of successive peaks in the displacement time series obtained from a free vibration test of an underdamped system. This approach begins with exciting the system, such as through an initial displacement or impact, and recording the resulting oscillatory response using sensors like accelerometers or displacement transducers. From the time-domain data, successive positive peak amplitudes x_1, x_2, \dots, x_n are identified, typically spanning multiple oscillation cycles to mitigate measurement noise. The logarithmic decrement \delta is then calculated as \delta = \frac{1}{n-1} \ln \left( \frac{x_1}{x_n} \right), where n represents the number of peaks (or equivalently, n-1 cycles).^[2]^[10] This formula assumes exponential decay of the amplitude envelope, allowing \delta to quantify the damping per cycle independently of the oscillation frequency.^[1] For enhanced precision, especially in noisy environments, the logarithmic decrement can be averaged over multiple single-cycle intervals within the record. Here, \delta for a single cycle is defined as \ln \left( \frac{x_i}{x_{i+1}} \right), and the multi-cycle average is computed as \delta = \frac{1}{m} \sum_{i=1}^m \ln \left( \frac{x_i}{x_{i+1}} \right), where m is the number of intervals (ideally m \geq 5). This averaging technique reduces the impact of random errors in peak identification. The method's simplicity stems from requiring only amplitude values, without needing precise time measurements or frequency determination, making it robust to variations in the damped period. Furthermore, it is insensitive to minor nonlinearities or perturbations that might affect the oscillation timing but not the overall decay rate.^[11]^[10] To ensure accuracy, measurements should encompass at least 5–10 cycles, as fewer cycles amplify the relative error from initial transients or noise, potentially leading to overestimation of \delta by 10–20% in lightly damped systems. Initial transients, arising from the excitation method, must be excluded by starting peak selection after the response has stabilized into quasi-periodic decay. In practice, data processing often involves filtering the signal to isolate the fundamental mode and visually or algorithmically detecting peaks to avoid spurious values from higher harmonics.^[2]^[1]^[11] A representative application occurs in cantilever beam experiments, where material damping is assessed by subjecting the beam to an impact excitation and capturing the decaying free vibrations at the free end. Successive peak displacements are extracted from the accelerometer data, and \delta is computed over 8–12 cycles to yield the damping ratio via \zeta \approx \delta / (2\pi) for light damping, enabling characterization of internal friction in the beam material.^[13]^[14]

Simplified Variation for Light Damping

In cases of light damping, where the damping ratio ζ is less than 0.1, the damped period T_d closely approximates the natural period T_n due to minimal influence from damping on the oscillation frequency.^[15] This approximation simplifies the calculation of the logarithmic decrement δ, yielding δ ≈ (1/n) ln(x_0 / x_n), where x_0 is the initial amplitude and x_n is the amplitude after n cycles.^[16] The procedure involves recording the free vibration response of the system and identifying the peak amplitudes at the start and after a chosen number of cycles n, typically several to average out minor variations. The logarithmic decrement is then computed directly as δ = [ln(x_0 / x_n)] / n, bypassing the need to measure and ratio individual successive peaks.^[1] This method assumes a constant decrement per cycle, which holds when the phase shift introduced by damping is minimal, as occurs in lightly damped conditions. This variation offers advantages in practical measurements, particularly by reducing errors associated with precisely identifying individual peaks in noisy or low-resolution data. It proves especially useful for high-Q systems, such as quartz tuning forks, where oscillations persist over many cycles with very low damping.^[17] For small damping, the logarithmic decrement relates approximately to the damping ratio as δ ≈ 2πζ.^[16] However, the approach becomes inaccurate for moderate damping levels (ζ > 0.1), where the shift in the oscillation period due to damping significantly alters the cycle counting and amplitude decay interpretation.

Fractional Overshoot Approach

The fractional overshoot approach offers an alternative to free-vibration methods for estimating the logarithmic decrement in second-order underdamped systems by analyzing the transient response to a step input. In these systems, the fractional overshoot M_p, which is the ratio of the peak deviation above the steady-state response to the steady-state value itself, relates directly to the damping ratio \zeta through the formula

M_p = e^{-\pi \zeta / \sqrt{1 - \zeta^2}}.

This expression arises from the enveloped decay of the oscillatory transient, where the time to the first peak approximates half the damped period, leading to the exponential term capturing the damping effect over that interval. To derive the logarithmic decrement \delta from M_p, first solve for \zeta using the inverse of the overshoot equation, which typically requires numerical methods but simplifies for small \zeta. The damping ratio \zeta serves as an intermediary, linked to \delta by

\delta = \frac{2\pi \zeta}{\sqrt{1 - \zeta^2}}.

For light damping where \zeta < 0.3, the approximation M_p \approx e^{-\pi \zeta} holds, yielding \zeta \approx -\ln(M_p)/\pi and thus \delta \approx 2\pi \zeta \approx -2 \ln(M_p). This direct approximation avoids explicit computation of \zeta and leverages the near-linear relation between damping and decay for low \zeta. The procedure entails applying a step input to the system—for instance, a sudden voltage step to an electric motor or a force step to a spring-mass damper—and recording the response. Identify the steady-state value y_{ss}, the maximum peak y_{\max}, and compute M_p = (y_{\max} - y_{ss}) / y_{ss}. Substitute into the relations above to obtain \delta. This forced-response technique is advantageous in scenarios where free vibration is challenging to initiate, such as active control systems or operational machinery under constant loading, as it utilizes readily observable step transients. This approach bridges the damping inferred from forced transient behavior to the free-decay logarithmic decrement, providing equivalent characterization of viscous damping with optimal accuracy for \zeta < 0.3, beyond which higher-order effects degrade the approximations.

Applications

In Mechanical Vibrations

Logarithmic decrement serves as a fundamental tool for assessing damping in mechanical structures such as beams, shafts, and vehicle components, facilitating predictions of resonance behavior and fatigue accumulation under dynamic loads. In beam-like structures, it quantifies energy dissipation changes that signal fatigue progression, allowing engineers to monitor structural integrity during random loading scenarios. For shafts and vehicles, it evaluates viscous and structural damping to avoid critical speeds that could amplify vibrations leading to material failure.^[18]^[19] A practical example arises in automotive suspension systems, where logarithmic decrement is determined from acceleration responses during road tests to optimize shock absorber tuning for enhanced ride comfort and handling. Experimental data from quarter-car models driven over urban roads and bumps yield damping ratios around 0.22 (corresponding to a logarithmic decrement of approximately 1.4), for light damping conditions that balance vibration isolation and stability. This measurement helps calibrate dampers to mitigate excessive oscillations from road irregularities.^[20] In modal analysis of mechanical systems, logarithmic decrement integrates with frequency response functions to extract mode shapes and map damping distribution across structural modes, enabling precise identification of vibration characteristics in complex assemblies like vehicle frames. This approach supports the design of tuned vibration absorbers to target specific modes, reducing overall dynamic response.^[21] Logarithmic decrement features prominently in ISO standards for characterizing damping in materials, such as polymers used in structural components, where values exhibit significant variation with temperature due to viscoelastic effects. These standards outline methods to measure decrement over temperature ranges, aiding selection of materials for vibration-prone environments like engine mounts. In design applications, a higher logarithmic decrement effectively lowers peak amplitudes at resonance by enhancing energy dissipation, though it can shorten settling times in transient responses, influencing trade-offs between stability and responsiveness.^[22]^[23]^[19]

In Control Systems and Other Fields

In control systems, the logarithmic decrement serves as a key metric for evaluating stability margins in feedback loops, particularly through ring-down tests that analyze the decay of oscillatory responses to assess damping and prevent instability.^[24] This approach is commonly applied in rotordynamic systems, where the logarithmic decrement quantifies damping levels during full-load, full-pressure operation to ensure stable performance, with values indicating the system's resistance to self-excited vibrations.^[25] In electrical engineering analogs, the logarithmic decrement measures damping in RLC circuits, where it directly informs the quality factor (Q) of resonant systems, with small values indicating underdamped behavior and high selectivity in filters.^[9] For instance, in quartz crystal oscillators, the method evaluates the exceptionally low damping inherent to the piezoelectric material, yielding logarithmic decrements on the order of 10^{-5}, which correspond to Q factors exceeding 10^5 and enable high-frequency stability in timing circuits.^[26] Beyond these domains, the logarithmic decrement finds applications in acoustics, where it quantifies damping in string instruments like guitars, helping to characterize the decay of vibrational modes in soundboards and strings for tonal quality assessment.^[27] In biology, it models damped oscillations in neuromuscular signals, such as velocity profiles during limb movements, revealing how viscoelastic properties of muscle tissue contribute to stability and positioning with logarithmic decrements reflecting inherent biological damping.^[28] Aerospace engineering employs it in flutter analysis for aircraft wings and blades, where ring-down data from flight tests determine aerodynamic damping via logarithmic decrement to predict and mitigate aeroelastic instabilities.^[29] In seismology, the parameter assesses ground motion decay during earthquakes, informing earthquake engineering designs by estimating damping ratios typically 0.02-0.2 for structures, higher (up to 0.3 or more) for soil damping depending on type, from impulse response functions of structures under seismic loading.^[30] An emerging application appears in additive manufacturing, where logarithmic decrement evaluates vibration damping in 3D-printed lattice structures and metamaterials, optimizing designs for enhanced energy absorption and fatigue resistance in dynamic environments.^[31]

References

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[PDF] Log Decrement - andrew.cmu.ed
The logarithmic decrement represents the rate at which the amplitude of a free damped vibration decreases. It is defined as the natural logarithm of the ratio ...
[2]
[PDF] Extracting Damping Ratio From Dynamic Data and Numerical ...
The logarithmic decrement method assumes an SDOF time harmonic oscillator and is best applied to underdamped exponentially decaying data. It is commonly used in ...
[3]
[PDF] Vibration Mechanics
Jun 25, 2024 · Next, we take the natural log of both sides to get the logarithmic decrement, denoted by δ: δ = ln x1 x2. = ln x(t1) x(t1 +T). = 2πζ p. 1−ζ2.
[4]
[PDF] m k x k x
The logarithmic decrement δ is defined as the natural logarithm. of the ratio of any two successive peaks, i.e., δ = ln⇣X1. X2 ⌘ (See Figure 2.4.)
[5]
Estimation of natural frequencies and damping ratios from measured ...
Estimation of natural frequencies and damping ratios from measured response: the logarithmic decrement ... 3:26like the simple pendulum, has a natural frequency.<|control11|><|separator|>
[6]
Nonlinear Damping of the Linear Pendulum - Department of Physics
Using the period T = 2p /w, the logarithmic decrement of the motion is given by . ... viscous damping coefficient is a constant, involving only the viscosity of ...<|control11|><|separator|>
[7]
Temperature Measurements
### Summary of Logarithmic Decrement and Related Concepts
[8]
[PDF] DAMPED HARMONIC OSCILLATIONS - CUTM Courseware
Equation (7) represents damped harmonic oscillation with amplitude Ae ... What is logarithmic decrement? Find the ratio of nth amplitude with 1st ...
[9]
[PDF] Transients and Oscillations in RLC Circuits
With damping, the frequency of oscillation is shifted to a smaller value. The logarithmic decrement, δ , is the natural log of the ratio of the charge (see ...
[10]
[PDF] Low-Frequency Attenuation Measurements of Fluids APPROVED ...
of attenuation are the attenuation coefficient α, the quality factor Q, and the logarithmic decrement δ. These quantities are related by. 1. Q. = 𝛼v. 𝜋f. = 𝛿. 𝜋.
[11]
[PDF] ME 440 Intermediate Vibrations
Logarithmic Decrement used to gauge the value of damping ration ζ. ▫. To this end, measure the decrement of successive peak amplitudes. Log of this decrement is ...
[12]
[PDF] EXPERIMENTAL STUDY ON DETERMINING DAMPING RATIO OF ...
Log-decrement equations are used with filtered damped free vibration records, which are obtained from eight vibration tests. Mean and median values of damping ...
[13]
[PDF] Cantilever Beam Static and Dynamic Response Comparison with ...
The logarithmic decrement of vibrational decay (δ) is a measure of internal friction and can be expressed in the form (for free vibrations) of Equation (7) ...
[14]
[PDF] Nonlinear Vibrations of Cantilever Beams and Plates - VTechWorks
Jul 3, 2003 · The modal damping factors ζn were determined experimentally using the logarithmic decrement method. The first four damping factors are found ...
[15]
[PDF] Dynamics of Simple Oscillators (single-degree-of-freedom systems)
If 0 < ζ < 1, the conjugate roots λ1,2 = −ζωn ± ωn q ζ2 − 1 = −ζωn ± iωd. (35) are complex-valued, and where ωd is called the damped natural frequency.
[16]
[PDF] Control Design and Performance Evaluation of a Hybrid Flexure ...
The gyro is based on quartz tuning fork technology which ... Damping estimates, via the logarithmic decrement method [128], were obtained from the free re-.<|control11|><|separator|>
[17]
Fatigue Damage Detection in Beam-Like Structures under Random ...
Oct 15, 2025 · The natural frequency and damping ratios—estimated using both the logarithmic decrement method and the half-power bandwidth method—were ...Fatigue Damage Detection In... · 1. Introduction · 2. Materials And Methods
[18]
None
Below is a merged summary of the four segments on "Logarithmic Decrement, Applications, Resonance, and Fatigue in Structural Vibration Analysis" by C. E. Beards. To retain all information in a dense and organized manner, I will use a combination of narrative text and a table in CSV format for detailed data points (e.g., formulas, examples, and problem-specific details). The narrative provides an overview, while the table captures specific details for clarity and completeness.
[19]
[PDF] Vibration Analysis of Suspension System for 3DOF Quarter ... - ijmerr
Mar 15, 2024 · Logarithmic decrement method is used to extract damping ratio from the time domain responses of the experimental test. These responses are plots ...Missing: typical | Show results with:typical
[20]
Modal parameter identification using the log decrement method and ...
This paper presents a time-domain technique for identifying modal parameters of test specimens based on the log-decrement method.
[21]
[PDF] international standard iso 6721-1
May 15, 2001 · The logarithmic decrement is expressed as a dimensionless number. It is used as a measure of the damping in a viscoelastic system. Expressed ...
[22]
Damping in Structural Dynamics: Theory and Sources | COMSOL Blog
Mar 14, 2019 · Decay of a free vibration for three different values of the damping ratio. Another measure in use is the logarithmic decrement, δ. This is ...
[23]
Stability of Control System - an overview | ScienceDirect Topics
Equation (3.15) is referred to as the 'logarithmic decrement'. The settling time, τs, is the time taken for the oscillatory response to decay below a ...
[24]
[PDF] rotordynamic stability measurement during full-load, full-pressure ...
This technique measures the rotor's logarithmic decrement (log dec), which indicates the level of stability, or damping, in the rotor.<|separator|>
[25]
MYTH: LOGDEC….WHO STOLE MY DAMPING?
Feb 14, 2023 · Logdec is a good stability indicator for rotordynamics. There is even some consensus as to the appropriate limits of stability values.
[26]
[PDF] Quartz Crystal Resonators and Oscillators - SOS electronic
... logarithmic decrement” method) of measuring Q. The relationship between Q and decay time is also relevant to oscillator startup time. When an oscillator is ...
[27]
Lutherie Myth/Science: Damping is Bad? - Liutaio Mottola
Haines1 did some early work in this area, providing damping values using the logarithmic decrement method. ... Damping plays an important role in the sound ...
[28]
Damping Actions of the Neuromuscular System With Inertial Loads
Our chosen measure of damping was the logarithmic decrement (LD) in velocity, calculated for every cycle of oscillation within a trial. Because the ...
[29]
[PDF] A Historical Overview of Flight Flutter Testing
In-flight analysis was usually limited to log decrement analysis of accelerometer decay traces on strip charts to determine damping. The F-111 program is an ...
[30]
Correlation between Ground Motion and Building Response using ...
Nov 1, 2015 · The damping ratio is then assessed by applying the logarithmic decrement method to the impulse response function (Clough and Penzien 1993) ...
[31]
On the damping and fatigue characterization of additively ...
Additive manufacturing (AM) is a process in which components are ... The logarithmic decrement is the natural log of the amplitude ratio between ...Short Communication · 3. Experimental Equipment... · 4. Results And Discussion