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Semi-log plot of the Internet host count over time shown on a logarithmic scale

A logarithmic scale (or log scale) is a method used to display numerical data that spans a broad range of values, especially when there are significant differences among the magnitudes of the numbers involved.

Unlike a linear scale where each unit of distance corresponds to the same increment, on a logarithmic scale each unit of length is a multiple of some base value raised to a power, and corresponds to the multiplication of the previous value in the scale by the base value. In common use, logarithmic scales are in base 10 (unless otherwise specified).

A logarithmic scale is nonlinear, and as such numbers with equal distance between them such as 1, 2, 3, 4, 5 are not equally spaced. Equally spaced values on a logarithmic scale have exponents that increment uniformly. Examples of equally spaced values are 10, 100, 1000, 10000, and 100000 (i.e., 101, 102, 103, 104, 105) and 2, 4, 8, 16, and 32 (i.e., 21, 22, 23, 24, 25).

Exponential growth curves are often depicted on a logarithmic scale graph.

A logarithmic scale from 0.1 to 100
The two logarithmic scales of a slide rule

Common uses

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The markings on slide rules are arranged in a log scale for multiplying or dividing numbers by adding or subtracting lengths on the scales.

The following are examples of commonly used logarithmic scales, where a larger quantity results in a higher value:

A logarithmic scale makes it easy to compare values that cover a large range, such as in this map.
Map of the Solar System and the distance to Proxima Centauri, using a logarithmic scale and measured in astronomical units.

The following are examples of commonly used logarithmic scales, where a larger quantity results in a lower (or negative) value:

Some of our senses operate in a logarithmic fashion (Weber–Fechner law), which makes logarithmic scales for these input quantities especially appropriate. In particular, our sense of hearing perceives equal ratios of frequencies as equal differences in pitch. In addition, studies of young children in an isolated tribe have shown logarithmic scales to be the most natural display of numbers in some cultures.[1]

Graphic representation

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Various scales: lin–lin, lin–log, log–lin, and log–log. Plotted graphs are: y = 10 x (red), y = x (green), y = loge(x) (blue).

The top left graph is linear in the X- and Y-axes, and the Y-axis ranges from 0 to 10. A base-10 log scale is used for the Y-axis of the bottom left graph, and the Y-axis ranges from 0.1 to 1000.

The top right graph uses a log-10 scale for just the X-axis, and the bottom right graph uses a log-10 scale for both the X axis and the Y-axis.

Presentation of data on a logarithmic scale can be helpful when the data:

  • covers a large range of values, since the use of the logarithms of the values rather than the actual values reduces a wide range to a more manageable size;
  • may contain exponential laws or power laws, since these will show up as straight lines.

A slide rule has logarithmic scales, and nomograms often employ logarithmic scales. The geometric mean of two numbers is midway between the numbers. Before the advent of computer graphics, logarithmic graph paper was a commonly used scientific tool.

Log–log plots

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Main article: Log–log plot
A log–log plot condensing information that spans more than one order of magnitude along both axes

If both the vertical and horizontal axes of a plot are scaled logarithmically, the plot is referred to as a log–log plot.

Semi-logarithmic plots

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Main article: Semi-log plot

If only the ordinate or abscissa is scaled logarithmically, the plot is referred to as a semi-logarithmic plot.

Extensions

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A modified log transform can be defined for negative input (y < 0) to avoid the singularity for zero input (y = 0), and so produce symmetric log plots:[2][3]

for a constant C=1/ln(10).

Logarithmic units

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A logarithmic unit is a unit that can be used to express a quantity (physical or mathematical) on a logarithmic scale, that is, as being proportional to the value of a logarithm function applied to the ratio of the quantity and a reference quantity of the same type. The choice of unit generally indicates the type of quantity and the base of the logarithm.

Examples

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Examples of logarithmic units include units of information and information entropy (nat, shannon, ban) and of signal level (decibel, bel, neper). Frequency levels or logarithmic frequency quantities have various units are used in electronics (decade, octave) and for music pitch intervals (octave, semitone, cent, etc.). Other logarithmic scale units include the Richter magnitude scale for earthquakes and the pH value for acidity or basicity.

In addition, some industrial measures are logarithmic, such as most wire gauges used for wires and needles.

Units of information

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Units of level or level difference

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Further information: Level (logarithmic quantity)

Units of frequency level

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Table of examples

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Unit Base of logarithm Underlying quantity Interpretation
bit 2 number of possible messages quantity of information
byte 28 = 256 number of possible messages quantity of information
decibel 10(1/10) ≈ 1.259 any power quantity (sound power, for example) sound power level (for example)
decibel 10(1/20) ≈ 1.122 any root-power quantity (sound pressure, for example) sound pressure level (for example)
semitone 2(1/12) ≈ 1.059 frequency of sound pitch interval

The two definitions of a decibel are equivalent, because a ratio of power quantities is equal to the square of the corresponding ratio of root-power quantities.[citation needed][4]

See also

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Scale

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Applications

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References

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Logarithmic scale

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from Grokipedia
A logarithmic scale is a method of scaling measurements or graphical representations where the positions or values are proportional to the logarithms of the numbers being represented, rather than the numbers themselves. This approach transforms multiplicative relationships into additive ones, allowing data spanning vast ranges—often several orders of magnitude—to be displayed compactly and analyzed effectively without losing detail on smaller scales. Unlike linear scales, which increase additively, logarithmic scales expand proportionally with each successive power of the base (typically 10), compressing large values while stretching small ones to reveal patterns in exponential or power-law phenomena. Logarithmic scales originated from the invention of logarithms by Scottish mathematician in 1614, who developed them to simplify astronomical calculations by converting multiplications into additions. Napier's work, published in Mirifici Logarithmorum Canonis Descriptio, laid the foundation for logarithm tables and later tools like the , which physically embodied logarithmic scaling for rapid computations. Over time, refinements by mathematicians such as Henry Briggs introduced the common base-10 logarithm, enhancing its utility in science and engineering. In modern applications, logarithmic scales are essential across disciplines for handling extreme value ranges. In , the measures earthquake magnitude as the base-10 logarithm of seismic energy released, where each whole-number increase represents about 31 times more energy. Acoustics employs the scale, which logarithmically quantifies relative to a reference level, capturing human perception's nonlinear response. Chemistry uses the scale to express concentration logarithmically, with each unit change indicating a tenfold variation in acidity or basicity. In and visualization, log scales facilitate plotting , such as population models or financial , and are standard in tools like semilog graphs. These scales also appear in for growth rates, in , and in physics for stellar magnitudes in astronomy.

Fundamentals

Definition

A logarithmic scale is a scale of measurement in which the position of a point on the scale is proportional to the logarithm of the value being represented, rather than the value itself. This means that equal distances on the scale correspond to equal ratios (multiplicative factors) in the magnitude of the variable, making it particularly useful for spanning multiple orders of magnitude. To understand logarithmic scales, it is essential to recall that a logarithm is the inverse operation to : if by=xb^y = x, then y=log⁡bxy = \log_b x, where bb is the base of the logarithm, a positive not equal to 1. Common bases include 10 for the (often denoted log⁡x\log x) and e≈2.718e \approx 2.718 for the natural logarithm (denoted ln⁡x\ln x). On a logarithmic scale with base bb, the positions of values xx are marked proportionally to y=log⁡bxy = \log_b x. In contrast to a , where equal intervals represent equal absolute additions (, such as increments of 1 from 1 to 10), a logarithmic scale represents equal intervals as multiplications by a constant factor (, such as doubling from 1 to 2 to 4 to 8). This distinction allows logarithmic scales to compress large ranges of values into a manageable visual or numerical space, avoiding the distortion caused by linear representation of exponentially varying data. For a simple visual comparison, consider a number line from 1 to 1000. On a , the distance from 1 to 2 is the same as from 999 to 1000 (both 1 unit), emphasizing small differences at high values. On a base-10 logarithmic scale, the distance from 1 (10010^0) to 10 (10110^1) equals that from 10 to 100 (10210^2), and from 100 to 1000 (10310^3), each spanning one logarithmic unit and highlighting proportional growth equally across the range.

Mathematical Properties

The logarithmic transformation underlying a logarithmic scale is defined by the function y=log⁡bxy = \log_b x, where b>0b > 0, b≠1b \neq 1 is the base, and x>0x > 0 is the input value from the . This maps the positive reals to the entire real line, effectively compressing wide ranges of multiplicative data into a more manageable additive scale. A key property is its effect on products, where log⁡b(xy)=log⁡bx+log⁡by\log_b (xy) = \log_b x + \log_b y for x>0x > 0, y>0y > 0, transforming multiplicative relationships into additive ones, which is particularly useful for data spanning orders of magnitude. Logarithmic scales exhibit additivity with respect to ratios, as log⁡b(x/y)=log⁡bx−log⁡by\log_b (x/y) = \log_b x - \log_b y, allowing differences in the transformed space to directly represent relative changes. The domain is restricted to , rendering the function undefined for zero or negative values, with a vertical at x=0x = 0. As xx approaches 0 from the right, log⁡bx\log_b x approaches −∞-\infty, creating a singularity at the origin that emphasizes the scale's sensitivity to small values. The change of base formula, log⁡bx=log⁡kxlog⁡kb\log_b x = \frac{\log_k x}{\log_k b} for any valid base kk, facilitates computation and equivalence across different logarithmic bases. This scale is the inverse of an exponential scale, where x=byx = b^y, highlighting their complementary roles in representing growth and compression. In constructing a logarithmic scale, the physical or graphical distance between consecutive marks for values xix_i and xi+1x_{i+1} (with xi+1>xi>0x_{i+1} > x_i > 0) is proportional to log⁡b(xi+1/xi)\log_b (x_{i+1} / x_i ), ensuring that equal ratios in the original data correspond to equal intervals on the scale. This derivation follows from positioning points at yi=log⁡bxiy_i = \log_b x_i, so the interval length is ∣log⁡bxi+1−log⁡bxi∣=∣log⁡b(xi+1/xi)∣|\log_b x_{i+1} - \log_b x_i| = |\log_b (x_{i+1} / x_i)|.

Graphical Representations

Semi-logarithmic Plots

A semi-logarithmic plot, commonly referred to as a , features one axis scaled logarithmically and the other linearly, making it ideal for visualizing data that follows or decay patterns. Typically, the vertical (y) axis employs the logarithmic scale to accommodate wide-ranging values, while the horizontal (x) axis remains linear for uniform time or independent variable progression. This configuration transforms multiplicative changes in the dependent variable into additive ones, facilitating the analysis of relative rates over absolute magnitudes. To construct a , the logarithmic axis is marked with ticks at intervals corresponding to powers of the base (often 10), such as 1, 10, 100, and 1000, where the physical spacing between these marks is equal despite the increasing numerical differences. Subdivisions between major ticks represent finer logarithmic increments, like factors of 2 or 3 within each , ensuring the scale compresses large values while expanding small ones. The linear axis, in contrast, uses standard equal intervals for its ticks, allowing direct plotting of the original data points without transformation on that . Data points are then connected or fitted, often revealing patterns obscured in linear plots. In interpretation, a straight line on a with logarithmic y-axis signifies an underlying exponential relationship, such as y=a⋅bxy = a \cdot b^x, where the line's corresponds to log⁡b\log b, indicating the constant relative growth or decay rate per unit change in xx. Deviations from highlight shifts, like saturation or inhibition, in the process. This simplifies calculations for rate constants and enables straightforward or comparison across datasets with vastly different scales. A classic example is plotting bacterial population growth over time, where the exponential phase appears as a straight line on a semi-log graph, allowing easy determination of the growth rate from the slope, as cell numbers double at constant intervals under ideal conditions. Similarly, in radioactive decay, the activity or remaining nuclei versus time yields a linear semi-log plot, with the negative slope reflecting the half-life, enabling precise estimation of decay constants from experimental counts spanning multiple orders of magnitude. The primary advantages of semi-log plots lie in their ability to clearly depict or relative changes, compressing wide dynamic ranges into a readable format without losing detail on slower variations. This makes them invaluable in for growth curves, in physics for decay processes, and in for tracking prices over extended periods, where a semi-log scale accurately represents returns as steady trends rather than distorting curves. By linearizing exponentials, they enhance trend identification and statistical fitting compared to linear scales.

Log-log Plots

A log-log plot is a graphical representation in which both the horizontal (x) and vertical (y) axes are scaled logarithmically, enabling the effective display of spanning multiple orders of magnitude. This dual logarithmic scaling is particularly suited for exhibiting power-law relationships, where quantities vary multiplicatively rather than additively. The construction of a log-log plot involves marking ticks on both axes according to a logarithmic progression, commonly using base 10 to denote decades—such as positions at 0.1, 1, 10, 100, and so on—allowing uniform spacing for exponentially increasing values. Data points are then plotted by taking the logarithm of both variables, which transforms the coordinates without altering the relative relationships. Mathematically, if the underlying relationship follows a power law of the form y=axby = a x^b, applying logarithms to both sides yields: log⁡y=log⁡a+blog⁡x\log y = \log a + b \log x or, denoting x′=log⁡xx' = \log x and y′=log⁡yy' = \log y, y′=log⁡a+bx′.y' = \log a + b x'. This results in a straight line on the plot, where the slope bb represents the scaling exponent and the y-intercept is log⁡a\log a. The linearity confirms the presence of a power-law dependence, facilitating parameter estimation through simple linear regression on the transformed data. In interpretation, the slope of the line directly quantifies the exponent bb, providing insight into the nature of scaling phenomena across disciplines. For example, in biology, log-log plots are used to analyze allometric scaling, where traits like metabolic rate scale with body mass according to a power law, often with exponents around 0.75 for mammals. Similarly, plotting earthquake magnitude against energy release on a log-log scale reveals the power-law relationship, with energy EE approximating 101.5M10^{1.5M} where MM is magnitude, highlighting how small increases in magnitude correspond to disproportionately large energy jumps. Another illustrative case is Zipf's law for city sizes, where log-log plots of city rank versus population show linear trends with an exponent approximately -1, reflecting self-similar patterns in the distribution of city sizes. Log-log plots excel at accommodating wide dynamic ranges, compressing vast scales into a manageable visual form without losing proportionality. This double-logarithmic is especially valuable for multiplicative processes, such as those generating power-law distributions in natural and social systems, by converting or decay into straightforward linear trends for analysis.

Extensions and Variations

Variations of logarithmic scales adapt the transformation to specific data types, such as probabilities or values near boundaries. The scale applies the logit transformation, defined as log⁡(p1−p)\log\left(\frac{p}{1-p}\right) where pp is a probability between 0 and 1, to linearize sigmoid relationships in plots of binary outcomes or proportions. This scale expands regions near 0 and 1, facilitating comparison of probabilities across a wide range without compression at the extremes. Similarly, the scale uses the inverse of the standard , Φ−1(p)\Phi^{-1}(p), to transform probabilities, often employed in dose-response curves or Q-Q plots to assess normality or model cumulative probabilities. Reversed log scales invert the direction of the logarithmic progression, which can emphasize differences in small values near zero by allocating more visual space to lower magnitudes in certain visualization contexts. Extensions of logarithmic scales address limitations like handling negative values, zeros, or discontinuous ranges. The sym-log (symmetric logarithmic) scale combines a linear region near zero with logarithmic scaling elsewhere to accommodate negative values, zero, and wide dynamic ranges while preserving continuity. Its construction follows the formula y=\sign(x)log⁡10(1+∣x∣10C),y = \sign(x) \log_{10} \left(1 + \frac{|x|}{10^{C}}\right), where CC (linthresh exponent) sets the crossover point between linear and logarithmic behavior, typically chosen based on data spread. Broken log scales integrate linear segments for low-value regions with logarithmic segments for higher values, creating a piecewise transformation to avoid distortion in mixed-range datasets. For shifted data that does not include zero, double-log plots with offsets apply logarithmic scaling to adjusted variables, such as log⁡(x+a)\log(x + a) versus log⁡(y+b)\log(y + b), where offsets aa and bb shift the domain appropriately. These extensions find use in scenarios requiring robust visualization of heterogeneous data. Sym-log scales are effective for histograms containing outliers, such as income distributions, where they reveal both modal clusters near zero and extreme tails without clipping or excessive compression. Broken scales suit engineering plots with sparse low values and dense high values, like signal intensities, by providing clarity across segments. Double-log with offsets aids analysis of bounded or offset datasets, such as growth curves starting from a baseline. Developments in these scales stem from needs in computational tools; for instance, sym-log originated in visualization software to extend log capabilities, with implementing it based on Webber's bi-symmetric transformation for broad applicability in scientific plotting. Similar adaptations appear in libraries like , enhancing interactive graphics for wide-range data.

Common Applications

In Data Visualization

Logarithmic scales play a crucial role in data visualization by compressing wide-ranging data into a more manageable and interpretable format, allowing viewers to discern patterns across orders of magnitude that would otherwise be obscured on linear scales. For instance, when plotting values spanning from 1 to , a logarithmic scale transforms the axis to represent multiplicative changes equally, preventing smaller values from being compressed to near-zero and invisible while highlighting relative growth rates. This approach is particularly effective for datasets exhibiting exponential or power-law distributions, such as income distributions or network traffic, where linear scales distort the visual emphasis on lower-end values. In practice, logarithmic scales are applied in various chart types to handle skewed data effectively. Histograms using log scales can reveal the distribution of species abundance in , where rare events dominate without overwhelming the view of common occurrences. Similarly, scatter plots with log-log axes facilitate the analysis of relationships in multiplicative processes, like city sizes versus densities, by linearizing power-law trends. Bar charts on log scales are useful for comparing volumes across sites, where a logarithmic y-axis ensures that sites with 10 visits are not dwarfed by those with millions. Best practices for implementing logarithmic scales emphasize clarity and user accessibility to mitigate their counterintuitive nature for non-experts. Axes should be labeled with actual data values at tick marks rather than logarithmic indices, enabling direct reading of magnitudes without mental conversion. Designers must also include annotations or legends warning that equal intervals on the scale represent multiplicative rather than additive differences, as this can lead to misinterpretation of growth rates. For datasets with zeros or negative values, transformations like adding a small constant or using a log(1+x) variant are recommended to avoid undefined points. Software tools simplify the adoption of logarithmic scales in visualizations. In Python's library, the yscale('log') function automatically applies a logarithmic y-axis to plots, supporting seamless integration with dataframes for exploratory analysis. R's package offers scale_y_log10() for customizable log transformations in layered graphics, ideal for statistical reporting. Tableau provides drag-and-drop options for axis scaling, with automatic log detection for measures like sales data, enhancing interactive dashboards. Similarly, in Microsoft Excel, a logarithmic scale can be applied to the Y-axis to address issues where small values appear nearly invisible on a linear scale due to compression when mixed with large values; this resolves the problem by providing proportional spacing for multiplicative changes, improving visibility across wide-ranging data. To enable it, select the chart, navigate to the Format Axis pane under Axis Options, and check the Logarithmic scale box. Note that logarithmic scales cannot handle zero or negative values. A notable example is the early visualization of COVID-19 case growth during the 2020 pandemic, where logarithmic scales on line charts effectively illustrated exponential phases across regions, making it easier to compare trajectories from dozens to thousands of cases without the curves flattening misleadingly on linear axes.

In Scientific and Engineering Contexts

In physics, logarithmic scales are essential for spectrum analysis, particularly in examining frequency responses where data spans multiple orders of magnitude. For instance, power spectra are frequently plotted using decibels (dB), a logarithmic unit that compresses wide dynamic ranges into a manageable view, allowing clear identification of signal components in electronics and optics. In control systems, Bode plots utilize a logarithmic frequency axis to represent the magnitude and phase of a system's transfer function, facilitating the analysis of stability and response characteristics across frequencies from hertz to kilohertz. In engineering applications, especially , logarithmic scales enable precise quantification of gain in amplifiers through the scale, where a 3 dB increase corresponds to a doubling of power, simplifying the handling of ratios in and audio systems. The in exemplifies this in , defining magnitude as the base-10 logarithm of the maximum of seismic waves recorded by a seismograph, adjusted for distance, which captures the exponential increase in release—each whole number step represents about 31 times more . Logarithmic scales prove invaluable in scientific modeling of complex phenomena involving scale-invariant or exponential processes. In studies, Kolmogorov's theory describes energy cascades across scales, with logarithmic scaling observed in the inertial subrange of velocity profiles, where the logarithmic layer begins around 100 times the Kolmogorov length scale, aiding predictions of dissipation rates. For , semi-logarithmic plots of plasma drug concentrations versus time reveal first-order elimination kinetics, where the log-linear decay simplifies dosing calculations for exponential clearance. Specific examples highlight these applications: in chemistry, the scale measures acidity as pH=−log⁡10[H+]\mathrm{pH} = -\log_{10} [\mathrm{H}^+], where [H+][\mathrm{H}^+] is the , compressing the 10^{-14} to 10^0 range into a 0-14 scale for practical analysis of solutions. In astronomy, stellar magnitudes use a logarithmic system where mm relates to brightness flux FF by m=−2.5log⁡10(F/F0)m = -2.5 \log_{10} (F / F_0), with F0F_0 a reference, allowing comparison of stars differing by factors of 100 in brightness across a five-magnitude interval. These scales offer key advantages in scientific and contexts by accommodating over vast orders of magnitude—such as from to kilovolts in signals—enabling equitable comparisons without distortion from linear plotting. They also align with natural laws exhibiting inverse-square or exponential dependencies, like gravitational or radiative fluxes, transforming multiplicative relationships into additive ones for easier modeling and interpretation.

In Human Perception and Everyday Uses

Human perception of sensory stimuli often follows logarithmic relationships, as described by the Weber-Fechner law, which states that the in a stimulus is proportional to the magnitude of the stimulus, leading to a logarithmic scaling of perceived intensity. This psychophysical principle, formulated by and Gustav Theodor Fechner in the , explains why equal ratios in physical stimuli produce equal perceptual increments across various senses. In auditory perception, is quantified using the sone scale, where perceived grows linearly with the logarithm of , such that a 10-phon increase roughly doubles the subjective . For , is approximately the logarithm of , allowing the to discern a wide range of light levels from dim twilight to bright on a compressed scale. Similarly, intensities in and smell follow power laws close to logarithmic transformations, where perceived strength increases more slowly than the physical concentration of stimuli, as established in sensory scaling experiments. Everyday tools leverage these logarithmic principles for practical computation and measurement; for instance, the , a historical invented around 1622 by , uses aligned logarithmic scales on sliding rulers to perform and division by addition and subtraction of log values. Audio equipment often features decibel markings on volume controls and meters, a logarithmic unit where a 10 dB increase corresponds to a tenfold rise in , aligning with human hearing sensitivity. Modern consumer devices incorporate logarithmic scales to enhance usability; smartphone spectrum analyzer apps, such as FrequenSee, display frequency responses on logarithmic axes to mimic human auditory perception, providing clearer visualization of audio spectra from low bass to high treble. Fitness trackers, like those from , log data in zones that reflect exponential increases in effort and physiological response, using percentage-based thresholds derived from maximum to track progress in a perceptually relevant manner. Culturally and historically, logarithmic scales underpin , where octaves represent a of 2:1, perceived as equivalent pitches across registers due to the logarithmic nature of auditory discrimination. In , logarithmic azimuthal projections, developed by Torsten Hägerstrand in the mid-20th century, compress distances from a central point using logarithms to better represent global connectivity and migration patterns on flat maps.

Logarithmic Units

General Concept

Logarithmic units are dimensionless measures that express the magnitude of a relative to a specified reference level using a logarithm of their , ensuring the result is inherently relative and scale-invariant. These units arise naturally when quantifying s of quantities with identical dimensions, such as power or intensity, where the logarithm transforms multiplicative relationships into additive ones. A prototypical form is the level of a , given by the L=10log⁡10(PP0),L = 10 \log_{10} \left( \frac{P}{P_0} \right), where PP is the measured power and P0P_0 is the reference power; this expression yields a value in decibels (dB). Such units are dimensionless because the ratio P/P0P / P_0 cancels out the physical dimensions, leaving a pure number scaled by the logarithmic factor. The foundational unit is the bel (B), defined as the common (base-10) logarithm of the ratio of two powers, introduced in the context of telecommunications to quantify signal attenuation and gain. The decibel, as its decimal submultiple (1 B = 10 dB), provides a more practical scale for measurements, with the general formula for level differences between two quantities following ΔL=10log⁡10(P2/P1)\Delta L = 10 \log_{10} (P_2 / P_1) in dB. This approach circumvents the need for arbitrary scaling when expressing ratios, promoting consistency across applications, and is prevalent in domains like acoustics, optics, and communications engineering. Logarithmic units offer significant advantages in handling concatenated systems, where values add directly—for instance, the total gain in dB of cascaded amplifiers is the sum of gains, simplifying calculations that would otherwise involve products. They also compress vast dynamic ranges into a compact scale, preventing numerical overflow in computations involving orders-of-magnitude variations, such as signal strengths spanning microwatts to kilowatts. In contrast to absolute units like watts, which quantify the intrinsic value of a quantity in a specific physical dimension (e.g., energy per time), logarithmic units such as the bel or dB solely capture relative differences or ratios without reference to . For example, 3 dB represents a doubling of power regardless of the baseline wattage, underscoring their in comparative assessments rather than standalone measurements.

Units in Information Theory

In , logarithmic units quantify the uncertainty or of probabilistic events, with the base of the logarithm determining the specific unit. The foundational unit is the bit (short for binary digit), introduced by as the information associated with a binary choice between two equally likely outcomes, equivalent to log⁡22=1\log_2 2 = 1 bit. This measure arises from the self-information of an event with probability pp, defined as I(p)=−log⁡2pI(p) = -\log_2 p bits, where lower probabilities yield higher due to greater surprise. For a discrete with outcomes xix_i having probabilities pip_i, the average , or H(X)H(X), is given by the formula H(X)=−∑ipilog⁡2piH(X) = -\sum_i p_i \log_2 p_i bits, representing the expected needed to specify an outcome. This formula, derived in Shannon's seminal 1948 paper, provides the theoretical limit for efficient encoding of sources. Other logarithmic bases yield alternative units: the nat, based on the natural logarithm ln⁡\ln (base ee), measures information in natural units where 1 nat corresponds to the information from an event with probability 1/e1/e; the dit (decimal information unit), using base-10 logarithm, aligns with decimal digits; and the hartley, also base-10, quantifies information in terms of decimal choices, with 1 hartley equaling log⁡1010=1\log_{10} 10 = 1. These units relate via conversion factors, such as 1 nat ≈1.4427\approx 1.4427 bits and 1 hartley ≈3.3219\approx 3.3219 bits, allowing flexibility in mathematical or practical contexts. Logarithmic units find key applications in data compression, where sets the minimum bits per symbol required for lossless encoding, as per , enabling algorithms like to approach this bound. In channel capacity, the maximum reliable transmission rate is measured in bits per second, limited by the channel's noise characteristics, as formalized in . For instance, file sizes are often expressed logarithmically, such as kilobits ( 2102^{10} bits) or megabits, reflecting the in storage needs and aligning with entropy-based compression efficiencies. Similarly, in algorithmic complexity, terms like O(log⁡n)O(\log n) describe logarithmic scaling in operations such as binary search, where the grows as log⁡2n\log_2 n bits to resolve nn possibilities.

Units in Physics and Acoustics

In physics, logarithmic scales are employed to quantify ratios of physical quantities, particularly in fields like acoustics, , and , where wide dynamic ranges are common. The (dB) is a widely used dimensionless unit for expressing such ratios, defined for power as L=10log⁡10(PP0)L = 10 \log_{10} \left( \frac{P}{P_0} \right), where PP is the measured power and P0P_0 is the reference power. For amplitude quantities, such as voltage or pressure, the formula adjusts to L=20log⁡10(AA0)L = 20 \log_{10} \left( \frac{A}{A_0} \right), reflecting the quadratic relationship between and power. Variants like the dBm express power relative to 1 milliwatt (P0=1P_0 = 1 mW), facilitating comparisons in and applications. The (Np) serves as another logarithmic unit, based on the natural logarithm and often applied in analysis and wave propagation. It is defined for ratios as L=ln⁡(AA0)L = \ln \left( \frac{A}{A_0} \right), providing a measure of or gain in systems like electrical cables or acoustic waveguides. One corresponds to approximately 8.686 dB, allowing conversions between the two scales in and contexts. In acoustics, logarithmic units address the perceptual scaling of sound, where human hearing responds nonlinearly to intensity. The unit quantifies perceived level, defined as the sound pressure level in decibels of a 1 kHz tone that matches the subjective of the sound in question; for example, a 60 level equals the of a 60 dB SPL tone at 1 kHz. This ties to equal-loudness contours, emphasizing the logarithmic dependence on intensity. The unit provides a more linear measure of perceived magnitude, with 1 defined as the of a 1 kHz tone at 40 dB SPL; in sones approximately doubles for every 10 increase, following S=2(Lp−40)/10S = 2^{(L_p - 40)/10}, where LpL_p is the level. Beyond acoustics, logarithmic scales appear in diverse physical measurements. The Richter scale for earthquakes measures magnitude as M=log⁡10(AA0)M = \log_{10} \left( \frac{A}{A_0} \right), where AA is the maximum amplitude recorded and A0A_0 is a reference , capturing the logarithmic growth in . In chemistry and environmental physics, pH quantifies acidity as pH=−log⁡10[H+]\mathrm{pH} = -\log_{10} [H^+], where [H+][H^+] is the concentration in moles per liter, such that each unit decrease represents a tenfold increase in acidity. For stellar magnitudes in astronomy, mm relates to FF via m1−m2=−2.5log⁡10(F1F2)m_1 - m_2 = -2.5 \log_{10} \left( \frac{F_1}{F_2} \right), a base-10 logarithmic scale inverted so brighter objects have smaller (more negative) values. Logarithmic units also describe frequency intervals in wave phenomena. An octave represents a frequency ratio of 2, defined such that log⁡2(f2/f1)=1\log_2 (f_2 / f_1) = 1, commonly used in acoustics and for where the upper frequency is twice the lower. A decade, analogously, denotes a of 10, with log⁡10(f2/f1)=1\log_{10} (f_2 / f_1) = 1, aiding in the visualization of broadband spectra in and studies.

Comparative Table of Examples

The following table provides a comparative overview of selected logarithmic units and scales, highlighting their definitions, applications, and interpretations. All entries represent dimensionless ratios, facilitating comparisons across wide dynamic ranges in their respective fields.
Unit NameBase/FormulaField of UseReference ValueExample Value Interpretation
Decibel (dB)10 log₁₀ (power ratio); 20 log₁₀ (amplitude ratio)Acoustics and signal processing10^{-12} W (1 pW) for sound power level in acoustics; 1 mW for electrical power (dBm) in signal processing; 20 μPa for sound pressure (human hearing threshold)+60 dB indicates sound pressure 1,000 times the reference (10³ ratio), perceived as significantly louder
Bit (bit)log₂ (probability ratio)Information theory1 bit distinguishes 2 equally likely outcomes (probability 1/2)8 bits represent 256 possible states (2⁸ ratio), sufficient for one byte of data
pH-log₁₀ [H⁺] (molar concentration)Chemistry (acidity)pH 7 equals 10⁻⁷ M H⁺ (neutral water at 25°C)pH 4 indicates 1,000 times more acidic than neutral (10³ higher [H⁺])
Richter magnitude (M_L)log₁₀ (amplitude ratio)SeismologyM_L 0 equals 1 μm ground displacement at 100 km distanceM_L 6 represents 1,000 times larger amplitude than M_L 3 (10³ ratio), releasing approximately 31,000 times more energy (≈31³ ratio, where each unit increase is ~31 times energy)
Astronomical magnitude (m)-2.5 log₁₀ (flux ratio)Astronomy (brightness)m = 0 for Vega's visual flux (reference star)m = -1 is 2.512 times brighter than m = 0 (flux ratio ≈ 2.512); difference of 5 magnitudes equals 100-fold flux change
Octavelog₂ (frequency ratio) = 1Music and acoustics (frequency intervals)One octave doubles the reference frequency (e.g., 440 Hz to 880 Hz)3 octaves span 8-fold frequency increase (2³ ratio), as in musical notes from C to high C
Decadelog₁₀ (frequency ratio) = 1Engineering (frequency response)One decade multiplies reference frequency by 10 (e.g., 100 Hz to 1 kHz)Filter roll-off of 20 dB/decade means amplitude halves every decade, common in first-order systems
Bel (B)10 log₁₀ (power ratio)General signal levels1 B equals 10-fold power increase over reference1 B corresponds to a 10:1 power ratio, rarely used alone due to large steps
Neper (Np)ln (amplitude ratio)General field quantities (e.g., voltage, wave amplitude)1 Np equals e-fold (≈2.718) amplitude increase over reference1 Np ≈ 8.686 dB for power-equivalent comparisons in transmission lines
These units share a common foundation as logarithmic measures of ratios, rendering them unitless and ideal for compressing exponential variations into linear perceptions, such as human sensory responses or vast physical ranges. Differences arise primarily in logarithmic bases—base 10 for decimal-friendly scales like dB and Richter, base 2 for binary-aligned information like bits, and natural base e for nepers—and in scaling factors, such as the 20 log₁₀ multiplier for amplitude-derived dB (reflecting squared power relationships) versus 10 log₁₀ for direct power. Conversion between units is straightforward via logarithmic identities; for instance, 1 bel = 10 dB by definition, and 1 neper ≈ 8.686 dB due to ln(10) ≈ 2.3026, with 20 log₁₀(e) ≈ 8.686 for amplitude-to-power equivalence. This unitless nature ensures interoperability across domains, though reference values must be contextually defined for absolute interpretations.

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