Source: https://jmrplens.github.io/phonometry/devices/electroacoustics/microphones/ # Microphone Characterisation (IEC 60268-4) Every acoustic measurement in this documentation starts at a microphone, and IEC 60268-4 is the standard that fixes how the device itself is described: what its sensitivity refers to, how its directional pattern is stated, and how its self-noise and overload point bound the levels it can read. This guide covers the conventions where datasheet comparisons go wrong (the sensitivity references and the noise weightings), the directional patterns with their directivity index, and the **rated-characteristics report** that renders the standard's data sheet from a measured free-field response, with every panel drawn to the IEC 60263 scale conventions. The distortion and frequency-response measurements themselves live in [Electroacoustics](https://jmrplens.github.io/phonometry/devices/electroacoustics/electroacoustics/); the loudspeaker counterpart has [its own guide](https://jmrplens.github.io/phonometry/devices/electroacoustics/loudspeakers/). ## 1. Sensitivity and its references (IEC 60268-4 clause 11) **Microphones (IEC 60268-4 clause 11).** The sensitivity $M$ is the output voltage per unit sound pressure, quoted in mV/Pa; the sensitivity *level* is $L_M = 20\log_{10}(M / 1\,\text{V/Pa})$ dB, which is negative for every real microphone (a 50 mV/Pa studio condenser sits at −26 dB re 1 V/Pa). The classic pitfall is the reference: −26 dB re 1 V/Pa and +34 dB re 1 mV/Pa are the *same* microphone, 60 dB apart on paper, so a sensitivity level without its reference is meaningless. The standard also distinguishes free-field, diffuse-field and pressure sensitivity; for the same capsule they diverge at high frequency, so the stated type matters as much as the number. Behind the number sits the transducer. A **condenser** capsule (externally polarised or electret) reads the sound pressure as a capacitance change and needs powering, but converts with high sensitivity, typically some tens of mV/Pa for a studio diaphragm and a few mV/Pa for a small measurement capsule; virtually every measurement microphone is a condenser because the stretched-membrane capsule is stable enough to calibrate. A **dynamic** (moving-coil) microphone generates its voltage by induction, needs no power and survives abuse, but converts far less efficiently, around 1-3 mV/Pa, which is why a dynamic on a quiet source runs out of preamplifier gain before a condenser does. The rated sensitivity of the standard is quoted at the 1 kHz reference frequency (clause 11.3), into the rated load, and the stated type (free-field, diffuse-field or pressure) is part of the rating, not a detail. ## 2. Directional pattern and the directivity index (clause 13) The directional response is stated as the pattern of sensitivity against the angle of incidence, normalized to the reference axis. The classic first-order family mixes an omnidirectional (pressure) term and a cosine (pressure-gradient) term, $M(\theta) \propto (1 - b) + b\cos\theta$: $b = 0$ is the omnidirectional capsule, $b = 1/2$ the cardioid with its null at 180°, and $b = 1$ the figure-of-eight with nulls at ±90°. One number condenses the pattern: the **directivity index** $D = 20\log_{10}(M_0/M_\mathrm{diff})$ compares the reference-axis sensitivity with the diffuse-field sensitivity obtained from the clause 11.2.2 a) integral over a rotationally symmetric pattern. An omni scores 0 dB, the ideal cardioid and figure-of-eight both $10\log_{10} 3 = 4.8$ dB: in a diffuse room field, a cardioid picks up three times less reverberant power than an omni of equal axial sensitivity, which is exactly the ratio a talker-to-microphone distance calculation wants. Real patterns hold their textbook shape only over the middle of the band; at high frequency the capsule's own size makes any microphone directive, which is also why the free-field and diffuse-field sensitivities of one capsule diverge there. ## 3. Inherent noise: dB(A) and dB(468) (clause 17) A microphone's electronics and the air load on its diaphragm set a noise floor, expressed as the **equivalent noise level**: the sound pressure level whose output would equal the weighted inherent-noise voltage, $L_\mathrm{N} = 20\log_{10}\bigl((U_\mathrm{N}/M)/20\,\mu\text{Pa}\bigr)$. The standard states it with two weightings, and the numbers are far apart on paper. The **A-weighted** figure (RMS detector) is the one most datasheets quote, e.g. 14 dB(A) for a good studio condenser. The **ITU-R BS.468-4** figure weights the same noise with the curve that peaks +12.2 dB near 6.3 kHz and reads it with a quasi-peak detector, so it penalises the hiss and crackle the ear actually notices; for typical microphone noise it comes out roughly 10 dB above the A-weighted number for the same capsule. Neither is wrong: they are different weightings of the same voltage, and comparing a dB(A) figure from one datasheet with a dB(468) figure from another silently flatters the first by that margin. The signal-to-noise ratio re 1 Pa (94 dB SPL) is derived from the same equivalent noise level, and the overload sound pressure level (clause 15.2) bounds the usable range from above. The BS.468 weighting curve itself is exposed as `itu_r_468_weighting` in the [electroacoustics distortion set](https://jmrplens.github.io/phonometry/devices/electroacoustics/electroacoustics/), where it also weights THD. The quasi-peak detector itself is not implemented: an inherent-noise level computed from `itu_r_468_weighting` and an r.m.s. sum is not a dBqps figure. ## 4. Microphone characteristics report (IEC 60268-4) The microphone companion of the [loudspeaker report](https://jmrplens.github.io/phonometry/devices/electroacoustics/loudspeakers/): the rated characteristics IEC 60268-4 defines around a measured free-field frequency response are gathered into a single **microphone characteristics** result that renders the standard's rated-characteristics data sheet. Four of the numbers are computed from the standard's own definitions rather than merely repeated: - **Sensitivity level** (11.1). The rated free-field sensitivity $M$ (mV/Pa, at the 1 kHz reference frequency of 11.3) as a level, $L_M = 20\log_{10}(M / 1\,\text{V/Pa})$: 12.5 mV/Pa is $-38.1$ dB re 1 V/Pa. - **Effective frequency range** (12.2). The band over which the response, normalized to 0 dB at the reference frequency, stays within the stated $\pm$ tolerance; the edges are the interpolated tolerance crossings. - **Directivity index** (13.2.2). $D = 20\log_{10}(M_0/M_\mathrm{diff})$ with the diffuse-field sensitivity from the 11.2.2 a) integral over a rotationally symmetric pattern; the ideal cardioid returns $10\log_{10} 3 = 4.8$ dB. - **Equivalent noise level** (17.2). The weighted inherent-noise voltage over the rated sensitivity as a sound pressure level, $L_\mathrm{N} = 20\log_{10}\bigl((U_\mathrm{N}/M)/20\,\mu\text{Pa}\bigr)$, with the signal-to-noise ratio re 1 Pa (94 dB SPL) derived from it. The overload sound pressure level (15.2) is read from a distortion-against-level curve at the stated THD limit. ```python import numpy as np from phonometry import ( MicrophoneDirectivity, MicrophoneElectrical, MicrophoneNoise, MicrophoneOverload, ReportMetadata, microphone_characteristics, ) freqs = np.geomspace(20, 20000, 400) response = -10 * np.log10(1 + (30.0 / freqs) ** 4) # low-frequency roll-off response -= 10 * np.log10(1 + (freqs / 19000.0) ** 8) # high-frequency roll-off response += 2.0 * np.exp(-(np.log2(freqs / 9000.0) ** 2) / 0.3) # presence region angles = np.linspace(0, 179, 359) cardioid = 20 * np.log10((1 + np.cos(np.radians(angles))) / 2) noise_f = np.geomspace(20, 20000, 31) result = microphone_characteristics( freqs, response, 12.5, tolerance_db=3.0, # 12.5 mV/Pa at 1 kHz directivity=MicrophoneDirectivity(polar=(angles, cardioid), frequency=1000.0), noise=MicrophoneNoise( # A-weighted, V voltage=1.25e-6, spectrum=(noise_f, 6.0 + 12.0 * np.log10(1000.0 / noise_f)), ), overload=MicrophoneOverload( distortion=(np.linspace(100, 140, 81), 0.5 * 10 ** ((np.linspace(100, 140, 81) - 130.0) * 0.08)), thd_percent=0.5, ), electrical=MicrophoneElectrical( rated_impedance=150.0, minimum_load_impedance=1000.0, powering="Phantom P48 (IEC 61938)", supply_current_ma=3.1, ), ) print(round(result.sensitivity_level_db, 1)) # -38.1 dB re 1 V/Pa print(tuple(round(x) for x in result.effective_range)) # Hz print(round(result.directivity_index_db, 1)) # 4.8 dB (cardioid) print(round(result.equivalent_noise_level_db, 1)) # dB(A) result.report("microphone.pdf", metadata=ReportMetadata(measurement_standard="IEC 60268-4")) ``` `microphone_characteristics` returns a `MicrophoneCharacteristics` with the computed `sensitivity_level_db`, `effective_range`, `directivity_index_db`, `equivalent_noise_level_db`, `max_spl_db`, `signal_to_noise_ratio_db` and `diffuse_field_sensitivity_level_db`, and a `.report()` that writes the fiche. The free-field response is drawn with its tolerance band, the reference-frequency marker and the effective-range markers to the IEC 60263 proportion (one frequency decade equal to 25 dB), and the directional pattern on the IEC 60263 25 dB reference circle; the inherent-noise spectrum and the distortion-against-level curve (with the THD limit and the overload level marked) feed the secondary panels. A `requirement` in the metadata is checked as a maximum permitted equivalent noise level. What the fiche cannot supply is the laboratory behind the numbers: the anechoic room and its qualification, the reference microphone and its calibration certificate, the high-level source that produces a 140 dB field and the turntable are what the reported numbers mean, and the acquisition itself is not implemented here — the functions reduce the curves they are handed. The rated characteristics are also available interactively through `.plot()`, which draws **one concept per figure** with the same panel code the report composes, selected by `quantity`. Passing an axes draws on it: ```python result.plot() # free-field response (default) result.plot(quantity="directivity") # polar pattern on the 25 dB circle result.plot(quantity="noise") # inherent-noise band spectrum result.plot(quantity="distortion") # THD vs sound pressure level ```

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```python import matplotlib.pyplot as plt import numpy as np from phonometry import microphone_characteristics freqs = np.geomspace(20, 20000, 400) response = -10 * np.log10(1 + (30.0 / freqs) ** 4) # low-frequency roll-off response -= 10 * np.log10(1 + (freqs / 19000.0) ** 8) # high-frequency roll-off response += 2.0 * np.exp(-(np.log2(freqs / 9000.0) ** 2) / 0.3) # presence region result = microphone_characteristics(freqs, response, 12.5, tolerance_db=3.0) result.plot() # quantity="response" (the default) plt.show() ```

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```python import matplotlib.pyplot as plt import numpy as np from phonometry import MicrophoneDirectivity, microphone_characteristics freqs = np.geomspace(20, 20000, 400) response = -10 * np.log10(1 + (30.0 / freqs) ** 4) angles = np.linspace(0, 179, 359) cardioid = 20 * np.log10((1 + np.cos(np.radians(angles))) / 2) result = microphone_characteristics( freqs, response, 12.5, tolerance_db=3.0, directivity=MicrophoneDirectivity(polar=(angles, cardioid), frequency=1000.0), ) result.plot(quantity="directivity") plt.show() ```

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```python import matplotlib.pyplot as plt import numpy as np from phonometry import MicrophoneNoise, microphone_characteristics freqs = np.geomspace(20, 20000, 400) response = -10 * np.log10(1 + (30.0 / freqs) ** 4) noise_f = np.geomspace(20, 20000, 31) result = microphone_characteristics( freqs, response, 12.5, tolerance_db=3.0, noise=MicrophoneNoise( voltage=1.25e-6, spectrum=(noise_f, 6.0 + 12.0 * np.log10(1000.0 / noise_f)), ), ) result.plot(quantity="noise") plt.show() ```

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```python import matplotlib.pyplot as plt import numpy as np from phonometry import MicrophoneOverload, microphone_characteristics freqs = np.geomspace(20, 20000, 400) response = -10 * np.log10(1 + (30.0 / freqs) ** 4) spl_axis = np.linspace(100, 140, 81) result = microphone_characteristics( freqs, response, 12.5, tolerance_db=3.0, overload=MicrophoneOverload( distortion=(spl_axis, 0.5 * 10 ** ((spl_axis - 130.0) * 0.08)), thd_percent=0.5, ), ) result.plot(quantity="distortion") plt.show() ```

The example fiche, regenerated with `make reports`, is kept rendered in the repository. Click the preview to open the PDF: [](https://raw.githubusercontent.com/jmrplens/phonometry/main/.github/reports/iec60268_4_microphone_example.pdf) *Microphone characteristics fiche (`MicrophoneCharacteristics.report`), the IEC 60268-4 rated-characteristics table beside the free-field response, with the cardioid directional pattern, inherent-noise and THD-against-level panels drawn to the IEC 60263 25 dB-per-decade and 25 dB reference-circle conventions.* ## See also - [Electroacoustics](https://jmrplens.github.io/phonometry/devices/electroacoustics/electroacoustics/): the IEC 60268-3 distortion set, THD+N and SINAD, the ITU-R 468 weighted THD and the H1/H2 frequency-response estimators that produce the measured curves. - [Loudspeaker Characterisation (IEC 60268-5)](https://jmrplens.github.io/phonometry/devices/electroacoustics/loudspeakers/): the companion rated-characteristics report for the reproducing side of the chain. - [Calibration and dBFS](https://jmrplens.github.io/phonometry/signals/metrology/calibration/): turning a microphone's rated sensitivity into calibrated sound pressure levels. - [Build a sound level meter](https://jmrplens.github.io/phonometry/signals/sound-level-meter/): the measurement chain a characterised microphone feeds. - API reference: [`electroacoustics.microphone`](https://jmrplens.github.io/phonometry/reference/api/electroacoustics/microphone/). ## References - Beranek, L. L., & Mellow, T. J. (2012). *Acoustics: Sound fields and transducers*. Academic Press. ISBN 978-0-12-391421-7. [doi:10.1016/C2011-0-05897-0](https://doi.org/10.1016/C2011-0-05897-0). The transducer physics behind sections 1 and 2: condenser and moving-coil transduction and first-order directional patterns. - International Telecommunication Union. (1986). *Measurement of audio-frequency noise voltage level in sound broadcasting* (Recommendation ITU-R BS.468-4). [itu.int](https://www.itu.int/rec/R-REC-BS.468/en). The weighting network and quasi-peak detection behind the dB(468) inherent-noise figure of section 3. ## Standards IEC 60268-4:2014, *Sound system equipment – Part 4: Microphones*: the rated microphone characteristics of this guide, namely the free-field sensitivity and its level re 1 V/Pa (11.1/11.3), the frequency response and effective frequency range (12.1/12.2), the directional pattern and the directivity index through the 11.2.2 a) diffuse-field integral (13.1/13.2), the overload sound pressure level (15.2), the equivalent sound pressure level due to inherent noise (17) and the rated impedances and power supply (9/10); since revised as IEC 60268-4:2018, the 2014 edition is the implemented one. IEC 60263:1982, *Scales and sizes for plotting frequency characteristics and polar diagrams*: the scale proportions of the characteristic graphs, one frequency decade equal to 25 dB on the ordinate (clause 2) and the polar diagram on a 25 dB reference-circle radius (clause 3). ITU-R BS.468-4: the weighting-network nominal response behind the dB(468) noise figure.