The number of microphones in a microphone device: a single microphone or several microphones forming a microphone array.
Microphone SpecificationsWhat type of microphone to choose for recording an orchestra, a singer, a snare drum or a guitar? Should you choose a cardioid, omnidirectional, or possibly a gun microphone? What about their price? Is a $20,000 microphone 200 times better than a $100 microphone or 20,000 times better than a $1 microphone? What if I tell you that a $1 no-name lavalier mike placed on the person you are interviewing is better than a 20,000 Neumann placed on your camera 5 meters away from the sound source? You will probably be able to answer these questions if you understand microphone specifications.
As an example, let us look at the specifications of a Shure PGA48 cardioid dynamic microphone:
- Sensitivity at 1 kHz, open circuit voltage: –53.5 dBV/Pa (2.10 mV) 1 Pa=94 dB SPL
- Frequency Response: 70 to 15,000 Hz
- Polar Pattern: cardioid
- Output Impedance: 600 Ω
- Connector: Three-pin professional audio (XLR), male
We will take a closer look at them below.
[size=0.7]An iSK BM-800 condenser microphone, its frequency response graph and polar pattern
Sensitivity and Transfer FactorThe microphone is a transducer that converts the sound pressure into the output voltage. Its sensitivity is the amount of electrical output for a given sound pressure input. It indicates how well the microphone can do this job. I high-sensitivity microphone creates more voltage for the same sound and hence needs less amplification at the mixer or recording device. At the same time, it should be noted that the sensitivity is in no way an indication of the microphone quality.
The sensitivity can be expressed as a transfer factor in terms of voltage units per pressure units, namely, in millivolts per pascal into an open circuit or into a 1 kiloohm load at 1 kHz frequency.
The microphone sensitivity is usually expressed in logarithmic units (decibels) and measured with 1 kHz sine wave at 1-pascal pressure (1 Pa = 1 N/m² = 10 dynes/cm² = 10 µbars), which is equal to 94 dB sound pressure level (SPL). Some microphone manufacturers use another reference level — 74 dB SPL, which equals to 0.1 Pa or 1 dyne/cm². However, 94 dB SPL is recommended and used much more often because 74 dB SPL is too close to typical noise levels.
The magnitude of the signal from the microphone is a measure of its sensitivity. The higher this value the greater the sensitivity. Because our hearing works on a logarithmic scale, the microphone sensitivity is often measured in decibels referring to a 1V/Pa standard value resulting in negative values because 1V/Pa is a very high sensitivity, much higher than any microphone can provide. In this unit converter the following formulas are used to convert the sensitivity in dB into the transfer factor and vice versa:
[size=1.2]SdB re 1V/Pa = 20 log10(TFmV/Pa/1000 mV/Pa)
or
[size=1.2]TFmV/Pa = 1000 mV/Pa × 10(SdB re 1V/Pa/20).
Here:
SdB re 1V/Pa is the sensitivity in dB relative 1 V/Pa,
TFmV/Pa is the transfer factor in mV/Pa and
1000 mV/Pa = 1 V/Pa is the reference output — 1 V of the voltage produced per 1 pascal of pressure.
The logarithmic sensitivity in decibels with the stated reference value is known as an “absolute” value and it can always be converted into mV/Pa or another linear value.
Why do we always see these two values — 94 and 74 decibels in all articles about the sensitivity of microphones? This is because the absolute threshold of human hearing equals to 2·10⁻⁵ N/m² or 20 µPa for a sound sine wave at a frequency of 1 kHz. This is the quietest sound a young human in good health can detect. The sound pressure level in decibels for 1 Pa pressure PSPL, which is often used for measuring the sensitivity of microphones, is determined as
[size=1.2]PSPL = 20×Log₁₀(P/P₀),
Where P = 1 Pa is the absolute hearing threshold P₀ = 2·10⁻⁵ Pa. That is,
[size=1.2]PSPL = 20×Log₁₀(1/2×10⁻⁵) = 93.979 dB.
If, on the other hand, the pressure of 1 dyn/cm² = 2·10⁻⁴ Pa is used as the reference level, then we have
[size=1.2]PSPL = 20×Log₁₀(1/2×10⁻⁴) = 73.979 дБ.
Note that a higher dB value indicates greater sensitivity, so a microphone with a sensitivity of –50 dB is more sensitive than a –65 dB microphone. The sensitivity of a hydrophone is usually expressed in decibels relative to 1 V/µPa.
Though the sensitivity is not an indication of the microphone quality, it is a vital specification if you are recording very tiny sounds in the field such as recording the sounds of chicken embryos. At the same time, if you want to record power hammer sounds, the highly sensitive microphone will probably overload the preamplifier or the mixer, producing distortion. Sensitive microphone capsules are used in gun microphones to record week distant sounds whereas less sensitive microphone capsules are installed in microphones like Shure PG48 used for recording spoken word when the microphone is placed only a couple of inches from the singer’s mouth. The microphone sensitivity is only one factor among many, considered in choosing a microphone for a particular application.
[size=0.7]The impedance of the balanced microphone input of this Behringer XENIX 802 Mixer Console is approximately 2.6 kΩ, which is high enough for connecting various microphones without loss of the microphone signal
In microphone specifications, the sensitivity is usually stated for the open circuit, that is, without an electrical load. There are several reasons for measuring the open-circuit sensitivity. First, the microphone performance can be calculated for any load if the two values are known: the open-circuit sensitivity and the microphone impedance. Second, in most modern audio equipment, for effective use, microphones are usually connected to a high-impedance load, for example, a 200-ohms microphone should be connected to the load equal or greater than 2 kΩ. This may be regarded as a microphone working into an open circuit. The open circuit sensitivity value is convenient for comparing sensitivities of different microphones.
When comparing sensitivities of microphones made by different manufacturers, keep in mind that they can use different reference units — 94 dB SPL and 74 dB SPL mentioned above. For example, a Shure PGA48 microphone with a transfer factor of 2.1 mV/Pa has the sensitivity of –73.5 dB re 1V/dyn cm² and –53.5 dB re 1 V/Pa. You can see that the difference in these values is exactly 20 dB. So, to compare specifications of microphones from different manufacturers, use our converter to convert them to the same reference level.
The table below shows typical sensitivity values for different transducer technologies in dBV/Pa and mV/Pa.
Dynamic microphones | 0.3 to 4 mV/Pa | –70 to –48 dBV/Pa |
Ribbon microphones | 1 to 3 mV/Pa | –60 to –50 dBV/Pa |
Condenser microphones | 1 to 18 mV/Pa | –60 to –35 dBV/Pa |
Electret condenser microphones | 6 to 18 mV/Pa | –45 to –35 dBV/Pa |
MEMS microphones | 6 to 18 mV/Pa | –45 to –35 dBV/Pa |
Power SensitivityIn old literature dealing with dynamic microphones, we can find microphone power sensitivity specifications that were developed during the early days of radio broadcasting when the matched impedance concept was common. According to this concept, the microphone was loaded with an impedance equal to the internal impedance of the microphone. Later on, the idea of voltage matching was adopted and this concept is retained for all microphone types. That is, nowadays, the impedance of any preamplifier is usually at least ten times higher than the microphone internal impedance. Therefore, the concept of the microphone power sensitivity is only of historical interest and is not discussed here.
[size=0.7]A Shure PG48 cardioid dynamic vocal microphone, its frequency response curve and polar pattern
Frequency ResponseA frequency response diagram shows the microphone sensitivity over the 20 Hz to 20 kHz range of frequencies, which is the range of human hearing, and often with different distances between the sound source and the microphone. This diagram is obtained by testing the microphone in an acoustic anechoic chamber, which is designed to completely absorb reflections of sound. A microphone being tested is installed in front of the calibrated loudspeaker playing a pink noise, in which the power of a signal falls off at 3 dB per each octave. The output signal from the microphone is analyzed and a frequency response curve is charted. The horizontal axis in the microphone frequency response indicated frequency on a logarithmic scale and the vertical axis represents the relative output level in decibels.
[size=0.7]
Pink noise
Directionality (Polar Pattern)A microphone’s directionality indicates its sensitivity to the sound arriving at different angles relative to its central axis. It is also called a polar pattern because it is represented in the polar coordinate system in which each point on a plane is determined by a distance from a reference point and an angle from a reference direction. The most common types of directionality are omnidirectional, subcardioid, cardioid, hypercardioid, and supercardioid. There are also bidirectional microphones with the polar pattern in the form of a three-dimensional figure of eight.
[size=0.7]A Behringer XM8500 dynamic cardioid vocal microphone and its frequency response curve
Internal (Output) ImpedanceInternal impedance is an electrical characteristic that describes the electrical resistance of the magnet coil or diaphragm in the case of ribbon microphones or the output resistance of the amplifier circuitry in the case of a condenser microphone. It varies over a wide range of various types of microphones, from about 1 ohm for a ribbon microphone to many megohms for a condenser microphone, though a condenser microphone usually has an internal preamplifier with its own output impedance that is much lower than the impedance of the microphone itself.
Until mid-fifties, sound engineers matched impedances of microphones and amplifiers. However, nobody matches the impedance of analog audio signals anymore because typically the output impedance of a microphone itself or its preamplifier is relatively low whilst the input impedance of an audio mixer or a power amplifier is relatively (usually more than ten times) high.
[size=0.7]A Boya BY-PVM1000L condenser shotgun microphone, its frequency response curve and polar pattern; note that the polar pattern shows more directivity because of hollow interference tube installed in front of the capsule
Thermal Noise and Equivalent Noise LevelWe can hear a low-level hissing noise from microphones and amplifiers, which is the thermal or Johnson–Nyquist noise resulting from the Brownian motion of ionized molecules within a resistance. This noise is fundamental and cannot be eliminated. Present day microphones have impedance 150 to 300 ohms and this resistance will generate noise even in the absence of any sound. Amplifiers, to which microphones are connected, also generate noise from semiconductor devices and resistors. This noise is also unavoidable and cannot be eliminated. A low noise level is especially useful when working with very quiet sound levels because the sound can “drown” in the unavoidable microphone noise.
The noise value is usually given in the microphone specifications as a signal to noise ratio in dB or as a self-noise also known as the equivalent noise level. For example, the self-noise of an iSK BM-800 condenser microphone is 16 dBA. Here dBA or dB(A) (decibel A-weighted, absolute amplitude ratio) is the sound pressure measured with the weighing filter A referenced to the sound pressure 20 μPa (human auditory threshold). The A-filter is used for measuring noise with very low amplitude and for filtering out low-frequency noise. When using this scale, good results are usually below 15 dBA. There is another noise-measuring scale, which uses a different weighting. For this scale, good (very low) noise result can be below 30 dB.
[size=0.7]A trombone can generate sound with very high peak levels, up to 140 dB SPL; the iSK BM-800 microphone (max. input sound pressure level of 132 dB) discussed above cannot be used to record the sound of this instrument because it cannot handle this sound pressure without distortion.
Sound Pressure Level Handling CapacityDuring sound recording, it is necessary to know the maximum sound pressure level (SPL) that the microphone you are using can handle without increasing its total harmonic distortion (THD) above the reference level (usually 0.5, 1 or 3%) and of course without clipping the signal when sine waveforms become squares. 0.5% distortion can be measured, but not heard. For example, the maximum input SPL of iSK BM-800 condenser microphone is 132 dB at 1 kHz and THD less than 1%.
Dynamic RangeThe dynamic range of a microphone is defined as the interval in decibels between its maximum SPL and the A-weighed equivalent noise level of the microphone (noise floor). For example, the dynamic range of iSK BM-800 condenser microphone can be calculated as 132 dB – 16 dB = 116 dB. It should be noted that many microphone manufacturers ignore this rating.
Proximity EffectEvery directional microphone has the proximity effect, which is an emphasis of lower frequencies when a source of the sound is moving closer to the microphone. The proximity effect is absent in omnidirectional microphones and is very pronounced in cardioid dynamic vocal microphones where the bass boost can be up to 16 dB and even more when the vocalist is touching the microphone with their lips. The proximity effect is usually shown in the frequency response curves of microphones. Radio broadcasters often use the proximity effect adding a sense of depth to their voice. At the same time, the proximity effect can compromise intelligibility of speech.
[size=0.7]Radio broadcasters often use the proximity effect adding a sense of depth to their voice
Harmonic DistortionIn acoustics, the total harmonic distortion (THD) of a signal is defined as the ratio of the sum of powers of all harmonic components to the power of the fundamental frequency and is a characteristic of the linearity of the audio system. This measurement is usually presented as a percentage. If the total harmonic distortion is small, the components of the acoustic system (a microphone, preamplifier, mixer, amplifier, and loudspeaker) produce a more accurate reproduction of sound. To calibrate a microphone, a pure sine sound is emitted by a test loudspeaker and the microphone response is analyzed for the presence of the first five harmonics of the fundamental frequency.
[size=0.7]Most common types of microphone connectors; 1 — male and female 3-pin XLR connectors, 2 — 6.35 mm stereo connector, and 3 — 3.5 mm stereo connector
Type of the Microphone ConnectorConsumer microphones usually use the ¼” (6.35 mm), 3.5 mm or 2.5 mm stereo or mono phone connector. The most common connector used on professional microphones is the 3-pin XLR connector for transferring balanced audio signal. Other types of connectors are used for specific applications, for example on amateur radio equipment and communication equipment.
The connector that deserves special mention is the three-pin XLR connector used for balanced audio interconnection through a shielded twisted-pair cable. The great majority of professional microphones use it. The balanced lines allow using of long cables because they reduce susceptibility to external electromagnetic interference. The cable carrying the audio signal has two wires — one wire is in-phase with respect to the microphone signal (pin 2) and another wire carries anti-phase signal, which is exactly opposite in polarity to the in-phase signal (pin 3). The two wires are connected to the input of a differential amplifier that amplifies the difference in voltage between the two lines and suppresses the noise that is identical on both wires. Twisting of wires in the microphone cable reduces electromagnetic interference caused by electromagnetic induction. The third wire is the cable shield and is connected to the ground and pin 1 of the XLR connector.
ConclusionWe hope that after reading this article you will be able to read the specifications of microphones, compare them and select the microphone you need. Keep in mind, however, that the specifications provide only objective information of microphone electro-acoustic performance and they cannot give you the total picture of how the microphone will sound. They cannot tell the whole story of a microphone quality. They will definitely not tell you about, for example, the quality of soldering components on the preamplifier board or the quality of a diaphragm or the microphone capsule.
And what about the price? Just remember that the established microphone manufacturers use exactly the same snob appeal technique as cosmetic and fashion industries. “Buy Neumann and distinguish yourself from amateur productions!”
References:
В. М. Сапожков. Акустика. М. — «Книга по требованию»
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发表于 2018-8-3 13:59:29
[size=0.7]The surface mounted (SMD) MEMS microphone S298 in an iPhone 4S headset. Note that the microphone grate is for show only. Just to show the users that this is a microphone. There is no hole in the plastic under the grate! The acoustic port hole of the SMD case is aligned with the hole in the printed circuit board (PCB). 1 — headset; 2 — MEMS microphone (enlarged); 3 —microphone grate (enlarged); 4 — microphone port hole in the printed circuit board (PCB) covered with a piece of fine metal mesh; 5 and 7 — volume buttons; 6 — central button; 8 —microphone grate (not used); 9 — MEMS microphone
[size=0.7]First microphones were used in telephones and radio transmitters
[size=0.7]The proximity effect is very pronounced in cardioid dynamic vocal microphones when the vocalist is almost touching the microphone with their lips