Microphones: Noise 1
In the sixth installment of his on-going series about microphones, Greg Simmons begins an exploration of noise and how it affects your choice of microphones and preamps...
In the previous installment we started an in-depth look at microphone specifications, beginning with Sensitivity. We discussed the importance of choosing a microphone with appropriate Sensitivity to place the signal within the preamplifier’s ‘Goldilocks Zone’, where the signal level coming from the microphone is high enough to avoid requiring too much gain (which would make any noise more apparent), but low enough to avoid risking distortion.
Sensitivity is easy to understand and simple to measure, as explained in the previous installment. It tells us how much signal (measured as a voltage) will come out of the microphone for a known amount of sound (94dB SPL, which is 1 Pascal) at the diaphragm. Despite its relative simplicity, there are two units of measurement used to specify Sensitivity: mV/Pa and dBV. The previous installment showed how to convert between mV/Pa and dBV so that meaningful Sensitivity comparisons can be made between different microphones. Other specifications are not so easy to measure or compare, as we’re about to see…
Anyone who has spent time trying to decipher and compare the varying specifications given by different microphone manufacturers might be surprised to learn that there is an in-depth International Standard that’s been in place since the 1960s, published by the IEC (International Electrotechnical Commission) and regularly updated by the AES (Audio Engineering Society). This International Standard says precisely how every microphone specification should be measured and the units those measurements should be presented in – which should make objective comparisons simple and straightforward. It’s all contained in a document titled IEC60268-4, with the latest version being updated in 2018. The following is how the IEC’s on-line store describes it:
“IEC 60268-4:2018 specifies methods of measurement for the electrical impedance, sensitivity, directional response pattern, dynamic range and external influences of sound system microphones, and also details the characteristics to be specified by the manufacturer. It applies to sound system microphones for all applications for speech and music. It does not apply to measurement microphones, but it does apply to each audio channel of microphones having more than one channel, for example for stereo or similar use. It is also applicable to flush-mounted microphones and to the analogue characteristics of microphones with digital audio output. For the purposes of this International Standard, a microphone includes all such devices as transformers, pre-amplifiers, or other elements that form an integral part of the microphone, up to the output terminals specified by the manufacturer.”
Despite the existence of a defined set of standards published by a major industry standards organisation that is regularly updated by people who know what they’re talking about, and readily available on-line for a few hundred Euros, many manufacturers deviate from those standards – thereby creating their own ‘standards’ and justifying their deviations with claims that they produce figures that are ‘more realistic’ or ‘more informative’, while conveniently sidestepping the facts that a) every deviation just so happens to make their product look better than it would if measured in accordance with the International Standard, and b) every deviation makes it harder, if not impossible, to compare their microphones against others to expose shortcomings or inadequacies – thereby defeating the purpose of standards in the first place. As the old saying goes, “the greatest thing about standards is that there are so many to choose from…”
In this and the following installment we’re going to briefly lift the lid on the biggest can of worms in audio electronics: noise. We’re going to look at what causes noise in microphones and preamplifiers, and explore ways of minimising it in our captured sounds. Along the way we’ll look at how noise should be specified, why it often isn’t specified how it should be, and when you can and cannot make valid noise comparisons. We’ll see how noise is interconnected with microphone Sensitivity, microphone Impedance and preamplifier gain, and we’ll see how we can minimise noise by choosing the right microphone and preamplifier for the job – landing the microphone’s output signal within the preamplifier’s Goldilocks Zone to avoid excessive noise.
SIGNAL OR NOISE?
Before going any further we need to understand the fundamental difference between ‘signal’ and ‘noise’ from a microphone’s point of view, and how it applies to the electrical signals that pass through our audio equipment.
A signal is correlated energy. For example, a note from a musical instrument passing through the air results in many air particles moving together in the same direction at the same time. When those air particles connect with the surface of a microphone’s diaphragm or ribbon element they all cause it to move in the same direction because they are all individually moving in the same direction. Their individual movements are correlated – they all reinforce each other – and from those correlated movements the microphone creates the electrical ‘signal’.
Noise is uncorrelated energy. To continue with the microphone example given above, one form of noise is due to air particles that are moving randomly with no particular order among them – otherwise known as ‘Brownian motion’. When those air particles connect with the surface of a microphone’s diaphragm or ribbon element some will try to move it in one direction and some will try to move it in the other direction because their individual movements are uncorrelated – they do not all reinforce each other. They create what is referred to as ‘Brownian Noise’ because it is due to the Brownian motion of air particles.
Noise is also caused by the random and uncorrelated motion of free electrons within an electrical component or circuit due to thermal agitation, causing noise currents that result in noise voltages. This is known as ‘Thermal Noise’ because it is related to temperature.
For the purposes of this installment we’ll define a signal as being ‘correlated energy’ and noise as being ‘uncorrelated energy’. As shown above, there are two sources of noise that affect our audio signals: Brownian Noise and Thermal Noise. They are ultimately represented in two of the specifications we’ll be exploring in this installment: the microphone’s ‘Self Noise’ (also known as its ‘Equivalent Noise Level’) and the preamplifier’s ‘Equivalent Input Noise’ (usually abbreviated to ‘EIN’). As we’ll soon see, if you’re capturing a sound with a microphone then there’s going to be noise; if the level of that noise is high enough to interfere with the audibility of the signal, then it’s a problem. The relative difference between the level of the signal and the level of the noise is represented in the specification known as ‘Signal To Noise Ratio’; the higher the value, the less noise. We’ll return to those noise specifications after looking closer at the sources of noise itself.
TYPES OF NOISE
It’s common to describe different types of noise based on how their audible spectrum parallels the visible spectrum (i.e. light and colour), giving us descriptions like ‘white noise’, ‘pink noise’, ‘red noise’ and so on. Each of these noise colours has a definition that describes its ‘Power Spectral Density’ (PSD), which basically tells us how bright or dull that type of noise is.
You can hear numerous noise colours on this Wikipedia page: https://en.wikipedia.org/wiki/Colors_of_noise
As mentioned earlier, Brownian Noise is caused by air particles randomly impacting with, or rubbing against, the diaphragm. Brownian Noise is not very significant; in fact, microphone and preamplifier designers become very excited when they encounter a product designed for real world applications that allows them to hear the Brownian Noise on a microphone’s diaphragm. Nonetheless, any discussion of noise in microphones would not be complete without mentioning it.
Although it is sometimes referred to as ‘brown noise’, Brownian Noise does not take its name from any similarities with the light/colour spectrum – it was named after botanist Robert Brown, who identified the random motion of air particles while studying the movement of pollen from one flower to another. If compared to the colour spectrum, Brownian Noise is closest to ‘red noise’.
What does Brownian Noise sound like? You can hear it on the Wikipedia page linked above. It contains considerably less high frequency energy than the noise we usually call ‘hiss’ (which is more like the white noise used in synthesisers) and even less high frequency energy than pink noise (used in synthesisers and spectrum analysers); its sound has been likened to heavy rain, which is also a good physical analogy for air particles on a diaphragm. Your chances of hearing Brownian Noise within a microphone signal are very remote because, in terms of level and frequency content, it’s easily masked by Thermal Noise (see below).
One interesting aspect of Brownian Noise is that it’s less significant in larger diaphragms than smaller diaphragms. Why is that? If we double the surface area of the diaphragm it will capture twice as much correlated sound energy (signal) and twice as much uncorrelated sound energy (Brownian Noise). The correlated sound energy adds completely, causing a 6dB increase in signal level (+6dB = x 2). This doesn’t happen with the uncorrelated sound energy; some of it adds and some of it subtracts, resulting in a 3dB increase in the level of Brownian Noise (+3dB = x √2). So, doubling the diaphragm’s surface area results in 6dB more signal but only 3dB more Brownian Noise. If we matched the diaphragm’s signal levels before and after doubling its surface area, we’d find that the signal from the larger diaphragm will be accompanied by 3dB less Brownian Noise than the signal from the smaller diaphragm.
If that larger diaphragm is being used in a condenser microphone it will be providing 6dB more signal level into its internal electronics than the smaller diaphragm on the same sound source, but with only 3dB more Brownian Noise. This provides a higher and relatively quieter signal into the condenser microphone’s impedance converter circuit, ultimately allowing an overall higher output signal with less noise.
As we’ll soon see, if you’re capturing a sound with a microphone then there’s going to be noise…
Also known as Johnson Noise, Nyquist Noise or Johnson-Nyquist Noise, this is the ‘hiss’ that is found in all audio circuits. It’s due to the effect of temperature on the circuit’s internal electronic components, hence ‘thermal’ noise. It sounds very similar to white noise.
The amount of Thermal Noise generated by any given circuit component is dependent on its impedance (see below) and the temperature – the higher the impedance and/or the higher the temperature, the higher the Thermal Noise. A complex audio circuit contains more individual components than a simple audio circuit and is therefore likely to have more Thermal Noise at its output, although that depends on how those circuit components are interconnected and if amplification is involved.
If we know the impedance, the temperature and the required bandwidth (typically 20kHz for audio work), we can calculate the Thermal Noise as a voltage and convert it to a dBu value – which is how noise voltages are often expressed. You’ll find all the mathematics for making those calculations and conversions at the end of this installment (see ‘Noise Maths’). For perspective, the Thermal Noise created by an impedance of 150 ohms at a temperature of 20°C and within a bandwidth of 20kHz is -130.9dBu; we’ll be seeing that figure again soon.
NOISE IN PASSIVE MICROPHONES
As we saw in the second installment of this series, a passive ribbon microphone is a simple circuit that consists of the ribbon element and a transformer. In the third installment of this series we saw that the passive dynamic microphone is also a simple circuit, consisting of a coil of wire attached to the diaphragm and then usually coupled to the output via a transformer. Both microphone designs have relatively low impedances and therefore both have relatively low Thermal Noise.
For example, Shure’s SM57 has an impedance of 310 ohms at 1kHz. As calculated in ‘Noise Maths’ (at the end of the next installment), its Thermal Noise is -127.8dBu. Electro-Voice specifies the impedance of the RE20 dynamic microphone as being 150 ohms, giving it a Thermal Noise of -130.9dBu (the same value we calculated earlier for 150 ohms, of course). Royer Labs specify the impedance of the R121 ribbon as being 300 ohms at 1kHz ‘nominal’, which gives it a Thermal Noise of -127.9dBu, and AEA quote the impedance of the R44C ribbon microphone as being 270 ohms ‘broadband’ which gives it a Thermal Noise of -128.4dBu. (We’ll discuss the meaning of those qualifying words ‘nominal’ and ‘broadband’ later in this installment when we look at Impedance, because they help us understand how the values were determined and whether or not we can make valid comparisons between them.)
Despite their measurement differences these are all typical low noise levels for passive dynamic and passive ribbon microphones, and much lower than the noise levels from condenser microphones. They’re so low that most manufacturers of passive ribbon and passive dynamic microphones don’t even publish them. In any application that requires so much gain that it would make the passive microphone’s Thermal Noise apparent, the preamplifier’s Thermal Noise will be so high in comparison that the mic’s Thermal Noise becomes insignificant. We’ll look at this phenomenon in the next installment when we explore EIN.
NOISE IN CONDENSER MICROPHONES
As we saw in the fourth installment of this series the condenser microphone’s diaphragm creates a varying voltage that represents the signal, but the diaphragm has a very high impedance and is incapable of providing an output on its own. It is therefore connected to an active impedance converter circuit (built inside the microphone) that provides a low impedance output signal suitable for connecting to a preamplifier.
The impedance converter circuit is considerably more complex than the circuitry found inside passive ribbon and passive dynamic microphones, and creates significantly higher Thermal Noise. However, the diaphragm’s higher Sensitivity ultimately means the condenser microphone provides a higher signal voltage to the preamplifier and therefore requires less gain, which, in turn, results in less noise from the preamplifier. Speaking of which…
NOISE IN PREAMPLIFIERS
A microphone preamplifier’s fundamental role is to amplify the very small signal from the microphone up to a useful level for the rest of the signal path. It requires an input circuit to accept the signal from the microphone, and any Thermal Noise created by that input circuit adds to the incoming signal from the microphone and is amplified with it. The more amplification required, the more apparent the preamplifier’s noise becomes – which is one of the reasons why it is important to choose a microphone with appropriate Sensitivity and noise specifications for the signal. We’ll explore preamplifier noise in the next installment of this series.
WHICH MIC IS QUIETER?
Around the turn of the century Rupert Neve wrote a four-part series for AudioTechnology magazine called Rupert’s Word. In the second part of his series he discussed noise in microphones, saying “The dynamic microphone is, therefore, mostly dependent on the console’s microphone amplifier [i.e. preamplifier] for its noise performance.” He went on to compare a typical dynamic microphone with a typical condenser microphone and concluded that in a typical recording studio application the level of noise in each microphone’s signal was ultimately very similar after the process of preamplification. He was being kind to the dynamic microphone by comparing it to a condenser microphone with a relatively high Self Noise of 17dBA – typical of a small diaphragm condenser. Nonetheless the take-away was that the advantage of the dynamic mic’s low Thermal Noise was lost to its low Sensitivity because it required more gain from the preamplifier, which, in turn, resulted in more noise overall. And this continues to be the case…
One of the problems when measuring noise is specifying it in a way that is meaningful to the listener. Noise can contain frequencies that extend above and below the range of human hearing, which in most cases are irrelevant to the listener. Furthermore, within the range of human hearing the ear is more sensitive to some frequencies and less sensitive to others. Therefore it makes sense to apply filters so that only the noise that matters to the listener is measured. The process of filtering the noise to isolate what matters is known as ‘weighting’, and one of the most common forms of weighting is known as ‘A-weighting’.
You will often see noise specifications and similar with an upper case ‘A’ added to the end. The ‘A’ indicates that the measurement was made after passing the signal through a filtering network (i.e. a combination of different filters) that was based on an early version of the ISO standard Equal Loudness Contour for a level of 40 Phons (i.e. when the volume is calibrated so that 1kHz has an SPL of 40dB at the listening position).
Essentially, the filtering network exhibits roll-offs below and above 3kHz that approximate the frequency response of human hearing at 40 Phons – so that the measuring device ‘hears’ the noise in a similar way as humans hear it. This is known as an ‘A-weighting’ filtering network, and measurements made with it are referred to as being ‘A-weighted’. They’re typically specified as ‘dBA’, ‘dB A’ or ‘dB(A)’.
One of the problems with the A-weighting network is that it was designed for measuring ‘signal’ rather than ‘noise’, and does not provide consistently meaningful results on different types of noise within the audible range, such as analogue tape hiss or the background noise of a radio broadcast. In fact, the A-weighting network’s upper midrange roll-off tends to hide a lot of audible noise.
To overcome the A-weighting’s inconsistencies a different weighting network was developed, based on research initially started in the 1960s by the BBC and others. Formerly known as CCIR 468 and still referred to as ‘CCIR’ by many microphone manufacturers, the latest version is correctly referred to as ‘ITU-R 468’ after the ITU (International Telecommunications Union) took it over from the CCIR (Consultative Committee on International Radio). The ITU-R 468 weighting network provides consistently meaningful results with different forms of noise, primarily by allowing more of the noise content above 1kHz into the measurement process. Microphone noise specifications quoted as ‘CCIR’ or ‘ITU’ are typically around 11dB higher than the same specifications quoted for A-weighting, so be sure to check which figures you’re using when comparing the noise specifications of different microphones.
The weighting networks described above are reassuring with their well-defined curves, but the results given by any manufacturer don’t take into account the effects of any other filtering used in the test process. For example, all digital test equipment requires a low pass filter on its analogue inputs to prevent aliasing (as with any analogue to digital conversion), and that means frequencies above the filter’s cut-off point are eliminated from the signal before it is measured.
One popular audio test device uses ‘brickwall’ filters (i.e. filters with slopes that are essentially vertical) to restrict the bandwidth of the tested signal from 22Hz to 22kHz. A signal that was rich in noise below 22Hz or above 22kHz could measure a slightly lower noise value on this device compared to an old-school analogue measuring device that allows a wider bandwidth before applying the weighting filters. The difference in measured results might only be fractions of a decibel and it’s tempting to dismiss them as insignificant, but when we consider that many of the noise values that are worth comparing exist in a relatively small 5dB ‘window’ between -127dBu and -132dBu, and will typically be subjected to 20dB (i.e. x 10) or more of gain, those fractions of a dB become significant.
What does this tells us about weighting, and specifications in general? Assuming a manufacturer is consistent with their measurement tools and techniques, their specifications should allow valid comparisons to be made within their own product range. When comparing specifications between different manufacturers there is always likely to be some inconsistencies unless they’re using the same test equipment and procedures. If they’re using different filters and weighting networks then the chances of making meaningful comparisons are blowing in the wind.
There are two noise specifications to consider when choosing the right microphone to capture a sound: the microphone’s ‘Self Noise’ (for condenser microphones) or ‘Impedance’ (for passive microphones), and the preamplifier’s ‘EIN’ (which stands for ‘Equivalent Input Noise’). Along with the microphone’s Sensitivity (discussed in the previous installment of this series), these specifications allow us to determine the total amount of electrical noise in the captured and amplified signal – before any EQ, compression, AD conversion or other processing is done to it. A third specification called ‘Signal to Noise Ratio’ is sometimes provided for condenser microphones but it’s really just another way of describing Self Noise, as we’ll see in the next installment…
This specification is also referred to as ‘Equivalent Noise Level’ and sometimes as ‘Equivalent Self Noise Level’, but we’re going to stick with the simpler term ‘Self Noise’ to avoid confusion with the preamplifier’s ‘Equivalent Input Noise’ specification, which is discussed in the next installment of this series.
A condenser microphone’s Self Noise specification represents the total noise created by the microphone itself, appearing at the microphone’s output and embedded into the signal. It’s the combination of the Brownian Noise from the microphone’s diaphragm and the Thermal Noise from the microphone’s internal circuitry, although the latter is far more significant. Self Noise is often quoted as A-weighted, CCIR, or both. Neumann uses both figures; according to their website, their U87Ai in cardioid mode has a Self Noise of 12dBA and 23dB CCIR, while in omni mode those figures increase by 3dB to 15dBA and 26dB CCIR. Sennheiser also quote both; their MKH8040 cardioid has a Self Noise of 13dBA and 22dB CCIR, while its omni counterpart the MKH8020 offers 10dBA and 21dB CCIR. Note the significant differences between the two weightings, and be sure not to mix them up when making comparisons.
To put these figures into perspective, a Self Noise rating of 10dBA is equivalent to having a speaker producing 10dBA of noise at the microphone’s diaphragm – hence the alternative term ‘Equivalent Noise Level’. For perspective, a Self Noise of 10dBA is very low. It’s considerably less than the background noise level of most recording studios and performance spaces where microphones are used, and can therefore be considered quiet enough for most recording applications. However, we must remember that Self Noise is broad bandwidth noise that extends throughout the range of human hearing and can therefore be more obvious than the background noise of a studio or performance space – especially when capturing sounds that do not contain much upper midrange energy to mask (i.e. hide) the noise at frequencies where the ear is most sensitive. People who record very quiet sound sources, such as nature sounds for documentaries, might prefer a microphone with even lower Self Noise than 10dBA. For those situations they’ll choose a microphone with low Self Noise and high Sensitivity so it provides a higher signal level to the preamplifier, thereby requiring less overall gain and resulting in less noise from the preamplifier as well. The table below shows a selection of microphones with low Self Noise and high Sensitivity, making them all good choices for capturing very quiet sounds and/or making very quiet recordings.
Your chances of hearing Brownian Noise within a microphone signal are very remote…
Each of these microphones offers very low Self Noise to begin with, while their high Sensitivity means they do not require as much gain from the preamplifier – thereby minimising noise even further. It’s also worth noting that all of these microphones are condensers, and, with the exception of Sennheiser’s MKH800, all are large diaphragms and therefore deliver a higher signal level to their internal electronics than a smaller diaphragm would, making the Thermal Noise less significant. With that in mind, let’s compare the Self Noise and Sensitivity specifications of the large diaphragm condensers listed above with some typical small diaphragm condensers from the same manufacturers.
With the exception of Sennheiser’s MKH8020, all of the small diaphragm condensers listed below have considerably higher Self Noise than the larger diaphragm condensers listed above. In addition, they all have considerably lower Sensitivity than the larger diaphragm condensers listed above, meaning they will require more gain from the preamplifier – and that can result in making the microphone’s Self Noise and the preamplifier’s noise more apparent. [Sennheiser’s MKH series are the exceptions in these two lists because they use RF technology, as explained in the fourth installment of this series; among other things, RF technology allows a small diaphragm microphone to achieve Sensitivity and Self Noise figures comparable to large diaphragms.]
Now let’s look at some small diaphragm condenser microphones that offer similar Self Noise figures as the small diaphragm condensers listed above, but at much smaller physical sizes. They are all electret condensers and, interestingly, offer Self Noise figures that are typical for small diaphragm condensers but Sensitivity figures closer to those found with large diaphragm condensers. It should be noted that their higher Sensitivity is achieved by the electronic circuitry built into the microphone, rather than the diaphragm capturing more sound energy from the air. In essence, they provide more gain from the microphone’s internal circuitry (i.e. its impedance converter) so that less gain is required from the preamplifier. The higher Sensitivity minimises the contribution of noise from the preamplifier, as it does with large diaphragm condensers, but their combination of high Sensitivity and high Self Noise figures make the noise from most decent preamplifiers irrelevant anyway.
The three microphones listed above are popular choices with nature recordists, field recordists and sound effects recordists who need to carry their recording equipment to remote locations, and their small size makes them appealing to people who need to hide microphones for film, video and theatrical applications. Of the three, DPA’s 4060 would be considered too noisy for making quiet recordings but it can capture much louder sounds than the other two (as explored in the next installment), and in those applications the microphone’s Self Noise isn’t a problem. It’s common for people who need very small microphones to have a pair of LOM Usi Pros or Clippy EM272s (they’re very similar), along with a pair of DPA 4060s, and choose whichever is most appropriate for the application.
In electronics there is a parameter known as ‘resistance’, which represents a component’s or circuit’s opposition to the flow of an electrical current. It is represented by the letter ‘R’, measured in Ohms and symbolised with ‘R’ or the Greek letter ‘Ω’.
‘Impedance’ (represented by the letter ‘Z’, measured in Ohms and symbolised with ‘R’ or ‘Ω’) is used to refer to any resistance that varies with frequency due to having reactive components such as inductance (e.g. a coil of wire, as used in a dynamic microphone or a transformer) or capacitance (e.g. as used in the diaphragm of a condenser microphone, or as an electronic component in EQ and filter circuits). Impedance is often quoted in the input and output specifications for audio equipment. Because it is frequency-dependent, every Impedance value should be accompanied by the frequency it was measured or calculated at. For audio applications, if no frequency is specified it is assumed to be 1kHz (in ‘old school’ parlance 1kHz was known as ‘mid frequency’). In this installment we’re focusing on noise, so we’ll start by looking at how the Impedance specification relates to the noise of passive dynamic microphones and passive ribbon microphones.
Earlier in this installment we explored the concept of Thermal Noise and saw that Impedance was a contributing factor. We saw that the calculated Thermal Noise of Shure’s SM57 microphone, using an Impedance of 310 ohms (which is its Impedance at 1kHz), was -127.8dBu.
The graph above is the Impedance curve for Shure’s SM57, and show’s how its impedance changes with frequency. (The shape of the curve is typical of many dynamic microphones but the actual values will vary.) It starts at 20Hz with an Impedance of 295 ohms, rises to almost 550 ohms at around 150Hz, drops to just over 300 ohms between 1kHz and 2kHz, then rises to over 800 ohms at 20kHz. If the Thermal Noise specification is dependent on Impedance, and Impedance is dependent on frequency, how much would the SM57’s specified Thermal Noise value change if the Impedance at a different frequency was used? The following list shows the calculated Thermal Noise values using some extreme points on the curve:
The maximum difference between them is 4.39dB. Which value should we use? The most impressive value occurs at 20Hz, but let’s not forget that it’s the same noise. It’s not getting any louder or softer; we’re just calculating different values to represent it by using different points on the microphone’s impedance curve.
Shure quotes the SM57’s 310 ohm impedance as being ‘actual’, and this is shown on the graph to be the impedance at 1kHz. Recognising that impedance changes with frequency, some manufacturers prefer to use terms like ‘broadband’, ‘nominal’ and similar. It’s unclear what those terms actually mean but ‘broadband’ implies some kind of averaged value throughout the microphone’s bandwidth, while in loudspeaker design the term ‘nominal’ is used to refer to the lowest Impedance of a speaker and that approach is perhaps being appropriated for microphone Impedance as well – after all, lower Impedance values provide more impressive looking specifications.
Many dynamic microphone manufacturers habitually quote an Impedance of 150 ohms, even when the mic’s impedance curve shows that it never gets down that low – as shown in this snapshot from the spec sheet for a popular dynamic microphone that’s impedance curve never falls below 295 ohms.
Quoting 150 ohms for a microphone that’s Impedance value never gets lower than 295 ohms (essentially twice the quoted value) is something that Penn and Teller would’ve called “bullshit”. Notice that the better looking but non-existent specification of 150 ohms is listed first, while the actual figure follows it in brackets like an apologetic afterthought for those capable of interpreting specs and/or measuring it themselves. This type of behaviour is very deceiving for anyone trying to calculate Thermal Noise. It’s also deceiving when choosing preamplifiers because their noise specification (known as ‘EIN’) is dependent on the microphone’s output impedance – as we’ll see in the next installment…
Furthermore, knowing a passive microphone’s output Impedance allows us to determine the minimum input impedance required from the preamplifier to avoid affecting the microphone’s tonality (as discussed in the second installment of this series about ribbon microphones). The rule-of-thumb is that a preamplifier’s input impedance should be at least five times higher than the microphone’s output Impedance; it’s generally referred to as ‘the 5:1 rule’. If the microphone mentioned above really did have an Impedance of 150 ohms, as stated in the specifications, we could mistakenly assume that a preamplifier with an input impedance of 750 ohms (i.e. 5 x 150) would be sufficient, which is not the case. To stick to the 5:1 rule, we should be using a preamplifier with an input impedance of at least 1,475 ohms (i.e. 5 x 295) to ensure it doesn’t affect the tonality of the microphone. It’s a safe bet that this particular microphone is going to sound different – and most likely worse – when connected to a preamplifier with an input impedance of 750 ohms (5 x its stated Impedance) than it will when connected to a preamp with an input impedance of 1,475 ohms (5 x its lowest actual Impedance). Most contemporary preamplifiers offer at least 1500 ohms input impedance so it’s an academic point for this example, but it raises the question, “What other specifications are microphone manufacturers willingly misrepresenting, and why?”
Some vintage preamplifiers have very low input impedances (typically between 300 ohms and 600 ohms) based on an older electrical design concept known as ‘maximum power transfer’, where the goal was to give the preamplifier the same input impedance as the microphone’s output impedance. Some contemporary preamplifiers offer switchable input impedance values so that the user can intentionally alter the tonality of passive microphones.
It’s also worth noting that the condenser microphone’s active internal circuitry gives it a more stable output impedance that is also considerably lower than most dynamic and ribbon microphones, meaning their tonality is rarely affected by the preamplifier’s input impedance. We’ll be taking a closer look at that in the next installment of this series as we continue our exploration of noise…
Special thanks to Steve Dove and Terry Demol for their invaluable insights, clarifications and patience when putting together this and the following installment related to noise in microphones and preamplifiers.