When judging a microphone’s frequency response, we often use the terms “flat” or “coloured.” These general adjectives give us a broad understanding of how a microphone’s outputted audio signal will sound relative to the sound source it is capturing.
What is a flat frequency response? A purely flat microphone frequency response means the microphone will be equally sensitive to all audible frequencies of sound (20 Hz – 20,000 Hz) along its primary axis (where the microphone “points”). Flat mics sound very natural since they do not accentuate any particular frequencies over any others.
What is a coloured frequency response? A coloured microphone frequency response means the microphone will be more sensitive to some audible frequencies than it will be to others. Coloured microphones accentuate some frequencies and can be thought of as being “EQed” to suit some sound sources better than others.
In this article, we’ll take a deeper look into what makes a microphone “flat” or “coloured” and look at examples of microphones with flat and coloured frequency responses.
Flat Versus Coloured Frequency Response
Before getting into a deeper conversation about flat and coloured microphone frequency responses, let’s have a look at the main differences between the two:
|Flat Frequency Response||Coloured Frequency Response|
|Natural sound||More characteristic sound|
|Often requires extra tuning and capsule design||Microphones are naturally coloured|
|Great for nature, ambient, and room recordings||Often specialized for specific sound sources|
|More common in condenser microphones||More common in dynamic and ribbon microphones|
The term “flat” comes from the way the frequency response is drawn in the mic’s frequency response graph. These graphs have relative sensitivity (in dB) on the Y-axis and frequency (in Hz) on the X-axis. Therefore, a “flat” frequency response resembles a flat horizontal line on the graph. Note that “flat” mics have nothing to do with sounding flat in pitch or in timbre.
The term “coloured” comes for the microphone “colouring” the sound of the intended sound source. These responses show up as wavy or otherwise non-flat on their graphs.
To learn more about microphone frequency response, please check out my in-depth article Complete Guide To Microphone Frequency Response (With Mic Examples).
A Quick Note On EQ And Frequency Response
As mentioned in the opening paragraphs of this article, coloured microphones may sound as if they are permanently EQed, if we think of the mic signal in the context of an audio mix.
In other words, coloured microphones may be preferred on certain sound sources that would benefit from the cuts and boosts the microphone’s frequency response offers.
For example, a boost in the presence range (3-6 kHz) helps to accentuate speech intelligibility and can help to push vocals forward in an audio mix.
This presence boost can be found in [coloured] vocal mics like the Shure SM58, which we’ll discuss later in this article in the section “Coloured Frequency Response Microphone Examples.”
Flat Frequency Response
A flat frequency response, in a perfect world, means that a microphone is equally sensitive to all audible frequencies.
Of course, the real world is not perfect and so a flat frequency response really means a microphone is approximately sensitive to all audible frequencies equally.
That is to say, flat microphones may have slight variations in sensitivity along their frequency response ranges.
Many microphones exhibit a roll-off in their low-end frequency response. These mics may be considered as having flat frequency responses so long as their responses flatten out at a reasonable setpoint (ie: 100 Hz or less).
Though it’s extremely difficult to generalize, there are some microphone types that often have flat frequency responses. They include:
- Small-diaphragm condenser microphones.
- Large-diaphragm condenser microphones.
- Measurement/calibration microphones (which are most-often small-diaphragm condensers).
Small-diaphragm condenser microphones and often “flatter” than large-diaphragm condensers due to fewer resonant cancellations at high frequencies (shorter wavelengths) of sound.
To learn more about the differences between small and large-diaphragm condenser microphones, check out my article Large-Diaphragm Vs. Small-Diaphragm Condenser Microphones.
Achieving A Flat Frequency Response
To achieve a flat frequency response, a microphone’s diaphragm and capsule must be tuned carefully.
Diaphragms and capsules naturally have resonant frequencies. These components are more sensitive to certain frequencies (known as resonant frequencies) and will move more easily when subjected to sounds at their resonant frequencies.
To learn more about microphone capsules, check out my article What Is A Microphone Capsule? (Plus Top 3 Most Popular Capsules).
Therefore, the audible resonant frequencies will cause more movement in the mic diaphragm and will be overrepresented in the mic signal. Thus, resonant frequencies cause naturally colouration to the mic frequency response and so the mic capsule, as a whole, must be tuned and designed to effectively counteract these natural resonances.
This is done through proper diaphragm tensioning and acoustic labyrinths around the mic diaphragm. Proper tension helps to adjust the natural resonance frequency. Acoustic labyrinths and pockets provide counteracting standing wave frequencies around the diaphragm that help to balance the frequency response.
Another important factor in achieving a flat frequency response is that the diaphragm itself is equally moved by all sound wave frequencies. Moving-coil dynamic microphone diaphragms, for example, have relatively heavy coils attached to them that make it difficult for high-frequency sounds to move them. Therefore it’s largely impractical to make a dynamic mic “flat.”
As we can imagine, it takes some special designing to achieve a truly flat frequency response.
The Reality Of Flat Frequency Responses
It’s worth noting here, once again, that a microphone frequency response is measured on the microphone’s primary axis.
Depending on the polar pattern, a microphone will have varying sensitivities to sounds coming from angles. These polar patterns, however, are frequency dependent. This essentially means that even a flat microphone will likely sound coloured if a sound source is off-axis.
Note that microphones naturally tend to be more directional at higher frequencies.
To learn everything you need to know about microphone polar patterns and directionality, check out my article The Complete Guide To Microphone Polar Patterns.
It’s important to consider that microphones are rarely, if ever, perfectly flat. Perfectly flat would mean that the microphone is equally sensitive to all frequencies between 20 Hz – 20,000 Hz.
This is incredibly difficult to achieve in the real world and to further the difficulty, sound sources are not always pinpointed on a mic’s primary axis (sound sources often produce sound from multiple points or from an area rather than a point).
So a flat frequency response is somewhat subjective in that it refers to a nearly flat frequency response.
To really understand flat frequency responses, we should look at real-world examples. This brings us to our next section:
Flat Frequency Response Microphone Examples
Let’s take a look at some examples of microphones with flat frequency responses. We’ll look at two small-diaphragm condensers (one of which is a measurement mic) and one large-diaphragm condenser.
The DPA 4006A (link to check the price at B&H Photo/Video) is a small-diaphragm condenser microphone with an incredibly flat frequency response. It has an omnidirectional polar pattern.
As we can see above, the 4006A’s response is extremely flat from 20 Hz to about 8,000 Hz. This frequency range hosts all fundamental frequencies of musical instruments. The upper ranges of human hearing host many of the upper harmonics of sounds and the “brilliance” or “air” of sound sources and ambient environments.
There are two different response lines on the graph of the DPA 4006A. The dotted line (with the upper-frequency boost) shows the response when the high-boost grid is attached to the microphone while the straight line represents the stock soft-boost grid. These grids effectively alter the acoustic labyrinths around the mic’s diaphragm, which, in turn, alter the high-end response of the 4006A.
The frequency response of the 4006A shows a steep high-end roll-off. However, if we look closely, we’ll see that this roll-off happens after 20,000 Hz which is the upper limit of human hearing and the cutoff at which we stop measuring frequency response.
DPA actually goes beyond what is expected and provides us with this above-20 kHz information. As we’ll see in the following examples, most microphone manufacturers only show the necessary 20 Hz – 20 kHz in their frequency response graphs.
The Earthworks M50 (link to check the price on Amazon) is a small-diaphragm condenser measurement microphone. Naturally, this microphone has a very flat frequency response, extending even further than the audible frequency spectrum (from 3 Hz all the way to 50 kHz). It has an omnidirectional polar pattern.
Measurement microphones are often used to accurately capture frequencies in and outside of the audible spectrum. In the case of the Earthworks M50, the frequency response ranges from 5 Hz (infrasound) to 50,000 Hz (ultrasound).
As we see from the graph, the response is beautifully flat in the audible range of 20 Hz – 20,000 Hz. There is a slight decrease in sensitivity below 20 Hz that rolls off to about -1 dB at 5 Hz. In the +10 kHz range, there are slight variations in the frequency response that we should be aware of, particularly when using this microphone for measurements.
Note that the frequency response specifies that the sound source is at 12 inches from the microphone capsule. Positioning the mic at this distance with the sound source in the direction of its primary axis will yield the results stated in the frequency response graph.
AKG C 414 XLS
The AKG C 414 XLS (link to check the price on Amazon) is a multi-pattern large-diaphragm condenser microphone. Though this microphone has a “bumpier” frequency response than its small-diaphragm counterparts mentioned above, it still certainly has a flat frequency response.
It’s also worth noting that the C 414 XLS has 9 selectable polar patterns. Each of these exhibits a slightly different frequency response yet they are all considerably flat.
To end our short list of flat frequency response mic examples, I figured I should show a large-diaphragm microphone.
Large-diaphragm condensers have greater variation in their polar patterns across their frequency responses. Additionally, they are generally more coloured in terms of flatness in their on-axis frequency responses.
So when we look at the frequency response graph of the AKG C 414 XLS, we see a fairly straight horizontal line. Though there is certainly wavering in the response line, the mic can still be considered “flat” relative to many other microphones.
I wanted to show the AKG C 414 XLS to argue that a microphone’s frequency response line does not need to be absolutely horizontal for the microphone to sound accurate.
As an aside, we see 4 different lines that represent different high-pass filters in the C 414. Though we could still call this microphone fairly flat with the high-pass filters engaged, the sound of the microphone would certainly change (especially at the 160 Hz cut-off). At that point, we could definitely argue that the microphone is no longer flat.
For more information on microphone high-pass filters, check out my article What Is A Microphone High-Pass Filter And Why Use One?
With a high-pass filter at 160 Hz, the C 414 may be considered coloured, which brings us to our next section!
Coloured Frequency Response
A coloured frequency response means that a microphone effectively “colours” the sound it captures. In other words, a coloured mic accentuates some frequencies more than others and, in doing so, adds a certain character that benefits some sound sources and applications while being a detriment to others.
Though it’s tough to generalize, there are some microphone types that typically have coloured frequency responses. They include:
- Moving-coil dynamic microphones.
- Ribbon dynamic microphones.
- Some condenser microphones.
Moving-coil microphones have relatively heavy conductive coils attached to their diaphragms. This makes it difficult for high-frequency sounds to effectively move the diaphragm which results in rather sharp high-end roll-offs at low cutoff points.
Moving-coil mics also typically suffer from many resonant frequencies (in their cartridges, diaphragms, coils, magnets). Though there are many design strategies to counteract these resonances, it is often overly complicated and expensive to rid of all of them.
Ribbon microphone diaphragms have relatively low tension. Generally, this brings their resonant frequency below the audible spectrum. However, combined with the corrugation and transducer induction, these microphones nearly always portray a gentle roll-off of high-end frequencies.
Finally, some condenser microphones (particularly the inexpensive ones) also have coloured frequency responses due to their resonant frequencies and even their inner electronics.
In general, there are 4 basic deviations a frequency response graph line can take from being horizontal:
- Low-end roll-off
- High-end roll-off
Let’s talk about each of these in a bit more detail, shall we?
A low-end roll-off is essentially a reduction in sensitivity below a certain frequency setpoint.
Although some microphones benefit from a low-end roll-off to reduce rumble and noise in their signals, it also takes away from the most accurate capture possible and, therefore, “colours” the sound.
High-pass filters essentially produce a low-end roll-off at a set cut-off point and colour the sound of a microphone as well.
A high-end roll-off is a gradual or sudden reduction in sensitivity above a certain cutoff point.
As discussed earlier, moving-coil dynamic mics often have sudden high-end roll-offs while ribbon dynamic mics typically have gentle high-end roll-off that take place over a longer range of frequencies.
Resonant frequencies and standing waves within the acoustic labyrinths of microphones may cause boosts in mic sensitivity. These boosts can be acute or cover a larger range of frequencies in a mic’s response.
Conversely, the design of a microphone may cause a dip in mic sensitivity in specific frequencies or frequency ranges.
Coloured Frequency Response Microphone Examples
Let’s take a look at some examples of microphones with coloured frequency responses. We’ll look at two moving-coil dynamic mics (one large-diaphragm and one smaller-diaphragm). We’ll also discuss a ribbon microphone and a condenser mic.
Shure Beta 52A
When I think of a coloured frequency response, the first microphone that comes to mind is the Shure Beta 52A (link to check the price on Amazon). This microphone is extremely coloured and is designed with a frequency response that is tailored specifically for kick drums. It has a supercardioid polar pattern.
As we see above, the Shure Beta 52A frequency response is far from flat. In fact, it’s the most coloured microphone mentioned in this article. Where do we even begin?
Well, first, there is no real low-end cut-off. This microphone will capture sound down to 20 Hz.
However, there are 4 different frequency response lines drawn in the low-end of this graph. They each have a distance assigned to them that takes into account the microphone proximity effect.
Basically, the proximity effect states that in directional microphones (like the supercardioid Beta 52A), the closer the mic is to a sound source, the more bass response that mic will exhibit.
There is quite a dip in the lower-mid frequencies (300-400 Hz). The severity of which is largely determined by the mic’s distance to the sound source.
Around 4 kHz is a major boost in mic sensitivity.
The mic also exhibits a steep high-end roll-off at about 10 kHz. Though this sharp roll-off is somewhat typical of dynamic mics, the cut-off at 10 kHz is very low.
This somewhat extreme colouration makes the Shure Beta 52A an incredible kick drum microphone (it’s featured on My New Microphone’s Best Kick Drum Microphones) but not a great choice on much else.
The AEA R84 (link to check the price on Amazon) is a high-end ribbon microphone with a typical coloured ribbon mic frequency response. By typical, I mean to say that this ribbon mic has a gentle high-end roll-off. It has a bidirectional polar pattern.
The AEA R84 behaves like a typical ribbon in that it has a gentle high-end roll-off that is drawn out over a long range (200 Hz to 20,000 Hz).
Next, we notice that AEA actually provides two frequency response lines: one for the front of the microphone and one for the rear. This is because the microphone has a bidirectional polar pattern but is not absolutely symmetrical in its response.
We notice a slight roll-off of frequencies in the low-end but this would not be overly noticeable to the naked ear.
Overall, this frequency response a fairly jagged. However, this is more so a product of AEA’s thorough calculation of the frequency response rather than a series of noteworthy boosts and cuts.
The Shure SM58 (link to check the price on Amazon) is a legendary moving-coil dynamic microphone and has been mentioned above in this article. This microphone exhibits both low-end and high-end roll-offs and benefits greatly from a boost in the presence range. It has a cardioid polar pattern.
The Shure SM58 features both a low-end and high-end roll-off.
This mic is often used for live vocals. The low-end roll-off helps to improve gain-before-feedback by reducing low-end rumble and handling noise in the mic signal. The high-end roll-off helps to attenuate cymbal wash and high-end room frequencies.
The main selling point of the SM58, though, is the presence boost of roughly 5 dB. This boost, which is caused by the resonance in the mic design, helps to provide vocal clarity in loud and/or dense audio mixes. Speech intelligibility is in and around 4 kHz and so vocal mics (like the SM58) benefit greatly from a presence boost.
The Shure SM58 is featured in my article on the Best Microphones For Live Vocal Performances.
Neumann M 150
The Neumann M 150 (link to check the price on Amazon) is a microphone based on the legendary Neumann M 50. This small-diaphragm condenser is, by nature, fairly flat. However, relative to many other condenser microphones, this mic can be considered pretty coloured. It has an omnidirectional polar pattern.
As we see, the M 150 is actually quite flat. However, the broad high-end boosts give it a bright character that has become beloved in the world of audio since the original M 50.
This “bright character” certainly suggests that this microphone is coloured (though not nearly as coloured as the aforementioned Shure Beta 52A).
What is a good frequency response for microphones? “Good” frequency response for a microphone largely depends on the intended use of that microphone. Certain situations call for flat, natural-sounding microphones while others benefit most from coloured mics.
I talk a lot about frequency response when recommending microphones on my Recommended Microphones page (click here to check it out).
What is a directional microphone? A directional microphone refers to any mic that is more sensitive to sound in one direction more than another direction. These mics have both sides of their diaphragms open to sound and can effectively be pointed toward their intended sound source.