Why do some microphones sound better than others? Why do some microphones capture certain instruments better than other? Although there are many factors to determine a microphone’s characteristic sound, the frequency response is the paramount specification!
So what is microphone frequency response? Microphone frequency response is the frequency-specific output sensitivity of a microphone. It shows the relative output levels of the frequencies a microphone is able to reproduce from external sound pressure. Frequency response is specified as both a range and as a detailed chart.
The frequency response of a microphone is the single most determining factor of its characteristic sound. If you’re interested in utilizing frequency response specifications to better understand and utilize your microphones, please continue reading this in-depth article!
What Is Microphone Frequency Response?
Frequency response is simply what it sounds like: the microphone’s response to frequencies.
More specifically, the frequency response is the microphone’s frequency-specific sensitivity to vibrating air molecules around its diaphragm. How well does a microphone capture certain sound frequencies in the air, process them, and reproduce them in its output audio signal?
There are two general ways of expressing microphone frequency response:
- The range of frequencies a microphone will reasonably reproduce.
- A graph showing the relative sensitivity of a microphone to frequencies within its “range.”
The frequency response range of a microphone doesn’t provide much valuable information. Let’s say a microphone’s frequency range is from 20 Hz – 20,000 Hz (the range of human hearing). There’s no telling if the microphone is more sensitive at 20 Hz than 20,000 Hz. And there’s no explicit data on the peaks and valleys of the frequency response within that range. These peaks are valleys are prominent factors in the signature sound of a microphone.
Sometimes manufacturers will offer a range with a measure of tolerance. For example 20 Hz – 20,000 Hz ± 3 dB. This is a bit better. We’re told there are no severe peaks or valleys (although there is a 6 dB difference between the range’s most prominent and least prominent frequency). However, it still does not paint a proper picture of where these sensitivities lie on the frequency spectrum.
The frequency response graph is a much better representation of a microphone’s frequency response. These 2-dimensional graphs lay the audible frequency spectrum along the X-axis (in Hertz) and the relative output sensitivity along the Y-axis (in dB).
Some manufacturers provide more in-depth graphs than others. It is sometimes difficult to express exact values for each frequency since the frequency spectrum is a continuum and microphones have many tiny peaks and valleys in their truest frequency response. Quality microphones from esteemed manufacturers typically provide the best charts.
The Determining Factors Of Microphone Frequency Response?
Frequency response is the most significant determinant of a microphone’s characteristic sound. It is a function of many variables both inside and outside the microphone itself.
The frequency response of a microphone is influenced by the following factors:
- Weight of the diaphragm
- Size of the diaphragm
- Shape of the diaphragm
- Tension of the diaphragm
- Size and shape of the capsule/baffle
- Directionality of the capsule
- Resonant frequencies of the microphone body
- Sound source distance from the microphone
- Output impedance versus load impedance between mic and preamp
The Weight Of The Diaphragm
The weight of the microphone diaphragm is a limiting factor on the high-frequency response. The inertia of a heavy diaphragm makes it less sensitive to smaller wavelengths of sound (higher frequencies). For this reason, the relatively heavy moving-coil diaphragms have poorer high-frequency sensitivity than their lighter condenser and ribbon counterparts.
The Size Of The Diaphragm
The diameter of a circular diaphragm plays a big role in determining its microphone’s high-frequency response. If a sound wave has a wavelength equal to the diameter of a diaphragm, it will apply equal amounts of both positive and negative pressure, effectively cancelling itself out.
Sound waves shorter than this wavelength get quite “phasey,” especially when both sides of the diaphragm are exposed to external sound pressure. This further reduces the clarity of the high-frequency response. The shorter the wavelength, the higher the frequency, and so smaller diaphragms are physically able to reproduce higher frequencies than large diaphragms.
The Shape Of The Diaphragm
The shape of the diaphragm is one factor in the resonant frequencies of the microphone. Typical moving-coil and condenser diaphragms are circular and therefore are susceptible to resonant frequencies (think standing waves). A wavelength having exactly twice the length of the diaphragm diameter causes a sort of standing wave on the diaphragm, where the nature of the diaphragm shape slightly accentuates this wavelength’s frequency. Integer multiples of the wavelength behave interestingly on the diaphragm, causing either self-cancellation or resonant standing waves.
Ribbon microphones are different. Their long, corrugated ribbon diaphragms typically do not have strong resonant frequencies due to their irregular shape!
Moving-coil diaphragms often have leaf slots and indents where the coil attaches. This irregular shape also affects the microphone’s frequency response.
The Tension Of The Diaphragm
The tension of a diaphragm affects the inertia of the diaphragm and the frequency response of the microphone.
Think of tuning a snare drum. The tighter we stretch the skin, the higher the resonant frequency of the snare drum. Similarly, the tighter a microphone diaphragm, the higher its resonance frequency due to tension. Just don’t hit microphone diaphragms with drumsticks!
In small diaphragm condenser microphones, this tension may cause resonance way above the audible range of human hearing.
Large diaphragm condensers will usually have a high-frequency boost due to diaphragm tension.
Ribbon diaphragms are often loose enough for their resonant frequency (due to tension) to be in the sub-bass region or even below the audible range of human hearing.
For more information on microphone diaphragms, check out my article What Is A Microphone Diaphragm?
The Damping Material And Space Around The Capsule
Microphones typically have protective grilles around their capsules. Within the grille and around the capsule there is often dampening acoustic foam. There is a space between the grille, foam, and capsule.
The damping material dampens help protect the capsule from plosives while damping higher frequencies.
The space within the mic has the potential to promote short standing waves.
Although minor and terribly complicated, these seemingly small factors are all serious parts of microphone design and frequency response.
Generally speaking, diaphragms are damped at −6 dB per octave in order to produce a natural sounding frequency response.
Directionality Of The Capsule
Yes, even the directionality of a microphone affect its frequency response. This is particularly true when moving off-axis from directional microphones.
In a directional microphone, the capsule is designed with a specific “path” that sound must travel to get from the front of the diaphragm to the back of the diaphragm. This distance affects the high-frequency roll-off of microphones.
The frequency roll-off starts at a peak point: At a frequency with a wavelength twice that of the path length from front to back of the diaphragm. At this frequency, there is a maximal pressure difference between the two sides of the diaphragm, causing a peak in frequency response.
The pressure difference above this cut-off frequency will get smaller and smaller. The amplitude difference will also get smaller with higher frequencies. These both cause a decrease in frequency response.
Note that microphones are more directional at high frequencies and become increasingly omnidirectional at low frequencies. Therefore, the off-axis frequency response of a directional microphone will have relatively less highs and more lows compared to the on-axis response. As we move the sound source further off-axis, the microphone becomes worse at reproducing high-frequencies.
Resonant Frequencies Of The Microphone Body
All physical objects have resonant frequencies (think tuning forks). Microphones are no different. Quality microphone bodies are designed with this in mind and special care is taken to minimize the presence of resonant frequencies. But the fact remains that these resonant frequencies of the microphone body will affect the frequency response.
Sound Source Distance From The Microphone
Though not a part of the microphone anatomy, sound source distance plays a role on the frequency response of directional microphones. Specifically on the low-end. This is due to the proximity effect, which we’ll discuss in more detail later.
The closer the sound source is to a microphone, the more low frequencies the microphone will reproduce.
Output Impedance Versus Load Impedance
Voltage transfer between microphone and preamp increases as the load impedance increases compared to the mic output impedance. A load impedance at least 5 times that of the output impedance is preferable.
However, a microphone’s output impedance is frequency specific and is often much greater at low frequencies. Therefore, low load impedances may cause a loss of low-frequency response.
At the impedance converters (whether step-up transformers or circuitry), high frequencies can be lost due to the increase in the mic signal impedance.
For more information on microphone impedance, check out my article Microphone Impedance: What Is It And Why Is It Important?
How To Read A Frequency Response Chart
If you have spent time in Digital Audio Workstations, you may have seen a parametric equalizer. A frequency response diagram looks very similar!
A microphone’s frequency response diagram has two axes:
- X-Axis: frequencies (Hz)
- Y-Axis: relative sensitivity (dB)
Humans hear frequencies logarithmically. That is to say that each doubling of a given frequency is heard as an octave above.
Therefore, the X-Axis is set up as a logarithmic scale. Each octave takes up the same amount of horizontal space along the X-Axis. In other words, the space between one frequency value and the next frequency value get smaller and smaller as you move from the left to the right on the graph.
The Y-Axis values are technically set up linearly, although decibels are a logarithmic ratio.
A full explanation of decibels would require a separate long-form article.
Basically, if there’s a peak above O dB on the graph, the microphone is sensitive to those frequencies (it reproduces more of those frequencies in its electrical signal than there are in the acoustic source).
Conversely, if there’s a dip below 0 dB, the microphone does not do such a great job at capturing those frequencies (it reproduces less of those frequencies in its electrical signal than there are in the acoustic source).
Decibels are relative values, and so the graph’s Y-axis can be thought of as being relative to itself rather than to any fixed point. Although some manufacturers do link the Y-Axis decibel rating to a certain fixed point, in order to read the graph, we only need to know that the decibel values are relative to one another!
The Frequency Response Line
Of course, to complete the diagram, we must actually have a line that matches the relative sensitivity of the microphone to frequencies along the audible spectrum.
By having this line drawn, we can effectively read and understand a microphone’s frequency response.
Note that sometimes there will be multiple response lines drawn on the graph. Typically these will relate to a particular “mode” of a microphone. For example, a high-pass-filter switch or a switch in the polar pattern (if the microphone has these features).
Reading The Frequency Response Chart
With a frequency response diagram, we can clearly see the roll-offs at the lower and upper limits of a mic’s frequency response range.
But we also see particularities between these limits:
- At which frequencies is the mic more sensitive?
- At which frequencies is the mic less sensitive?
If there’s a peak in the upper frequencies, we could probably infer that the microphone has a bright “character.”
If there’s a peak in the lower frequencies, the microphone may provide a better “coloration” to a male voice, kick drum, or bass guitar cabinet.
With a trained eye, an audio engineer can look at a series of microphone frequency response diagrams and know how to maximize their potential.
However, even with a trained eye, it’s difficult know whether or not you’ll subjectively enjoy the character and coloration of a microphone in a certain situation until you actually put the microphone in that situation!
How Do Humans Hear Frequencies?
Yes, our ears also have a frequency response!
The outer limits of human hearing, as we’ve discussed, are 20 Hz on the low end, and 20,000 Hz on the high end.
We’ve evolved to have a sensitivity between 2,000 Hz and 5,000 Hz. It’s no coincidence that this range contains a lot of information relating to the human voice and speech intelligibility.
As we get lower on the spectrum, heading toward 20 Hz, we become less and less sensitive to SPL levels. We actually feel these sub-bass frequencies (20 Hz – 60 Hz) frequencies more than we hear them. Think of the kick drum and sub-bass at a loud EDM show!
On the higher end of the spectrum, we slowly lose sensitivity as we get older and repeatedly damage our hearing. For example, from spending so much time playing music in loud bands and attending loud shows, I personally have a hard time hearing anything above 16,500 Hz…
Protect your ears!
To better understand the complicated frequency response of human hearing, check out the Fletcher-Munson curves.
A Possible List Of The Frequency Bands
Before we move on, I’d like to give a bit more information on the frequency bands (ranges) and how we hear them:
Note that these ranges are simply rough guidelines. There is no standard here.
The frequency ranges are:
- ≤ 60 Hz = Sub Bass
- 60 Hz – 250 Hz = Bass
- 250 Hz – 500 Hz = Low Mids
- 500 Hz – 2 kHz = Midrange
- 2 kHz – 4 kHz = High Mids
- 4 kHz – 6 kHz = Presence
- ≥ 6 kHz = Brilliance
≤ 60 Hz = Sub Bass
This frequency band is felt more than it’s heard (check out the Fletcher-Munson curves). Most instruments and sounds lack information in this range.
Pay special attention to the sub bass frequency response in the microphones for kick drums, bass guitar amplifiers, and tubas.
60 Hz – 250 Hz = Bass
This is where most of the “musical” bass information is. The fundamental frequencies of many instruments are in this range including the fundamental frequency of most human voices!
250 Hz – 500 Hz = Low Mids
This range contains the stronger harmonics of the bass instruments and the fundamentals of some higher pitched instruments. To much response in this band may cause the microphone to sound “muddy.” Not enough response in this band may cause the mic to sound too thin.
500 Hz – 2 kHz = Midrange
The human ear starts getting more sensitive in this range. This range contains weaker harmonics from bass instruments and stronger harmonics from higher pitched instruments.
2 kHz – 4 kHz = High Mids
The human ear is the most sensitive in this range. A microphone that has a substantial boost or cut in this band will not recreate the timbre of a sound very accurately.
4 kHz – 6 kHz = Presence
If a microphone is sensitive in this range, it may add more presence to the sound, or it may sound harsh. There’s a fine line.
If there’s a dip in a microphone’s frequency response in this band, it can make the source sound transparent or further away than it actually is.
≥ 6 kHz = Brilliance
Many dynamic microphones drop off somewhere in the “brilliance” frequency band (even though they may be rated to capture up to 20 kHz).
This band is part of the reason why condenser microphones, in general, sound more “hi-fi” than their dynamic counterparts. Condenser microphones typically do a great job at picking up frequencies in this range, which encompasses all the upper harmonics of sounds as well as the “air” and “sparkle” of a sound (those are my technical terms).
Ribbon microphones tend to gently roll-off in the brilliance band, therefore reproducing a “warm” sound.
How Do Microphones “Hear” Frequencies?
Different sound frequencies vibrate the air at different rates. Frequency is measured in Hertz (Hz), as we’ve discussed, which is cycles or vibrations per second.
Sound vibrations in the air are caused along longitudinal waves (sound waves). These waves have peaks (max compression) and troughs (max rarefaction) just like other waves. The wavelength of these longitudinal waves is inversely proportional to the frequency (assuming a constant speed of sound).
The lower frequencies of sound have longer wavelengths. Lower frequencies naturally have a greater amplitude (a fundamental frequency of an instrument, for example, has a greater amplitude than its harmonics).
But these “quieter” harmonics are added to the overall vibration of the air, resulting in all sorts of interesting wave forms. So we have slower moving waves (low frequencies) and faster moving waves (higher frequencies) effectively summed together to vibrate the air in complicated ways.
Our ears pick up on this and send an electrical signal to our brains.
Microphone diaphragms are very similar!
The airwaves around a microphone’s diaphragm will vibrate the diaphragm in a similar way as they do the air.
Dynamic moving-coil microphone diaphragms seem to resist this vibration a bit more than the capacitor diaphragms of condenser microphones. This is especially true at higher (often weaker) frequencies. This helps to explain why condenser mics generally have a better high-frequency response than their dynamic counterparts.
Some microphone intentionally “roll-off” the low end of their frequency response, or add a high-pass-filter switch so you can choose whether to roll-off the low end or not.
As we’ve discussed, the sub-bass frequencies don’t necessarily contain a lot of “musical information.” Removing these from the frequency response of the microphone can reduce rumble and unwanted low-frequency noise while still picking up what we really want to here!
So microphones are very similar to our ears, they:
- Transduce acoustic (mechanical) energy into electrical energy.
- Have a specific frequency response.
How Is Frequency Response Measured In Microphones?
The process of properly calculating a microphone’s frequency response is simple to conceptualize. However, microphone manufacturers need expensive equipment in order to actually measure correctly. This equipment includes:
- An anechoic chamber.
- A perfectly calibrated loudspeaker.
An anechoic chamber is absolutely acoustically dead. There is no ambient noise in an anechoic chamber nor are there any reflective surfaces in the room itself. It’s not uncommon to have negative dBA values when measuring the room noise of an anechoic chamber. This is incredible!
Next, a perfectly calibrated loudspeaker is needed. This loudspeaker needs a flat frequency response and must able to output 20 Hz to 20,000 Hz evenly. Like the anechoic chamber, a truly incredible amount of design detail must go into this loudspeaker.
The microphone in question is placed in front of the loudspeaker, connected via XLR to a calibrated spectrum analyzer, and the test is ready to take place.
Pink noise is played through this loudspeaker and captured by the microphone.
Pink noise is used because it has equal energy across all octaves in the human range of hearing. And so if the resulting frequency response diagram is not flat, it has something to do with the microphone’s response to particular frequencies.
The microphone signal is routed into a spectrum analyzer and a frequency response chart is then produced.
This sounds expensive, and it is!
An Alternative Way To Measure Frequency Response In Microphones
Instead of using all of this expensive equipment to produce an accurate frequency response, manufacturers (and audio enthusiasts alike) compare against a known reference.
By analyzing the microphone in question against another mic with a known frequency response, we calculate the differences and deduce a frequency response for the unknown microphone!
In this option, we need to ensure that everything but the microphones are the same in both tests:
- The room.
- Everything in the room.
- The position of the microphones.
- Distance to the speaker.
- The same pink noise is to be used and at the same volume.
This is a cheaper option. The room doesn’t need to be anechoic since both microphones will be subjected to the same stimulus. Though acoustically dead rooms are better since reflections of sound within the room will give a skewed result.
Also, the loudspeaker doesn’t need to be perfect, but should be calibrated so we know the pink noise is as flat as it can be.
We find the differences in frequency response between the two microphones using frequency analyzers. From that, we chart out a new frequency response diagram based on the known diagram.
Even that sounds tedious…
And it is, but it’s worth having for a spec sheet to better serve the customer.
However, some manufacturers do more “guesswork” than others when it comes to their frequency response charts. So sometimes the frequency response diagram isn’t necessarily accurate.
Another way these charts lack accuracy is in the scale of the graph. Oftentimes the response line is smooth and sums the average sensitivity (rather than being very precise and jagged). This is fine to purvey a general sense of where the mic naturally boosts and cuts in the frequency spectrum but lacks the true detail of the real frequency response.
So basically there are 3 ways of measuring a microphone’s frequency response:
- An anechoic chamber with a calibrated loudspeaker.
- Comparison to a known microphone.
Wow, the precision really dropped off quick!
Is A Flat Frequency Response Always Best?
Why would we want anything other than a perfectly flat frequency response from 20 Hz to 20,000 Hz?
This is a good question, and in many applications, this is exactly what we want.
If we are tasked with capturing natural sound at a sporting event, or recording a wide range instrument like a piano or harp, a wide and flat frequency response is what we would want in our microphone!
However, a “non-flat” frequency response gives a microphone colour and character and is beneficial in capturing certain sounds.
For example, the Shure Beta 52A is a microphone I love using to capture the sound of kick drums. It has a boost in bass frequencies, a significant cut in the mids, and a slight boost in the upper midrange before dropping off completely at 10,000 Hz. This frequency response is radically different than a perfectly flat 20 Hz to 20,000 Hz.
But this makes the Beta 52A a great candidate for miking kick drums! It has the low boosts for the “thuds,” a mid cut to reduce the “boxiness” of the kick drum, a slight boost in the upper midrange for the “thack” of the kick drum, and no response to frequencies higher than 10,000 Hz which effectively reduces the bleed from a drum kit’s cymbals.
Of course, microphone signals can be “fixed in the mix,” but I’d argue it’s best to capture sounds the way you want to from the beginning. Choosing microphones for a job is a fun task, too!
Choosing The Microphone With The Proper Frequency Response
So there is great variation in the possible frequency responses of microphones. There are some microphones out there with “flat” frequency responses across the audible spectrum; some microphones with a narrower band of responsiveness; some with wild variation and “colour” across the frequency spectrum; and others that are specifically tailored for particular instruments.
Choosing the microphone with the proper frequency response depends on what we’re planning to record. The response of some microphones is better suited for certain sound sources. It’s best to first understand the inherent qualities of what you’re recording and then to compliment the sound with a suitable microphone.
For example, a piano has a wide range of fundamental frequencies and a strong harmonic profile. Microphones with a flat frequency response and a wide 20 Hz – 20,000 Hz range would work magnificently well on pianos.
When recording vocals, perhaps a presence boost between 3 kHz – 6kHz would increase the clarity of voice.
When miking a kick drum, a strong low-end response would be beneficial along with a slight presence boost. Conversely, a cut or lack of responsiveness in the high-end and mid-range frequencies would reduce the extraneous noise and muddiness of the kick drum recording, respectively.
Choosing a microphone with a complimentary frequency response comes with the practice of recording various instruments. Once again, a solid strategy includes just 2 steps:
- Understand the frequency range (fundamentals and harmonics) of an instrument.
- Choose a microphone with a frequency response that enhances the important frequencies of that instrument.
The Proximity Effect
The proximity effect is a microphone phenomenon in which moving a mic closer to a sound source actually boosts its sensitivity to low-end frequencies. A microphone’s frequency response, therefore, changes depending on its proximity to the source it is capturing.
How can this be?
First, there are two kinds of microphone principles: the pressure principle and pressure-gradient principle. The proximity effect only affects pressure-gradient microphones. Pressure-gradient microphones are directional (cardioid and figure-8 polar patterns and all the variations thereof).
Because the pressure difference between the back and front plates is what causes the signal, and the difference is physically greater in high frequencies than it is in low frequencies (due to phase differences in shorter wavelengths). Microphones dampen the diaphragm to help balance out the signal and capture a “flatter” frequency response.
This works great at a fair distance.
However, when the sound source gets close to the capsule, things change. Let’s say there’s a distance D between the front and back of a mic’s diaphragm and the sound source is a distance D from the front of the diaphragm.
This means the source is 2D from the back and therefore the SPL is four times louder at the front than it is at the back.
At these distances, the difference in pressure between the front and the back is barely dependant on frequency and much more dependent on amplitude. And so the dampening actually boosts the bass frequencies rather than equalizing them. This boost results in a variation of the frequency response in the low end of a microphone and is known as the proximity effect!
For more information on microphone proximity effect, check out my article What Is Microphone Proximity Effect And What Causes It?
Frequency Response Generalizations Between Transducer Types
There are 4 main types of professional microphones:
- Dynamic moving-coil microphones.
- Dynamic ribbon microphones.
- True Condenser microphones.
- Electret Condenser microphones.
Generalizations are made about the typical frequency response of each type of microphone:
Dynamic moving-coil dynamic microphones have relatively poor high-frequency response due to the inertia of their heavy diaphragms. Their diaphragms also tend to have a resonant frequency in the low-mid or bass frequency region. Dynamic microphones are often regarded as having the most “coloured” frequency response with many peaks and valleys.
For more information on moving-coil dynamic mics, check out my article Moving-Coil Dynamic Microphones: The In-Depth Guide.
Dynamic ribbon dynamic microphones have a gentle, natural sounding roll-off of high frequencies, which makes them sound “warm” in the digital age. Their loose diaphragms tend to have a resonant frequency well below the human range of hearing. Generally speaking, the frequency response of a ribbon microphone is greatly affected by the proximity effect and by load impedances.
For more information on dynamic ribbon mics, check out my article Dynamic Ribbon Microphones: The In-Depth Guide.
True Condenser microphones are known for their wide, flat frequency responses. The small diaphragm condenser mics have a typical diaphragm resonant frequency above the audible spectrum, whereas large diaphragm condensers typically have a resonance peak in the brilliance range. Due to their extended high-end response, condensers may sound a tad harsh in digital recordings. However, this well-represented high-end does wonders on analog tape!
Electret Condenser microphones are the most common microphone made today. When electret technology is used in professional microphones, the mics end up sound quite similar to their “true” counterparts.
Of course, take the above generalizations only as generalizations!
Do speakers and headphones have a frequency response? Speakers and headphones, like microphones, have frequency responses. The frequency response of an audio output device is determined mostly by the size of the speaker(s), transducer type, circuitry, and resonant frequencies.
How do I change a microphone’s frequency response? Although an innate characteristic of a microphone, frequency response can be altered by the following:
- Proximity effect
- Off-axis sound source
- Cupping the microphone
- Introducing or removing grilles and acoustic foam
- Supplying varying amounts of phantom power (to active microphones)
- Changing the load impedance
- Switching filters/patterns on/off (if the microphone has switchable options)
For information on all the possible microphone specification, please continue to my article Full List Of Microphone Specifications (How To Read A Spec Sheet).