There are many specifications that help us to understand the nature and performance of a microphone. Transient response is one such specification that is difficult to measure and does not show up on data sheets but is, nonetheless, important to understand if we are to comprehend the true nature of a microphone.
What is microphone transient response? Transient response, as the name suggests, refers to the response a microphone’s diaphragm has to sound wave transients. A transient is a fast peak in the amplitude of a sound best described as the initial hit of a drum. The transient response is how fast or slow the mic responds to the transient.
In this article, we’ll discuss microphone transient response in more detail, covering the factors that make up a mic’s transient response and looking into the general transient responses of various microphone types.
Microphone Transient Response
Before we get into our discussion on transient responses, it’s critical we understand what a transient is.
What Are Sound/Audio Transients?
Transients, in sound and audio, are momentary increases in amplitude in either sound waves (mechanical wave energy) or audio signals (electrical energy).
It’s easier to visualize a transient than it is to explain a transient with words, so let’s have a look at a simple audio signal:
Sounds (particularly percussive sounds) will tend to have transients at the beginning of their sound waves.
For example, a snare drum will have a large transient relative to the body of its sound.
A trumpet, for example, could have a small transient if it’s played softly or a loud transient if played loud and staccato. The transients in most instruments depend largely on the method/style in which they are played. The same is true of the human voice, where a short shout would have a pronounced transient while “oo-ing and ah-ing” would likely produce low transients.
Synthesizer pads are often designed so that their transients are barely recognizable if noticeable at all.
Speaking of synths, if you’re familiar with ADSR synthesizer envelopes (attack, decay, sustain and release), you can envision the transient as being the point between the attack and sustain (though in synthesizers we can effectively rid of the transient by having the sustain amplitude equal to the max amplitude of the synth patch).
What Is A Microphone’s Transient Response?
So now that we have a solid idea of what constitutes an audio/sound transient, we can discuss microphone transient responses with greater clarity.
Sound waves change the local pressure as they travel through a medium. As a sound wave reaches a microphone diaphragm, it causes alterations in the pressure at the diaphragm.
A difference in pressure between the front and back of the mic diaphragm causes the diaphragm to move. This movement coincides with the sound waves that hit mic diaphragm, which allows the microphone to effectively convert these sound waves into electrical audio signals with astonishing accuracy and clarity.
To learn more about how microphones work to convert sound to audio, check out my articles How Do Microphones Work? (A Helpful Illustrated Guide) and Microphone Types: The 2 Primary Transducer Types + 5 Subtypes.
Transients, as discussed, are large variations in pressure at the onset of a sound. Therefore, a microphone’s transient response tells us how a mic’s diaphragm will react to transient pressure spikes and how accurate the mic will be at outputting an audio signal that represents transient sounds.
If a microphone has an accurate transient response, it will recreate transient sounds in its audio signal with tremendous accuracy. If the transient response is too close, it will colour the sound (much like compression does). If the transient response is too fast, it will likely cause an inaccurate representation of the sound in the audio signal.
A microphone’s transient response does not only refer to its initial reaction to a transient sound. A microphone’s transient response has a lot to do with its diaphragm’s ability to return to rest position after the initial transient has caused the movement.
In other words, the transient response has to do with the inertia of a microphone’s diaphragm. Diaphragms with higher inertia will begin moving more slowly and take longer to return to resting position.
Slow Transient Response And Its Similarity To Signal Compression
Some microphones, like many moving-coil dynamic mics, have slow transient responses.
More on dynamic mics later.
A noticeably slow transient response certainly colours the sound. Let’s break down the reasons:
First, the high inertia of the diaphragm means that the mic will react a bit slow to transients. The transient sound will hit the diaphragm but the diaphragm will not react immediately.
Second, once the diaphragm begins moving (which happens slowly), it will take some extra time to stop moving. Even after the short transient of sound subsides, the diaphragm may still be oscillating with some relatively high amplitudes.
To sum it up, a slow transient response means a microphone’s signal will effectively recreate the sound transient with a longer attack and more sustain. The mic signal will produce a stronger tail of the transient which acts to colour the sound.
This added “sustain” is essentially what an audio compressor does when it ducks the volume on the transients of an audio signal. Whether we bring the transients down in amplitude (like the compressor) or bring the tail up in amplitude (like with a slow transient response), we essentially reach the same conclusion: a compressed audio signal that exaggerates the tail and reduces the sharpness of the transient.
This can help to make transients sound less harsh and more full and punchy. Conversely, its colouration can deaden the character of some instruments, leading to dull and poorly-defined results.
Transient Response Overshoot
Overshoot, in electrical signals, refers to the event when a signal exceeds its target. With microphones, overshooting is when the diaphragm reacts too fast and/or moves too far when subjected to a transient sound.
This may seem like a good thing. Don’t we want a fast transient response in order to accurately reproduce the sound waves as audio signals?
Let me assure you this is not as good as we immediately think it is.
What basically happens when a microphone has overshoot issues is this:
The sound transient reaches the diaphragm and the diaphragm moves very quickly. So far so good. However, the diaphragm then moves back due to physical limitations before the sound wave’s transient causes a decrease (or increase) in pressure to move the diaphragm.
So the diaphragm oscillates faster than the sound wave should cause it to oscillate. This is often unnoticeable, but it does cause distortion in the mic signal.
The phase issues that come from overshoot can cause a somewhat “tinny” high-end response.
Inexpensive condenser mics will sometimes have overshoot.
More on condenser mics later.
Inertia And Other Factors Of Microphone Transient Response
As mentioned earlier, diaphragm inertia is the leading factor in a microphone’s transient response.
Intertia is a physical body’s resistance to change in motion. In the case of microphone transient response, we’re concerned with the tendency of the mic diaphragm to stay in resting position or, alternatively, to keep oscillation (once the diaphragm is in motion).
But what factors influence the inertia of a microphone’s diaphragm? There are 2 main factors:
- Weight of the diaphragm.
- Tension of the diaphragm.
Weight Of The Diaphragm
The diaphragm weight is a major factor in a microphone’s transient response.
Generally speaking, the heavier the diaphragm, the higher the inertia and the slower the transient response. Conversely, lighter diaphragms typically mean faster, more accurate transient responses but may also exhibit overshoot.
Tension Of The Diaphragm
The tension of the diaphragm is another big factor in a microphone’s transient response.
In generally, tightly tensioned diaphragms will react quickly to sound and return to resting position promptly. Looser diaphragms will often react more slowly and will nearly always take longer to stop oscillating once they have been moved from resting position.
To learn more about microphone diaphragms, check out my article What Is A Microphone Diaphragm? (An In-Depth Guide).
Typical Transient Responses Of Various Mic Types
When learning about any new topic, it’s great to know the generalities and have examples. Let’s now focus on the typical transient responses of the following 4 types of microphones.
- Moving-coil dynamic.
- Ribbon dynamic.
- Small-diaphragm condenser.
- Large-diaphragm condenser.
Note that there are many other microphone types. However, these are the four types that have notable differences when it comes to transient response.
Typical Moving-Coil Dynamic Microphone Transient Response
Moving-coil dynamic microphones have relatively heavy conductive coils attached to their diaphragms. This makes their diaphragms, as a whole, quite inert.
The typical transient response of a moving-coil mic is slow and gives dynamic mics a somewhat compressed sound. Depending on the size and weight of the diaphragm, a dynamic mic may even sound dull and lifeless (which is actually beneficial in some situations like kick drum miking).
One extreme example of a moving-coil dynamic microphone with a slow transient response is the Shure Beta 52A (link to check the price on Amazon). This mic sounds great on kick drums due to its full low-end and dull overall response (read about it here). However, it is typically a terrible choice otherwise.
For everything you need to know about moving-coil dynamic microphones, check out my article Moving-Coil Dynamic Microphones: The In-Depth Guide.
Typical Ribbon Dynamic Microphone Transient Response
Ribbon microphone diaphragms are oddly shaped. They are extremely thin; rectangular in shape; corrugated, and attached at either side of their length (rather than around their perimeter like other diaphragms).
These diaphragms are very lightweight but are tensioned very loosely.
The result is typically a very accurate transient response. In general, ribbon mics do not exhibit overshoot nor do they sound compressed or “slow.”
One such example of a natural-sounding ribbon mic with an accurate transient response is the famous Royer R-121 (link to check the price on Amazon):
For everything you need to know about ribbon microphones, check out my article Dynamic Ribbon Microphones: The In-Depth Guide.
Typical Small-Diaphragm Condenser Microphone Transient Response
Small-diaphragm condensers (SDCs) are typically lightweight and are tensioned quite tightly.
These mics account for some of the most accurate measurement mics in the world and tend to have very accurate transient responses.
A great example of a natural-sound SDC with a fast transient response is the Neumann KM 184 (link to check the price on Amazon):
Note that some inexpensive SDCs exhibit overshoot and may output distorted transient information.
Typical Large-Diaphragm Condenser Microphone Transient Response
Large-diaphragm condensers (LDCs), as the name would suggest, have larger diaphragms than their SDC counterparts.
These larger diaphragms are slightly looser and heavier and so LDC transient responses are typically not as spot-on as SDC responses.
That being said, LDCs are often tuned with accuracy in mind when it comes to transient response and so many LDCs sound very accurate when reproducing transients.
A great example of an LDC with a fast transient response is the multi-pattern AKG C 414 XLII (link to check the price on Amazon):
To learn more about the differences between SDCs and LDCs, check out my article Large-Diaphragm Vs. Small-Diaphragm Condenser Microphones.
What is the sensitivity of a microphone? Microphone sensitivity refers to the strength of the output signal (in mV or dBV) a mic will produce when subjected to a certain sound pressure level (in Pa or dB SPL). Sensitivity ratings are typically measured with a 94 dB SPL tone of 1000 Hz at the mic capsule.
For everything you need to know about microphone sensitivity, check out my article What Is Microphone Sensitivity? An In-Depth Description.
What is a microphone frequency response? Microphone frequency response refers to the frequency-dependent sensitivity of the microphone. The response range tells the general range of audible frequencies (from low to high) that a microphone will reproduce. A graph tells us the variation in sensitivity across this range.
For everything you need to know about microphone frequency response, check out my article Complete Guide To Microphone Frequency Response (With Mic Examples).