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What is Frequency Response?
Frequency response can be defined as the sensitivity of a microphone over a range of frequencies. A frequency response curve is often generated by microphone manufactures to allow customers to visualize a microphone’s ability to capture signals at different frequencies.
Some brands will publish the frequency response of a microphone to show the range of frequencies that can be captured, such as 20 Hz to 20 kHz. This is inaccurate as the level that can be captured at each frequency can vary wildly. A more accurate frequency response graph will show the frequency response plotted using the signal’s measured frequency for the x-axis and the measured signal level for the y-axis.
Having a flat frequency response means that a microphone captures sound at an equal level for all frequencies. This means that a sound source is captured by the microphone accurately with little change from the original sound. It is extremely difficult to have a perfectly flat frequency response, so the term “flat” is also used for a frequency response that has a small dB variation over a frequency range.
What is the Importance of Frequency Response?
Before buying a microphone, you should consider what you’ll be using the microphone for. Typically, a microphone that captures a flat frequency response isn’t always good for recording speech. This is because there is a lot of unnecessary frequency content at the low and high ends that aren’t present in the human voice.
Low frequencies, especially when recording a podcast, should be avoided as this is where noises such as tapping a table and other vibrations are easily picked up. Whereas recording an instrument, like a piano, can benefit from a flat frequency response to record the range of frequencies present.
Of course, unwanted frequencies can be filtered out using EQ, however, having a microphone that has a certain characteristic for different situations can reduce the need to EQ and the complications that can come with it, which are especially noticeable when miking a live venue because of the bleed that can occur from the close proximity of multiple sound sources.
There is no ‘best’ frequency response, it is based on the uses required for each microphone. It is also subjective; some listeners may prefer the way a microphone colors a sound, like how a pair of headphones sound better to some because of increased bass response, even if its effects are counter-intuitive to having a flat frequency response. See the examples below to find out what microphone polar patterns may be naturally better suited to different situations.
Frequency Response Examples
Dynamic and Condenser Microphone Frequency Response Comparison
It is more common for a condenser microphone to have a flatter frequency response compared to a dynamic microphone, which typically has a shaped response.
Generally, a small-diaphragm microphone (SDM) and large-diaphragm microphone (LDM) have the flattest frequency responses on a condenser microphone. Measurement microphones, which have to accurately capture a wide frequency range with a very flat frequency response, use a very small diaphragm.
It is common for a condenser microphone with a flat frequency response to cost more than one with a frequency response which isn’t as flat. This is because of the cost of solving the issues of achieving a flat frequency response. Resonant frequency, where the diaphragm naturally vibrates with more energy at a frequency relative to its size and mass, therefore increasing the levels in and around a certain frequency, must be overcome with testing and intricate design solutions.
Dynamic microphones use a heavy coil to capture sounds and so higher frequencies struggle to vibrate the coil resulting in higher frequencies being captured at a lower level.
Take a look at the frequency response examples below for some popular microphones. Remember, the published frequency responses give a general idea of the frequency response only and the manipulation of data has not been stated by any of the following manufacturers, as is discussed in the section that follows the examples.
Popular Microphone Frequency Response Examples
|Viewable on: https://pubs.shure.com/guide/SM58/en-US||Shure SM58 (Dynamic Vocal Microphone)||Small frequency range. Low-frequency roll-off and increased vocal pickup|
|Viewable on: https://pubs.shure.com/guide/SM7B/en-US||Shure SM7B (Cardioid Dynamic Microphone)||Flat and wide frequency range. Little sound coloration|
|Viewable on: http://recordinghacks.com/microphones/Rode/NT1-A||Rode NT1-A (Condenser Microphone)||Wide frequency range. Slight increase in response around 3 kHz for increased vocal pickup|
Can You Trust the Published Microphone Frequency Response?
Some microphone brands do not state how they determined the frequency response of their microphones. Potentially, microphones could be measured in conditions favorable to how the microphone is to be marketed; a microphone being sold to have a flat frequency response could have been measured in a more reverberant room than normal to increase lower frequency sensitivity to closer match the higher frequencies.
There is also the issue of smoothing. Published microphone frequency responses are almost always smoothed to remove small variations in levels. The smoothing amount is also rarely stated, meaning that a frequency response can be significantly manipulated to hide any weaknesses in the frequency response.
In this example below, I have increased the smoothing and removed data points to make the frequency response appear to have less variation and potentially be more favorable to consumers. I have not increased or decreased any individual data points. What is to stop a microphone brand from doing this if they do not state that the measurement has not been manipulated?
Why Measure the Frequency Response of Your Microphone?
If you have a microphone that you already own, you should consider measuring the frequency response. Knowing the frequency response of your microphone can give a greater indication of what frequencies you may want to EQ in your DAW.
Measuring frequency response could highlight any issues with your microphone that could be causing spikes or pits in the frequency response that you weren’t aware of. You could also compare your microphone to the published frequency response to test how trustworthy a brand is or find out if you’ve been getting your money’s worth compared to the published frequency response.
How to Measure the Frequency Response of Your Microphone
Measuring the frequency response is usually a delicate procedure that requires many expensive resources to get an accurate measurement. Within this tutorial, you will be able to ascertain a general frequency response measurement of your microphone.
- First, set up the microphone you want to measure the frequency response of for recording. Professionally, this measurement would be performed in an anechoic chamber where there is high soundproofing and little sound reflection. To most closely replicate this, try to use a place that is quiet and with high absorption.
- Approximately one meter away from the direction the microphone is pointing, place a loudspeaker facing the microphone. Ideally, this loudspeaker would have a perfectly flat frequency response. For this experiment, we’re going to have to assume that your speaker has a flat frequency response.
If you do know the frequency response of your loudspeaker, which could be from an online source, you could in theory normalize the results with this information, this basically means correcting the data to compensate for the lack of flatness from the loudspeaker.
- Play pink noise through the loudspeaker. This can be easily done with sourced pink noise found online (https://www.youtube.com/watch?v=WJ9Go1PnAVA). Pink noise is used as it creates a sound audible across the entire frequency spectrum and matches the logarithmic nature of human hearing, so will sound of roughly equal loudness at different frequencies.
- Ensure the levels are not clipping on your setup and record approximately 10 seconds of the pink noise.
- Now that you have pink noise recorded, import the audio file into Room EQ Wizard, which is free software available on a range of operating systems, downloadable from https://www.roomeqwizard.com. This can be done by selecting File > Import > Import Audio Data.
Next, navigate to your audio file, select it, then press “Open”.
I didn’t actually have a microphone available at the time of this test myself, so for this example, I’m using some pink noise recorded through my phone’s microphone, so let’s see how (badly) it fairs.
- The audio file will be displayed with phase. Click “All SPL” to show the SPL only.
- You can make out a slight frequency response curve, but to make it clearer, select the “Controls” button to the right of the program. This brings up a control panel with multiple features.
- If you’d like, select “Show frequency bands” to get a better understanding of the range of frequencies.
Select the drop-down button to apply smoothing. Select 1/48 smoothing and then click “Apply smoothing”. This will present a clearer frequency response.
- You can save an image of the frequency response by selecting “Capture”, which can be found just above the top left of the graph. This will open up the export settings. These settings do not need to be changed to export the image, although I recommend you use a resolution of at least 1500. Press the Save icon, highlighted in blue.
Name the file FR 1 48 Smoothed to note the smoothing of your graph.
- Capture the frequency response again but with 1/1 smoothing and you will find a curve style similar to that of most published frequency responses. Take a look at how different the two are and consider where people may be misled due to the effects of smoothing.