Audio Global Viewpoint – May 2018
How Frequency Response Affects the “Sound” of Microphones
Image Azden
A microphone is a transducer that converts analog sound pressure waves into electrical impulses. We all know, however, that what goes into a microphone is not identical to what comes out. A lot of this can be attributed to frequency response, which can “color” the audio. Understanding frequency response is essential to understanding sound recording.
Every microphone colors to some degree the sound it picks up.
The frequency response of any given microphone affects the color of the audio coming out of it. It not only reveals the difference between a $5 crystal mic and $3,000 condenser studio mic, but it also helps determine the right microphone to use for a given project.
A new white paper produced by Azden, a Japanese company that has specialized in pro audio for over 60 years, tackled the subject of frequency response and tried to explain it to the rest of us. Frequency, the company said, refers to how many cycles a soundwave undergoes in a second. This measurement is called Hertz (Hz).
The range of human hearing generally spans from 20 to 20,000 Hz. The frequency of a soundwave determines its perceived pitch. In general, low frequencies are heard as bass and high frequencies as treble. The audible frequency spectrum can be effectively broken down into segments that are commonly referred to as bass, low mids, mid range, upper mids and highs. This is a rule of thumb breakdown, however, and there are no objective standards for these segments.
Loudness, wrote Azden, is the other factor to consider in frequency response measurements. The issue centers around how much louder or softer a particular frequency is once it passes through a microphone. The whole loudness issue can get confusing.
Loudness is measured in decibels, or dB. There are also different kinds of dB. With microphones, dBV is often used. That is dB with a reference of 1 volt = 0 decibels. For the loudness of sound in air, dB SPL is used. It stands for “sound pressure level.”
Most of the time when viewing a microphone’s frequency response specs all one sees is 20 Hz – 20 kHz. This, of course, lacks detail. The missing information is the mic’s sensitivity to each frequency in that range, especially at the fringes. A chart which plots the mic’s output level across the frequency spectrum is far more helpful.
For example, if there is a bump up in sensitivity in the upper mids and some of the highs this frequency response is suited very well to accentuate the hard consonants in speech (sounds from T, S and CH), which is useful if the user is trying to pick up dialog in a noisy environment. It is also the range where sibilance occurs.
If the response remains fairly flat throughout the bass, low mids and mid range, it can be concluded that the microphone should produce a well-rounded sound that isn’t too bass heavy or thin sounding.
Shure SM58 microphone frequency response.
The range of human hearing generally spans from 20 to 20,000 Hz. The frequency of a soundwave determines its perceived pitch. In general, low frequencies are heard as bass and high frequencies as treble. The audible frequency spectrum can be effectively broken down into segments that are commonly referred to as bass, low mids, mid range, upper mids and highs. This is a rule of thumb breakdown, however, and there are no objective standards for these segments.
Loudness, wrote Azden, is the other factor to consider in frequency response measurements. The issue centers around how much louder or softer a particular frequency is once it passes through a microphone. The whole loudness issue can get confusing.
Loudness is measured in decibels, or dB. There are also different kinds of dB. With microphones, dBV is often used. That is dB with a reference of 1 volt = 0 decibels. For the loudness of sound in air, dB SPL is used. It stands for “sound pressure level.”
Most of the time when viewing a microphone’s frequency response specs all one sees is 20 Hz – 20 kHz. This, of course, lacks detail. The missing information is the mic’s sensitivity to each frequency in that range, especially at the fringes. A chart which plots the mic’s output level across the frequency spectrum is far more helpful.
For example, if there is a bump up in sensitivity in the upper mids and some of the highs this frequency response is suited very well to accentuate the hard consonants in speech (sounds from T, S and CH), which is useful if the user is trying to pick up dialog in a noisy environment. It is also the range where sibilance occurs.
If the response remains fairly flat throughout the bass, low mids and mid range, it can be concluded that the microphone should produce a well-rounded sound that isn’t too bass heavy or thin sounding.
To determine the full range of sensitivity, view the difference between the highest and lowest points on the chart. These may be about -36 dBV and -44 dBV, respectively. This is a range of eight dB. From this we could state the following: the frequency response is 20 Hz to 20 kHz, ±4 dB.
Wouldn’t it be best if a microphone picked up all frequencies equally? If the goal is to create a reference microphone that can accurately measure sound levels across all frequencies, then the answer is “yes.” Of course, there are reference microphones created specifically for this purpose.
But when it comes to recording speech, unique musical instruments or various environmental sounds, recordists want a microphone that’s suited for each situation. Mics with a boost in the upper mids and highs are excellent for recording dialog, while a mic designed for a kick drum would be best with a boost in the bass for extra thump. Microphones are built today to record specific sounds.
Having a mic tailored for a given situation uncomplicates recording and frees users from having to EQ the sound. Manufacturers tend to sculpt a microphone’s frequency response to compliment the sound source they intend to record.
This is why the term “mic cabinet” is used by recordists with a collection of microphones — each designed for specific voices, instruments and sound effects. Microphones are similar to an artist’s paint brush and a pallet of colors. Each is a tool to create a unique and signature sound.
Editor note: Frank Beacham has a wealth of tutorial articles posted on The Broadcast Bridge. You can find them via the home page search box, searching for his last name.
Here are three of his most recent articles you may enjoy.
Setting up New Audio Monitors in a Studio