How Microphone Frequency Response Relates to Recorded Sound
Testing in an anechoic chamber
One of the most popular microphone specs is frequency response. However, is the frequency response spec really an accurate description of a microphone’s sound characteristics?
Generally, specs like 20 Hz – 20 kHz are not very useful. A frequency response diagram, on the other hand, will give recordists much more information, because it illustrates the microphone’s sound balance.
For instance, if you see a rise in the upper frequencies of, say, 6 dB, you can expect the respective microphone to sound quite bright.
However, a frequency response plot won’t say if you are going to like the character or “texture” of this brightness, whether it will sound silky or harsh to your ears. That’s where the mystery comes in the recording process.
In an essay on the subject, Neumann, the German microphone manufacturer, suggests we look at the X-axis. The frequencies are usually displayed on a logarithmic scale, which is closer to human perception than a linear scale.
The logarithmic scale is spaced in octaves (which you get by doubling the frequency): the distance from 100 Hz to 200 Hz is just as wide as from 1000 Hz to 2000 Hz or from 10,000 Hz to 20,000 Hz.
Neumann suggests this rough idea of the various frequency ranges of microphones:
- Below 40 Hz: Sub Bass, apart from a kick drum, the sub bass range contains little musical or voice information.
- 40-200 Hz: Bass frequencies, the foundation. The lowest note of a four-string bass is about 40 Hz. The lowest note on a guitar is about 80 Hz. The lowest note of a male singer (baritone) is about 100 Hz, although we rarely hear such low notes. Most pop singing by men and women is above 150 Hz.
- 200-500 Hz: Low Mids, where the human voice has most of its energy. And it is the body of most musical instruments.
- 500-3000 Hz: Mid Range, an area that is crucial for the sound character, because it’s where the human ear is most sensitive to even the smallest details. The telephone transmits little below or above this range, yet we are able to recognize callers by their “hello.”
- 3000-7000 Hz: Presence, the range is important for sound definition and good intelligibility. It is the area of many speech consonants.
- 7000-14000 Hz: Treble, the area crucial for our sense of brightness. However, too much energy in this area can sound harsh and distracting. This is the area of speech sounds such as “S” and “T,” of cymbals, but also of string noises.
- Above 14,000 Hz: Air band, the area that’s important for recordings that want to sound “expensive” and “super-hi-fi.” It gives voices and stringed instruments an airy feel, hence the name. It does not contain much musical information.
When comparing frequency plots, always take note of the how the Y-axis is scaled. Some manufacturers use a very wide scale to make a jagged frequency response look nice and smooth.
Also, some less reputable manufacturers more or less make up their frequency plots, as they don’t own the necessary gear to measure them. An anechoic room and measurement equipment is an expensive investment for small manufacturers. A pencil is much cheaper.
Typically, Neumann writes, a studio microphone is more balanced than a stage mic, but not exactly linear, either. Most microphones show a rise in the upper frequencies, i.e. in the presence and treble regions.
While frequency diagrams could be helpful to compare the location and magnitude of those boost frequencies, the published charts are often unreliable or mere wishful thinking, especially on lower end products.
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