Getting familiar with frequencies, the frequency spectrum, and frequency response is a baseline audio concept. Frequency plays a role in every part of the recording, mixing, and mastering processes. It’s pivotal to live sound reinforcement, integral to musical notes and pitch, and even tells us a bit about the science of sound and how acoustic waves propagate. Many of us have at least a rudimentary understanding if we’ve ever adjusted the equalizer on a stereo, so it usually isn’t a completely foreign concept. Parameters like bass, middle, and treble, each make use of the frequency spectrum and frequency response. If you intend to dive further into audio, comprehending frequency response on a deeper level is essential.
What is Frequency Response?
Frequency response refers to how a piece of audio equipment reproduces sound for recording or playback. This includes how much of the actual frequency spectrum it covers, as well as the relative volume levels of those frequencies at different points in the spectrum. This is measured in ± decibels (dB) using 0dB as a reference.
A full-range frequency response in audio is 20 Hz to 20,000 Hz (or 20 kHz). This just so happens to be the complete range of human hearing. Anything above or below that range can exist, it just won’t be perceptible to people. Dogs can hear up to 40,000 Hz!
In pro audio, you’ll commonly find frequency response measurements included with microphones, headphones, and speakers. Frequency response is also an important concept in acoustics. It can tell us how certain rooms enhance or accentuate specific frequencies based on the space’s dimensions.
You might sometimes hear “frequency response” used interchangeably with “frequency content.” The latter more accurately refers to the frequencies contained within a recorded signal, or the timbre of a voice, instrument, or sound.
Why is Frequency Response Important?
Audio engineers have to understand frequency response so they can act accordingly for the best possible sound quality. A good recording or live sound engineer can determine the most appropriate microphone to use on a sound source just by understanding the basics of frequency response.
For example, if an engineer is miking a trumpet, its frequency content is bright, bordering on harsh. They normally wouldn’t want to pair it with a microphone whose frequency response includes a boosted high-end. Instead, they’d opt for a microphone with a mellower frequency response. This might include a rolled-off top-end that reduces brightness altogether.
The same concept applies at the mixing stage. Mixing the frequencies of multiple instruments involves managing low-end, keeping the midrange defined without being harsh, and including enough high-end to sound clear without being shrill. It’s all a balancing act in which frequency content — among many other factors like volume, panning, spatial dimension, and more — plays a significant role.
When it comes to musicians and music theory, the specific notes they play have a corresponding frequency. For instance, Middle C on a piano has a measurable frequency of 262 Hz, while the A above middle C, used as tuning reference, has a frequency of 440 Hz. Music theory also tells us that doubling a frequency gives us the same note an octave higher. Halving a frequency gives us the same note an octave lower (e.g., 880 Hz is an octave above 440 Hz; 220 Hz is an octave below; they’re all As in different registers).
How Frequency Response is Measured
Sound consists of acoustic energy (i.e., waves) that radiates through the air. We measure the speed of those waves in cycles per second, or Hertz (Hz), as we mentioned earlier. The fewer cycles a wave completes per second, the lower the tone, or frequency. Higher pitches move incredibly fast, such as the case of a 20,000 Hz wave completing 20,000 cycles every second. When we’re looking at a frequency response graph from 20 to 20, 20 Hz is a low tone and 20 kHz is a high tone.
In terms of frequency response for pro audio equipment, manufacturers use specialized instruments and testing to measure the frequency response of their gear. They then plot the results on a chart showing frequency in Hz on the X-axis and level in dB on the Y-axis. It shows us the range of frequencies in addition to the volume of those frequencies across the spectrum as values above or below 0dB.
In recording and mixing, many EQ plugins helpfully feature a real-time spectrum analyzer displaying the frequency content of a signal, so engineers have a visual reference to refer to when equalizing to taste.
What is a Good Frequency Response?
0dB on a frequency response chart is considered the perfect value. Any sort of deviation above or below 0dB isn’t an outright flaw, but a colorful quirk of the given piece of equipment or signal. Plus or minus 1dB is a perceptible but very subtle deviation, while ± 3dB is more noticeable but still within reason. Higher values, like big peaks and dips, could cause issues on playback devices if the engineer doesn’t compensate for them in the mix.
Additionally, a “good” response for headphones or speakers is different than a “good” response for a microphone. When you’re critically listening while mixing or mastering, you’d ideally have headphones or speakers with as little variation as possible. Thus, a frequency response stated as 20 Hz – 20 kHz ± 3dB is quite good — it’s stating that there’s no greater peak or dip than 3dB for any frequency across the spectrum. On the other hand, people like colorful microphones with distinguished sonic characteristics. A big + 6dB boost at 10 kHz can add sparkle and air to an otherwise dull sound source.
Flat vs. Shaped Frequency Response
Flat Frequency Response
A flat frequency response means that the curve is a relatively straight line on a frequency response graph. There are no major peaks or dips where a group of frequencies gets very loud or very quiet. There’s no such thing as perfect, but minimal deviation above or below 0dB is considered flat. A microphone with a flat frequency response means it’s going to do a good job representing the source naturally, just how it is. A pair of headphones or speakers with a flat response ensures you can listen to a mix just as it is, too.
Curved Frequency Response
If something has a curved frequency response, it means there are likely big peaks or dips in certain frequency areas. You can have a big low-end roll-off resembling a high-pass filter, where the low-end frequencies don’t become “flat,” or reach 0dB, until 50 Hz or higher. You can also see boosts in the presence area around 6 kHz, for example. Microphones with a curved response usually have it for aesthetic or practical reasons. A bass roll-off near 50 Hz reduces microphone handling noise, unwanted rumble, and plosives.
On dynamic microphones, it can also help mitigate proximity effect — an unnatural bass boost that occurs the closer the microphone is to the sound source. You might also find that a slightly hyped midrange on a mic can help vocals or guitars cut through a mix, and a peak between 8 and 12 kHz can open up the top. Headphones and speakers with a curved response are where engineers can get into trouble. If you’re mixing a track on headphones with a big low-end boost, you might inadvertently end up cutting too much bass from the mix itself to compensate. The result is a mix not translating to other playback systems.
Consumer headphones or speakers, on the other hand, benefit from curved frequency responses. It’s common to see a bass boost and a treble boost on these types of products, because it can flatter the song. Not only that, but it’s more fun to listen to!
Frequency Response: Frequently Asked Questions
Is frequency response 20kHz good?
20 Hz to 20 kHz is exceptional. It accounts for the entire range of human hearing. Just be aware of a product’s ± variation, and have a look at the frequency response graph yourself if you can. That will reveal more detail about the actual sound reproduction.
How many Hz should a normal person hear?
The average person doesn’t actually hear 20 to 20 with perfect accuracy. With age and inevitable hearing loss, a person’s high-end perception decreases drastically over time. Someone in their 20s who’s seen a few concerts might only hear up to 16 or 17 kHz. A middle-aged person might be lucky to get to 13 or 14 kHz, reducing gradually over time.
What is a good frequency response for bass?
Sub-bass frequencies range from about 20 to 60 Hz, with the bulk of the perceptible low-end living between 60 and 250 Hz.
Conclusion
As you can see, frequency response is everywhere when it comes to audio. It determines what we’re able to record, what we hear once we’ve recorded it, and how something translates once it’s left the studio and played back on different systems and devices. Frequencies represent different notes and pitches in music. They also represent general zones when it comes to equalization and mixing. These include sub-bass, bass, low midrange, midrange, upper midrange, presence, and brilliance (or air). In any case, frequency is the backbone of all things audio, and an indispensible concept to learn when working with sound in any capacity.
