Frequency response describes the specification of equipment like microphones and speakers. As defined, frequency response is the measured output of a device when given an input signal. The response is charted as amplitude over frequency. These are expressed as graphs known as frequency response charts. In the audio world, frequency response is similar to how color accuracy and color gamut apply to workflows and equipment design and calibration. In other words, frequency response is about consistently defining, measuring and maintaining accuracy across the creative process.
For a deeper dive, head on over to our article on the audio spectrum.
Measuring frequency response
Standardized and accurate data gathering is crucial to building a consistent, trustworthy system of measurements. This methodology allows frequency response to function as a like-for-like means of comparing a variety of audio equipment like microphones and speakers.
If you happened to read our guide on acoustics, you’re already familiar with the measurement methodology. A reference signal is sent through the device input that sweeps through the frequency spectrum. The reference signal must use constant amplitude. There are also several types of digital and analog signal types available for use.
Flat vs. shaped frequency response
Audio frequency response curves come in two distinct types, flat and shaped. Both curve types feature in microphone and speaker designs. Adjustable curves that use digital signal processing (DSP) have been crossing over into consumer theatre systems for some time now. While not a distinct curve design, the technology has matured over the last fifty years and is worth considering.
Shaped response curves
What does a shaped frequency response sound like, and how does it impact my work? Both of these are great questions. The answers to which should make the user experience more meaningful.
A microphone’s exact shaped response is the byproduct of its design and electrical component selection. This is a big part of why microphones have different timbral qualities — also explaining their ability to add texture and color to a recording.
For example, take the Shure SM7B and its reputation for delivering rich, smooth vocal recordings. The SM7B is a dynamic cardioid microphone. Its dynamic side makes it capable of taking a beating, and vocalists can really project into it without fear of distortion. That’s not to say it is a slouch at regular volumes. The cardioid pattern focuses the pickup pattern towards the front and makes it a great fit for broadcasts by rejecting most of the sound around it. More on this microphone later.
Shaped response curves are common across most speakers and microphones for several reasons. These factors are primarily down to product and consumer category, which directly determine the cost and the device’s intended use.
Flat response curves
Let’s refer back to the color gamut and accuracy analogy. Flat frequency response in speakers is necessary for the accurate reproduction of sound. Just like their visual counterparts, a good pair of studio speakers deserve to go into a suitable room. Nonflat response speakers are most commonly used in consumer systems because they can get away with less accuracy. So much so that most people prefer a shaped curve for day-to-day listening because flat response curves can sound boring or lacking drama to casual listeners.
Achieving a flat response comes with one drawback: cost. The cost factor comes down to electrical components, bias and tolerance. Flat frequency curves are not naturally occurring and are purely artificial.
Microphones with a flat response also exist but make up a smaller portion of models overall. Model types include reference vocal microphones and calibration microphones. Their primary use is for vocal auditions and room calibration, respectively.
The Behringer ECM8000 is an affordable calibration microphone with an XLR interface. It’s used for measuring and calibrating rooms and can pair with Behringer’s DEQ2496 EQ analyzer processor.
Home theatre systems opt for a more portable interface in the form of a custom or USB connector for their calibration microphones.
One area that has gained significant traction outside pro audio circles in the last decade is automatic speaker calibration. The idea behind it is to allow sound systems to adjust their output frequency response to compensate for the room’s frequency response. Digital calibration debuted on studio monitor speakers before crossing over to home theater systems.
Adjustable calibration is particularly interesting because of its possible applications. In one instance, calibration can further increase the accuracy of studio monitors by better tuning them around the listening environment.
Another use of calibration works around the electrical bias in lower cost and less accurate speakers to improve the frequency response, using DSP to flatten the shaped curve. While this can sound more than a little cheeky and with a hint of possible false advertising, the benefits are clear. Adjustable speaker response has become another tool in the quest for better-sounding speakers. We cannot deny that the average consumer-grade speaker sounds massively better than what came before.
There are limits to calibration technology, and it is not a substitute for room treatment or poor speaker design.
Auto calibration at its best is a form of fine-tuning that enhances accuracy and helps smooth out rough spots. Quality varies across price points, with a good indicator being the supplied calibration microphone. A low-quality calibration microphone bodes less positively for the rest of the process, to the extent where the calibrated result may sound different instead of sounding subjectively or quantifiably better.
High-end home theater receivers, like the Anthem MRX 740, include more capable calibration functionality and microphones.
This opens up several possibilities, and their success depends on several factors. There are limits to what automatic calibration can achieve based on the quality of the calibration software, hardware and room quality.
Interpreting frequency response charts
How does one explain the Shure SM7B’s ability to flatter with deeper and smoother-sounding dialogue? The answer is: quite easily with the use of a frequency response chart. Reading a chart is easy, with frequency in Hertz occupying the x-axis and amplitude in decibels along the y-axis.
Let’s take a look at the below frequency response chart for the Shure SM7B and see what the story is. Starting with its design, the SM7B is a large-diaphragm dynamic microphone with a cardioid pattern. It also has a party piece in the form of three adjustable frequency response profiles: flat response, bass roll-off and presence boost.
The potential for the proximity effect is real, which is caused by addressing a microphone too closely. This is an easy habit to form and is generally best avoided. There is no need to get right against the foam filter or grill when speaking into a microphone. This will result in exaggerated low-frequency pickup, known as the proximity effect.
It doesn’t take long to see where the potential for smooth-sounding dialogue originates. Using the flat response setting, we start with a short and shallow dip of around -4 dB in the 50 Hz range that flattens out by 100 Hz. In this configuration, the microphone is barely rolling off and has low frequencies.
Moving on, we immediately note a mostly flat trend of the 0 to -1 dB all the way to the 4.5 kHz range. There is no real attempt to boost or cut the lower and mid-frequency ranges, which results in their prominence in the absence of any additional air or presence.
There is a slight notch upward of around +2 dB at most in the 5 to 6 kHz range that in turn gives way to a dip of the same size from 7 to 9 kHz before rolling off the highest frequencies altogether. This last bit seals the fate of the Shure SM7B by significantly cutting out the frequencies responsible for a sense of airiness in a recording.
This is helpful in broadcast and streaming setups by focusing solely on dialogue and with less sensitivity to environmental transient sounds.
Flipping the bass roll-off switch cuts the 50 Hz response by -10 dB. The rolled-off slope does not reach 0 dB until 3.5 kHz. This significant reduction cuts the perceived loudness of sounds in the low and low-mid frequency by about a third.
As for the presence boost setting, it contributes a +2 to +4 dB bump, starting at 1 kHz and extending all the way to 7 kHz This setting brings out many core frequencies in the human voice and can lend clarity to dialogue that initially sounds muddy.
In its own way, the presence boost allows the Shure SM7B to function more like its conventional siblings, like the SM58. While the former has a strong following among broadcasters and streamers alike, it holds a special place for vocalists in the studio. The SM7B handles louder vocals with grace and shines in various applications ranging from hip-hop to rock vocals.
Understanding frequency response will give you more comprehensive control over your audio equipment. Choosing and calibrating the right mic or speaker system can make a big difference in the quality of your audio, so it’s worth your while to get to know this fundamental aspect of audio recording technology.