Microphone Correction Curves - Do You Need Them?

Microphone Correction Curves - Do You Need Them?

Many measurement microphones come with a calibration file, otherwise known as a correction curve. Even Rational Acoustics' RTA-420 has a version that comes with its own correction curve. What is the purpose of a correction curve? More importantly: Do you even need one do begin with?

What Exactly is a Correction Curve?

A correction curve is a text file that contains frequency-level data designed to complement an individual microphone's frequency response. When applied, this correction curve flattens the frequency response of the microphone for spectrum and transfer function measurements. 

A manufacturer creates these curves for each individual microphone to offset its unique frequency response. Even across the same model microphone, correction curves are not interchangeable, and correspond to the mic's serial number. Mics that come with a correction curve often ship with a USB drive or a card with information on how to download the appropriate curve off the manufacturer's website. 

For instance, Rational Acoustics sells two versions of the RTA-420: a version that comes with a correction curve tailored to the individual mic, and one that doesn't. If you purchase a mic with a correction curve but lose the file and/or included USB drive, you can download the file from Rational Acoustics' correction curve database. Since they are unique to each mic, you'll need your mic's serial number to search for its corresponding file. iSEMcon has a similar feature on their website, as well.

Are Microphone Correction Curves Necessary?

While correction curves are included with many models of measurement microphones, they aren't included with all microphones. This begs the question: why buy a microphone with a correction curve? Are they even necessary? The answer is, much like the answer to many measurement questions: it depends.

Measurement microphones (especially high-end measurement microphones) are engineered to have a largely flat frequency response. Otherwise, they wouldn't be very useful for measurement. As such, any microphone-specific frequency phenomenon that would be flattened by a correction curve would be incredibly subtle. You would likely only need a correction curve in a measurement scenario requiring incredibly granular accuracy, such as in a laboratory or manufacturer setting. In scenarios where you're taking spatial averages to measure a space, you're less concerned with these smaller frequency anomalies.

That being said, if you're using multiple microphones, it can be helpful to make a correction curve in order to match the frequency response(s) of any additional mics to your main mic. Having microphones with matched frequency response can save valuable time, placing each of them once instead of having to move a single mic multiple times. These would need to be created for your specific set of mics, and therefore be different from manufacturer-created mic correction curves.