What to expect when using lower quality measurement microphones

The main challenge with using lower quality measurement microphones is that performance issues can be too faint to notice, unless you are constantly monitoring the microphone sensitivity using a calibrator. This doesn’t mean there are no differences, but rather that the flaws can be subtle and may have a significant impact on the accuracy - and reliability - of your measurements over time, especially under specific conditions.

(Note: Actual performance can vary from data sheet specifications.)

One of the most common issues with measurement microphones is particle contamination, which affects cleanliness and performance. In precision microphones, the space between its backplate and the diaphragm is around 20 µm, defining the microphone’s condenser capacity. To put this in perspective, a strand of hair ranges from 60 to 100 µm in diameter, which is 3 to 5 times thicker.

Any particles trapped inside the microphone cartridge can work their way under the diaphragm, impacting the capacity and/or the polarization voltage, thus altering the microphone’s sensitivity and the noise floor. Changes in temperature and the humidity can exacerbate this by causing condensation around the particles, which amplifies sensitivity drift and raises the noise floor.

A company has published a counterview after exposing their microphones to high humidity tests. They claim that although their microphones showed changes in sensitivity during the tests (more than 1.5 dB variation), its sensitivity only experienced a small change when dried out (+0.3 dB from the initial sensitivity). While this may be true, at HBK we do not consider this acceptable performance.
Our experience indicates that the microphones weren’t clean enough and will not maintain stable sensitivity.

During an experiment, we exposed forty microphones to a week of high humidity. The result was 40% exhibited a sensitivity change greater than 0.2dB, with some losing more than 4dB, while the HBK precision measurement microphones showed non-measurable sensitivity change.

What are your thoughts on the long-term stability of the microphones, with a sensitivity change of >0.2 dB, when exposed to humidity?

In short, not really.

The sensitivity of the microphone will drift throughout the day, requiring constant recalibration. Correcting the sensitivity won’t solve all the issues because changes in polarization voltage will also affect the diaphragm tension, impacting the high-frequency response of the microphone.

Figure 4. Healthy vs unhealthy microphone response before and after sensitivity correction.

They might appear simple, but high-quality microphones rely on carefully selected materials to ensure stability under changing temperatures and static pressures. Without this meticulous construction, microphones are prone to hysteresis, meaning that exposure to temperature variations or static pressure shifts parts of the cartridge, such as the diaphragm, into new positions altering the equilibrium point, causing lasting sensitivity changes.

In a poorly designed microphone, sensitivity changes can persist, even after returning to ambient conditions, causing unreliable results in measurement situations.

Inconsistent microphones produce inconsistent results, with errors becoming more pronounced over time and in varying environmental conditions. With lower-quality measurement microphones, you may experience:

• Reduced Accuracy - resulting in unreliable data.
• Higher Noise Floor - decreasing signal to noise ratio.
• Limited Frequency Response – causing inaccuracy within the required frequency range for specific kinds of measurements.
• Calibration Issues - these microphones will not hold their calibration, leading to drift over time and the need for more frequent recalibration.

The microphone is the first link in the measurement chain and certainly the most critical. The signal quality captured by the microphone must be the best possible; a microphone with high uncertainty will result in a poor measurement setup and uncertainties for the entire measurement chain. 

Is saving a small amount today, with a lower quality microphone, worth the risk of compromising long-term measurement precision and accuracy?