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Inherent noise of a recording chainInherent noise (self-noise) in theory
Noise performance is extremely important in speech research. We usually record speech at normal conversational levels (50-60 dB SPL); we also record quiet speech, including whisper. Suppose we want to set the peak level at around -12 dBFS. With a low-sensitivity microphone (below 2 mV/Pa), you might have to turn the field recorder gain all the way up. If your recorder has average noise performance, you are likely to get significantly degrated recordings, with the characteristic "hissing" noise present. Therefore, it is important to be aware of the self-noise issue and make informed equipment purchasing decisions. Suppose we want to buy the Fostex FR-2LE recorder (review) and the Sennheiser HSP2 omnidirectional microphone (review). The two noise values that some manufacturers do publish most of the time are:
The ENL value for the Sennheiser HSP2 microphone is 28 dB SPL, and the EIN value for the Fostex FR-LE2 value is -129 dBu. Fostex actually does not publish the EIN value, but it was independently measured by the Avisoft Lab. How do we make sense of these two values? How can we tell if the HSP2 and the FR-2LE are a good match in terms of noise performance? To make the calculation easier, I created a look-up table, which helps you convert the microphone's ENL value into dBu, so you can directly compare it to the EIN value of your recorder. Follow the following steps:
In our example, the Fostex FR-2LE's A-weighted EIN is -129 dBu, while the Sennheiser's ENL value is -117.77 dBu, so the recorder seems to just be able to provide adequate gain without too much noise degradation. Microphones of higher sensitivity should give even better noise performance. One word of caution. You may have concluded that if you have a microphone of sufficiently high sensitivity, you can use a noisier recorder and still get very good results. While this holds true in theory, in practice, noisier pre-amplifiers are typically cheaper, and therefore built of lower quality components. They are likely to be more prone to distortion, even at seemingly low gain levels (e.g., see my review of the Marantz PMD660). I encourage you to read the materials available at the Avisoft Lab website for some interesting noise floor comparisons among popular field recorders. Finally, what about dynamic microphones? Since they can be considered virtually noiseless due to their design and operation principles, one might conclude that they should have decent noise performance with most field recorders. This, regrettably, is not true. Dynamic microphones are typically at least ten times less sensitive than condenser microphones, so they require significantly more pre-amplifier gain for equivalent signal levels. This inevitably results in higher noise. For example, the Sennheiser HMD25-1's ENL value is approximately -135 dBu, A-weighted, while its sensitivity value is 1 mV/Pa. It would require a recorder of an EIN value of at least -145 dBu for clean gain. This is very hard to achieve, by any field recorder. This is one of the reasons why I typically recommend condenser microphones for field recording. The influence of phantom power on self-noiseWhen using condenser microphones, we often forget that phantom power adds another potential source of noise and/or distortion. It is relatively easy to design and build a phantom power supply unit operating on 110 V AC, but it is significantly more difficult to build one operating on two AA batteries and achieve the same level of quality. This is why, we should always make sure that the phantom power supply unit is of high quality, especially when it is built into the recorder itself. Figure 1 shows a comparison of two spectra of the Audix HT5 self-noise with the APS911 ded icated battery power unit (left panel) and the Zoom H4N's onboard phantom power (right-panel) the RMS is left at its original settings (H4N's recording level of 80). The onboard phantom power supply adds a significant amount of broadband noise to the recording. I am very glad that the Audix HT5 is available with the APS911 unit, which not only is going to save your recorder's battery power, but is going to provide the microphone with clean voltage.
Figure 1. A comparison between two spectra of self-noise of the Audix HT5 and the Zoom H4N recorder with the APS 911 battery power unit (left panel) and the Zoom H4N's onboard phantom power Inherent noise in the real worldThe theoretical self-noise calculations are only a part of the picture. It is surprisingly easy to overestimate the value and usefulness of technical specifications; noise performance being influenced by a number of factors beyond what the EIN value alone might indicate. We should be able to have a more tangible idea of the noise levels captured by our recording chain in a typical speech recording scenario. Of course, the same theoretical principles apply, but we should try to estimate the actual noise levels when recording speech in a quiet environment at typical conversational levels (calibrated to peak values of around -12 dBFS). I have, therefore, set up a simple test to estimate real-world self-noise levels. All the microphone tests publish on this website, have been tested by this methodology. I use a sound-treated booth, I record with the Sound Devices USBPre (one of the quietest field recorders), into a laptop computer running on battery power (to avoid possible ground loop problems). The microphone is at the distance of about 4 feet from the laptop computer, which is the only appreciable source of noise. The laptop uses SSD storage and its fan is disabled. Some level of noise from the environment (especially low-frequency rumble) is unavoidable. The setup is meant to introduce a source of low, but controlled of noise into the environment, and to always calibrate the pre-amplifier's gain to the same signal RMS across all the tested microphones (equivalent of RMS of 70 dB SPL, measured on a typical, sentence-long voice recording sample at normal vocal effort and conversational volume). You can read more about volume calibration techniques here. The resulting noise in the digital audio file is a mix of environmental sound picked up by the microphone, the noise made by the electronics of the pre-amplifier, quantization noise from the A/D converter (a negligible amount in this case), and self-noise generated by the electronics of the microphone itself. All sources of noise except the microphone's self-noise and pre-amp noise, are kept constant across all tested microphones. Pre-amplifier noise tends to be more severe at high gain settings, particularly on low-quality, inexpensive portable recorders. In this test, we measure not just the levels of noise, but also its power spectrum (FFT). Figure 2 shows a comparison of self-noise spectra of Opus 55 MkII (left panel) and Shure Beta 53 (right panel) with the SoundDevices USBPre, tested according to the above principles. The spectra allow you to study self-noise levels and energy distributions of each microphone in a real-world recording situation.
Figure 2. Comparison of ambient noise spectra of Opus 55 MkII (left panel) and Shure Beta 53 (right panel) |
Every recording chain produces some noise due to electrostatic activity. This kind of noise is often referred to as "inherent noise" or "self-noise." Let's take a simple field recording chain as an example. When matching a microphone with a recorder, it is important to make sure that the recorder (or its microphone pre-amplifier, to be exact) does not appreciably degrade the microphone's noise performance. The task is often difficult because of the lack of standard protocols and nomenclature in describing self-noise. To make matters worse, some manufacturers do not even publish noise floor data for their products.
