Internal machinery noise levels are probably one of the most ignored aspects of any testing paradigm. In the most-simplest of cases- sound field audiometry- internal noise is governed by the environmental noise level in the audiometric test booth. For very low frequency sounds, the audiometric test booth attenuation ability is quite limited. One would need a wall diameter in excess of a foot to provide significant attenuation for sounds below 125 Hz. Given that many audiometric sound booths have wall thickness on the order of 3-4”, realistically we may only be able to obtain sound field thresholds down to about 200 Hz. However, for most uses, this is more than sufficient.
Another factor that rears its ugly head in audiometric test booths is vascular noise which is the audible sound of blood rushing through the arteries. Again, this is very low frequency, and for most uses, is below the level that would typically concern us.
It may however be a different story with real ear measurement testing such as with a probe tube microphone system. The following figure shows the internal noise floor for my clinical probe tube microphone system.
To obtain this noise spectrum, one calibrates the real ear measurement system in the normal fashion, and then performs a Real Ear Unaided Response (REUR) in the conventional manner. Once done, for an “aided” run, either clamp the probe tube with your fingers tightly, or use a pair of pliers. The result will be a measurement of the internal noise floor of the probe microphone device and nothing to do with the test environment or room. All measures must be at least 10 dB greater than this noise floor or else there will be doubt whether the measurement is real or an artifact of measuring the probe microphone’s internal noise spectrum.
In practice, this is resolved by using a higher stimulus level than what would normally be used for hearing aids. Instead of using a level of 55 dB SPL (characteristic of quiet speech), one would use a level of 70 or 80 dB SPL for the measurement of the attenuation of hearing protectors.
For example, if the hearing protector provides 35 dB of attenuation at a certain frequency, one wants to ensure that the stimulus level – attenuation is greater than the noise floor. If a 55 dB SPL stimulus were to be used, then the measured level would be at 20 dB SPL (55 dB – 35 dB). If the internal noise was around 20 dB SPL at that test frequency, the measured results would be in error. However, if a 75 dB SPL stimulus were to be used with this same hearing protector, the measured value would be 40 dB SPL (75 dB – 35 dB) which most certainly would be above the noise floor.
In cases of the musicians’ earplugs such as the ER-15, there is certainty that the measured attenuation would be several magnitudes of order higher than the internal noise floor.