Some modalities in hearing diagnostics involve the measurement of sound pressure in the ear canal. Measuring a correct sound pressure in the first place is also the foundation for estimating other related quantities, e.g., the sound pressure at the tympanic membrane, acoustic intensity, or sound power transmitted to the inner ear.
Modalities in hearing diagnostics involving the measurement of sound pressure in the ear canal include among others otoacoustic emissions (OAE). In OAE measurements, a sound produced by well-functioning hair cells in the cochlea that is loud enough to be recorded by a microphone positioned in the ear canal of a patient (i.e., a test-subject) is measured. The OAE measurements may provide relevant information of a patient's hearing capabilities and may assist in identifying damage of the ear canal potentially providing indication of a hearing loss. Especially, OAE measurements are useful when testing infants and/or younger children.
For assisting in the recording of OAEs in the ear canal, a diagnostic tool, configured to provide objective information of different pathologies of the ear by use of a series of measurements, is used. In more detail, such diagnostic tool comprises a handle element and an acoustic unit, such as a probe unit configured to create stimuli and/or sound signals in the ear canal of, e.g., a human test-subject. The probe unit generally consists of at least one output unit, such as a receiver and a sound input unit, such as a microphone. In addition, such diagnostic tool may further comprise a pressure unit configured to cause a changing pressure in the ear canal of a test-subject, such as in pressurized OAE measurements. The probe unit as such is an element, which is configured to output stimuli signals for provoking an ear canal response, where the input unit, such as a microphone, measures ear-canal responses.
Thus, when performing diagnostic measurements related to measuring an accurate sound pressure in the ear canal, it is important that the input sound unit (e.g., a microphone) records the most accurate and correct measurements as possible. One factor that may influence such recording is the sensitivity of the microphones used in probe units, and it is therefore of importance to know the sensitivity of the microphone, when evaluating different diagnostic measurements.
The microphones used for probes (i.e., probe units) in hearing diagnostics are mostly of a commercial type (same as in hearing aids) where variations in sensitivity across the frequency spectrum is to be expected. Furthermore, there are often differences between the sensitivity of different microphones of the same type used in different probes. Such microphones can therefore not be assumed to have a flat frequency response. It should be noted that reference microphones used for calibration of the sensitivity of probe microphones as for instance described in the following do normally not suffer from such problems. Accordingly, for obtaining the most accurate response from the ear canal when exposed to a stimulus and recorded by a probe microphone, a calibration is often carried out to obtain the complex probe microphone sensitivity that relates the voltage output from the microphone to the sound pressure at the tip of the probe unit.
Currently, the most precise way to obtain the probe microphone sensitivity is by placing the probe microphone in an acoustic free-field next to a reference microphone and thereby assuming that the two microphones are subject to the same sound pressure. This is a rather extensive measurement since it, preferably, requires an anechoic chamber or a sound box, is quite sensitive to noise and not very convenient in the context of a calibration procedure for a diagnostic probe.
A more simple method is to insert the probe in a small coupler (e.g., the commercially available G.R.A.S. 0.4 cc coupler) and assuming that the sound pressure at the probe microphone and a reference microphone of the coupler is equivalent. This is, however, not the case towards higher frequencies due to standing waves in the cavity and the probe being inserted opposite to the reference microphone. The result of these standing waves is a large error towards the ¼-wavelength resonance of the coupler since the sound pressure at the probe cancels out. A rough sketch of such setup is depicted in FIG. 1.
To reduce the effect of this cancellation (i.e., the error caused by standing waves), one option is to reduce the size of the coupler thereby transitioning the notch (i.e., the error) towards higher frequencies. This has been tested using a special insert in the 0.4 cc coupler reducing the volume to approximately 0.13 cc. This transitioned the notch to approximately 17 kHz, but significant errors were still present due to impedance mismatch towards the notch. Smaller cavities are not feasible due to limitations in the physical size of the reference microphone.
Rasetshwane and Neely [2] describe a method in which a probe-tube microphone is initially calibrated at the end of a waveguide using a reference microphone. The probe for which the microphone sensitivity is to be found then replaces the reference microphone and the probe-tube microphone is now used as the reference microphone. This procedure is, however, also quite extensive since it besides the reference microphone also requires an additional external microphone and sound source.
Hence, there exists a need for a microphone calibration method that makes it possible to obtain the sensitivity of a microphone, such as a probe microphone used for instance in hearing diagnostics, over a very wide frequency range without the need of an acoustic free-field measurement or any external transducers other than a reference microphone. There further exists a need for a microphone calibration method that does not require a coupler or similar device, the physical dimensions of which are not so small that it poses a problem due to limitations of the size of the reference microphone.