14.10.2009
Demand for solid precision instruments for objective audiometry rose sharply after the German government's decision to include newborn hearing screening in the catalogue of public health insurances in 2009. Recently, PTB's assessment of the hearing thresholds of high-efficiency "Chirp" test signals has enabled reliable calibration of hearing screening equipment and fostered the sale of large quantities.
The basic concept of electroencephalography (EEG), i.e. measuring the brainwaves by means of electrodes connected to the scalp, can be applied to an objective test of hearing ability. It is even possible to examine very young children while they are asleep. Because the sense of hearing is an essential prerequisite for learning to speak, newborn hearing screening was introduced as a routine test in Germany in 2009.
In order to produce clearly detectable brainwaves, even if presented very softly to the babies' ears, the test stimuli should be as efficient as possible in evoking the brain responses. Therefore, click trains have always been applied, instead of the pure tones normally used in conventional audiometry. Furthermore, newly developed chirp signals are even more efficient than click trains.
Hearing ability is described by means of a scale based on the mean hearing threshold of otologically normal, young people. The "zero line" is called "reference hearing threshold", and it is specific to each headphone model and each acoustic stimulus. Therefore, it is necessary to determine the reference hearing thresholds, separately for each combination of headphone model and acoustic stimulus, by means of expensive experiments with groups of test subjects.
In cooperation with a German medium-sized audiometer manufacturer, the reference hearing thresholds of various chirp signals were determined at PTB. In the course of the investigations, a number of basic characteristics of these signals was explored. The reference hearing thresholds provide the basis for a precise calibration of the most recent screening audiometers, and, consequently, the test results of different medical centers and physicians become comparable and diagnostically conclusive.
Click the buttons and listen to the signals. Can you hear the difference?
Figure 1: Earphone terminal voltage of a click signal (left) and a chirp signal (right)
By the way: The chirps do not really sound like birds' twittering - they would, if played back in "slow-motion". Actually, both click and chirp - when played in quick repetition series - sound like a rattling noise. Astoundingly, they sound very much alike, despite the chirps' much higher efficiency for the brain response.
Contact person:
Johannes Hensel, FB 1.6, AG 1.61, E-Mail: Johannes.Hensel@ptb.de