Masking in audiometry
In audiometric testing, pure-tone stimuli of different frequencies and intensities are presented to the listener, but if a stimulus to one ear is loud enough, it can cross through the head or radiate out to the opposite ear. This isn’t ideal as the patient could respond to the unintentional stimulation of the non-test ear which may result in the stored threshold being better than it would be if the signal came from its intentional destination.
To prevent this from happening, we use something called audiometric masking - a technique used during hearing tests to ensure the accurate measurement of hearing thresholds.
Why is audiometric masking used?
Audiometric masking is used to ensure the listener is only responding to the target stimulus in the intentional test ear and not to the signal that crosses over to the non-test ear. This is done by presenting a noise or tone in the non-test ear, which prevents that ear from detecting any of the signal that crosses over.
This leads to accurate and reliable test results as the masking noise is presented at a sufficient intensity level to mask the crossing over of sounds, while also making it possible for the listener to still hear the test stimulus.
When to use audiometric masking
Masking is needed where the difference between unmasked air conduction thresholds is 40dB or more when using supra-aural headphones, or 55dB if using insert headphones. These values correspond with a transducers’ associated Interaural Attenuation (IA) value.
Interaural attenuation (IA) is the amount of signal loss caused by a travelling stimulus throughout a medium – in this case the patients’ head. As supra-aural headphones are a larger speaker, the IA value is lower as less energy is lost by this travelling signal.
For Bone Conduction transducers there is a 0dB Interaural attenuation, and it should be assumed that a non-masked response is elicited from the best hearing cochlea, as both are stimulated equally.
Where your local authority has guidelines for audiology procedures such as audiometry, it is recommended that clinicians follow masking guidance, which will often suggest how masking should be applied according to different rules. It is recommended that this information is sought depending on your location, as guidance can vary.
As an example of this, here are the BSA Audiometry guidelines which detail masking guidance and rules in section 8.6: https://www.thebsa.org.uk/wp-content/uploads/2018/11/OD104-32-Recommended-Procedure-Pure-Tone-Audiometry-August-2018-FINAL-1.pdf
How to perform audiometric masking
Here are the basic steps involved in audiometric masking:
- Determine the need for masking: audiometric masking is only needed when the sound presented to one ear might interfere with the listener’s ability to hear sounds presented to the other ear. This typically occurs when the listener’s hearing threshold in one ear is significantly better than the other ear
- Select the masking noise: the masking noise should be chosen to match the frequency of the target stimulus as closely as possible. For pure tone stimuli, this is typically narrowband noise
- Determine the masking level: the masking level should be set to the minimum requirement to mask the interfering sound without interrupting the patient’s ability to hear the target stimulus in the test ear
- Present the test stimulus: once the masking level has been determined and applied, the test stimulus can be presented to the patient in the ear being tested. It’s worth instructing the patient to focus on the stimulus here and not the masking noise
- Identify the threshold and increase the masking noise: the test stimulus should be altered to find the threshold (which may move in the presence of masking) and also increase the masking noise to find a plateau, this will ensure accurate measurement of the listener’s hearing threshold.
It’s important to note that audiometric masking should only be performed by a trained audiologist or hearing healthcare professional to ensure accurate and safe testing.
Diagnostic audiometer with masking
The Model 270+ is a powerful two-channel diagnostic audiometer with enhanced technology for a more accurate and efficient testing experience.
The device ensures completely accurate measurements, featuring an integrated masking assistant function which informs the operator when to apply masking levels, whilst also assessing the volume of the stimulus as either ideal, too loud, or too soft.
When required, the Model 270+ can also be set to automated testing where it will apply masking levels automatically, for both Air Conduction (AC) and Bone Conduction (BC).
For more information about the audiometer please visit our Model 270+ webpage, contact our customer support team on +44 (0)1865 880 846 or email.