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Measurement of subjective hearing effort: "ACALES" - a new method for hearing acoustics

What is hearing effort and how can it be measured?

Hearing effort has been discussed in scientific circles for many years. This refers to the fact that conversations in a sociable environment, such as a café, are perceived as more strenuous than in a quiet environment, even though most of what is being said can be understood. In order to understand everything despite the background noise, increased concentration is necessary. This is the case for both people with good hearing and those with hearing loss. In most situations where persons understand most of what is being said, but speak with increased listening effort, the speech volume is higher than the volume of the background noise. The typical listening situations for this have a positive signal-to-noise ratio (SNR) between +5 and +15 dB. Although high individual speech intelligibility can be achieved in these positive SNR ranges, hearing-impaired individuals in particular report that it takes increased effort to understand this speech. For affected individuals, hearing effort plays an important role and is one of the reasons why unprovided hearing impaired individuals withdraw from social life. However, hearing effort is also of great importance for hearing impaired people, as it can vary with different hearing systems and settings and thus additionally contributes to differentiation in the selection process.

In science, both objective and subjective methods have been developed to measure auditory effort. The objective methods include physiological methods such as the measurement of alpha waves in the brain (EEG measurements) and the determination of the change in pupil width (pupillometry), as well as the so-called dual-task paradigms, in which cognitive tests are performed in parallel with the listening task. The secretion of stress hormones such as cortisol is also used to measure listening effort. It is also measured cumulatively as exhaustion ("fatigue"), as a result of recurrent effort, at the end of a day.
Diese Verfahren sind zwar schnell und flexibel einsetzbar, dafür aber nicht sensitiv genug, um kleinere Effekte, wie beispielsweise zwischen unterschiedlichen Hörsystemen oder verschiedenen Einstellungen von Hörsystemen zu zeigen.
Since these methods are associated with a large measurement effort, subjective methods have been developed in recent years to determine the hearing effort as close to practice as possible. These methods mainly work with scales and questionnaires. For example, hearing effort in everyday life can be assessed using various gradation scales or questionnaires can be used to record the overall situation before and after a hearing system fitting.

Although these procedures are fast and flexible, they are not sensitive enough to show smaller effects, such as between different hearing instruments or different settings of hearing instruments.
Until now, therefore, there was no procedure with which a hearing care professional could measure hearing effort quickly and without additional equipment in a valid and reproducible manner in everyday professional life.

A new method: Adaptive Categorical Listening Effort Scaling (ACALES)

The Oldenburg research group around Melanie Krüger and Michael Schulte, have been working for several years on the scaling method - "ACALES: Adaptive CAtegorical Listening Effort Scaling - to measure subjective listening effort. This method is ready to be used as a measurement tool in the everyday life of hearing care professionals. When measuring subjective listening effort, the client rates how strenuous it is to follow a speaker in noise using a 14-point scale from "effortless" to "extremely strenuous" with an additional category of "noise only." During the measurement, the client is presented with two sentences from the Oldenburg Sentence Test in succession in a background noise.

Figure 1: Customers independently rate the subjectively perceived hearing effort on a 14-step scale from "effortless" to "extremely strenuous" with the additional category "noise only".A separate tablet or smartphone can be used, which is connected via Bluetooth.


Based on the client's individual discomfort threshold, the hearing care professional can set the maximum presentation level. It is also possible to select the start level of the speech material individually (default is 65 dB). It is important for the comparability of the results at the end to set the start level identically for all measurements. After a written instruction, a short training takes place, during which the client gets acquainted with the measurement procedure and the background noise. The training duration depends on the response behavior of the customer and lasts between 30 seconds and 2 minutes.
Das Ergebnis der Messung ist eine Höranstrengungsfunktion, die den individuellen SNR-Bereich für „mühelos“ bis „extrem anstrengend“ umfasst.
The special feature of the adaptive measurement procedure is the automatic individual calculation of the level variations to be tested (SNR), which are generated by the changed ratios of speech volume to ambient noise volume.
Based on the previous assessment of perceived listening effort, the SNR is recalculated and different effortful listening situations are created that mimic everyday life. The result of the measurement is a hearing effort function that encompasses the individual SNR range for "effortless" to "extremely strenuous".
Since ACALES allows a comparison of the listening effort of different hearing systems and even of different algorithms of a hearing system, but can also illustrate the development of the listening effort over time, the results of a total of up to four measurements can be clearly displayed in a graph. The user can choose from different representations, namely representation of the hearing effort 1.) over all hearing situations, 2.) for difficult and simple hearing situations (optimally suited for clients) and 3.) the detailed results for the audiological expert and interested client, cf. Figure 2 with an example of a reference measurement (unaidable) and a measurement with hearing aid A.

Figure 2: At the top you can see the client modes once for all hearing conditions and separately for difficult hearing situations as well as the expert mode, for audiological experts and interested clients.


With the help of ACALES, the hearing care professional can easily and comprehensibly present the client with the influence of a hearing system fitting or a fine-tuning. The measurement of subjective listening effort is particularly suitable when speech intelligibility measurements cannot show any differences between the hearing systems or the maximum of 100% speech intelligibility has been reached.


Case studies from everyday life

For the use of the method in everyday life, we would like to explain the range of application and practicality with the help of three case studies.


1. Influence of a hearing aid fitting on hearing effort
This client is 78 years old at the time of measurement and has been wearing hearing instruments for 11 years. He has a moderate hearing loss. With hearing instrument fitting, he has an improvement in speech understanding in the Oldenburg Sentence Test of -1.7 dB SNR (SRT). The positive effect of the hearing system fitting is not only evident in speech intelligibility, but also in the subjective hearing effort measurement. The hearing effort is reduced by the fitting over the entire SNR range, in some cases very significantly. Let's take a look at the SNR of 4.5 dB. In this hearing situation, the subjective hearing effort in the fitted condition is perceived as "effortless", but without hearing instruments with almost 5 ESCU (Effort Scaling Categorical unit), which corresponds to a hearing effort of "little effort".

Figure 3: Case study for a comparison of a supplied and unsupplied measurement. The presentation of results is an excerpt from a scientific study.


2. Comparison of different hearing systems and comfort functions
This client is 67 years old at the time of measurement and has been fitted with hearing systems for two years and has a moderate hearing loss. He tests two different hearing systems for which the result after measuring speech intelligibility is almost identical (hearing system A: SRT -7.9 dB SNR; hearing system B: SRT -7.8 dB SNR. For the measurement of individual listening effort, speech was presented frontally and background noise was presented from obliquely behind (135°). Speech-simulating background noise was used to simulate a conversational situation.
The figure shows that the subjectively perceived hearing effort differs visibly with the two hearing instruments and the client perceives less hearing effort with hearing instrument B. Depending on the SNR range, the hearing effort is reduced by up to three rating categories. This means that the client / patient is exposed to less hearing effort throughout the day by wearing hearing aid B and consequently the cumulative effort at the end of the day is lower.

Figure 4: Case study comparing two hearing systems. The presentation of results is an excerpt from a scientific study.


3. In the course of fitting cochlear implants and hearing instruments
The third case study illustrates that hearing effort measurement is also suitable for cochlear implant wearers. This patient was implanted at the University ENT-Kllink Oldenburg by Director Prof. Dr. Radeloff and followed in a longitudinal study. She is 70 years old at the time of implantation and had hearing loss in the left and right ear for many years and received a cochlear implant in the right ear. The following plot shows the changes in individual hearing effort function at different time points before and after surgery.
The dark dashed hearing effort curve shows how hearing effort is assessed before CI implantation with beidohr hearing aid fitting. It should be taken into account that signals are perceived more quietly due to the inadequate fitting. Immediately after the initial adjustment of the cis, the ACALES measurement is repeated. It becomes clear that the subjective hearing effort has increased significantly. This is not surprising due to the new sound perception. It is interesting to see that the hearing effort is already reduced after three months. We are now waiting with anticipation to see how the hearing effort continues to change after six, nine and twelve months.

Figure 5: Case study for the measurement of hearing effort in a patient at different time points of CI fitting. The presentation of the results is an excerpt from a scientific study.



The ACALES measurement procedure makes it possible to measure the mental 'listening load' or listening effort that a person must expend to understand speech in noise. This is an additional dimension to speech intelligibility.
First, differences in perceived listening effort with and without hearing aid fitting can be shown. This allows the user to demonstrate the influence of the hearing aid fitting on the subjectively perceived hearing effort and to use this for hearing education. With the help of ACALES, the client/patient evaluates both everyday and challenging listening situations. Furthermore, the subjective added value of comfort features, such as noise algorithms or directional microphones, can be measured. Based on the evaluation, the hearing care professional can determine metrologically with which hearing systems affected persons not only understand best, but can also hear most easily. At the same time, the visualization of the personal hearing effort provides the person affected with an important decision-making aid in the finding phase for selecting the hearing system with the least hearing effort. The procedure extends the fitting process to include the component of subjective everyday benefit and thus provides support for client counseling at the individual level.
Another area of application is the documentation of the habituation effect to hearing aids over time or the rehabilitation process after cochlear implant surgery, in which the subjective hearing effort is determined at regular intervals.
The recording of the individual hearing effort with ACALES can supplement the classic speech tests with a component of subjective everyday use.

Further reading

Krueger, M., Schulte, M., Brand, T., & Holube, I. (2017). Development of an adaptive scaling method for subjective listening effort. The Journal of the Acoustical Society of America, 141(6), 4680-4693.

Krueger, M., Schulte, M., Zokoll, M. A., Wagener, K. C., Meis, M., Brand, T., & Holube, I. (2017). Relation between listening effort and speech intelligibility in noise. American journal of audiology, 26(3S), 378-392.

Krüger, M., Schulte, M. , Holube, I (2015). Entwicklung einer adaptiven Skalierungsmethode zur Ermittlung der subjektiven Höranstrengung. 18. Jahrestagung der Deutschen Gesellschaft für Audiologie, Bochum.

Krüger, M., Korte, J., Schulte, M. , Holube, I (2016). Evaluation einer adaptiven Skalierungsmethode zur Ermittlung der subjektiven Höranstrengung für schwerhörige Versuchspersonen. 19. Jahrestagung der Deutschen Gesellschaft für Audiologie, Hannover.

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