Here a summary of finished projects are found.

Children and Hearing Aid Gain – A study at Alviksskolan, Stockholm (2008)

Aim

The project was undertaken to document the hearing aid gain used by school-aged hearing-impaired children.

Procedure

In total 35 children with sensorineural hearing loss (19 boys and 16 girls, median age 9.9 years, ranging from 7.3 to 14.6 years) participated in the study. The basic goal was to measure the gain the children normally use. During a short excerpt from a TV program, containing a dialogue between a female and a male talker at a presentation level of 65 dB SPL, the participants selected hearing aid program and changed the volume control so that the speech was as clear as possible and with the appropriate loudness. Using an Aurical test equipment, real ear insertion gain measurements were performed using modulated speech-weighted noise at 65 dB SPL. These measured insertion gain results were compared to individually calculated NAL-RP gain targets. Coupler gain measurements were performed using modulated speech-weighted noise at 50, 65, and 80 dB SPL and output measurements were performed for pure tones at 90 dB SPL. All gain measurements started after a 15 second pre-conditioning period.

Main findings

In the frequency range 250-2000 Hz the median gain used resembled that of the NAL-RP prescription (apart from the fact that the “NAL-boost” at 1 kHz was not mirrored). At higher frequencies, the median gain used was less than that of the NAL-RP prescription. The coupler gain measurements were used to calculate two static “compression ratios” at each frequency. The calculated compression ratios were fairly low, ranging from 1.1 to 1.7. Test-retest data on the program choice and VC setting showed that most participants produced hearing aid settings that were highly reproducible.

Hearing aid noise reduction – Measured long-term average gain reduction for 12 modern hearing aids (2008)

Aim

The aim of this project was to get a rough picture of the differences between various modulation-based noise reduction algorithms implemented in a number of hearing aids on the market 2008.

Procedure

Long-term average gain measurements were performed in a dedicated hearing aid test box. Gain-curves from the hearing aids were first measured by setting most of the features to OFF and then another set of corresponding gain-curves with the noise reduction set as default suggested by the different fitting software (ON). The gain reduction accounted for by the noise reduction algorithm was then calculated from the two corresponding gain-curves (OFF – ON). The hearing aids were programmed with three different audiometric hearing losses. To study the dependence on the Signal to Noise Ratio (SNR) of the input signal, the hearing aids were presented with various signal mixes with different SNRs. The input signal levels were set to have a fixed speech level at 62 and 75 dB SPL. As speech signal the new ISTS signal produced by the ISMADHA-group was used. The ISTS is a real speech signal mix of different female speakers reading the same text in six different languages. The speech-signal used here is a draft submitted to both IEC and ANSI for standardization. The noise signal was IRCA1, an un-modulated speech-weighted noise signal.

Main findings

Gain and Acclimatization – Measured long-term average gain for12 modern hearing aids (2008)

Aims

The aims of the study were

1. to document differences in prescribed gain for a selection of hearing aids implementing the NAL-NL1 prescription.
2. to document how acclimatization stages are used to adjust the prescribed gain.

Procedure

The hearing aids were set to two different audiogram configurations and measured in an acoustic test chamber using a 2cc-coupler. Two different ICRA noises were presented at three different input levels: ICRA4 at 50 and 65 dB SPL and ICRA8 at 85 dB SPL. All hearing aids were tested in two software conditions, the lowest and highest degree of user experience.

Main findings

The study provides two main conclusions.

1. The measured gain for hearing aids programmed with the same generic prescription, NAL-NL1, differed with over 20 dB for certain input levels. This means that some implementations of the NAL-NL1 are unacceptable, and it is impossible for an audiologist to know what a “NAL-NL1 fitting” represents.
2. Hearing aid manufacturers have chosen to implement gain acclimatization stages in very different ways. Some manufacturers do not employ gain acclimatization at all, whereas others reduce the gain for a first-time hearing aid wearer up to 10 dB. Most manufacturers who have implemented acclimatization stages reduce the gain in a uniform way across input level, but three manufacturers reduces the gain more for low-level sounds than for high-level sounds.