Audio amplification apparatus

1. A method of adjusting frequency-dependent amplification in an audio amplification apparatus, the audio amplification apparatus including:

a forward transfer path connectable to an output transducer, the forward transfer path including a frequency transposing element; the method including the steps of:

presenting stimuli to the output transducer at a plurality of frequencies;

adjusting the stimulus level at each frequency to meet a predefined loudness perception level or detection threshold of the listener;

deriving an equal loudness contour of output transducer levels from the adjusted stimuli levels; and

deriving the frequency-dependent amplification of levels of input signals at each frequency from the equal loudness contour at the corresponding transposed frequencies.

2. A method according to claim 1 , wherein the frequency-dependent amplification at each frequency is derived by subtracting the magnitude of a standardised input signal component at that frequency from the magnitude of the stimulus level at a transposed frequency.

3. A method according to claim 2 , wherein the magnitude of the input signal component at each frequency is the level of international long-term average speech spectrum (ILTASS) at an overall level of 70 dB SPL at that frequency.

4. A method according to claim 1 , wherein the stimuli are applied directly to the output transducer.

5. A method according to claim 1 , wherein the predefined loudness perception level of the listener is selected from the group of: very soft, soft, comfortable but slightly soft, comfortable, comfortable but slightly loud, loud but OK, uncomfortably loud and extremely uncomfortable.

6. A method according to claim 5 , wherein the predefined loudness perception level of the listener is comfortable but slightly soft.

7. A method according to claim 1 , wherein the predefined loudness perception level of the listener is determined by:

the listener choosing a descriptor of the stimulus level at each frequency; and

adjusting the stimulus level until the chosen descriptor meets the predefined loudness perception level of the listener.

8. A method according to claim 1 , wherein the predefined loudness perception level of the listener is determined by:

presenting a reference stimulus to the output transducer at one of the plurality of frequencies, the stimulus level meeting the predefined loudness perception level of the listener;

presenting stimuli to the output transducer at the other frequencies; and

adjusting the stimulus level at the other frequencies to match the listener's loudness perception level to that of the reference stimulus.

9. A signal processing device for use in an audio amplification apparatus, the device acting to receive digitised sound signals and generate output transducer signals, the sound processing device including:

processing means for use in performing a method according to claim 1 .

10. A signal processing device according to claim 9 , wherein the processing means is implemented in digital signal processing (DSP) technology.

11. A method of adjusting frequency-dependent amplification in an audio amplification apparatus, the audio amplification apparatus including:

a forward transfer path connectable to an output transducer, the forward transfer path including a frequency transposing element; and

a feedback path adding a feedback signal to the forward transfer path, the feedback path being disconnected from the forward transfer path during fitting; the method including the steps of:

applying a standardised input signal to the forward transfer path so as to apply stimuli to the output transducer at a plurality of frequencies; and

adjusting the frequency-dependent amplification at each frequency so that the stimulus level meets a predefined loudness perception level or detection threshold of the listener.

12. A method according to claim 11 , wherein the magnitude of the input signal component at each frequency is the level of international long-term average speech spectrum (ILTASS) at an overall level of 70 dB SPL at that frequency.

13. A method according to claim 11 , wherein the predefined loudness perception level of the listener is selected from the group of: very soft, soft, comfortable but slightly soft, comfortable, comfortable but slightly loud, loud but OK, uncomfortably loud and extremely uncomfortable.

14. A method according to claim 13 , wherein the predefined loudness perception level of the listener is comfortable but slightly soft.

15. A method according to claim 11 , wherein the predefined loudness perception level of the listener is determined by:

the listener choosing a descriptor of the stimulus level at each frequency; and

adjusting the stimulus level until the chosen descriptor meets the predefined loudness perception level of the listener.

16. A method according to claim 11 , wherein the predefined loudness perception level of the listener is determined by:

presenting a reference stimulus to the output transducer at one of the plurality of frequencies, the stimulus level meeting the predefined loudness perception level of the listener;

presenting stimuli to the output transducer at the other frequencies; and

adjusting the stimulus level at the other frequencies to match the listener's loudness perception level to that of the reference stimulus.

17. A method of determining activation levels for feedback suppression in an audio amplification apparatus, the audio amplification apparatus including:

a forward transfer path connectable to an output transducer;

a feedback path adding a feedback signal to the forward transfer path; and

feedback suppression means for selectively compensating for the presence of an undesired feedback signal component when a signal output level is greater than a predetermined activation level; the method including the steps of:

determining a listener disturbance threshold level D at each frequency;

determining the amplification H at each frequency;

determining feedback path transfer function G at each frequency; and

determining the activation level A at each frequency from the disturbance threshold levels, amplifications and feedback path gains according to:

| A|=|D|−|H|−|G|.

18. A method according to claim 17 , wherein the disturbance threshold level is a hearing threshold level.

19. A method according to claim 17 , wherein the forward transfer path is disconnected from the output transducer during fitting, and wherein the feedback path transfer function G at each frequency is determined by:

presenting stimuli to the output transducer at a plurality of frequencies;

recording output transducer signal components and feedback signal components at the plurality of frequencies; and

deriving the feedback path transfer function G at each frequency from the output transducer signal components and feedback signal components.

20. A method according to claim 17 , wherein the feedback path transfer function G at each frequency is determined by:

adjusting the amplification at each frequency;

deriving the feedback path transfer function G at that frequency from the lowest amplification at which feedback oscillation is detected.

21. A method according to claim 17 , wherein the forward transfer path includes a frequency transposing element, and wherein the activation level A at each frequency is determined from the amplification and feedback path gain at that frequency and from the disturbance threshold level at a transposed frequency.

22. A method according to claim 17 , wherein the forward transfer path is disconnected from the output transducer during fitting, the method further including the steps of:

presenting stimuli to the output transducer at a plurality of frequencies;

adjusting the stimulus level at each frequency to meet a predefined loudness perception level or detection threshold of the listener; measuring feedback signal components resulting from the stimuli at the plurality of frequencies;

determining the amplification at the plurality of frequencies from the stimuli and standardised input signal components; and

determining the activation level at each frequency from the levels of stimuli, feedback signal components, disturbance threshold levels and amplifications.

23. A method according to claim 22 , wherein the activation level As at each frequency is determined according to:

15 AS=S+{D−[Fs+H]}

where at each frequency, S is the magnitude of the stimulus, D is the listener's hearing threshold level, F S is the feedback signal component resulting from the stimulus and H is the amplification.

24. A method according to claim 23 , wherein the forward transfer path includes a frequency transposing element, and wherein the activation level As at each frequency is determined from S, F S and H at that frequency and D at a transposed frequency.

25. A method of fitting an audio amplification apparatus, the audio amplification apparatus comprising a forward transfer path connectable to an output transducer, the forward transfer path including a frequency transposing element; a feedback path adding a feedback signal to the forward transfer path; and feedback suppression means for selectively compensating for the presence of an undesired feedback signal component when a signal output level is greater than a predetermined activation level, the method comprising the steps of:

adjusting frequency-dependent amplification in the audio amplification apparatus by presenting stimuli to the output transducer at a plurality of frequencies; adjusting the stimulus level at each frequency to met a predefined loudness perception level or detection threshold of the listener; deriving an equal loudness contour of output transducer levels from the adjusted stimuli levels; and deriving the frequency-dependent amplification of levels of input signals at each frequency; and

determining activation levels for feedback suppression in the audio amplification apparatus by: determining a listener disturbance threshold level D at each frequency; determining the amplification H at each frequency; determining feedback path transfer function G at each frequency; and determining the activation level A at each frequency from the disturbance threshold levels, amplifications and feedback path gains according to:

| A|=|D|−|H|−|G|.