Amplification and consonant modulation spectra.

Clinicians and researchers who work with hearing aids are most concerned with ensuring that the acoustic frequency spectrum is audible and not distorted. For example, it is assumed that applying high-frequency gain will improve speech recognition for a listener with high-frequency hearing loss, because that will improve audibility of the acoustic frequency spectrum. Clinicians are less likely to concern themselves with the modulation spectrum, which represents the dynamic variations in level within each acoustic frequency band. To understand the difference between the acoustic frequency spectrum and the modulation spectrum, consider a simple example: a 1 kHz carrier tone modulated by a 4 Hz sinusoid. The acoustic frequency spectrum (level as a function of acoustic frequency) would have a component centered at 1 kHz, with the width of that component determined by the duration and onset/offset of the tone. The modulation spectrum (modulation depth as a function of modulation frequency) would have a component at 4 Hz with a modulation depth of 100% (and a modulation index of 1). A useful description of modulation rates for speech is provided by Rosen (1992). Envelope (2-50 Hz) conveys information about consonant manner, voicing, and vowel identity. Periodicity (50-500 Hz) and aperiodicity convey information about voicing, manner, and prosodic cues such as stress and intonation. Fine-structure (600 Hz and higher) conveys information about consonant place of articulation. Thus, we can think of speech in terms of a modulation spectrum, in which a range of modulation rates is present within each carrier frequency band. How does the modulation spectrum relate to speech recognition? Gallun and Souza (2008) related a simplified representation of the modulation spectrum to consonant errors. If a nonsense syllable had a modulation spectrum that was quite similar to another nonsense syllable, listeners were likely to confuse the two. In that study, acoustic frequency was substantially degraded to force listeners to rely more heavily on modulation spectra, so we do not know whether the same confusions would have resulted had acoustic frequency cues been readily available (i.e., in spectrally-robust speech). We also do not know whether altering the modulation spectrum for a particular syllable (e.g., making two syllables more or less similar) might have changed error patterns. However, the idea that it is important to preserve the modulation spectrum has been discussed in the research literature for some time. For example, preserving modulation characteristics at the output of a system is the basis of the modulation transfer function used to measure room acoustics (Houtgast & Steeneken, 1985). Some investigators suggested that changing the modulation spectrum could reduce speech clarity or quality (Grant, Elhilali, Shamma, Walden, Surr, Cord & Summers, 2008; Huber & Kollmeier, 2006; Payton, 1999). Completely removing some range of modulation frequencies across all carrier bands had a significant negative effect (Drullman, Festen & Plomp, 1994) but such drastic manipulations would be unlikely to occur in real life. Regarding more realistic effects, we would expect the multichannel wide-dynamic range compression (WDRC) strategies widely used in digital hearing aids to alter modulation spectra as they change the amplitude variations of the signal in specific frequency bands. Stone and Moore (Stone & Moore, 2007; Stone & Moore, 2008) noted that fast-acting compression with a relatively high (∼3:1) compression ratio had the following negative effects on the envelope: (1) Compression reduced the similarity of slow modulations (envelope) across channels. They proposed that such an effect could reduce the ability to fuse frequency channels of a target in the presence of a distractor. (2) Compression altered the envelope of the target signal and (3) Compression caused the modulations of the target and distractor to be more similar to one another, potentially making them more difficult to separate. Because the various measures embedded all envelope changes into a single value, Stone and Moore commented that they were unable to parse out the components of the modulation spectrum that had been most affected. Also, note that all of their data were collected for vocoded signals in which listeners were limited to use of modulations below 100 Hz (with envelope smoothing filter set at 45 Hz); that is, periodicity/aperiodicity cues were not available. That processing removed fine-structures cues, such as fundamental frequency, that would normally have been available to hearing-impaired listeners. Such cues would have aided talker separation (Arehart, Souza & Miller, submitted) and might have offset the impact of the envelope changes. Previous work (Noordhoek & Drullman, 1997) showed that as long as acoustic frequency (fine-structure) cues were available, reducing modulation by reducing peaks via amplitude compression was less detrimental than “filling” valleys via reverberation. In summary, then, it has not been shown whether using clinically-realistic amplitude compression to alter the natural modulation characteristics of spectrally-robust speech, such as would be the case with most individuals with hearing loss, will necessarily have a detrimental effect. Instead, different means of altering modulation spectra might have different effects (positive, negative, or neutral) in terms of their impact on speech recognition. Therefore, the following questions were addressed in the current study: (1) what is the effect of clinical amplification strategies on the consonant modulation spectrum? (2) Is there a relationship between the modulation spectrum and consonant errors for spectrally-robust, amplified speech presented to listeners with hearing loss?