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IEEE SIGNAL PROCESSING MAGAZINE [122] MARCH 2015

insignificantly different

sig

t values in the nonenhanced case,

resulted in significantly higher

f -RMSE values. Such finding

shows the importance of short-time processing for CI users.

Interestingly, while PESQ and PEMO-Q have been shown to be

highly correlated with subjective quality ratings of NH listeners in

a number of telephony applications, poor performance was

obtained for CI users, particularly under speech enhancement.

For the nonintrusive measures, the SRMR-CI measure achieved

the best results with performance levels in-line with those

obtained with STOI, but with the advantage of not requiring a ref-

erence signal. In fact, when both noisy and enhanced conditions

were considered overall, the SRMR-CI metric outperformed STOI

across all four performance criteria. By incorporating CI process-

ing percepts into the original SRMR measure, significant gains

could be observed. Generally, the findings observed here resonate

with those reported in the literature showing the importance of

spectral envelopes for CI intelligibility.

HA: NFC CONDITIONS

For users of HAs with frequency lowering strategies, PEMO-Q-HI

and STOI attained insignificantly different

sig

t and f-RMSE

results. The HASQI measure, in turn, resulted in the highest

sig

t ,

but achieved a significantly higher

f-RMSE than the two afore-

mentioned metrics. This higher error may be a result of the range

of conditions used during training of the internal parameter (i.e.,

noise, linear, and nonlinear terms) mapping available in the

HASQI. Notwithstanding, given the burgeoning popularity of such

nonlinear frequency compression schemes for HI listeners with

severe to profound high frequency sensorineural hearing loss, our

results suggest that users have a few reliable intrusive metrics to

choose from. On the other hand, the tested nonintrusive measures

were not capable of correctly characterizing the perceptual arte-

facts caused by NFC in HA users. For example, none of the metrics

surpassed the correlation threshold of 0.8 established by ITU-T

during the competition that resulted in the P.563 Recommenda-

tion [20]. These findings motivate the need for more research on

the development of innovative nonintrusive quality measures for

frequency-lowering strategies. As an exploratory test, the modula-

tion energy thresholding and modulation filter bank compression

strategies implemented in the SRMR-CI metric (see the section

“SRMR-CI and SRMR-HA”) were tested on the original SRMR and

SRMR-HA metrics and significant improvements

.p 0051

could be observed (e.g., .080

sig

t = and f-RMSE .468= with the

so-called SRMR-HA

comp

). In fact, these newly obtained results were

in-line with some of the intrusive metrics, such as PEMO-Q, and

suggest that further improvements may be obtained with nonin-

trusive measures.

HA: NOISY AND ENHANCED CONDITIONS

Finally, for HA users in complex listening environments compris-

ing noise, reverberation, and noise-plus-reverberation, it was

observed that all intrusive measures achieved insignificantly differ-

ent

sig

and

-RMSE values (with the exception of HASPI, in the

0 0.2 0.4 0.6 0.8 1

100

PESQ

Quality

0 0.2 0.4 0.6 0.8 1

ModA

(a) (b)

Quality

Not Enhanced

Enhanced

Not Enhanced

Enhanced

[FIG3] Scatterplots of subjective quality versus objective scores for condition-averaged data points obtained from the (a) PESQ and

(b) ModA metrics for the HA reverberation/enhancement quality database.

[TABLE 4] A SUMMARY OF RECOMMENDED OBJECTIVE

METRICS FOR DIFFERENT CONDITIONS. THE METRICS IN

BOLD REPRESENT THOSE THAT ACHIEVED HIGHEST

sig

AND LOWEST

-RMSE. THE METRIC SRMR-HA

comp

CORRE-

SPONDS TO AN EXPLORATORY MEASURE DESCRIBED IN

THE SECTION “HA: NFC CONDITIONS.”

CI HA

CONDITION INTRUSIVE NON-

INTRUSIVE

INTRUSIVE NONINTRUSIVE

COMBINED STOI, NCM SRMR-CI PESQ, STOI,

PEMO-Q-HI,

NCM

ModA, SRMR-HA

NON-

ENHANCED

STOI SRMR-CI ALL EXCEPT

HASPI

(PEMO-Q)

ALL (ModA)

ENHANCED STOI SRMR-CI PEMO-Q-HI,

HASQI, PEMO-

ModA

NFC – – PEMO-Q-HI,

STOI

SRMR-HA

comp

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