Comparisons of speech privacy criteria

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Comparisons of speech privacy criteria

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Com pa risons of spe e c h priva c y m e a sure s

N R C C - 5 1 3 2 7

B r a d l e y , J . S .

A u g u s t 2 0 0 9

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Ottawa, Canada

INTER-NOISE 2009

Comparisons of Speech Privacy Measures

John S Bradley1

National Research Council 1200 Montreal Rd

Ottawa, K1A 0R6, Canada

ABSTRACT

Various speech privacy criteria have been proposed for use in both open-plan offices and enclosed rooms. They have been given in terms of: Articulation Index (AI), Speech Intelligibility Index (SII), and various signal-to-noise ratio values. In this paper, the results of previously published speech intelligibility tests were used to describe the relationships between the various measures in both types of office environments. Because increased speech privacy is usually assumed to relate to decreased speech intelligibility, the previous speech test results were used to interpret each criterion value in terms of the expected intelligibility of speech. The criterion values were also related to published estimates of the threshold of intelligibility.

1. INTRODUCTION

Speech privacy is usually considered to correspond to conditions with reduced speech intelligibility and hence is related to the same signal-to-noise ratio measures as speech intelligibility. This was included in the pioneering work of Cavanaugh et al.1 who first showed the Articulation Index (AI)2 to be a good measure of speech privacy. They proposed a criterion for ‘confidential’ speech privacy corresponding to AI ≤ 0.05. Below this value there was said to be, “zero phrase intelligibility with some isolated words being intelligible.”1

In open-plan offices, conditions corresponding to AI ≤ 0.15 have been described as

‘acceptable’ or ‘normal’ privacy3. Such conditions are said to be not too distracting. In practice AI ≤ 0.15 corresponds to a level of speech privacy that is achievable in a well designed open-plan office.

More recently Gover and Bradley4 evaluated several frequency weighted signal-to-noise ratios as predictors of speech intelligibility scores as well as the thresholds of audibility and intelligibility of speech transmitted through walls. In their approximately free field conditions, they found the threshold of intelligibility corresponded to a uniformly weighted signal-to-noise ratio (SNRUNI32) of -16 dB and the threshold of audibility of the speech sounds to an SNRUNI32 of

-22 dB. More recently they have shown that the threshold of intelligibility would correspond to an SNRUNI32 of –11 dB5,6 in conditions more representative of moderately reverberant meeting

rooms. SNRUNI32 values were shown to be useful over a broader range of speech privacy

conditions including those for very high privacy where AI values approach zero.

The present paper provides new information to help the interpretation of these criteria. It also provides information on how to convert between values of the various measures. For example, the AI measure was replaced by the Speech Intelligibility Index (SII) in 19977. Although intended to be an improvement on the original AI measure, SII leads to different values

1

for identical conditions. It would be useful to be able to describe the requirements for

confidential or normal privacy in terms of SII values and perhaps other measures too.

By re-analyzing data from 3 previous studies of both open-plan offices and enclosed offices, the interrelationships among the various measures were determined as well as their relationships with speech intelligibility scores. By using data for a broad range of conditions, it was hoped to give a better understanding of the meaning of each criterion value and provide a means for converting among the various speech privacy measures.

2. DATA FOR THE COMPARISONS

Data from 3 previous studies were used in the current analyses. One of the three studies8 included speech intelligibility tests in conditions representative of open-plan offices. The levels and spectra of the speech were modified to represent transmission from one workstation to an adjacent one based on measurements of this type of propagation. The source levels of the speech and the levels and spectra of the simulated ambient noise were also varied. For use in the current analyses, the original data were sorted in order of increasing speech privacy measures and aggregated to give 10 data points, each indicating the average of 10 original consecutive data points. The data from this study8 are referred to as the ‘open office data’ in this paper.

The second study4 included speech intelligibility tests for sound modified to represent transmission through 4 different walls. The speech source levels and simulated ambient noise were also varied. The study also determined the thresholds of audibility and intelligibility of the transmitted speech sounds defined as the point at which 50% of attentive listeners with good hearing could just detect some speech sound (threshold of audibility) or could just understand one word of a test sentence (threshold of intelligibility). For use in the current analyses, the original data were sorted in order of increasing speech privacy measures and aggregated to give 34 data points. These data are referred to as the “closed office I” data4.

The third study9 included speech intelligibility tests of speech transmitted through 20 different walls with STC (Speech Transmission Index) values ranging from 34 to 58. Speech intelligibility test scores as well as measures of the thresholds of intelligibility and audibility were available from the original results. The data from this study were aggregated by wall type in the original analyses to give 20 data points. The data9 are referred to as the “closed office II data” in this paper.

In all 3 studies recordings of the Harvard sentences10, spoken by a male talker, were used. They are phonetically balanced English sentences with content that is of low predictability. Speech intelligibility scores were the percentage of correctly identified words in each sentence.

Data for 6 different speech privacy measures were available from these studies. The 6 measures included the AI and SII described in the related ANSI standards2,7. They also included 3 different frequency-weighted signal-to-noise ratio measures SNRAI, SNRSII22 and SNRUNI32. In

general these were defined as4,

, (1)

(

∑

)

where wf are the importance weightings for each 1/3-octave band of centre frequency f, LS(f) and

LN(f) are the 1/3-octave band speech and noise levels.

For SNRAI the frequency weightings wf were those from the AI standard2. The summation

was made over the 1/3-octave band frequencies from 200 to 5k Hz as in the AI measure. For the SNRSII22 measure the summation was over the frequencies from 160 to 5k Hz. Although the SII

measure7 was defined with an upper frequency of 8k Hz, the 5k limit was specified to correspond to the usual upper limit of information on sound transmission in buildings. The frequency

weightings, wf, were scaled up so that the sum of all weightings from 160 to 5k Hz was the same

as the sum of the original weightings from 160 to 8k Hz. The 1/3-octave band level differences, (LS(f)-LN(f)), were clipped so that they were never less than –22 dB. The SNRUNI32 measure used

a frequency weighting of 0.0625 in each of the octave bands from 160 to 5k Hz. The 1/3-octave band level differences, (LS(f)-LN(f)), were clipped so that they were never less than –32

dB. The A-weighted signal-to-noise ratio, SNR(A) was the difference in A-weighted speech and noise levels at the position of the listener. SNR(A) is not a well-accepted speech privacy measure, but many are tempted to use it as a simpler measure as some early studies proposed11.

3. EXPLAINING THE MEANING OF CRITERIA IN TERMS OF SI SCORES

The speech intelligibility (SI) scores were first plotted versus AI values of the aggregate data from the 3 previous studies. The resulting plot in Fig. 1 makes it possible to associate speech intelligibility scores with the confidential and normal privacy criteria. This was done by fitting regression lines to the combined data, and separately to the open-plan office data.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0 20 40 60 80 100 Open office Closed office I Closed office II Confidential privacy Normal privacy Fit to open office Fit to all data

Speech Inte lligibility, % AI 79% normal privacy 39% confidential privacy

75% normal privacy (open office)

Fig. 1. Plot of mean speech intelligibility scores versus AI values for data from 3 previous studies4,8,9. The vertical dashed and dash-dot lines indicate confidential and normal privacy criteria respectively.

The following 5th order polynomial represents the best-fit regression line to the combined data from all 3 studies of speech intelligibility (SI) versus AI values shown on Fig. 1.

SI = 1059.6AI5 - 4427.9AI4 + 5758.8AI3 - 3315.4AI2 + 896.29AI + 1.9707, R2 = 0.9485 (2) From this regression line (equation (2)), determined from conditions for both open-plan and enclosed rooms, confidential privacy (AI = 0.05) corresponds to a speech intelligibility score of 39%. That is, 39% of words in sentences would be understood by an attentive listener with good hearing. Such conditions would make it quite difficult to follow the overheard speech.

The same regression line indicates that normal privacy corresponds to a speech intelligibility score of 79%. For the subset of the open-plan office cases, normal privacy corresponds to a speech intelligibility score of 75%. This was obtained by separately fitting a 5th order polynomial to the open office data as illustrated in Fig. 1 and is given by equation (3).

Speech intelligibility scores from both regression lines are given in Table 1 for AI intervals of 0.05. The same AI value corresponds to a slightly lower speech intelligibility score on average in the open-plan offices. This is a relatively small difference compared to the scatter in the data, and the separate open-plan results are not used in the remainder of these analyses.

For either regression line, normal privacy corresponds to a quite modest amount of privacy, but it does represent a substantial improvement over doing nothing to improve speech privacy. Speech would be less disturbing but mostly understandable. In open-plan offices, it is not realistic to achieve much better than this without unusual efforts.

However, greater privacy can be achieved for enclosed rooms to prevent eavesdroppers from understanding any words and in some cases making the speech from the adjacent room inaudible. These conditions would correspond to much higher privacy (lower AI values) than

confidential privacy. Clearly AI values are not very helpful for such very high privacy cases and measures such as the uniform weighted signal-to-noise ratio are more suitable.

AI ALL OPEN 0.00 2.0 -0.05 39.2 -0.10 63.8 54.0 0.15 79.1 75.2 0.20 87.9 85.8 0.25 92.6 90.6 0.30 94.7 92.6 0.35 95.6 93.7 0.40 96.1 94.9 0.45 96.7 96.3 0.50 97.5 97.6 0.55 98.3 98.4 0.60 98.6 98.2 SI, %

Table 1. Speech intelligibility scores in steps of 0.05 in AI from the regression lines in Fig. 1 (i.e. equations (2) and (3)). ‘All’ includes data from all 3 studies, ‘Open’, includes only open-plan office data.

4. OTHER SPEECH PRIVACY MEASURES

Speech intelligibility scores were also plotted versus the other 5 speech privacy measures. Fig. 2 shows the intelligibility scores plotted versus the uniformly-weighted signal-to-noise-ratio (SNRUNI32) values. Signal-to-noise ratio measures such as SNRUNI32 spread out the conditions at

lower intelligibility scores (i.e. for higher privacy) as seen by comparing Fig. 2 for SNRUNI32

values with Fig 1 for AI values. Because AI compresses values close to 0 and cannot rate conditions with AI < 0, SNRUNI32 values are preferred to AI for higher privacy conditions.

A number of measures predict the speech intelligibility scores almost equally well. These include: SNRUNI32, SNRSII22, SNRAI, AI and SII values. This is indicated by the associated R2 values,

given in Table 2, from regression line fits to plots of speech intelligibility scores versus each of these measures.

-25 -20 -15 -10 -5 0 5 0 20 40 60 80 100 Open office Closed office I Closed office II S pee ch In te llig ibil it y , % SNR_UNI32

Fig. 2. Plot of mean speech intelligibility scores versus SNRUNI32 values for data from 3 previous studies4,8,9.

However, SNR(A) values are not an acceptable speech privacy measure. Changes to the spectrum of the speech due to transmission through different wall transmission loss characteristics lead to different SNR(A) values for similar SI scores as illustrated in Fig. 3.

SNR(A) is not a universally applicable measure. If speech privacy is assumed to be the inverse of

speech intelligibility, then measures that relate well to speech intelligibility are needed.

Measure R2 Fit AI 0.949 Poly SII 0.945 Poly SNRAI 0.947 Boltzmann SNRSII22 0.946 Boltzmann SNRUNI32 0.936 Boltzmann SNR(A) ?

Table 2. R2 values for regression line fits to plots of mean speech intelligibility scores versus each of the speech privacy measures. Regression lines were either 5th order polynomial fits (Poly) or Boltzmann equations.

-20 -15 -10 -5 0 5 10 15 0 20 40 60 80 100 Open office Closed office-I Closed office-II S p e e c h I n tel ligibil ity, % SNR(A)

4. RELATIONSHIPS AMONG SPEECH PRIVACY MEASURES

Since several measures are equally well related to SI scores, we would expect these measures to be well related to each other. It is useful to describe these relationships and determine the equivalent values of other measures to the AI=0.05, confidential and AI=0.15, normal privacy criteria.

Fig. 4 shows a plot of AI values versus SII values. Since SII is an update of AI, it is not surprising that these values are strongly related. From Fig. 4 we can determine the SII values corresponding to AI=0.05 and AI=0.15. These are included in Table 3.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Open office Closed office I Closed office-II Confidential Normal AI SII

Fig. 4. Plot of AI values versus SII values for data from 3 previous studies4,8,9. AI = 0.0084SII4

- 0.5848SII3 + 0.8808SII2 + 0.5609SII - 0.0039, R2 = 0.995.

The horizontal solid and dash-dot lines indicate the confidential and normal speech privacy criteria respectively. From Fig. 5 we can determine the SNRUNI32 values corresponding to AI=0.05 and AI=0.15. Fig. 5

also illustrates how SNRUNI32 values are more spread out for lower intelligibility cases (higher

privacy). -25 -20 -15 -10 -5 0 5 10 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Open office Closed office-I Closed office-I Confidential Normal AI SNR_UNI32

Fig. 5. Plot of AI values versus SNRUNI32 values for data from 3 previous studies4,8,9.

AI = 2E-05x3

+ 0.0016x2 + 0.0419x + 0.3686, (where x=SNRUNI32), R2 = 0.935.

Table 3 lists the equivalent values of all 5 measures corresponding to the normal and confidential speech privacy criteria. The regression lines relating AI values to SNRAI and SNRSII22 values and

associated R2 values are as follows:

AI = 9·10-06x3 + 0.0013x2 + 0.0623x + 0.9101, (where x=SNRAI), R2 = 0.927

AI = -2·10-05x3 + 0.0004x2 + 0.0335x + 0.3952, (where x=SNRSII22), R2 = 0.981

Criterion SI, % AI SII SNR(A) SNRAI SNRSII22 SNRUNI32

Confidential 39 0.05 0.082 - -19.4 -8.9 -8.0

Normal 79 0.15 0.208 - -25.85 -14.3 -14.2

Table 3 Equivalent values of speech privacy measure values for confidential and normal speech privacy criteria. Table 3 does not include equivalent values in terms of SNR(A) because there was not a unique relationship between AI and SNR(A) values as illustrated in Fig. 6 below. The data from each of the 3 previous studies resulted in a different relationship.

-20 -15 -10 -5 0 5 10 15 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Open office Closed office I Closed office II AI SNR(A)

Fig. 6. Plot of AI values versus SNR(A) values for data from 3 previous studies4,8,9.

5. INTELLIGIBILITY AND AUDIBILITY THRESHOLD VALUES

Previous work4 established estimates of the thresholds of intelligibility and audibility of speech sounds transmitted through walls. These are repeated in Table 4 below.

Intelligibility Cadence Audibility

AI 0.0271 0.0033 -0.0001

SII 0.0547 0.0150 0.0047

SNRAI -15.87 -18.77 -20.09

SNRSII22 -15.86 -18.87 -20.13

SNRUNI32 -15.64 -20.05 -22.14

Table 4. Values of speech privacy measures that correspond to the thresholds of: intelligibility, cadence and audibility from reference4.

The data in Table 4 were from a study that included only 4 different wall transmission loss characteristics. The combined data in the current analyses include a much more varied range of transmitted speech spectra. From the plot of AI versus SNRUNI32 values of the combined data (see

in terms of their SNRUNI32 values. These values were very similar to those in the Table above

(e.g. AI=0.0281 compared to the original AI=0.0271 from Table 4 for the threshold of

intelligibility).

It is not useful to convert the SNRUNI32 values for the thresholds of Cadence and of

Audibility to AI values because they are very small values very close to or below AI=0. For the higher levels of privacy (i.e. low SNR), where these thresholds may be of interest, it is necessary to measure speech privacy in terms of SNRUNI32 values.

More recent work has shown that the combination of spatial and temporal effects leads to a different estimate of the threshold of intelligibility for sound transmission from enclosed rooms with moderately reverberant sound corresponding to an SNRUNI32 of –11 dB. The conversion

between AI and SNRUNI32 values from Fig. 5 indicates the confidential privacy criterion of

AI=0.05 corresponds to an SNRUNI32 of –13 dB. This is intermediate to the –16 dB and –11 dB

SNRUNI32 values for the threshold of intelligibility for free field and moderately reverberant

conditions respectively. An SNRUNI32 value of –13 dB would correspond to that expected5,6 for a

room with a reverberation time of about 0.5 s. That is, in many real office conditions, the

confidential privacy criterion is approximately equal to the threshold of intelligibility. 6. CONCLUSIONS

With the exception of A-weighted signal-to-noise ratios (SNR(A)), the other 5 speech privacy measures (AI, SII, SNRAI, SNRSII22 and SNRUNI32) were similarly accurate predictors of

speech intelligibility scores for the combined data that included conditions from barely intelligible to completely intelligible. As indicated in Table 2, regression line fits to plots of intelligibility scores versus each of these measures led to similar R2 values varying between 0.936 and 0.949. The standard deviations of the aggregate data about these regression lines were typically 7 to 8% in speech intelligibility scores.

The data for open-plan office conditions did indicate slightly lower speech intelligibility scores for the same AI values than the other data, and the criterion for normal privacy (AI=0.15) led to a speech intelligibility score that was 4% lower for open-plan office conditions. It was also shown that the frequency-weighted signal-to-noise-ratio measures such as SNRUNI32, can better

discriminate among conditions of high privacy and are capable of describing conditions below the point where AI approaches zero.

Confidential privacy was shown to correspond to an average speech intelligibility score of

39%. This was also found to be approximately equivalent to the threshold of intelligibility for typical offices and meeting rooms with reverberation times of approximately 0.5 s. The original description of confidential privacy, “zero phrase intelligibility with some isolated words being intelligible”1, is confirmed to be accurate.

Normal privacy was found to correspond to an average speech intelligibility score of 79%.

For the special case of open-plan office conditions, where this criterion is most relevant, normal privacy corresponded to an average speech intelligibility score of 75%. For such conditions, unwanted speech would be mostly intelligible but less distracting than unattenuated speech.

Because of the nature of the speech test material10, these results are thought to be representative of the expected intelligibility of overheard conversations in situations where speech privacy is required.

The results in Fig. 1 and 2 as well as Table 1 can be used to interpret the degree of privacy provided by other proposed speech privacy criteria.

REFERENCES

[1] Cavanaugh, W.J., Farrell, W.R., Hirtle, P.W., and Watters, B.G., “Speech Privacy in Buildings”, J. Acoust. Soc. Am. 34 (4), 475-492 (1962).

[2] ANSI S3.5-1969, American National Standard Methods for the Calculation of the Articulation Index, Standards Secretariat, Acoustical Society of America, New York, USA.

[3] CSA Standard Z412-00, “Guideline on Office Ergonomics, (2000).

[4] Gover, B.N., and Bradley, J.S., “Measures for assessing architectural speech security (privacy) of closed offices and meeting rooms”, J. Acoust. Soc. Am., 116, (6), 3480-3490 (2004).

[5] Bradley, J.S., Apfel, M. and Gover, B.N, “Spatial and Temporal Effects of Room Acoustics on the Speech Privacy of Meeting Rooms”, IRC/NRC Research Report, RR-265, (October 2008).

[6] Bradley, J.S., Apfel, M and Gover, B.N., “Some Spatial and Temporal Effects on the Speech Privacy of Meeting Rooms”, J. Acoust. Soc. Am. 125 (5) 3038-3051 (2009).

[7] ANSI S3.5-1997, “Methods for Calculation of the Speech Intelligibility Index”, American National Standard, Standards Secretariat, Acoustical Society of America, New York, USA.

[8] Bradley, J.S, and Gover, B.N, “Describing Levels of Speech Privacy in Open-Plan Offices”, Institute for Research in Construction, National Research Council, Ottawa, report, IRC-RR-138, (2003). http://irc.nrc-cnrc.gc.ca/pubs/rr/rr138/rr138.pdf

[9] Park, H. K., Bradley J.S. and Gover, B.N., “Evaluating Airborne Sound Insulation in Terms of Speech Intelligibility”, J. Acoust. Soc. Am., 123 (3) 1458-1471 (2008).

[10] I.E.E. Recommended Practice for Speech quality Measurements, I.E.E.E. Trans. On Audio and Electroacoust., 227-246 (1969).