Sound Metrics

Sound quality and psycho-acoustic metrics

Table of Contents

Chapter 1 Sound pressure level..........................................................................5 Section 1.1 Time domain sound pressure level ...................................................... 5

Chapter 2 Equivalent sound pressure level.......................................................7

Chapter 3 Loudness .............................................................................................9 Section 3.1 Stevens Mark VI ................................................................................ 10 Section 3.2 Stevens Mark VII ............................................................................... 11 Section 3.3 Zwicker Loudness ............................................................................. 12 Section 3.4 Time Varying Zwicker Loudness ........................................................ 14

Chapter 4 Sharpness......................................................................................... 17

Chapter 5 Roughness........................................................................................ 19

Chapter 6 Fluctuation strength ....................................................................... 21

Chapter 7 Pitch and Tonality ............................................................................ 23 Section 7.1 Pitch .................................................................................................. 23 Section 7.2 Tonality.............................................................................................. 24 Section 7.3 Table of Articulation indices .............................................................. 27

Chapter 8 Speech interference level (SIL, PSIL) ............................................ 29

Chapter 9 Noise evaluation criteria: NR, NC and NCB .................................. 31 Section 9.1 Noise Rating (NR) .............................................................................. 31 Section 9.2 Noise Criterion (NC) .......................................................................... 33 Section 9.3 Balanced Noise Criterion (NCB)......................................................... 36

Chapter 10 Tone-to-Noise Ratio and Prominence Ratio .................................. 39 Section 10.1 Tone-to-Noise Ratio........................................................................... 40 Section 10.2 Prominence Ratio .............................................................................. 40

Chapter 11 Reading list....................................................................................... 43

Fundamentals of acoustics - Sound quality and psycho-acoustic metrics - LMS International 2

Chapter 1 Sound pressure level


In This Chapter

Time domain sound pressure level.....................................5

The basic descriptor of sound signals is the sound pressure level (SPL) denoted

by :

where

is the RMS value of the measured acoustic pressure in Pascal (Pa)

is the RMS reference pressure, such that .

The stimulus of the sound pressure level needs to be interpreted as a hearing sensation and one approach consists of multiplying the frequency spectrum of the acoustic pressure signal with a weighting function before calculating the RMS level. Several weighting functions have been defined, of which the A-B-C and D weightings are the most widely used. They are based on experimentally determined equal loudness contours which express the loudness sensation of single tones as a function of sound pressure level and frequency.

Section 1.1 Time domain sound pressure level

This function calculates the frequency and time weighted sound pressure level according to the IEC 651 and ANSI SI.4-1983 standards.

Frequency weighting can be applied to the time signal using the A, B or C weightings described in section 4.1 of the "Acoustic Quantities" theory document. The time signal is then exponentially averaged to arrive at the sound pressure level (cf. section 4.3 of the "Spectral Processing" theory document). An exponential weighting factor, denoted by t is called the time constant. The values of t depend on the type of signal (mode) and three default (standardized) values are supplied:



t = 35ms for impulse (peaky) signals t = 125 ms for fast changing signals t = 1000 ms for slow changing signals.

By selecting the type of signal (mode) then the appropriate time constant is applied.

When the signal contains spikes and is therefore defined by the mode "impulse" an additional peak detector mechanism is implemented. In this case when an increase in the averaged signal is detected, then the signal is followed exactly. When the signal is decreasing, then exponential averaging is used with a long time constant, set by default to 1500 ms. The time constant used in this situation is termed the decay time constant.


Chapter 2 Equivalent sound pressure level

Chapter 2 Equivalent sound pressure level The ISO standards: ISO1996/1-1982 and ISO1999:1990 provide a definition for the 'equivalent A- weighted sound pressure level in decibels' identified as

.

This function gives the value of the A-weighted sound pressure level of a continuous, steady sound that, within a specified time interval T, has the same mean square sound pressure as the sound under consideration whose level varies with time. This leads to the expression:

where

is the equivalent continuous A-weighted sound pressure level, in

decibels, determined over a time interval starting at and ending at

is the reference sound pressure (20mPa);

is the instantaneous A-weighted sound pressure of the sound signal

In practice with sampled data the equivalent sound pressure level is computed by a summation of the sampled values of the pressure level, in dB over the number of samples required. As a generalization, you can apply the same

formula to a non-A-weighted sound pressure signal to obtain .


Chapter 3 Loudness

Chapter 3 Loudness

In This Chapter

Stevens Mark VI.................................................................10 Stevens Mark VII................................................................11 Zwicker Loudness ..............................................................12 Time Varying Zwicker Loudness.........................................14

The equal loudness contours shown in Figure 4-2 in the document "Sound quality" are the result of large numbers of psycho-acoustical experiments and are in principle only valid for the specific sound types involved in the test. These curves are valid for pure tones and depict the actual experienced loudness for a tone of given frequency and sound pressure level when compared to a reference tone. The resulting value is called the 'loudness level'.

The loudness level itself is expressed in Phons. 1 kHz-tones are used as the reference, which means that for a 1 kHz tone, the Phon value corresponds to the dB sound pressure level. The equal loudness contours for free field pure tones and diffuse field narrow-band random noise are standardized as ISO 226-1987 (E).

A linear unit derived from the (logarithmic) Phon values is the Sone (S), which is related to the Phon (P) in the following way :

The Sone scale's linear relationship to the experienced loudness makes it easier to interpret. A loudness of 1 Sone corresponds to a loudness level of 40 Phons. A tone which is twice as loud, will have double the loudness (Sone) value, and a loudness level which is 10 Phons higher.

When broadband or multi-tone sounds are being considered, the frequency spectrum of the loudness is made in terms of critical bands instead of the total value. Critical bands and barks are described in Table 4.1 in the chapter on"Sound quality". In this case the terminology 'specific loudness' is used, expressed in Sones/Bark.

For steady state sounds, standardized calculation procedures have been defined by Zwicker and Stevens and are accepted as ISO standards {12, 13, 14}. A more recent procedure by Stevens {15} has not yet been accepted as an ISO standard.

They are both based on :

a convention for the relation between octave band sound pressure levels and octave band partial (specific) loudness descriptions

a convention to combine the specific loudness values into a global loudness, taking into account masking effects.

For temporally varying sounds, Zwicker has also proposed an approach taking


Chapter 3 Loudness

into account temporal effects {16}. The latter evolved until its acceptance as DIN 45631/A1 {56}.

Note: Please refer to Time Varying Zwicker Loudness (on page 14) for more information.

Section 3.1 Stevens Mark VI

The Stevens (Mark VI) method, standardized as ISO 532-A-1975 and ANSI S3.4-1980, starts from octave band sound pressure levels. Their loudness is compared to that of a critical band noise at 1 kHz. It is only defined for diffuse sound fields with relatively smooth, broadband spectra. Through a set of standardized curves, each octave band level is converted into a partial loudness index (s) see Figure 5-1. The partial loudness values are then combined into a total loudness (in Sones), using equation 5-3.

where :

= the total loudness in Sones

= the greatest of the loudness indices, in Sones

=

=

the sum of the loudness indices of all bands, in Sones

fractional loudness contribution factor, reflecting masking effects. It depends on the type of octave measurement (0.3 for

1/1 octaves, 0.15 for 1/3 octaves).


Chapter 3 Loudness

Figure 3-1 Loudness (Mark VI)

Section 3.2 Stevens Mark VII

A more recent calculation scheme is Stevens Mark VII {15, 17}, which uses a more refined partial loudness calculation ( see Figure 3-2 ), as well as a level dependent calculation for F in equation 5-3. The reference frequency is 3150 Hz. Apart from the loudness (in Sones), the logarithmic unit 'perceived


Chapter 3 Loudness

loudness level' (PLdB) is used here, which is 32 dB for a loudness of 1 Sone at 3150 Hz. PLdB values will be about 8 dB lower than the loudness level in Phones. Examples are discussed in {5} and {17}.

Figure 3-2 Loudness (Mark VII)

Section 3.3 Zwicker Loudness

Loudness assessment using the Zwicker method (standardized as ISO 532B) starts from 1/3 octave band sound pressure level data, which can originate from either a free or diffuse sound field. It is capable of dealing with complex broadband noises, which may include pure tones.

The method takes masking effects into account. Masking effects are important for sounds composed of multiple components. A high level sound component may 'mask' another lower level sound which is too close in frequency. An example of masking is shown below {5}. A 50 dB, 4 kHz tone (marked +) can be


Chapter 3 Loudness

heard in the presence of narrow-band noise, centered around 1200 Hz, up to a level of 90 dB.

If the noise level rises to 100 dB, the tone is not heard.

Figure 3-3 Masking effects of narrow band noise (5)

The method uses different sets of graphs for diffuse and free fields that relate loudness level to sound pressure level and that take the masking into account by a sloping-edge filter characteristic for each octave band. This way, dominant and hence masking frequency bands will show their influence over a large frequency range and prevent masked sounds contributing to the total level. Figure 3-4 shows an example of the Zwicker method. The 1/3 octave band data are transferred to the appropriate Zwicker diagram.


Chapter 3 Loudness

Figure 3-4 Example loudness calculation according to Zwicker's method {5}

The partial loudness contours are computed for each defined segment (global evaluation) or frame (tracked evaluation) using a classical Zwicker loudness calculation. The frame or segment size should be selected to ensure that the spectral resolution needed for the FFT-based octave band analysis can be achieved. The frame size can be used to restrict the analysis to time periods over which time- varying signals can be regarded as stationary.

The Zwicker loudness analysis allows you to distinguish between unmasked and masked contours thus allowing you to see that certain levels are either wholly or completely masked by previous ones.

The total loudness is calculated as the surface under the enveloping partial loudness contours and can be expressed in Sones, or as loudness level in Phones as a function of time. This is presented as a single value in the global evaluation and a trace of values for the tracked evaluation.

Section 3.4 Time Varying Zwicker Loudness

The time-varying Zwicker loudness is particularly useful to calculate the loudness of non-steady-state noises, or time-variant sounds. The time-varying loudness is calculated in compliance with DIN 45631/A1 {56}. The calculation scheme of Time Varying Loudness is illustrated in figure 3-5.

Figure 3 5. Calculation of Time Varying Loudness.

The recorded sound p(t) in free or diffuse filed - is filtered every 2ms into 28 third octave bands. The specific loudness N is calculated for each critical (Bark)


Chapter 3 Loudness

band according to the Zwicker method (ISO 532B DIN 45631, see Zwicker Loudness (on page 12)). Third order non-linear filters NL account for temporal post-masking effects. The filtered specific loudnesss are summed along the Bark scale, and the result is sent to two parallel low-pass filters (LP). The resulting signal is the time varying loudness N(t).(From Zwicker and Fastl, Psychoacoustics, p.237, 3rd edition, Springer, 2007). The time varying loudness can be understood as the Zwicker loudness (ISO 532B) calculated every 2ms and accounting for both spectral and temporal masking as well as other temporal effects. Figure 3-5 illustrates the full data processing for Time Varying Loudness.

Temporal effects are linked to our hearing perception, which is not able to instantly account for a sudden change of level (e.g. impulsive sounds). For instance, considering a 1kHz tone burst of constant amplitude and of long enough duration (see figure 3-6), the loudness initially grows fast and then slows down to finally reach a plateau. This illustrates that the hearing system perceives the total loudness only after some time and in two steps. This build-up or fade in effect is simulated by two parallel low-pass filters (LP) at the end of the time varying loudness calculation chain (see figure 3-5). One filter has a short time constant to account for the fast growth and the second one has a longer time constant to account for the slower growth. The weighted average of the two filtered data yields the time varying loudness N(t).

Figure 3 6. Examples of Time varying loudness in free field for three different 1 kHz tone bursts of constant amplitude (70dB RMS).

The shorter burst (10ms) shows the fast growth in the first milliseconds, the second burst shows the slower growth after several milliseconds and the third and longest burst shows that the total loudness (8 Sones GF for a 1kHz tone at 70dB) is finally reached after 300ms. The slow fade-out or decay time of each loudness curve is due to temporal post-masking. (From DIN 45631/A1)


Chapter 4 Sharpness

Chapter 4 Sharpness A sensation which is relevant to the pleasantness of a sound is its 'sharpness', allowing you to classify sounds as shrill (sharp) or 'dull'. The sharpness sensation is strongly related to the spectral content and center frequency of narrow-band sounds and is not dependent on loudness level or the detailed spectral content of the sound.

Roughly, it corresponds to the first spectral moment of the specific loudness, with a pre-emphasis for higher frequencies. A quantitative procedure has been proposed, expressing the sharpness in the unit 'acum'. The reference sound of 1 acum is a narrow-band noise, one critical band wide, and at a center frequency of 1 kHz and having a level of 60 dB.

The dependency of sharpness on the center frequency and bandwidth of the noise is shown in Figure 5-5 {6}. The middle curve represents a noise of one critical bandwidth as a function of center frequency, the upper and lower curves representing the sharpness of noises with respect to fixed upper (10 kHz) or lower (0.2 kHz) cut-off frequency as a function of the other cut-off value. Higher frequency noises produce higher sharpness.

Figure 5-5 Loudness of bandlimited noise

The specific sharpness calculation (S'(z) ) is made according to:

where:


Chapter 4 Sharpness

N'(z) is the specific Zwicker loudness g(z) a weighting function that pre-stresses higher frequency components

(Figure 5-6)

g(z) has unit value below 16 Bark and rises exponentially as

Figure 5-6 Sharpness calculation weighting function

The total sharpness S expressed in 'acums' is obtained by integrating the specific sharpness.


Chapter 5 Roughness

Chapter 5 Roughness The roughness or harshness of a sound is a quality associated with amplitude modulations of tones. When this modulation frequency is very low (15 Hz), the actual time varying loudness fluctuations can be perceived. This fluctuation sensation is discussed in section 5.6.

At high modulation frequencies (above 150-300 Hz), three separate tones can be heard. In the intermediate frequency range (15-300 Hz), the sensation is of a stationary, but rough tone, which renders it rather unpleasant. This sensation is often associated with engine noise, where fractional orders can cause the modulation effects.

Roughness increases with degree of modulation and with modulation frequency, and is less sensitive to the base or carrier frequency. The unit used to describe roughness is the "asper"; 1 asper being produced by a 100%, 70 Hz modulated 1 kHz tone of 60 dB.

The dependency relationship between modulation depth and frequency is however not straightforward. An important element is that the temporal variations of the loudness can cause masking effects, and a temporal masking

depth is introduced, representing the difference between maximum and minimum in the actually perceived time dependent loudness pattern. Due to post masking, this masking depth is smaller than the modulation depth, with the difference becoming greater at higher frequencies. The roughness (R) of an amplitude modulated sound can then be approximated as

in which is the modulation frequency.

Quantitative procedures to calculate roughness have been proposed. They involve the calculation of "partial or specific roughness" in each critical band, based on modulation and depth, including masking effects and integrating them to obtain total roughness.


Chapter 6 Fluctuation strength

Chapter 6 Fluctuation strength When the sound functions have modulation frequencies below 20 Hz, they are perceived as changes in the sound volume over time. Typically, fluctuation signal sound louder (and more annoying) than steady state signals of the same rms amplitude. In this case, the intensity of the sensation is referred to as "Fluctuation strength" with the unit "vacil". A reference sound of 1 vacil corresponds to a 1 KHz tone of 60 dB with a 100 % amplitude modulation of 4Hz. The ear is most sensitive to fluctuations at 4 Hz. Quantitative models have been proposed for the fluctuation strength {6} which take into account the temporal masking effects due to the sound fluctuation.

The dependency of the fluctuation strength ( ) on the modulation frequency

( ) and masking depth is then the following

Figure 6 1. Illustration of Fluctuation Strength.

The hatched part corresponds to the modulated signal in dB (level ). The

black curve is a sinusoid-like curve on which the masking depth and the

modulation frequency can be measured to calculate the fluctuation strength according to equation 5-8. (from Zwicker and Fastl, Psychoacoustics, 3rd edition, Sringer, 2007).

In LMS Test.Lab, the fluctuation strength is derived from the Time Varying Loudness. This has the advantage to include spectral and temporal effects such as masking. These effects must be accounted for in the measurement of the

masking depth .


Chapter 7 Pitch and Tonality


In This Chapter

Pitch ..................................................................................23 Tonality ..............................................................................24

Section 7.1 Pitch

Pitch is a sound attribute that classifies sounds on a scale from low to high. For pure tones, pitch depends largely on the frequency of the tone, but it is also influenced by its level.

In a complex tone, consisting of many spectral components, one or more pitches can be perceived. These pitches also depend to a large extent on the frequencies of the constituent components, but also masking effects can occur, making some pitches more prominent than others. Pitches, both for pure and complex tones, which can be derived from the spectral content of the signals, are called spectral pitches.

It has been observed that in a complex tone, consisting of a fundamental frequency and a number of its harmonics, a pitch corresponding to the fundamental frequency is perceived, even when that fundamental frequency is filtered out of the signal. In this case, the perceived pitch does not relate anymore to a component actually present in the signal but relates to the difference between the higher harmonics. This type of pitch is called residue pitch or virtual pitch.

The pitch calculation is implemented according the method developed by Terhardt (J. Acoust. Soc. Am. Vol 71, pp 679-688, 1982). Both spectral and virtual pitches can be derived as well as the weight of each calculated pitch. These indicate how prominently the pitches are perceived. If, in the calculation the effect of the tone level on the pitch is taken into account, the calculated pitch is called true pitch. If the influence of level on the tone is neglected, it is called nominal pitch.

Pitch metrics are not presented on the frequency scale but on the Pitch Unit (p.u.) scale. The Pitch Units are closely related to frequencies. The difference is that Pitch Units account for Pitch Shifts. The latter correspond to small variations of a given frequency while studying the perceived pitch of a tonal sound. These variations can be of several percent and are dependent on the amplitude/level of the tone as well as on the masking pattern in the case of complex tones.



Section 7.2 Tonality

Tonality is a metric that is concerned with the tonal prominence of a sound. The purpose is to evaluate the presence or not of tones in the spectrum of a noisy sound. In a noise spectrum, a tone contribution at about 700Hz gives the maximum tonality impression. It follows that tonality is modeled as a frequency dependent function. Narrowband noises can also be perceived as tonal. The smaller the bandwidth is, the more tonal the noise seems.

The tonality metric is built from the model of W. Aures (PhD dissertation, TU Munich, 1984). To identify tonal components, the model uses the method developed by Terhardt (J. Acoust. Soc. Am., vol.71, pp.679-688, 1982) for pitch extraction.

First, the spectral lines that are at least 7dB higher than their two lower and higher neighbors are isolated. A new spectrum, free of tonal components, is built by removing the detected sequences of five spectral lines, considered as pure tones. From both spectra, the fraction of the total loudness due to tonal

components is calculated. This is denoted by

An extra weighting function, , is determined from the pitch weights of the tonal components relevant to the pitch perception. This is a frequency dependent function such that at 700Hz the perception of tonality is maximal.

Finally, a constant value C is added to scale the tonality results to standardize the result, i.e. such that a 1kHz sine tone at 60dB gives a tonality of 1 t.u. (tonality unit).

Finally the tonality is calculated as the combination of the three functions described above:



Articulation index (AI)

In This Chapter

Table of Articulation indices...............................................27

The Articulation Index is a parameter developed with a view to assuring speech privacy. Speech privacy can be defined as the lack of intrusion of recognizable speech into an area when background sound or noise then provides a positive quality of privacy.

The measure of interference caused by noise to the masking of speech can be calculated by weighting the noise spectrum (in 1/3 octave bands) according to its importance to the understanding of speech. From this weighted spectrum, the Articulation Index is derived.

A graphical equivalent of the calculation is given in Figure 5-7 (from {17}).

Figure 5-7 Graphical representation of the Articulation

The 1/3 octave bands relevant to speech are weighted by a number of dots. When the sound pressure level is plotted on this graph, the AI can be derived as the num ber of dots above the spectrum divided by the total number. Practical calculations are of course based on tables.



Figure 5-8 Intelligibility of sentences as a function of articulation index.

Figure 5-7 Graphical representation of the Articulation Index

This index can then be related to a per centage of syllables understood (see Fig ure 5-8 from {17}) For complete privacy, an AI of 0.05 is the limit, for semi-privacy to discuss non-confidential matters, an AI of 0.1 is acceptable {17}. Figure 5-8 Intelligibility of sentences as a function of articulation index.

There are two methods available:

Standard The calculation is based on the work of Beranek as set out in "The design of speech communication systems", Proceedings of the IRE, Vol 45, 880-884, 1947. The results of this method will lie in the range 0-100%

Modified or Open These calculations are based upon the AIM method which has been described in the work mentioned above, but which opens up the internal floating range of 30dB to a fixed range of 80dB between the limits of 20 and 100dB. The results of this method will lie in the range-107% to almost 160%



Section 7.3 Table of Articulation indices

Below is the table containing all Articulation and Open articulation indices per 1/3 octave band and per Sound Pressure Level (in dB).


Chapter 8 Speech interference level (SIL, PSIL)

Chapter 8 Speech interference level (SIL, PSIL) When the comprehension of speech is the goal, background sound or noise has the negative quality of interference. It can cause annoyance, and even be hazardous in a working environment where instructions need to be correctly understood. Therefore, a noise rating called 'Speech Interference Level' (SIL) was developed.

Beranek originally defined it as the arithmetic average of the sound pressure levels in the bands 600-1200, 1200-2400 and 2400-4800 Hz. Since the definition of the new preferred octave band limits, this definition was changed to the Preferred Speech Interference Level' or PSIL, defined as the average sound pressure level in the 500, 1000 and 2000 Hz octave bands {5,17}.

In 1977, the Speech Interference level was standardized as ANSI S3.14-1977(R-1986) {33}, which also included the 4 kHz octave band. This is in accordance with an ISO suggestion, described in ISO Technical Report TR 3352-1974. On average, the ANSI-SIL is about 1 dB higher than the original (Beranek) and about 2.5 dB lower than the PSIL {17}.

Before the standardised ANSI-SIL, another definition of the Speech Inteference Level was used and known as the SIL3, defined as the average sound pressure level in the 1, 2 and 4kHz octave bands.

The application of the SIL to the actual understanding of speech is presented in several graphs and tables {see 5,17}. These papers show the relationship between SIL and the conditions under which speech can be understood. As an example, Figure 5-9 shows the relationship between ease of face- to-face conversation with ambient noise level in PSIL, and separation distance in meters {5}.

Figure 5-9 Communication limits in the presence of background noise (after Webster)


Chapter 9 Noise evaluation criteria: NR, NC and NCB


In This Chapter

Noise Rating (NR) ..............................................................31 Noise Criterion (NC)...........................................................33 Balanced Noise Criterion (NCB) .........................................36

In this chapter, three methods to evaluate background noise in interior spaces are presented. These are:

the Noise Rating (NR) method, as given in the recommendation standard R-ISO 1996-1971;

the Noise Criterion (NC), introduced by Beranek (1957) and extended in ISO 9568:1993;

the Balanced Noise Criterion (NCB), proposed by Beranek (1989) as a substitute to the NC and standardized as ANSI S12.2-1995.

Interior spaces are various, such as offices, factories, theaters, conference rooms, classrooms, churches, houses

The three metrics are based on sound pressure level (SPL) measurements in 1/1 octave bands spectra.

Section 9.1 Noise Rating (NR)

NR curves, or Noise Rating curves, were introduced by Kosten and Van Os (National Physical Lab. Symp. 12 1962) to determine the acceptable indoor environment for hearing preservation, speech communication and annoyance. They are also used in many cases by machinery manufacturers to specify machinery noise levels.

Noise curves are generated to fit a tendency from peoples perception under noisy environment. The Noise Rating value in dB corresponds to the SPL above which environmental noise becomes annoying.



NR curves have given values for the nine octave bands between 31.5Hz and 8000Hz.

Figure - 10-1 (Blue) NR curves: The NR value of each curve corresponds to the SPL (in dB) above which environmental noise becomes annoying. (Red) 1/1 octave band spectrum of a noise that gives a NR value of NR-42.

According to the environment, the Noise Rating level for different uses should not exceed the Noise Ratings indicated in the table below:

Noise rating curve Application

NR 25 Concert halls, broadcasting and recording studios, churches

NR 30 Private dwellings, hospitals, theatres, cinemas, conference rooms

NR 35 Libraries, museums, court rooms, schools, hospitals operating theaters and wards, flats, hotels, executive offices



NR 40 Halls, corridors, cloakrooms, restaurants, night clubs, offices, shops

NR 45 Department stores, supermarkets, canteens, general offices

NR 50 Typing pools, offices with business machines

NR 60 Light engineering works

NR 70 Foundries, heavy engineering works

Section 9.2 Noise Criterion (NC)

Noise Criterion or NC curves were built from interviews of person in many indoor environments and in the presence of various background noises (Beranek, Noise Control, vol.3, pp.19-27, 1957).



NC curves have given values for the eight octave bands between 63Hz and 8000Hz. For the purpose of rating indoor noises emitted by heating, ventilating and air-conditioning (HVAC) and intrusive noise from specific electric or mechanical equipments like in theatres, review rooms and dubbing rooms, the Noise Criterion is extrapolated to the octave bands centred at 31.5Hz and 16000Hz. This extrapolation is standardized in ISO 9568:1993.

Figure 10-2 (Blue) NC curves (with ISO 9568:1993 extrapolation): The NC value of each curve corresponds to the SPL (in dB) above which environmental noise becomes annoying. (Red) 1/1 octave band spectrum of a noise that gives a NC value of NC-41.

The noise in different types of rooms should not exceed the Noise Criterion limits below:



Type of Room - Space Type Recommended NC Level NC Curve

Equivalent Sound Level dB(A)

Apartments 25-35 35-45

Assembly Halls 25-30 35-40

Churches 30-35 40-45

Courtrooms 30-40 40-50

Factories 40-65 50-75

Hotels/Motels

- Individual rooms or suites 25-35 35-45

- Meeting or banquet rooms 25-35 35-45

- Service and Support Areas 40-45 45-50

- Halls, corridors, lobbies 35-40 50-55

Offices

- Conference rooms 25-30 35-40

- Private 30-35 40-45

- Open-plan areas 35-40 45-50

- Business machines/computers 40-45 50-55

Hospitals and Clinics

- Private rooms 25-30 35-40

- Operating rooms 25-30 35-40

- Wards 30-35 40-45

- Laboratories 35-40 45-50

- Corridors 30-35 40-45

- Public areas 35-40 45-50

Schools

- Lecture and classrooms 25-30 35-40



- Open-plan classrooms 35-40 45-50

Movie motion picture theaters 30-35 40-45

Libraries 35-40 40-50

Legitimate theaters 20-25 30-65

Private Residences 25-35 35-45

Restaurants 40-45 50-55

TV Broadcast studies 15-25 25-35

Concert and recital halls 15-20 25-30

Sport Coliseums 45-55 55-65

Sound broadcasting 15-20 25-30

Section 9.3 Balanced Noise Criterion (NCB)

The Balanced Noise Criterion or NCB (Beranek, J. Acoust. Soc. Am., vol.86, pp.650-664, 1989) was dedicated to supersede the Noise Criterion (NC) and were standardized as ANSI S12.2-1995. Some reasons of using the NCB are that these are based on the ANSI (four-band) definition of the Speech-Interference Level (SIL) and that they extend the noise criterion down to the 31.5- and 16-Hz 1/1 octave bands.



Moreover, the high values of NCB in the 16- and 31.5- Hz octave bands correspond to areas in which vibrations in light-weight partitions and ceiling constructions are perceivable, from moderately noticeable to clearly audible.

Figure 10-3 (Blue) NCB curves: The NCB value of each curve corresponds to the SPL (in dB) above which environmental noise becomes annoying. (Red) 1/1 octave band spectrum of a noise that gives a NCB value of NCB-41.



The noise in different types of rooms should not exceed the Balanced Noise Criterion limits below:


Chapter 10 Tone-to-Noise Ratio and Prominence Ratio


In This Chapter

Tone-to-Noise Ratio ...........................................................40 Prominence Ratio...............................................................40

Tone-to-Noise ratio and Prominence ratio are two metrics related to the detection and evaluation of prominent discrete tones in noises emissions.

In both methods, a discrete tone may be partially masked by the broadband noise in a relatively narrow frequency band, the "critical band", centered at the frequency of the tone. The critical bandwidth is a function of the tone frequency and is the same for the Tone-to-Noise and Prominence ratios.

Within the "critical band", a discrete tone is just audible if its sound pressure level (SPL) is 4dB below the sound pressure level (SPL) of the masking noise in the critical band centered on the tone. To be prominent, a discrete tone should fulfill some criteria. These are different for the Tone-to-Noise and Prominence ratios method.

The methods of calculating both ratios comply with standard ECMA-74:2008.



Section 10.1 Tone-to-Noise Ratio

In the Tone-to-Noise ratio approach, the level of a prominent discrete tone must be at least 8dB above the level of the masking noise. This is valid for frequencies higher than 1000Hz and it should be slightly more for lower frequencies.

Figure 11-1 Tone-to-Noise ratio method. ft is the center frequency of the critical band [f1, f2]. Lt is the SPL of the tone, Ltot is the SPL of the critical band and Ln is the deduced SPL of the noise. The Tone-to-Noise ratio is DLT=Lt-Ln. (From ECMA-74:2008)

The level of the masking noise is related to the difference between the sound pressure level of the critical band and the sound pressure level of the discrete tone. The masking noise level is also dependent on the bandwidth of the critical band and the narrow band that corresponds to the tone itself. Tone-to-Noise ratio is illustrated in figure 11-1.

Tone-to-Noise ratio is mostly accurate in the case multiple tones are in adjacent critical bands. In the case multiple tones are within the same critical band, Prominence ratio may be more effective.

Section 10.2 Prominence Ratio

In the Prominence ratio method, a discrete tone candidate is said prominent if the average SPL of the "critical band" centered on the tone is at least 9dB higher than the average SPL of the adjacent critical bands (lower and upper). The SPL difference must be a greater amount for tones at frequencies lower



than 1000Hz. The adjacent critical bands have also frequency dependent bandwidths. Prominence ratio is illustrated in figure 11-2.

Figure 11-2 Prominence ratio method. ft is the center frequency of the middle critical band [f1,M, f2,M] whose SPL is LM. The average levels in the two adjacent critical bands (lower - LL - and upper - LU) are also calculated. They are required to determine the Prominence ratio DLp. For ft > 171.4Hz,

(From ECMA-74).

Prominence ratio is more effective than Tone-to-Noise ratio in the case multiple tones are present within a critical band. However, for multiple tones in adjacent critical bands, for example when strong harmonics exist, the Tone-to-Noise ratio is more accurate.


Chapter 11 Reading list

Chapter 11 Reading list Step 1

D.LUBMAN, Noise Quality, Toward a Larger Vision of Noise Control Engineering, Journal of Noise Control Engineering, ....

Step 2 J.BLAUERT, Spatial Hearing, MIT Press, Cambridge (MA), 1983.

Step 3 W.BRAY ET AL, Development and Use of Binaural Measurement Technique, Proc. Noise Con. `91, Tarytown (NY), July 14-16, 1991, pp 443-450.

Step 4 D.HAMMERSHOI, H.MOLLER, Binaural Auralisation : Head-Related Transfer Function Measured on Human Subjects, Proceedings 93rd AES Convention, Vienna (A), March, 24-27, 1992, 7pp.

Step 5 J.HASSAL, K.ZAVERI, Acoustic Noise Measurements, Bruel & Kjaer, DK2850 Naerum, Denmark, 1988

Step 6 E.ZWICKER,H.FASTL, Psychoacoustics, Facts and Models, Springer Verlag, Berlin (Germany), 1990.

Step 7 J.HOLMES, Speech Synthesis and Recognition, Van Nostrand Reinhold, Wokingham, Berkshire (UK), 1988.

Step 8 M.HUSSAIN, J.GOELLES, Statistical Evaluation of an Annoyance Index for Engine Noise Recordings, SAE Paper 911080, Proc. SAE Noise and Vibration Conference, Traverse City (MI), May 16-18 1991 pp 359-368.

Step 9 H.SHIFFBAENKER ET AL, Development and Application of an Evaluation Technique to Assess the Subjective Character of Engine Noise, SAE paper 911081, Proc. SAE Noise and Vibration Conference, Traverse City (MI), May 16-18 1991, pp 369-379.

Step 10 K.TAKANAMI ET AL, Improving Interior Noise Produced During Acceleration, SAE paper 911078, Proc. SAE Noise and Vibration Conference, Traverse City (MI), May 16-18 1991, pp 339-348.

Step 11 G.IRATO, G.RUSPA, Influence of the Experimental Setting on the Evaluation of Subjective Noise Quality, Proceeding of the second International Conference on Vehicle Comfort, Oct 14-16, 1992, Bologna (Italy), pp. 1033-1044.

Step 12 INTERNATIONAL ORGANIZATION FOR STANDARDIZATION, Method for Calculating Loudness Level, ISO-532-1975 (E)

Step 13 E.ZWICKER ET AL, Program for Calculating Loudness According to DIN45631 (ISO532B), Journal Acoustic Society Jpn (E), Vol. 12, Nr.1, 1991.



Step 14 S.J.STEVENS, Procedure for Calculating Loudness : Mark VII, J. Acoust. Soc. Am., Vol. 33, Nr.11, pp.1577-1585, 1961.

Step 15 S.J.STEVENS, Perceived Level of Noise by Mark VII and Decibel, J. Acoust. Soc. Am., Vol.511, Nr.2, pp. 575-601, 1971.

Step 16 E.ZWICKER, Procedure for Calculating Loudness of Temporally Variable Sounds, J. Acoust. Soc. Am., Vol. 62, Nr. 3, pp 675-681, 1977.

Step 17 L.L.BERANEK, Criteria for Noise and Vibration in Communities, Buildings and Vehicles in Noise and Vibration Control, revised edition, McGraw-Hill Inc., 1988.

Step 18 W.AURES, Berechnungsverfahren f r den Sensorischen Wohlklang beliebigen Schallsignale, Acustica, Vol.59, pp. 130-141, 1985

Step 19 M.ZOLLNER, Psychoacoustic Roughness. A New Quality Criterion, Cortex Electronic, 1992.

Step 20 W.AURES, Ein Berechnungsverfahren der Rauhigkeit, Acustica, Vol.58, pp. 268-280, 1985.

Step 21 M.F.RUSSEL, What Price Noise Quality Indices, Proc. Engineering Integrity Society Symposium on NVH Challenges - Problem Solutions, Oct.21, 1992.

Step 22 M.F.RUSSEL ET AL. Subjective Assessment of Diesel Vehicle Noise, IMechE paper 925187, Ref. C389/044, FISITA Conference Engineering for the customer, pp.37-42, 1992.

Step 23 D.G.FISH, Vehicle Noise Quality - Towards Improving the Correlation of Objective Measurements with Subjective Rating, I. Mech. E. - paper 925186, Ref. C389/468 FISATA-conference, Engineering for the customer, pp. 29-36, 1992.

Step 24 G.TOWNSEND, A New Approach to the Analysis of Impulsiveness in the Noise of Motor Vehicles, Proc. Autotech `89, paper 7/26.

Step 25 MOTOR INDUSTRY RESEARCH ASSOCIATION, Improving Correlation of Objective Measurements with Subjective Rating of Vehicle Noise, MIRA research report K3866326.

Step 26 F.K.BRANDL ET AL, A Concept for Definition of Subjective Noise Character - A Basis for More Efficient Vehicle Noise Reduction Strategies, - Proceedings Internoise-89, Newport Beach (CA), Dec. 4-6, 1989, pp.1279-1282.

Step 27



R.S.THOMAS, A Development Process to Improve Vehicle Sound Quality, SAE paper 911079, Proc, SAE Noise and Vibration Conference, Traverse City (MI), May 13-16 1991, pp. 349-358.

Step 28 G.R.BIENVENUE, M.A.NOBILE, The Prominence Ratio Technique in Characterizing Perception of Noise Signals Containing Discrete Tones, Proc. Internoise `92, Toronto, Canada, July 20-22, 1992, pp. 1115-1118.

Step 29 K.TSUGE ET AL, A Study of Noise in Vehicle Passenger Compartment during Acceleration, SAE paper 8509665, Proceedings SAE Noise and Vibration Conference, Traverse City (MI), May 15-17, 1985, pp. 27-34.

Step 30 T.WAKITA ET AL, Objective Rating of Rumble in Vehicle Passenger Compartment during Acceleration, SAE paper 891155, Proceedings SAE Noise and Vibration Conference, Traverse City (MI), May 16-18, 1989, pp. 305-312.

Step 31 W.YAGISHASHI, Analysis of Car Interior Noise during Acceleration Taking into Account Auditory Impressions, JSAE Review (E), Vol. 12, nr.4, Oct. 1991, pp. 58-61.

Step 32 K.FUJITA ET AL, Research on Sound Quality Evaluation Methods for Exhaust Noise, JSAE Review (E), Vol. 9, Nr. 2, April 1988, pp. 28-33.

Step 33 American National Standard, S.3.14-1977 (R886), Rating Noise with Respect to Speech Interference, Acoustical Society of America.

Step 34 H.STEENEKEN, T.HOUTGAST, RASTI, A Tool for Evaluating Auditoria, Bruel & Kjaer Technical Review, nr.3-1985, pp. 13-30.

Step 35 M.NAKAMURA, T.YAMASHITA, Sound Evaluation in Cars by RASTI Method, JSAE Review, Vol.11, Nr.4, Oct 1990, pp.38-41.

Step 36 H.MOLLER, Fundamentals of Binaural Technology, Applied Acoustics, Vol. 36, 1992, pp. 171-218.

Step 37 K.GENUIT, M.BURKHARD, Artificial Head Measurement System for Subjective Evaluation of Sound Quality, Sound and Vibration, March 1992, pp. 18-23.

Step 38 G.MICHEL, G.EBBIT, Binaural Measurements of Loudness as a Parameter in the Evaluation of Sound Quality in Automobiles, Proc. Noise Con. `91, Tarytown (NY), July 14-16, 1991, pp. 483-490.

Step 39 G.THEILE, The Importance of Diffuse Field Equalisation for Stereophonic Recording and Reproduction, Proc. 13-th Tonmeistertagung, 1984.



Step 40 D.S.MANDIC, P.R.DONOVAN, An Evaluation of Binaural Measurement Systems as Acoustic Transducers, Proc. Noise Con 91, Tarytown (NY), July 14-16, 1991, pp. 459-466.

Step 41 H.HAMMERSHOI, H.MOLLER, Artificial Head for Free Field Recording ; How Well Do They Simulate Real Heads ?, Proc. 14th ICA, Beijing, 1992, Paper H6-7 (2pp).

Step 42 K.GENUIT, H.GIERLICH, Investigation between Objective Noise Measurement and Subjective Classification, SAE Paper 891154, Proceedings SAE Noise and Vibration Conference, Traverse City (MI), May 16-18 1989, pp 295-303.

Step 43 H.MOLLER ET AL, Transfer Characteristics of Headphones, Proc. 92nd AES Convention, Vienna (A), March 24-27, 1992, 28 pp.

Step 44 Y.OKAMOTO ET AL, Evaluation of Vehicle SOunds Through Synthesized Sounds that Respond to Driving Operation, JSAE Review (E), Vol.12, Nr.4, Oct.1991,pp.52-57.

Step 45 S.M.HUTCHINS ET AL, Noise, Vibration and Harshness from the customer's Point of View, IMechE paper 925181, Ref. C389/049, Proc. FISATA-92 Conf, Engineering for the Customer.

Step 46 H.AOKI ET AL, Effects of Power Plant Vibration on Sound Quality in the Passenger Compartment During Acceleration, SAE paper 870955, Proc. SAE Noise and Vibration Conf., Traverse City (MI), Apr. 28-30, 1987, pp.53-62.

Step 47 K.C. PARSONS, M.J. GRIFFIN, Methods for predicting Passenger Vibration Discomfort, Society of Automotive Engineers Technical Paper Series 831921

Step 48 SM.J. GRIFFIN, Handbook of Human Vibration, Academic Press Ltd. ISBN 0-12-03040-4

Step 49 J.D. LEATHERWOOD, L.M BARKER, A User-Oriented and Computerized Model for Estimating Vehicle Ride Quality, NASA Technical Paper 2299 (1984)

Step 50 International Standard, Ref. No. ISO 2631/1 - 1985 (E)

Step 51 International Standard, Ref. No. ISO 5349 - 1986 (E)

Step 52 British Standards Institution, Measurement and evaluation of human exposure to whole-body mechanical vibration and repeated shock Ref. No. BS 6841 - 1987

Step 53 American National Standard, S3.14 - 1977 (R-1986), Rating Noise with Respect



to Speech Interference, order from the Acoustical Society of America.

Step 54 ANSI S3.5, Calculation of the Articulation index, American National Standards Institute, Inc., 1430 Broadway, New York, New York 10018 USA, 1969

Step 55 International Standard, Ref. No. ISO 532 - 1975 (E)

Step 56 German standard with amendment, Ref No. DIN 45631/A1


Index

A Articulation index (AI) 25

B Balanced Noise Criterion (NCB) 36

E Equivalent sound pressure level 7

F Fluctuation strength 21

L Loudness 9

N Noise Criterion (NC) 33 Noise evaluation criteria

NR, NC and NCB 31 Noise Rating (NR) 31

P Pitch 23 Pitch and Tonality 23 Prominence Ratio 40

R Reading list 43 Roughness 19

S Sharpness 17 Sound pressure level 5 Speech interference level (SIL, PSIL) 29 Stevens Mark VI 10 Stevens Mark VII 11

T Table of Articulation indices 27 Time domain sound pressure level 5 Time Varying Zwicker Loudness 10, 14 Tonality 24 Tone-to-Noise Ratio 40 Tone-to-Noise Ratio and Prominence Ratio

39

Z Zwicker Loudness 12, 14


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