## Acoustics research centre

# More metrics

Some less well known sound quality metrics or metrics with multiple definitions are listed and briefly explained below; this list is by no means exhaustive.

Index |
Descriptions and/or uses |

Tonal Colour, Brightness, Sensory Euphony, Timbre [1] | These can be partially quantified using the metrics sharpness, roughness, fluctuation strength, tonality and pitch. Brightness and sensory euphony are usually more closely linked with the calculation of sharpness, and timbre is usually more closely linked to roughness. |

Pitch/Pitch Strength [2] | A measure of the subjective impression of the frequency content of a sound. Pitch of complex tones must incorporate subjective effects such as virtual pitch. |

Harmonic content [3] | Examination of the spectrum to find the amplitudes of harmonics. The ratio of odd to even harmonics (or half to full harmonics). Strength of tones, “Spectrum peak level – local mean level”. Pitch and virtual or “perceived” pitch coupled in some way with masking effects. Used to assess vehicle interior noise, and vehicle exterior noise. |

Impulsivity/Impulsiveness [4] | A measure of a short burst of sound with a rapidly changing loudness. The ‘crest factor’ of the pressure of a wave form peak/rms measured over a 1 sec interval. Kurtosis: this is a measure of how sharply the function peaks around its mode. Standard deviation of \(L_{instmean}\). A method for measuring impulsive noise emitted by IT and telecommunications equipment can be found in Annex E BS EN ISO 7779:2001. Used in the assessment of vehicle interior noise. |

Irregularities [4] | Measures of unexpected sounds. Standard deviation of pressure peaks normalised by RMS of pressure signal, Variations in 10 ms RMS levels in dB(A) Used in the assessment of vehicle interior noise. |

Time varying or instationary loudness analysis [4, 5, 6]: | Measures of how the loudness varies with time. Statistical analysis, including the \(N_{10}\) (Value of loudness exceeded 10% of the time) \(N_{50}\), etc, Absolute loudness, or loudness relative to background level can be measured. Statistical instantaneous mean loudness: This has been used to calculate a combination metric for washing machine motors. Statistical instantaneous minimum loudness: This has been used to calculate a combination metric for hairdryers The prominence ratio: This has been used to calculate annoyance index for a vacuum cleaner. Also used in the Automobile Industry. |

Rhythm [2] | Assessed using a listener. |

Assessment by ear [1] | Listening for irregularities tones etc. Add a correction value to a combination metric if the noise contains a distinguishable, discrete, continuous note (whine, hiss, screech, hum, etc.); or the noise contains distinct impulses (bangs, clicks, clatters, or thumps); or the noise is irregular enough to attract attention. For example BS4142 (1997) adds a 5dB correction. |

Speech Interference level (SIL)/ Modified Articulation index (MAI)/ Articulation index (AI) | All are measures of the way speech interferences with speech/communication. SIL is calculated as the arithmetic mean of the sound pressure levels of the ambient noise in the four octave bands with the central frequencies 500Hz, 1000Hz, 2000Hz and 4000Hz. See BS ISO 9221-1:1996 for the SIL method. |

Attenuation Slope of a spectrum envelope [5] | The attenuation slope of the spectrum envelope beyond 1000 Hz has been used in the characterisation of vacuum cleaner noise. |

Spectral formant [5] | A measure of a frequency region with high amplitude. For example the spectral formant in the band 1500 to 3000 Hz is used in the characterisation of vacuum cleaner noise. |

Order analysis [4] | Sound levels compared with operation rate or r.p.m of the machine. Level differences between each ear can be identified. Used in car interior noise. Used by Hashimoto in the calculation of booming strength. |

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## Combination Metrics

Combination metrics take several different metrics together to give an overall quality score for an appliance. The individual metrics are weighted and then summed. Some examples of combination metrics are given below:

Index |
Descriptions and/or uses |

Annoyance index for vacuum cleaner [5] |
Using a combination of loudness (L), sharpness (S), roughness (R), fluctuation strength (F), tone to noise ratio (T) and prominence ratio (P) in varying quantities [17]. \[ \mbox{Annoyance} = 0.655L - 2.618S + 0.047R + 2.708F - 0.04T - 0.106P\] |

Vacuum cleaner characterisation [5] | Using the SPL in dB(A), after equalisation of subjective level of the sounds three acoustical Parameters emerge: Spectral Peak between 100 and 1500 Hz, Spectral formant (i.e. a frequency region of high amplitude) in the band 1500 to 3000 Hz, Attenuation Slope of the spectrum envelope beyond 1000 Hz. |

Washing machine motors [6] |
Used a combination of stationary loudness (L), statistical instantaneous mean loudness (\(L_{instmean}\)), and statistical sharpness standard deviation (\(S_\sigma\)) [18]. \[\mbox{Annoyance} = 87.817+126.66L + 0.047L_{instmean} + 644.94 S_\sigma\] |

Hairdryers [6] |
Used a combination of tone to noise ratio (T) and statistical instantaneous minimum loudness (\(L_{instmean}\)) [18]. \[\mbox{Annoyance} = 491.28+5.89T + 11.46L_{instmean} \] |

Sensory pleasantness [2] |
Is a combination of loudness (\(L\)), roughness (\(R\)), sharpness (\(S\)) and tonality (\(T\)) these are all expressed as relative values making sensory pleasantness a relative quantity [9]. \[{P \over P_0} = e^{-0.7 {R\over R_0}}e^{-1.08 {S\over S_0}} (1.24 - e^{-0.7 {T\over T_0}}) e^{-0.23 {N\over N_0}}\] |

Unbiased annoyance (UBA) [2] |
Is a combination of sharpness (\(S\)), fluctuation strength (\(F\)), \(N_{10}\) level, and includes a correction factor for time of day (\(d\)) [9]. \[UBA = d(N_{10})^{1.3} \left\{1+0.25(S-1)\log(N_{10} +10) + 0.3F\left({ 1+N_{10} \over N_{10} + 0.3} \right) \right\}\] where \[d = \left\{ \begin{array}{l l}1 & \mbox{from 6am to 10pm} \\1+\left({N_{10} \over 5}\right) ^{0.5} & \mbox{from 10pm to 6am} \end{array} \right.\ \] |

Rapid speech transmission index (RASTI) | The method for calculation can be found in IEC 268-16 |

Composite Rating of Preference (CRP) [3] | Combines low frequency sound and spectral balance i.e. modifiers for boom and high frequency. Used in the car industry. |

AVL annoyance index [3] | Used to quantify engine noise quality. |

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## References

[1] ‘Perceptive Characterisation of the Acoustical Quality of Real Complex Sounds – Validation with Synthesis’ Castellengo M, Guyot F, Viollon S, Acustica Vol 82 S78 (1996)

[2] ‘Psychoacoustics and Sound Quality Metrics’ Fastl H (Sound Quality Symposium 98)

[3] ‘An objective Approach to Vehicle Internal Noise Assessments’, MF Russell, Noise and the Automobile, Selected papers from Autotech ’93

[4] ‘NVH Reduction Trends’, Automotive Engineering International Online, (2000)

[5] ‘A comparative study on the sound quality of wet and dry type vacuum cleaners’ Altinsoy, Kanca & Belek

[6] ‘Design of Combination Metrics for Two Household Appliances’ Sobhi, Ladegaard