Page 43 - January 2007

Standards News
A NEW ANSI LOUDNESS STANDARD
Rhona P. Hellman
Department of Speech-Language Pathology and Audiology Northeastern University
Boston, Massachusetts 02115
Loudness depends on both the acoustic properties of the sound that impinges on the listener and on the lis- tener. In 2005 a new standard for calculating the loud- ness of steady sounds as perceived by listeners with normal hearing was approved by the American National Standards Institute (ANSI). The new standard replaces and supersedes the computational procedure used in the old ANSI S3.4- 1980 standard. To gain some insight into the new ANSI standard, several important distinctions between the old and new loudness standards are described below.
Comparison between new and old ANSI loudness standards
The old ANSI standard was based on the procedure pro- posed by S. S. Stevens (1957, 1961). This procedure was rela- tively easy to apply without the need for high-speed computers. It was used with success to predict the loudness of noises with relatively homogeneous spectra in the mid-frequency region (e.g., Scharf and Hellman, 1980). However, more complex sig- nals and technological advances required a standard with broader applicability. The following restrictions placed limita- tions on the applicability of the old ANSI standard. First, old ANSI S3.4-1980 is restricted to broadband stimuli that do not contain tonal components. Thus, it is unable to predict accu- rately the loudness of noises with sharp line spectral compo- nents such as transformer hum or fan noise [see ANSI S3.4- 1980 (R 2003); Hellman, 1991]. The new ANSI loudness stan- dard overcomes these limitations. Second, loudness calculated according to old ANSI S3.4-1980 is based heavily on linear loudness indices that do not reveal the detailed curvilinear shape and loudness magnitudes of the revised equal-loudness contours in ISO 226:2003 (International Organization for Standardization, Geneva); Suzuki and Takeshima (2004). In contrast, equal-loudness contours calculated according to ANSI S3.4-2005 predict equal-loudness contours that are in good overall agreement with those in ISO 226:2003. The agree- ment between the calculated and measured loudness levels is especially good in the frequency region below 500 Hz (Hellman, 2002, 2006). Third, old ANSI S3.4-1980 limits the description of the relation between loudness in sones and loud- ness level in phons to a loudness level of 20 phons, whereas the new ANSI S3.4-2005 standard extends the dynamic range for loudness calculations down to near threshold levels.
Figure 1 compares the relation between loudness in sones and loudness level in phons predicted by the old ANSI S3.4- 1980 standard to the relation predicted by the new ANSI S3.4- 2005 standard. According to Fig. 1, in old ANSI S3.4-1980 a simple power function in the form L = kIn describes the relation between loudness and sound intensity above 20 phons. This
relation is based on an obsolete and withdrawn ISO standard (ISO/R 131:1959). In contrast, the curve based on the new ANSI S3.4-2005 standard approximates a simple power func- tion down to 40 phons. Between 40 and 90 phons, the calculat- ed sone values do not deviate by more than 5% from the stan- dard sone values in ISO 532:1975. However, below 40 phons the loudness function based on ANSI S3.4-2005 becomes progres- sively steeper than a simple power function prediction approaching a limiting slope close to unity between loudness and sound intensity near threshold. This result means that threshold loudness cannot be zero in accord with experimental evidence (e.g., Hellman and Zwislocki, 1961, 1963; Hellman, 1976; Buus et al., 1998) and theoretical considerations (Zwislocki, 1965; Moore et al., 1997). Instead, consistent with the definition of threshold, threshold loudness must have a finite positive value. Given these basic improvements, ANSI S3.4-2005 was introduced and approved.
Relation between the new ANSI loudness standard and the ISO loudness standard
The new ANSI S3.4-2005 standard also updates ISO 532:1975 titled “Acoustics–Method for calculating loudness level.” ISO 532 consists of two sections. Section one describes method A and section two describes method B. Method A of ISO 532:1975 is basically the same as the replaced ANSI S3.4-
Fig. 1. Loudness in sones as a function of loudness level in phons. The dotted curve shows the loudness function based on the old ANSI S3.4-1980 standard; the dashed function shows the loudness function based on the new ANSI S3.4-2005 standard.
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