In environmental acoustics, the ground effect is described by its acoustic impedance. By analogy with the electrical or mechanical impedance, the ground impedance effect accounts for the reaction of the support (i.e., the ground) with a physical stress (i.e., acoustic wave). Many methods for measuring the acoustic impedance of materials have been developed in laboratories (e.g., impedance tubes/ kundt tubes). However, these invasive methods prove to be unsuitable for ground problems since several samples should then be taken in situ (in the original place) between source(s) and receiver(s). ASA/ANSI S1.18-2018 (R2023): Method For Determining The Acoustic Impedance Of Ground Surfaces describes procedures for obtaining ground acoustic impedance that do not include the use of an impedance tube.
Measuring Ground Acoustic Impedance
Acoustic impedance is the ratio of the pressure over an imaginary surface in a sound wave to the rate of particle flow across the surface. The real part of the specific acoustic impedance is specific acoustic resistance; the imaginary part is specific acoustic reactance. Experimental techniques to measure impedance include the use of an impedance tube (Kundt tubes), techniques that measure the sound pressure levels above a surface, and direct measurements of sound pressure and volume velocity.
The impedance tube is in common use to measure the acoustic impedance of porous materials—such as foams or aerogels. These materials generally have a low acoustic impedance but a high attenuation. The impedance tube obtains the normal incidence absorption coefficient of a layer of textiles with a small diameter (usually less than 10 cm). It has the advantage of a straightforward theoretical framework that allows direct determination of both the real and imaginary parts of the impedance. However, its application in the field to obtain ground impedance suffers from two major disadvantages. First, it requires an accurate measurement of the distance from the first interference minimum to an ill-defined test surface, and second, it is invasive. ASA/ANSI S1.18-2018 (R2023) does not recommend the use of an impedance tube for the measurement of the acoustic impedance of a ground surface.
Techniques that use measurements of sound pressure levels above a surface include several variations based on the type of excitation, angle of incidence, number of microphones, and fitting methods. The advantage here is that the measurement is performed on the ground in its natural condition. However, because of the spherical wave front, the theoretical framework is mathematically intricate.
The ASA/ANSI S1.18-2018 (R2023) Standard for Acoustic Impedance
ASA/ANSI S1.18-2018 (R2023) describes procedures for obtaining the real and imaginary parts of the normalized acoustic impedance ratio of ground surfaces. This is achieved by the following:
- In-situ measurements of the sound pressure levels at two vertically separated microphones using specified geometries
- The averaged values of the difference between the simultaneous, instantaneous sound-pressure signals at the two microphones
This acoustic impedance method enables the user to deduce parameters for a ground impedance model by fitting spectral data to templates. It also enables the user to obtain values of the normalized specific acoustic impedance ratio of the ground entirely from measurements. The method in ASA/ANSI S1.18-2018 (R2023) is applicable to all nominally flat, commonly occurring surfaces including grassland or snow covered ground. It is to be expected that the acoustic impedance of the ground surface will vary with location; as a result, the standard maintains that a sufficient number of measurements should be taken to establish the variability with position. The method, however, is not applicable to rough grounds where the variation in height is greater than half of the shortest wavelength of interest.
This standard does not consider the direct measurement of sound pressure and volume velocity.