The impedance tubes that are commercialized on the market have been designed and conceived for the acoustic characterization of materials in an air medium. There are specific applications where a foam or porous medium is not saturated with air, but with another fluid: water, seawater, diesel, kerosene or oils, for example. Pythmen R&D design and develop impedance tube specific for any application where the natural medium is not air, for example, seawater-infilled impedance tubes.

Submarine acoustics is a very relevant field for civil and defence applications, where acoustic materials are saturated by seawater and not by air. For the direct acoustic characterization of porous materials saturated by seawater, it is necessary the custom design of impedance tubes and the associated software. Besides, depending on the particular application, the frequency range of interest will be very different from the frequency range of interest in acoustics in air.

Submarine applications in the civil field are very important. For example, whales and other marine mammals, fish, and even some invertebrates depend on sound, which travels much farther in the water than light. Animals that use sound to find food and mates, avoid predators and communicate face a growing problem: Man-made noise in the oceans is drowning them out. The effects of ocean noise on marine organisms include mortality, hearing impairment, masking of communication, altered behaviour, or drastic reduction in reproductive capacity. The range of frequencies of interest goes from low frequencies, which affect species such as mysticetes or baleen whales, to frequencies of tens of kHz, which affect odontocetes that communicate at high frequencies, such as toothed whales, dolphins and porpoises. Frequencies of interest can range from 10 Hz affecting some fish to 100 kHz affecting some marine mammals and snapping shrimp.

In submarine defence applications, the frequencies of interest ranges from near-zero frequencies to 500 kHz. In addition, materials with extreme acoustic capacities are sought, since the acoustic invisibility of the submarine and its safety depend on it. Specific developments are necessary to acoustically characterize materials with extreme acoustic capacities.

In some industrial sectors, it is useful to use porous foams in diesel or kerosene tanks, or in hydraulic systems that work with oils, to reduce the noise generated and to act as acoustic energy sinks.

Each application, depending on the natural medium, the frequency range of interest and the acoustic capacity of the materials, will require the custom design of impedance tubes, the associated software for the analysis and the inverse characterization of materials, and other types of alternative measurement systems for material acoustic characterization when the development of impedance tubes is not physically possible.

En general, cuando hablamos de acústica nos referimos principalmente a una propagación de ondas mecánicas, que se propagan por el aire, y en donde la frecuencia de interés se corresponde al rango de frecuencias audibles por el oído humano, desde 50 Hz hasta 20 kHz. Se trata de enfrentar un problema de confort acústico, donde el objeto es el confort del ser humano.

Los tubos de impedancia que se comercializan en el mercado están concebidos para trabajar en medio aéreo, y tienen capacidad para medir en un rango de frecuencias entre 50 Hz y 6.4 kHz. Este rango de frecuencias suele cubrirse con al menos dos tubos, con distintos diámetros y con distintas distancias entre sensores. Frecuencias por encima de los 6.4 kHz requieren de diámetros demasiado pequeños, y por tanto de sensores y distancias entre sensores extremadamente pequeñas, siendo necesario el desarrollo de microsensores para medir ruido y proyectores de ruido de diámetros diminutos.

Para ciertas aplicaciones, tanto en medio aéreo como en otros medios fluidos, son de interés las frecuencias por debajo de los 50 Hz o las frecuencias superiores a 6.4 kHz, incluso frecuencias ultrasónicas hasta los 500 kHz.

En el campo de la defensa, ya no se trata de un problema de confort acústico para el ser humano, sino que se trata de un problema de capacidad de detección para poder identificar objetivos militares, o de un problema de baja observabilidad o invisibilidad acústica, para evitar ser detectado. En este caso el rango de frecuencias de interés no está relacionada con la capacidad del oído humano, sino con la capacidad y la sensibilidad de sónares activos y sónares pasivos. Esta capacidad es enorme y no deja de aumentar, pudiéndose detectar amplitudes muy débiles desde frecuencias casi nulas hasta frecuencias de hasta casi 500 kHz.

En el campo civil para especies marinas y por razones medioambientales, son de interés frecuencias que pueden ir desde los 10 Hz hasta los 200 kHz, ya que hay especies cuyo rango de frecuencias audibles es muy superior a las del ser humano. Además de las frecuencias de las especies marinas ya indicadas, la frecuencia audible para perros y gatos está por encima de los 40 kHz, y el murciélago hasta 200 kHz. Las palomas pueden captar frecuencias extremadamente bajas, de hasta 0.5 Hz.

Para cubrir frecuencias por debajo de 50 Hz, es necesario el diseño de tubos de impedancia a medida, de grandes diámetros, de varios metros de longitud, y con distancias entre los sensores elevadas. Además, tanto para la proyección de ruido, como para las medidas acústicas, se necesitan proyectores y sensores a medida, en función de la más baja frecuencia que deseemos poder medir.

Pythmen está especializada en el diseño a medida y en la fabricación de tubos de impedancia, y el software de análisis y de caracterización de materiales inversa asociado, para cubrir rangos de frecuencias que se salen de los rangos de frecuencias convencionales, tanto para hacer ensayos en aire como en otros medios fluidos. Se utiliza el modelo completo de Biot para la identificación inversa de materiales, con 21 ecuaciones diferenciales y 21 variables, y no el modelo simplificado con 5 ecuaciones y 5 variables que se suele utilizar para acústica en aire.

In ASTM E2611 is written the following:

“5.3 Transmission loss is not only a property of a material but is also strongly dependent on boundary conditions inherent in the method and details of the way the material is mounted. This must be considered in the interpretation of the results obtained by this test method.”

In the case of very flexible foams with low isolation capacity, the Transmission Loss poorly depend on the boundary conditions. Nevertheless, the higher the stiffness of the material and the higher the isolation capacity of the material, the higher the influence of the boundary conditions in the Transmission Loss.

The reason is very simple, since the ideal test must fulfil the following conditions:

• Ideal Condition 1. No acoustic leakage may occur in the boundary between the material sample and the tube. Let’s say, no sound must pass throughout the boundary.

• Ideal Condition 2. The sample must not receive any stress in the boundary, being the longitudinal displacement free to move.

Obviously, the previous ideal conditions cannot be fulfilled, let us say, the perfect test is merely and idealization, that cannot be put in practice.

If the material has not very high isolation capacity, potential leakage in the boundary is low in comparison with the transmission acoustic path throughout the material sample. Therefore, even if Ideal Condition 1 is not perfectly fulfilled, it barely produces modifications of the Transmission Loss. On the contrary, if the isolation capacity of the material is very high, any little leakage in the boundary will produce a reduction of the dB of the Transmission Loss, and therefore, much attention must be paid to obtain the test conditions as close as possible to Ideal Conditions 1. Besides, if the foam is very flexible, it can be cut with a diameter of 0.1 or 0.2 mm higher than the diameter of the tube. Since the foam is flexible, the foam itself could avoid leakage in the boundary.

The Ideal Condition 2 is also fulfilled naturally by the foam, since its own flexibility allow the longitudinal displacement, and boundary condition has only a very local and negligible effect in the longitudinal displacements.

The worse difficulties arise experimentally when the material is extremely stiff, and it has a high isolation capacity. Advanced materials, like metamaterials, could produce isolation capacities much higher than classic materials. Besides, materials formed by several layers, some of them formed by solid non porous materials, would be very stiff and with high isolation capacities.

At last, conventional impedance tube are limited to transmission loss around 70 dB. Much more sensitivity are required to test and characterize materials with extremely high isolation capacity.

Pythmen has high expertise to design impedance tube very reliable for any type of materials, including stiff materials with extremely high isolation capacity. Special devices to obtain ideal boundary conditions are required. Also, custom design to reduce leakage, drastically increase acoustic noise projection capacity, and custom design of sensor with extremely high sensitivity.

Pythmen is a technology-based company created in 2021. It was born from the background of the needs for acoustic characterization of materials for acoustic applications in the defence field. With the experience in an industrial sector where materials with extreme acoustic capacities are necessary, at frequencies from 1 Hz to 500 kHz, with different natural medias, knowledge has been generated that allows us to design custom-made impedance tubes for those situations where the conventional impedance tubes are not suitable. Although the company is of recent creation, the Professors and Researchers involved have extensive experience in this field.