An acoustic monitoring system for detecting a condition of an air filter, the acoustic monitoring system comprising: a first acoustic transducer upstream with respect to airflow over the air filter; a second acoustic transducer upstream with respect to airflow over the air filter; a third acoustic transducer downstream with respect to airflow over the air filter; a fourth acoustic transducer downstream with respect to airflow over the air filter; a control unit in communication with the first acoustic transducer, the second acoustic transducer, the third acoustic transducer and the fourth acoustic transducer; characterized in that: the control unit is configured to generate an acoustic test signal from at least one of the first acoustic transducer, the second acoustic transducer, the third acoustic transducer and the fourth acoustic transducer; and the control unit is configured to determine a filter attenuation value in response to one or more sound pressure level, SPL, values measured by at least one of the first acoustic transducer, the second acoustic transducer, the third acoustic transducer and the fourth acoustic transducer from the acoustic test signal; wherein the one or more SPL values includes a first direction cross-filter SPL value and a second direction cross-filter SPL value; wherein the first direction is opposite the second direction; wherein the one or more SPL values includes an upstream SPL value (u) in response to attenuation between the first transducer and the second transducer; and wherein the one or more SPL values includes a downstream SPL value (d) in response to attenuation between the third transducer and the fourth transducer. a first acoustic transducer upstream with respect to airflow over the air filter; a second acoustic transducer upstream with respect to airflow over the air filter; a third acoustic transducer downstream with respect to airflow over the air filter; a fourth acoustic transducer downstream with respect to airflow over the air filter; a control unit in communication with the first acoustic transducer, the second acoustic transducer, the third acoustic transducer and the fourth acoustic transducer; characterized in that: the control unit is configured to generate an acoustic test signal from at least one of the first acoustic transducer, the second acoustic transducer, the third acoustic transducer and the fourth acoustic transducer; and the control unit is configured to determine a filter attenuation value in response to one or more sound pressure level, SPL, values measured by at least one of the first acoustic transducer, the second acoustic transducer, the third acoustic transducer and the fourth acoustic transducer from the acoustic test signal; wherein the one or more SPL values includes a first direction cross-filter SPL value and a second direction cross-filter SPL value; wherein the first direction is opposite the second direction; wherein the one or more SPL values includes an upstream SPL value (u) in response to attenuation between the first transducer and the second transducer; and wherein the one or more SPL values includes a downstream SPL value (d) in response to attenuation between the third transducer and the fourth transducer. the control unit is configured to generate an acoustic test signal from at least one of the first acoustic transducer, the second acoustic transducer, the third acoustic transducer and the fourth acoustic transducer; and the control unit is configured to determine a filter attenuation value in response to one or more sound pressure level, SPL, values measured by at least one of the first acoustic transducer, the second acoustic transducer, the third acoustic transducer and the fourth acoustic transducer from the acoustic test signal; wherein the one or more SPL values includes a first direction cross-filter SPL value and a second direction cross-filter SPL value; wherein the first direction is opposite the second direction; wherein the one or more SPL values includes an upstream SPL value (u) in response to attenuation between the first transducer and the second transducer; and wherein the one or more SPL values includes a downstream SPL value (d) in response to attenuation between the third transducer and the fourth transducer.