The measurement of low frequency impact sound in rooms (below 100 Hz) is critical to determining human response and acceptability of floor ceilings [1], [2]. Measurement uncertainties generally increase at lower frequencies, and a lack of precision is a general problem that ratings of low-frequency noise must overcome. Impact noise uncertainties at low frequencies are reviewed. The effect on measurement uncertainty of changes to the measurement procedure (such as fixed vs. roving microphones, measurement duration, microphone position, number of tapping machine locations, etc.) is studied, with emphasis on the uncertainties in the 50-80 Hz third-octave bands. The measurement uncertainty will be related to the precision desired to accurately relate to human reaction.

The impact ball was recently standardized within the ISO standards as a low-frequency input impact source for impact testing of floor-ceiling assemblies. The input force due to an impact ball is not measured during the tests. A force measurement plate was created with three force transducers to measure the impact force due to ball drops on six different floor assemblies. The input force showed a really good comparison for heavy and lightweight floors in lower frequency but poor force excitation for all the floors above 100 Hz. The input force values were compared with a modified tapping machine's force input levels showing the tapping machine has poor low-frequency excitation (in comparison) but an improved high-frequency excitation. An alternate input method was developed where a force transducer is added to the impactor to make live measurements of the input force and the impact tip can be changed to modify the range of targeted frequencies for any test.

The current ASTM and International Standards Organization (ISO) standard im- pact test method for floor-ceiling assemblies does not require the test engineer to measure the input force during the impact. A standard tapping machine or a standard impact ball is used for these tests, but these input sources usually do not have any provision to measure the input force. In this work, the input force of these impact methods was measured on seven different assemblies, and a modal hammer was used as a “control” source. The input force and average floor impedance were measured and compared to each other. The peak input force levels due to the tapping machine vary by approximately 38 dB from one floor to the other, and the frequency excitation bandwidth varies widely based on the type of floor construction. This shows that there is a need to measure the input force and scale the output sound quantity with the input force to get a frequency response function (FRF)-like quantity to compare different floor-ceiling assemblies. For the impact ball, the variation in low-frequency input force levels is almost non-existent, but the impact ball has a poor measurement signal-to-noise ratio above 80 Hz one-third octave band. The impedance measured using the impact ball matches closely with the control impedance measured using the modal hammer.

Standard impact and airborne noise insulation testing is based on measuring the average sound level in a reverberant field in order to estimate the incident and/or radiated sound power. While this method is straightforward, the variation under reproducibility conditions is significant and attempt to reduce the uncertainty has so far proven unsuccessful. It is possible that a significant portion of the uncertainty is due to unavoidable variation in reverberation rooms, and that measuring the vibration on the surfaces of an assembly may be a better measurement of the impact or airborne isolation of the assembly. Preliminary investigations have been performed by measuring the vibration levels on wall and floor assemblies in situ while excited by airborne and impact sources, and the results are compared to conventional test methods.

In multistory buildings, the isolation of sound from floor to ceiling is a major concern. The building codes use an impact insulation class (IIC) rating defined by ASTM to characterize the acoustical performance of floorâ–“ceiling assemblies due to impacts. The measurement process defined in this standard has repeatability and reproducibility limitations due to low-frequency, non-diffuse sound fields in receiving rooms. A comparison method is proposed in this article that uses a reference sample with known sound power to calculate the room or path contribution to the measured sound pressure level, which is then used to calculate the sound power of the floorâ–“ceiling assembly. The proposed method is tested for a small-scale hardboard plate, and the test results are within 1 to 2 dB of baseline sound power values. A simply supported plate used as the reference plate showed MAC values higher than 0.9 for analytical and experimental mode shapes. The analytical natural frequencies are within 1% to 2% of experimental frequencies and analytical sound power values are within 1-2 dB of experimental data. This study showed that for a small-scale assembly, the new methodwas able to characterize the room contribution within 1 to 2 dB.