Impact Noise
1.0 Definition
Impact or “Impulse (loosely)” Noise includes all forms of high-intensity short-duration sounds. The energy of an impact is usually broadly distributed, but spectral concentrations of energy can occur at various frequencies throughout the audible range.
2.0 Impact versus Impulse Noise
2.1 Impact Noise
Impact Noise (also Reverberant B-wave) is normally produced by non-explosive means (i.e., metal-to-metal impacts in industrial plant processes).
- Reverberant industrial environments where impact noises are usually heard causes the duration of impact noises to be longer than associated impulse noises.
2.2 Impulse Noise
Impulse Noise (also Non-reverberant A-wave) is defined as a short-duration sound characterized by a shock front pressure wave-form (i.e., instantaneous pressure rise) produced by sudden release of energy (i.e., explosives or gun blasts).
- Impulse noises are typically generated in free-field environments where sound-reflecting surfaces that create reverberation are absent.
Figure 1: Schematic Representation of Two Basic Impulse Noise Pressure-Time Profiles (Source: [2])
3.0 Measurement of Impact Noise
- Record the pressure-time history of the impact
- Consider the response properties of the measuring device (rise time can be short as few microseconds and intensity high as 185 dB SPL)
- Common impulse SLM have time constant of (30 - 40 ms)
- Gunfire typically lasts for few hundred of microsecond (Common SLM underestimate impulse parameters)
- Typical industrial impacts may last for 200 ms or more (Common SLM are suitable)
4.0 Factors Affecting Impact Noise
4.1 Environmental Conditions
- Free field or reverberant enclosure
- Angle of incidence
- Extraneous noise and vibrations
4.2 Noise Signatures
- Industrial - reverberant (ringing) and conforms to law of acoustics
- Blast (Shock) waves - governs by physical principles that are different than the laws of acoustics
5.0 Floor Impact Noise
- Measured in laboratory
- Heavy slab with a framed 10 m^2 opening in the middle
- Flanking transmission is negligible
- Sole contribution of sound levels in receiving rooms from test floor radiation
- The test floor is excited using impact (tapping) machine
- One side note, impact noise on walls could be assessed similarly using a small impact machine (pendulous hammer) according to Swiss Engineers and Architects standards (SIA 181)
The impact noise level, L_n is given by:
Where:
- L_1 = mean sound pressure level in reception room [dB]
- S = area of floor specimen (10 m^2) [m^2]
- A = equivalent absorptive area in the reception room [m^2]
5.1 Measurement Standards
- ISO 140-6: (100-3150 Hz, recently attempting to extend towards low-frequency region until 50 Hz)
- ASTM E492: (125-4000 Hz)
6.0 Impact Noise Transmission
Affected by:
- Direct transmission through separating floor
- Flanking transmission by all walls linked to the separating floor
- Parasite transmission by leakages around a constructive element or duct/opening
6.1 Measurement:
- Using an impact machine at the emission room
- Measure the mean sound pressure level at the receiving room
- Standardize the result by reverberation time (SPL measured in receiving room is sensitive to room absorption)
Standardized impact noise level, L_{nT} is given by:
Where:
- L_1: mean sound pressure level in reception room [dB]
- T: reverberation time in receiving room [s]
- T_0: reference reverberation time (usually 0.5 s) [s]
6.2 Measurement Standards
- ISO 140-7
- ISO 15712-2
- ASTM 1007 (also account for ceiling underneath the floor)
7.0 Rain Impact Noise
- Significant on light roof elements
- Softer roof covering provide quieter rain impact noise
7.1 Test Procedure Standard
- ISO 140-18
8.0 Single Number Rating
- ASTM E989: Impact Insulation Class (IIC)
- ISO717-2: Normalized Weighted Impact Sound Index (L_{nw})
More reading between the two ratings here.
9.0 Impact Noise Reduction by Floor Coverings
- ISO 140-8: Laboratory Measurements of the Reduction of Transmitted Impact Noise by Floor Coverings on a Heavyweight Standard Floor
- ASTM E2179: Standards Test Method for Laboratory Measurement of the Effectiveness of Floor Coverings in Reducing Impact Sound Transmission Through Concrete Floors
10.0 Walking Noise
- Walking noise is assessed by performing the measurement in the same room.
- Measurement carried out using EN 16205 standard (L_{new})
References
[1] M. Asselineau, Building Acoustics. Boca Raton, Florida: CRC Press, 2015.
[2] D. A. Bies, C. H. Hansen, and C. Q. Howard, Engineering Noise Control, 5th ed. Boca Raton, Florida: CRC Press, 2017.
[3] M. J. Crocker, Handbook of Noise and Vibration Control. Hoboken, New Jersey: John Wiley & Sons, Inc., 2007