Low Frequency Noise Study

Project Category: 
Noise
Project Number: 
1

The Low Frequency Noise Study evaluated the perceptual impact of low frequency aircraft noise. It encompassed many factors, including the source level and its spectrum; atmospheric propagation; the impact on homes in the form of noise, vibration and rattle; subjective perception and annoyance; and the ability of metric calculations to predict the physical and perceived impact. Its goal was to enhance metrics currently used in the FAA's Integrated Noise Model, and to identify alternative algorithms that predict the impact of low frequency noise and its perceived annoyance.

Reasons for focusing on aircraft noise low-frequency components were:

  • as low-frequency sound encounters less absorption as it travels through the air than higher frequency sound, it persists for longer distances
  • the amount of sound transmitted from the outside to the inside of buildings is greater at low than at high frequencies
  • A-weighting metrics, commonly used in aircraft noise impact assessment, deemphasize low frequencies
  • standard airport noise assessment models neglect source noise below 50 Hz
  • prior research indicates that frequencies in the 20–80 Hz range influence perception of low-frequency noise

Major Project 1 findings were:

  • Start-of-takeoff-roll, runway acceleration, and thrust reversal generate high LFN levels (below 200 Hz) at critical distances from runways (around 3000 ft in the study), which can be annoying to people living around airports.
  • The Hubbard exterior sound pressure level threshold criteria should be used as a first assessment of the potential for low-frequency noise impact.
  • Assessment of impact should include both single and multiple events in areas where noise from multiple runways can impact a neighborhood simultaneously.
  • A-weighted Sound Pressure Level (LAmax) and C-weighted Sound Pressure Level (LCmax) metrics correlate well with laboratory based subjective response to indoor aircraft noise when LFN levels are low to moderate. Because these metrics are simple to implement, they should be used to predict subjective response to indoor aircraft noise when the levels are appropriate for A- and C-weightings and there are not high levels of low-frequency noise.
  • When high levels of LFN are present, Tokita & Nakamura thresholds can be used as indicators of the potential for annoyance due to LFN. C-Weighted Sound Exposure Level (LCE) metric should be used as a single-number metric for assessing the potential for annoyance. Data lower than 50 Hz is needed to assess vibration/rattle annoyance.
  • Overall the findings suggest that people are responding to the broad spectral content and any predictive metric should quantify the full broadband noise. Loudness algorithms should include frequency content below 50 Hz to optimally correlate with the perception of low frequency noise.
  • The risk of window rattle is lowered with preload and avoiding resonance response in the design. Outdoor-Indoor Transmission Class is a better rating for rattle prone applications than Sound Transmission Class commonly used in rating windows for transmission loss.

The results of this research offers valuable guidance for authorities in addressing noise-related issues at airports. The final report may be downloaded below.

Project 1 Questions and Answers

Participating universities

Pennsylvania State University
Purdue University
University of Central Florida

Lead investigators

Anthony Atchley, Graduate Program in Acoustics, Pennsylvania State University, atchley@engr.psu.edu
Kathleen Hodgdon, Applied Research Laboratory, Pennsylvania State University, kkh2@psu.edu

Project manager

Mehmet Marsan, mehmet.marsan@faa.gov

Downloads

  • Low Frequency Noise Study. (Project final report) Kathleen Hodgdon, Anthony Atchley, Robert Bernhard. April 2007. (Report No. PARTNER-COE-2007-001) Download pdf
  • Passive Sound Insulation: PARTNER Project 1.5 Report. Daniel H. Robinson, Robert J. Bernhard, Luc G. Mongeau. January 2008. Report No. PARTNER-COE-2008-003. Download pdf
  • Vibration and Rattle Mitigation: PARTNER Project 1.6 Report. Daniel H. Robinson, Robert J. Bernhard, Luc G. Mongeau. January 2008. Report No. PARTNER-COE-2008-004. Download pdf