Jet Aircraft EPNL

aircraft_noise_sources_mod-520x245

There are several tools for analyzing jet aircraft sound as measured on the ground.    The measurements would typically be made for takeoff and final approach at or near an airport.  Fly-over sound levels can also be recorded.

The tools begin with the unweighted, one-third octave sound pressure level (SPL).  One SPL should be taken for each 0.5 second increment.  Furthermore, each SPL should have an overall sound pressure level that is within 10 dB of the maximum overall level.

The tools build upon one another in this order:

  1.  Sound Pressure Level (SPL)
  2.  Perceived Noisiness (Noys)
  3.  Perceived Noise Level (PNL)
  4.  Tone Corrected Perceived Noise Level (PNLT)
  5.  Effective Perceived Noise Level (EPNL)

Each of the functions is in units of dB except for Noys. The Effective Perceived Noise Level is sometimes represented as EPNdB to emphasize that it is a decibel scale.   The functions are defined in Annex 16 of the ICAO International Convention on Civil Aviation, and in the US Federal Air Regulations Part 36.

Noy is a subjective unit of noisiness. A sound of 2 noys is twice as noisy as a sound of 1 noy and half as noisy as a sound of 4 noys.

The Matlab scripts for the EPNL processing are included in the GUI package at: Vibrationdata Matlab Signal Analysis Package

The function can accessed via:

>> vibrationdata > Select Input Data Domain > Sound Pressure Level

An alternative is to compute the A-weighted SPL.  This option is also available in the Matlab GUI package.  Nevertheless, the EPNL is used by convention for jet aircraft noise.

– Tom Irvine

Payload Fairing Foam Blankets

A spacecraft at launch is subjected to a harsh acoustic and vibration environment resulting from the passage of acoustic energy, created during the liftoff of a launch vehicle, through the vehicle’s payload fairing. In order to ensure the mission success of the spacecraft it is often necessary to reduce the resulting internal acoustic sound pressure levels through the usage of acoustic attenuation systems. Melamine foam, lining the interior walls of the payload fairing, is often utilized as the main component of such a system.

Here are some NASA reference papers:

29th_ATS_Absorption_Paper_23September2015_Final_as submitted to ATS

FINAL_29th_ATS_AMCNELIS_23Sept2015

TM-2014-218350 Noise Con 2014 on Melamine Foam Acoustic Testing

TM-2014-218127 ATS version of NEMFAT

– Tom Irvine

Boeing 717-200

1024px-AirtranJet

I recently flew as a passenger on a Boeing 717-200 aircraft similar to the one shown in the image.  This aircraft has two Rolls-Royce BR700 engines, with the following specifications:

Maximum Engine Rotational Speeds (Both Engines)

N1 Low Pressure Turbine = 6,195 RPM (103 Hz)
N2 High Pressure Turbine = 15,898 RPM (265 Hz)

I made an audio recording from inside the cabin during take-off and climb-out.

data_plot

The sound file Fourier transform for a 10-second segment is shown in the image.

The first peak is at 88 Hz, which is 85% of the maximum N1 speed.

The second peak is at 129 Hz and is unidentified.

Most of the higher frequency peaks are integer multiples of 88 Hz.

Complete audio file:  Boeing_717_200.mp3

The Fourier transform was taken from 40 to 50 seconds into the recording.

A “buzz saw” sound occurs due to shock waves at the turbofan blade tips which have a supersonic tangential velocity.

– Tom Irvine

Franken Vibroacoustic Method for Cylindrical Shells

The front end of a typical rocket vehicle contains avionics and a payload, enclosed by a cylindrical skin. Rocket vehicles are subjected to intense acoustic loading during liftoff. The external acoustic pressure causes the skin surfaces to vibrate. The skin sections are also excited by structural-borne vibration transmitted directly from the engine or motor. Nevertheless, the acoustic field is usually the dominant excitation source.

The references present an empirical method for calculating the vibration response of a cylindrical skin to an external acoustic pressure field. This method is based on data collected by Franken from studies of Jupiter and Titan 1 acoustic and radial skin vibration data collected during static firings.

The external acoustic pressure field may be due either to liftoff or to aerodynamic buffeting thereafter.

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References:

Vibration Response of a Cylindrical Skin to Acoustic Pressure via the Franken Method: Franken.pdf

Peter Franken, Methods of Space Vehicle Noise Prediction, WADC Technical Report 58-343, Volume II: Franken_acoustics.pdf

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The scripts for performing this calculation are given at:
Vibrationdata Signal Analysis Package

>> vibrationdata > Acoustics & Vibroacoustics > Vibroacoustics

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– Tom Irvine

Spann Vibroacoustic Method

Avionics components in aircraft and launch vehicles may be mounted to surfaces which are exposed to high intensity acoustic excitation. The external acoustic pressure field causes the panel and shell surfaces to vibrate. This vibration then becomes a base input to any component mounted on the internal side. Components must be designed and tested accordingly.

The component vibration input levels can be derived via analysis and testing for a given sound pressure level.

Acoustic testing of the structure can be performed in a reverberant chamber or using a direct field method. There is some difficultly in testing, however, because the simulated acoustic field in the lab facility may be different in terms of spatial correlation and incidence than that of the flight environment even if the sound pressure level can be otherwise replicated.

The vibroacoustic analysis techniques include finite element and boundary element methods, as well as statistical energy analysis. These are powerful tools, but they require numerous assumptions regarding external acoustic pressure field type, coupling loss factors, modal density, impedance, radiation efficiency, critical and coincident frequencies, distinguishing between acoustically fast and slow modes, etc.

As an alternative, simple empirical methods exist for deriving the structural vibration level corresponding to a given sound pressure level. Two examples are the Franken and Spann techniques. These methods may be most appropriate in the early design stage before hardware becomes available for lab testing and before more sophisticated analysis can be performed.

The Spann method provides a reasonable estimate of the acoustically excited component vibration environments when only the areas exposed to the acoustic environment and mass are known.

Spann_vibroacoustic_method.pdf

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The scripts for performing this calculation are given at:
Vibrationdata Signal Analysis Package

>> vibrationdata > Acoustics & Vibroacoustics > Vibroacoustics

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– Tom Irvine

Plate Vibration Response to Oblique Acoustic Pressure Field

oblique

The Steady-State Response of a Baffled Plate Simply-Supported on All Sides Subjected to Harmonic Pressure Wave Excitation at Oblique Incidence:  ss_plate_oblique_incidence.pdf

A related paper is:

The Steady-State Response of a Baffled Plate Simply-Supported on All Sides Subjected to Random Pressure Wave Excitation at Oblique Incidence:  ss_plate_plane_wave_random.pdf

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The scripts for performing this calculation are given at:
Vibrationdata Signal Analysis Package

>> vibrationdata > Acoustics & Vibroacoustics > Vibroacoustics

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See also:

Steady-State Response of a Rectangular Plate Simply-Supported on All Sides to a Uniform Pressure:  ss_plate_uniform_pressure.pdf

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– Tom Irvine

Webinar 37 – Acoustic Fatigue

PowerPoint Slides:  webinar_37_acoustic_fatigue.pptx

Audio/Visual File:

NESC Academy Acoustic Fatigue – Recommend viewing in Firefox with Sliverlight Plugin

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References:

Rainflow Fatigue Posts

Acoustic Fatigue of a Plate

Acoustic Power Spectra

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Matlab script: Vibrationdata Signal Analysis Package

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See also: Vibrationdata Webinars

Thank you,

Tom Irvine

Embraer ERJ 145 Acoustics

emb145

Figure 1.  ERJ (EMB) 145 Aircraft

Rolls-Royce_AE_3007_2

Figure 2.  Rolls-Royce AE 3007 Series Turbofan Engine

emb145_fft

Figure 3.  Fourier Magnitude, ERJ 145 Climb-out

I recently flew as a passenger in an EMB 145 jet similar to the one shown in Figure 1.

This aircraft has two Rolls-Royce AE 3007 series turbofan engines turbofan engines, as shown in Figure 2.

Here are the rotational speeds for each rotor.

Fan Speed                       7903 RPM   (132 Hz)
Gas Generator Speed   16013 RPM   (267 Hz)

Note that there are several variants of this engine with slightly different rotor speeds.

I made an audio recording from within the aircraft cabin during climb-out, while hearing some distinct sine tones against the background random noise. The audio file is: emb145.wav

A Fourier transform of the sound file is shown in Figure 3.  Spectral peaks occur at 133 and 267 Hz, which agree with the specifications for the engines.

Again, this recording was made from inside the cabin. So the fuselage walls would have attenuated some of the engine-generated acoustic energy, particularly at higher frequencies.

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– Tom Irvine

Nissan Versa Side Window Buffeting

02-2014-nissan-versa-note-fd

versa

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I recently drove a Nissan Versa rental car similar to the one shown above.

The Versa made a strong throbbing sound when I drove down a highway at 65 mph with the front side windows closed and the rear windows open.

Here is the sound recording that I made with my Samsung Android: Versa Sound

Please listen to the sound file using a headset with good low frequency response to hear the full effect.

The Fourier transform is shown above with its fundamental frequency at 21.2 Hz and integer multiples thereof.

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Dr. Stephen Granade explains:

That “whum whum WHUM WHUM” noise happens because the wind passing over the small window opening… forms tiny tornadoes as it moves past the front edge of that opening. When those tornadoes, or vortices, reach the opening’s back edge, they make a wave of pressure that pushes air into and out of the car.

Since sound is nothing more than waves of pressure, this makes noise… The vortices keep pressing on the air in your car just at the right time to make big pressure waves that we can feel and hear.

The technical term for this effect is the Helmholtz resonance, though car people call it “side window buffeting”…

…As you drive faster, the rate at which the whums occur speed up and the loudness goes up.

Interestingly enough, Granade goes on to theorize that “It’s more noticeable in modern cars because they’re more aerodynamic,” the thinking being that cracking a window is more disruptive to a smoothly-tuned airflow. If that’s true, it would mean cars with boxy shapes would suffer less from Hemholtz resonance.

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A more technical explanation is:

Buffeting (also known as wind throb) is an unpleasant, high-amplitude, low-frequency booming caused by flow-excited Helmholtz resonance of the interior cabin. Vortex shedding in the shear layer over the cavity opening (side window) couples with the cabin acoustics, leading to a self-sustained oscillation of shear layer and cabin pressure.

Reference

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– Tom Irvine