Architectural Helmholtz Resonators

Tapiola Lutheran Church in Espoo, Finland, Opened in 1965

Helmholtz-type resonators are built into the walls of the church.  The slots between brick pairs and cavities behind them act as both absorbers and diffusors of sound.  The absorption reduces the reverberation time as desired for speech intelligibility during sermons.  Some reverberation is desirable, however, to enhance organ music.

A Helmholtz resonator is a volume of air which is enclosed in a container with at least one opening. It is also called a cavity resonator.  The air in the container’s neck acts as a mass. The air in the volume acts as a spring. The Helmholtz resonator thus behaves as a mechanical spring-mass system.

Courtroom in Bankstown, NSW, Australia, with slotted panels for reverberation reduction.  https://decorsystems.com.au/

Slotted block absorbers in a gymnasium.

The BT240 slotted modular bass trap panel provides excellent absorption down to 65 Hz.  https://soundacoustics.com.au/

– Tom Irvine

Cuba Sonic Attack Analysis

A sound file from the attack on the U.S. Embassy in Cuba has now been made available on the Internet.  I did a spectral analysis of this file using my Matlab GUI scripts.  The sound source is still unknown.  The attacks have caused hearing, cognitive, visual, balance, sleep and other problems for embassy personnel.

Here is a quick look paper: Cuba_sonic_analysis.pdf

Here is the sound file: Cuba_sonic.mp3   Turn up the speaker volume to hear the sound.

– Tom Irvine

NASA SP-8072 Launch Vehicle Liftoff Acoustics

NASA SP-8072  Acoustics Loads Generate by the Propulsion System
(Alternate SP-8072 link)

The liftoff analysis has been added to the GUI package at:  Vibrationdata Matlab GUI

The function can be accessed via:

>> vibrationdata > Miscellaneous Functions I > Acoustics Vibroacoustics & SEA > acoustics > Launch Vehicle Liftoff Acoustics

Here is document which gives further details using an older C++ version: liftoff_notes.pdf

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The aerodynamic flow-induced pressure during the transonic and maximum dynamic pressure phases can be calculated using the follow tools:

Prediction of Sound Pressure Levels on Rocket Vehicles During Ascent: flow.pdf

This function can be accessed via:

>> vibrationdata > Miscellaneous Functions I > Acoustics Vibroacoustics & SEA > acoustics > Launch Vehicle Aerodynamic Flow

– Tom Irvine

Quieter Supersonic Passenger Jets

LA Times Article, Supersonic passenger jets might make a comeback, more than a decade after the last Concorde flight  link

Boom Technology Inc.

– Tom Irvine

Jet Aircraft EPNL

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

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.

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 & Alternate Link

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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

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 & Alternate Link

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– Tom Irvine