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Acoustics

"... acoustics is characterized by its reliance on combinations of physical principles drawn from other sources; and that the primary task of modern physical acoustics is to effect a fusion of the principles normally adhering to other sciences into a coherent basis for understanding, measuring, controlling, and using the whole gamut of vibrational phenomena in any material medium." Origins in Acoustics. F.V. Hunt. Yale University Press, 1978

The main sub-disciplines of acoustics are

Aeroacoustics is the study of how sound and gas flow interact and has particular application to aeronautics, examples being the study of sound made by jets and the physics of shock waves (sonic booms).

Architectural Acoustics is the study of how sound and buildings interact including the behavior of sound in concert halls and auditoriums but also in office buildings, factories and homes.

Bioacoustics is the study of the use of sound by animals such as whales, dolphins and bats.

Biomedical Acoustics is the study of the use of sound in medicine, for example the use of ultrasound for diagnostic and therapeutic purposes.

Engineering Acoustics or Electro Acoustics is the study of how sound is generated and measured by loudspeakers, microphones, sonar projectors, hydrophones and ultrasonic transducers and sensors.

Psychoacoustics or Psychological and Physiological Acoustics is the study of how people react to sound, hearing, perception, and localization.

Physical Acoustics is the study of the detailed interaction of sound with materials and fluids and includes, for example, sonoluminescence (the emission of light by bubbles in a liquid excited by sound ) and thermoacoustics (the interaction of sound and heat).

Speech Communication is the study of how speech is produced, the analysis of speech signals and the properties of speech transmission, storage, recognition and enhancement.

Vibration Acoustics Structural Acoustics and Vibration is the study of how sound and mechanical structures interact and includes the transmission of sound through walls and the radiation of sound from vehicle panels.

Ultrasonics is the study of high frequency sound, beyond the range of human hearing.

Musical Acoustics is the study of the physics of musical instruments

Underwater Acoustics is the study of the propagation of sound in the oceans.

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Table of contents

1 Sound pressure level SPL 2 Measurement methods 3 Reverberation and anechoic rooms 4 Helmholtz Resonator 5 Rectangular Boxes 6 More 7 External links

Sound pressure level SPL

The amplitude of a sound wave is most commonly characterized by its sound pressure. In a normal working environment, a very wide range of pressure can occur and so it is the convention that sound pressure is measured on a logarithmic scale using the decibel. If is the rms sound pressure amplitude then the sound pressure level (SPL) is defined as 20 times the logarithm of the ratio of the pressure to some reference pressure.

Sound pressure level SPL is calculated in decibels as

= 20 µPa in air and 1 µPa in water. (Pa = pascal = N / m²; N = newton)

There are two popular ways for scientists to perform acoustical measurements. They include a "direct method", and a "comparison method". The direct method computes sound power levels by computing an equation of environmental factors (such as room temperature, humidity, reverberation time, etc.) and sound pressure levels. A more precise implementation of this method can be found in the ISO3745 acoustics standard. The comparison method however, is conducted by measuring sound pressure levels from a reference sound source which emits a known, constant, sound power level, and then comparing that level with the sound pressure level of the object being recorded. Each way is equally valid and accurate.

Reverberation and anechoic rooms

Experiments such as the two methods mentioned above are sometimes performed in reverberation rooms, or in some cases, anechoic rooms. The design of a reverberation room is to create long lasting echoeses of sound waves. This helps create a highly averaged and omnidirectional sound level throughout the entire chamber. A typical example of rooms with characteristics similar to reverberation rooms are concrete tunnels, caves, etc. Anechoic rooms, such as hemi-anechoic rooms, or fully anechoic rooms are created to simulate what is called a free field. A free field is the representation of a theoretical infinite plan, in which no sound wave reflections, or echoes, take place. In rooms such as these, the only sounds which exist are being emitted directly from the source, and are not reflected from another part of the chamber. Anechoic rooms have the characteristic of being muted, muffled, and silent.

Helmholtz Resonator

A helmholtz resonator is a container with an open hole or neck.

• f = resonant frequency
• s = speed of sound in air
• a = area of neck
• l = length of neck
• v = volume

Rectangular Boxes

• f = frequency of standing wave of a rectangular box
• s = speed of sound in air
• x,y,z = dimensions of box
• Nx,Ny,Nz = any integers

• Structural acoustics
• Noise control
• Room acoustics
• Concert hall acoustics
• Musical instruments
• Underwater acoustics
• Audio signal processing
  • Audio storage
  • Sound synthesis
  • Speech processing
• Psychoacoustics

• Helmholtz Resonator
• Helmholtz Resonance
• Helmoholtz Resonator with oscilloscope

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