Introduction to Acoustics

• Acoustics involves the study of all mechanical waves, including vibration, sound, ultrasound, and infra-sound.

General Acoustic Terms

• Airborne noise refers to sound transmitted through the air.
• Attenuation is the gradual loss of sound intensity as it travels through a medium; a structure's ability to limit sound transmission.
• A baffle is device that reduces the strength of airborne sound.
• Audimutes acoustic baffles are designed to hang vertically from ceilings to absorb sound, reduce echo, and lower reverberation time.
• Clouds are free-hanging absorption panels designed to suspend horizontally from a ceiling.
• Decibel is a unit of measurement for the loudness of sound.
• Echoes occur when reflected sound reaches a listener's ear with a delay, often exacerbated by hard surfaces or large, open spaces.
• Flanking sound is sound that travels indirectly between two spaces, such as through vents.
• Flutter echo occurs when sound reflects off parallel surfaces and reaches the listener at different times, creating a fluttering sound.
• Footfall is impact noise created by walking, occurring when feet contact the floor, and is a common issue in multi-floor apartments.
• Frequency, in sound waves, is the number of waves in a given time period, generally correlating to the pitch of the sound.
• IIC (Impact Isolation Class) rating measures a structure's ability to limit impact noise and vibration transfer.
• Impact noise is structure-borne sound created when two objects come into contact.
• NRC (Noise Reduction Class) rating translates the amount of sound absorbed by an item, ranging from 0 (reflective) to 1 (perfectly absorptive).
• Reverberation is the persistence of sound after its initial creation as sound reflects off surfaces and decays.
• Reverberation time measures how long (in seconds) it takes for a sound to decay.
• Sound absorption is the process where sound waves are taken in by an item or structure rather than being reflected.
• A sound barrier is a material that can increase a structure's STC rating.
• Sound pressure is the pressure created in a space in the presence of sound.
• Soundproofing is the process of reducing sound pressure within a space.
• STC (Sound Transmission Class) rating is a numerical value representing how well a structure reduces airborne sound transmission; a higher value indicates better sound reduction, with a well soundproofed wall having a rating of 50 STC.

Sound Theory

• The sensation of sound results from air vibrations caused by pressure fluctuation, which can be measured by a sound level meter.
• A pure tone is the simplest form of vibration that is a one dimensional sinusoidal vibration with only one frequency, also called simple harmonic vibrations.
• Sound is measured using a sound level meter in Pascals (Pa), and sound intensity is measured in Watts/m².
• Amplitude is the maximum difference of a variable's extreme values over a single period, high amplitude refers to noise, and low amplitude refers to quiet.
• Frequency is measured in Hertz (Hz), the number of cycle periods repeated per second (1 Hz = 1 Cycle/sec).
• The human ear can respond to frequencies between 20 Hz and 20,000 Hz, with peak sensitivity between 3,000-4,000 Hz.
• Ultrasonic frequencies are above 20 kHz, while infrasonic frequencies are below 20 Hz.
• The measuring unit for sound is the decibel (dB), with the human ear capable of hearing from 0 dB to 120 dB (higher is dangerous).
• Noise is a type of undesired sound and may be subjective.
• Acoustic waves are longitudinal waves where particles oscillate along the direction of wave propagation.
• Transverse waves are waves where particles vibrate perpendicular to the direction of wave propagation.
• Temporal frequency is the number of occurrences of a repeating event per unit of time, measured in Hz (one per second).
• Velocity of sound is the distance through which a sound wave's point travels per unit of time, i.e. speed of sound = 343 m/sec.
• Wave propagation is the physics term for the movement of waves.
• All media have properties affecting sound propagation, including the relationship between density and pressure, the motion of the medium, and the viscosity of the medium.
• The speed of sound is approximately 0.33 km/sec in air, 1.5 km/sec in water, and 5 km/sec in steel.
• The equation V = f x λ defines the speed of a wave, where V is speed in m/s, f is frequency in Hz, and λ is wavelength in m.
• Wavelength is the distance between adjacent and identical parts of a sound wave.
• Sound intensity describes the rate of energy flow through a unit area.
• Free field propagation is characterized by a 6dB drop in sound pressure level.
• Inverse square law describes the reduction in sound intensity over distance.
• Sound pressure is the sound force acting on a surface perpendicular to the sound's direction, measured in Pascals (Pa) or N/m².
• Sound Pressure Level (PSL) measures pressure disturbance in the atmosphere under predefined conditions.
• Intensity is the power per unit area carried by a wave.
• Power is the rate at which energy is transferred by the wave.
• Sound absorption is the measure of energy removed from a sound wave as it passes through a given thickness.

Sound Absorption Principles

• Porosity in materials like glass-fiber or mineral-fiber blankets & boards converts indirect sound energy into thermal energy.
• Thickness significantly affects the efficiency of porous sound absorbers where 0 = no sound absorbed, 1 = perfect absorption.
• Standard test mountings are laboratory tests that determine sound absorption efficiency, replicating installation methods used on actual surfaces.
• Common absorbers used in building construction include spray-on cellulose coatings, mineral fiber blankets and boards, and glass fiber blankets and boards.
• Needle punched nonwoven mat can be used as facing material for sound absorbers.
• When plane pressure waves strike a plane surface between two media, reflection and transmission occur based on acoustic impedance.
• Acoustic impedance determines the amount of sound reflected and transmitted.
• Sound Absorption Coefficient (SAC) is used to evaluate a material's sound absorption efficiency as a ratio of absorbed energy to incident energy.
• Noise Reduction Coefficient (NRC) is an average rating of how much sound an acoustic product can absorb with .95 NRC meaning 95% of the sound is absorbed, and 5% reflected.

Types of Sound Absorbing Materials

• Porous absorbers are permeable materials where sound waves penetrate the surface and flow into the fibrous or cellular structure.
• Membrane/plate absorbers are air impervious, non-rigid, non-porous, and placed over an airspace.
• Resonate absorbers are used to combat sound in a narrow, yet defined frequency range to focus on issues related to bass frequencies.
• Cushions and pillows are porous sound absorbers with soft surfaces that can absorb sound waves.
• Wall hangings absorb sound waves in a room.
• Carpets and area rugs absorb impact noise and prevent sound transmission through the structure.
• Curtains and blankets help absorb and slow the passage of sound waves.
• Acoustic window film increases window density and absorbs the transmission of sound waves.
• Acoustic partitions act as sound barriers in open spaces.
• Acoustic foam panels are soft, porous material featuring notches and cups.
• Reverberation time is the time required for a sound to fade away or decay.