Behavior of sound in enclosed spaces

R.Narasimha SwamySenior consultant

narasimhaswamy@yahoo.com

Propagation of sound waves

• Reflection• Absorption• Refraction• Diffusion• Interference• Dissipation• Standing waves• Reverberation

Characteristics discussed

Analysis model• Ray model is considered

• Assumptions:– Rectangular room– Specular sound reflections– Frequency-independent reflections coefficients– No air absorption

• Image sources– Reflection of the actual sound source in the wall

Analysis• Sound waves are spherical waves as most of the

times it originates from a point source.

• For ease of analysis, it can be treated as rays.

Reflection• Sound reflects of hard flat surfaces just as light reflects

off a mirror. This is called specular reflection.

• We can think of reflected sound as coming from an image source - or an imaginary sound source behind the mirror.

• Specular reflections only occur when the wavelengths are significantly smaller than the reflecting surface and its surface irregularities.

Sound absorption• In room acoustics, sound is ‘absorbed’ when it escapes from

the room for ever through windows and doors.

• Also, sound is absorbed when it encounters:

• fibrous or porous materials• panels that have some flexibility• volumes of air which resonate• openings in the room boundaries (eg doorway)

• Materials readily absorb the energy of a sound wave depending on its physical structure.

• Absorption of sound by a particular material/panel depends on the frequency and angle of incidence of the sound wave.

• The absorption coefficient of a particular material/panel is function of frequency and hence depicted graphically.

Typical sound absorbers

• Acoustic panels• Membranes• Drapes• Foams• Carpets

Noise reduction coefficient [NRC]

NRC• NRC is a laboratory rating of a material's sound absorption quality.

• The NRC rating is an average of how absorptive a material is at only four frequencies (250, 500, 1000 and 2000 Hz). This industry standard ranges from zero (perfectly reflective) to 1.0 (perfectly absorptive). It is always expressed as a decimal rounded to the nearest 0.05.

• This is more of an industry standard and not necessarily enforced. This standard is widely used and accepted.

• Another industry standard used is absorption coefficient based on the method of Test Method ASTM C423. Unit Sabin/m. Instead of 4 spot frequencies, six are considered which includes 125 and 4000 Hz.

• NRC is being replaced by the Sound Absorption Average (SAA), which is described in the current ASTM C423-09a. The SAA is a single-number rating of sound absorption properties of a material similar to NRC, except that the sound absorption values employed in the averaging are taken at the twelve one-third octave bands from 200 Hz to 2500 Hz, inclusive, and rounding is to the nearest multiple of 0.01.

Material NRCBrick, painted 0.00 - 0.02Carpet, indoor-outdoor 0.15 - 0.20Carpet, heavy on concrete 0.20 - 0.30Concrete (smooth), painted 0.00 - 0.05Concrete (smooth), unpainted 0.00 - 0.20Concrete (block), painted 0.05Concrete (block), unpainted 0.05 - 0.35Drapery, light weight (10oz0.) 0.05 - 0.15Drapery, medium weight (14oz), velour draped to half 0.55Drapery, heavy weight (18oz), velour draped to half 0.60Fabric on Gypsum 0.05Fiberglass, 3-1/2" batt 0.90 - 0.95Fiberglass, 1" Semi-rigid 0.50 - 0.75Glass 0.05 - 0.10Gypsum 0.05Linoleum on Concrete 0.00 - 0.05Marble 0.00Plaster 0.05Plywood 0.10 - 0.15Seating (occupied) 0.80 - 0.85Seating (unoccupied), metal 0.30Seating (unoccupied), fabric upholstered 0.60Wood 0.05 - 0.15

Limitation of NRC• The NRC rating is only measured at 4 spot frequencies. Which is acceptable for

speech, but can be inadequate for music and other low frequency sounds.

• Because this rating is an average, two materials with the same rating might not perform the same.

• Ratings claimed by OEMs can be misleading for the following reasons:– The information provided is based on lab tests –a perfect environment.

– Some OEMs will quote absorption at the more desirable higher frequencies without clearly explaining the testing conditions. NRC is based only on absorptive characteristics at the 4 spot frequencies. Hence, one has to be sure of the product data.

– The mounting procedure used in the tests is to be considered for obtaining similar results. Ex, OEM of a wall carpet product provides an NRC rating of 0.80, which is extremely good. But, if one reads the fine print, this was achieved while the carpet was installed over fiberglass. With such mounting, the fiberglass, not the carpet, acts as the sound absorber. Without the acoustic material below, the wall carpet will probably only achieve an NRC of 0.20!

Resonance absorber with Vibrating panel

Perforated panel

Diffusion Diffusion is described as the process of spreading or

dispersing sound energy, so that, it is less direct or coherent.

For instance, the plastic cover over a fluorescent light acts as a diffuser. The covers make the light spread out in a randomized way so, it is less harsh.

Breaking up echoes into innocuous reflections, without introducing excessive absorption.

Nearly full band diffusion of sound requires a reflection phase grating (Schroeder Diffuser)

Specular reflection

Diffused reflection

QRD panel

Refraction

Diffraction

Interference

Standing waves• Standing waves can occur when sound from a source is

reflected back on itself.

• Resonance occurs when sound is repeatedly reflected back on itself.

• The superimposed waves create zones of cancellation and reinforcement - ie zones of quiet and loud.

• The pattern of these zones is different for each frequency, and is determined by the sound wavelength.

• Standing waves are dominant at lower frequencies

• Standing waves causes the coloration of the recorded music.

Thank you