bu 323 acoustics

8/3/2019 Bu 323 Acoustics

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BU 323 Building Utilities: ACOUSTICS

What is Sound?Sound is a form of energy that is both physics and biology.Physics: Sound is a longitudinal pressure wave. For humans, this wave is usually in air.

Biology: Sound is how we perceive and respond to the pressure wave.

Different sounds give us different kinds of feelings, e.g. relaxation, excitement,annoyance.

Noise can be useful too. Fire alarms and car horns warn us of danger.

The Physics Side:

Its a longitudinal wave the crests are areas of high pressure calledcompression.The troughsare areas of low pressure called rarefaction.

If we plot air pressure vs time on a graph we would see the sound wave.

The variations in air pressure are very small compared to the normal atmosphericpressure.

The Biology Side: how our ears perceive and respond to the sound

Frequency of sound is called pitchAmplitude of the pressure wave is related to loudness through some c

Sounds above human hearing are called ultrasonic

Sounds below human hearing are called infrasonic

Production of SoundSound waves are produced when a vibrating object causes small but rthe air pressure around it.

When layers of air are pushed close together, a compressionof theformed.

When the air layers are pulled apart, a decompression or rarefactionof thformed.

Parameters of Sound

Vibrations and WavesWaves transmit energy and information.

Sound and Light are both waves.

The Medium of TransmissionAny medium that has particles that can vibrate will transmit sound.Solids can transmit sound fastest followed by liquid then air.


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Compressions

regions where the air has a slightly increased pressure, as a result of the particlesbeing closer together.Rarefactions

regions in the medium where the particles are spread out. This results in a slightdecrease in air pressure.

Wavelength () for longitudinal waves, it is the distance between the centers of adjacent

compressions (or rarefactions), or the distance between adjacent regions wherethe particles that are moving are in phase.

Effect of Physical ConditionsTemperatureSound travels faster in higher temperatures

HumiditySound travels faster in high humidity

Pressure

Pressure has no effect on speed of sound

Sound waves travel faster as the medium it travels through becomes warmer.Speed of sound in various mediumsAir340 m/sWater1500 m/sIron5100 m/s

SOUND NEEDS A MATERIAL MEDIUM FOR ITS TRANSMISSION. IT WILL NOT TRAVEL IN A

VACUUM.

The speed of sound in a medium depends on the density of the medium.The speed of sound increases as the density of the medium increases.

PitchWhen describing a musical note as high or low, we are actually tapitch of the sound.Frequency affects pitchHigh frequency high pitchLower frequency lower pitch

requency and PitchPitchthe perception of frequency

Humans can detect a wide range of frequencies and are sensitive to aamplitudes.Humans (hearing) 20-20,000 HzHumans (making sounds) 85-1100 HzBats1000 120,000 HzDog whistle 20,000 24,000 HzWaves with frequencies greater than 20,000 Hz are called Ultrasonic.Waves with frequencies less than 20,000 Hz are called Subsonic.

Reflection of SoundEchoesWhen sound hits a hard surface, it will bounce off the surface. Threflected back to us is called an echo.

ReverberationsIt is the prolonged sound due to the merging of many echoes.Many echoes are produced because sound will be reflected off the diffroom.


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a) a flat, reflective rear wall in an auditorium will reflectsound back toward the speaker, this is called "slap-back".

b) parallel reflective walls can create a reflection betweenthe two surfaces, this is referred to as "flutter echo" or"standing wave".

y Concave surfaces - cause reflections to be concentrated rather than dispersed.- causes an abundance of reflection to be heard by the listeners

in the focal point, or the point at which all of the reflections arefocused.

- reflections can also travel along a concave surface bringing

delayed reflections around the room.

y Convex surfaces - the best surfaces fordistributing sound; provide a wide spread of reflectedsound.

Noise Reduction Coefficient (NRC)- a single-number index for rating how absorptive a particular material is.- gives no information as to how absorptive a material is in the low and high

frequencies, nor does it have anything to do with the materials barrier effect(STC).

Sound Transmission Class (STC)- a single-number rating of a materials or assemblys barrier

airborne soundtransfer at the frequencies 125-4000 Hz.- Higher STC values are more efficient for reducing sound transmis- In general, a higher STC rating blocks more noise from transm

partition.

Airborne sound

- travels through the air and can be transmitted through an obssound reaches an environment where it is unwanted, it becomes

Noise

- can come from both external (traffic noise) and internal sources.- can also pass under doorways, through ventilation, over, und

obstructions.


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Transmission Loss- a measurement of a partition's ability to block sound at a given frequency, or the

number of decibels that sound of a given frequency is reduced in passingthrough a partition.- Measuring Transmission Loss over a range of 16 different frequencies between

125- 4000 Hz, is the basis for determining a partitions Sound Transmission Class.-

Basic Principles of Noise ControlThere are three basic elements to be considered in controlling noise: Controlling noise at its source. Controlling noise along its path. Controlling noise at the receiver.

Direct Sound

An effective means of reducing direct sound is to install an effective acoustical barrierbetween the noise source and the receiver.

Indirect SoundThe most effective means of reducing reflected sound is to install absorptive materials onthe surfaces the sound strikes. Thus, when the sound strikes these surfaces, most isabsorbed and very l ittle reflected.Fiberglas insulations can absorb up to 99% of the sound striking their surface makingthem one of the most efficient sound absorptive materials available.

DIFFERE

NCE

BE

TWEE

N NRC

& STC


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Impact and Airborne Sound

Impact Sound Transmission sound arising from the impact of an object on a building element (wall, floor, or

ceiling). Typical sources are footsteps, jumping, and dropped objects. Impact

sound transmission occurs because the impact causes both sideelement to vibrate, which generates sound waves.

Impact Insulation Class rates the impact sound transmission performance of an assembly

The higher the IIC rating, the better the impact noise control of the eleme

Sound Transmission Class rates the airborne sound transmission of building element.

The higher the STC rating, the better the airborne noise control of the

measuredindecibels (dB). An STC of60dBisusuallyreported

asSTC60.

Optimum Reverberation Times


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