1. Final Project - Acoustics Flashcards
Define these 4 terms:
- NOISE
- PITCH
- VOLUME
- FREQUENCY
- NOISE
- Any unwanted sound
- PITCH
- Wavelength of sound
- VOLUME
- Amplitude of sound
- FREQUENCY
- # cycles/second (hertz)
What is a sound wave?
Sound waves are longitudinal pressure waves in the air or an elastic medium.
What is a Decibel (dB)?
Decibel (dB) is a unit for expressing the relative pressure or intensity of sounds on a uniform scale from 0 (threshold for hearing) to 130 (pain).
What is a frequency?
Frequency, sometimes referred to as pitch, is the number of times per second that a sound pressure wave repeats itself.
The range is between 15Hz to 20,000 Hz for human ear.
Hertz (Hz) is 1 cycle per second.
What is a reflecting surface?
Reflecting surfaces are nonabsorptive surfaces from which incident sound is reflected, used to redirect sound in a space.
It must have least dimension equal or greater than the wavelength of the lowest frequency of the sound.
What is diffracted sound?
Diffracted sound is airborne sound waves bent by diffraction around an obstacle in their path.
What is reflected sound?
Reflected sound is the return of unabsorbed airborne sound after striking a surface, at an angle equal to the angle of incidence.
When is reflected sound important?
In a room, the human ear always hears direct sound before it hears reflected sound.
As direct sound loses intensity, the importance of reflected sound increases.
What is attentuation?
Attenuation is a decrease in energy or pressure per unit area of a sound wave, occuring as the distance from the source increases as a result of absorption, scattering or spreading in three dimensions.
What is reverberation?
Reverberation is the persistence of a sound within an enclosed space, caused by multiple reflection of the sound after its source has stopped.
What is resonance?
Acoustic resonance is a phenomenon in which an acoustic system amplifies sound waves whose frequency matches one of its own natural frequencies of vibration.
What is an echo?
Echoes are the repetitions of a sound produced by the reflection of sound waves from an obstructing surface, loud enough and received late enough to be perceived as distinct from the source.
What is flutter?
Flutter is a rapid succession of echoes caused by the reflection of sound waves back and forth between two parallel surfaces.
What is focusing?
Focusing is the convergence of sound waves reflected from a concave surface.
What are 6 types of sound that can reach a listener? (2 types not shown below)
-
Incident Sound (mirror)
- reflects off of surfaces with low a (sound absorb)
- to be effective surface area>wavelength
- returns @ angle = incident angle
-
Diffracted Sound
- airborne sound waves bent by obstacle in path
-
Airborne Sound
- Travels directly from source to listener
-
Attenuation
- greater distance from source = lower amplitude as it spreads
-
Echoes
- reflected sound waves
- loud & late enough to be considered distinct
- occurs w parallel surfaces 60’(18m) apart
-
Focusing
- convergence of sound waves reflected from concave surface
What STC rating does NBC rqr for dwelling units?
STC 50
A17 - Define these 5 terms:
- ATTENUATION
- ARTICULATION
- SOUND INTENSITY
- SOUND ABSORPTION
- SOUND REVERBERATION
-
ATTENUATION
- reduction of sound
-
ARTICULATION INDEX
- measure of speech intelligability
- >.6 = good communication
-
SOUND INTENSITY
- dB
-
SOUND ABSORPTION
- ratio of sound absorbed by material
- unit = sabins
-
SOUND REVERBERATION
- persistance of sound after source stopped producing
Define these 3 measures of sound :
- NC
- NRC
- STC
- NC - Noise Criteria
- specifies max. background noise in a given space
- NRC - Noise Reduction Coefficient
- # rating system for ave. sound sbsoption coefficient of a material @ frequencies of 250/500/1000/2000 Hz
- STC - Sound Transmission Class
- Rating system to determine effectiveness of construction cross sections to stop sound transmission.
What is the typ. STC rating for :
- normal speech heard through a barrier
- loud speech heard through a barrier
- loud speech heard but not understood
- loud speech is faint through barrier
- loud sound other than speech heard through a barrier
- STC 25
- STC 30
- STC 35
- STC 42-45
- STC 46-50
A20 - What is the Inverse Square Law?
sound intensity = inversely proportional to square of distance to source
Decibles and hearing?
- What is the measure in dB of the threshold of human hearing
- What increment of dB is and is not noticeable?
- zero dB
- 1 dB is not noticeable
- 5 dB is noticeable
In what range of Hz does human conversation occur?
125Hz - 8000Hz
What relationship does TL (transmission loss) have to NR (noise reduction)?
NR = TL(db)
What is the function of noise criteria curves?
What is the PNC (preffered noise criteria) for each of the following spaces:
- concert hall
- bedroom
- lobbies
- kitchens
Balanced Noise Criteria (NCB) curves are used to specify acceptable background noise levels in occupied spaces and include noise from air-conditioning systems and any other ambient noise. They are intended to replace the NC curves.
- 12-20 dB (concert hall)
- 20-30 dB (bedroom/hospital/aptmt)
- 40-45 dB (lobbies/labs)
- 45-55 dB (kitchens/light maint. shops)
