ACOUSTICS Flashcards
What do you measure velocity of sound in?
Ft/sec
Symbol for coefficient of absorption
Lowercase a
Symbol for wavelength
Lowercase w
Symbol for power
Uppercase W
What is amplification?
Increased intensity of sound by mechanical or electrical means
What is articulation index?
A measure of speech intelligibility calculated from the number of words read from a selected list that are understood by an audience.
What is a low articulation index and what is the desired situation?
0.15 or less, for privacy
What is a high articulation index and what is the desired situation?
Above 6.0 is desired for good communication
Attenuation
The reduction of sound
Decibel - definition and formula
10 times the common logarithm of a the ratio to a reference quantity of the same kind, such as power, intensity, or energy density. … Often the unit of sound intensity in this formula:
IL = 10*log(I/Io)
Symbol and measure for sound intensity
Capital I… W/cm squared
Symbol for minimum sound intensity audible to the average human ear
Io … 10 to the negative 16’s W/cm squared
Symbol and measure for sound intensity level
IL measured in dB
Symbol and measure for noise reduction
NR …dB
Symbol and measure for acoustic power
P … W
Symbol and measure for area of barrier or component between rooms
S … Feet squared
Symbol and measure for coefficient of transmission
t
Symbol and measure for total acoustical absorption
A … Sabins
dBA
The unit of sound intensity measurement that is weighted to account for the response of the human ear to various frequencies
Frequency
The number of pressure fluctuations recycles occurring in one second, expressed in hertz (Hz)
Hertz
The unit of frequency; one cycle per second equals one hertz
Impact insulation class (IIC)
A single number rating of a floor-ceilings impact sound transmission performance at various frequencies
Intensity
The amount of sound energy per second across a unit area
Intensity level
10 times the common logarithm of the ratio of a sound intensity to a reference intensity… The formula for decibel describes this
Noise
Any unwanted sound
Noise criteria (NC)
A set of single number ratings of acceptable background noise corresponding to a set of curves specifying sound pressure levels across octave bands. Noise criteria curves can be used to specify continuous background noise, achieve the sound isolation, and evaluate existing noise situations.
Noise insulation class (NIC)
A single number rating of noise reduction
Noise reduction
The arithmetic difference, in decibels, between the intensity levels in two rooms separated by a barrier of a given transmission loss. Noise reduction is dependent on the transition loss of the barrier, the area of the barrier, and the absorption of the surfaces of the receiving room.
Noise reduction coefficient (NRC)
The average sound absorption coefficient to the nearest 0.05, measured at the four one third octave band center frequencies of 250, 500, 1000, and 2000 Hz
Octave band
A range of frequencies in which the upper frequency is twice that of the lower
Phon
A unit of loudness level of a sound equal to the sound pressure level of a 1000 Hz tone judged to be as loud
Reverberation
The persistence of a sound in a room after the source has stopped producing the sound
Reverberation time
The time it takes the sound level to decrease 60 dB after the source has stopped producing the sound
Sabin
The unit of absorption; theoretically, one square foot of surface having an absorption coefficient of 1.00
Sabin formula
The formula that relates reverberation time to a room’s volume and total acoustical absorption.
T = 0.05 (V / A)
T is reverberation time
V is room volume
A is total acoustical absorption
Sound absorption coefficient
The ratio of the sound intensity absorbed by material to the total intensity reaching the material. Theoretically, 1.00 is the maximum possible value of the sound absorption coefficient.
Sound power
The total sound energy radiated by a source per second, in watts
Sound transmission class (STC)
A single number average over several frequency bands of a barrier’s ability to reduce sound. The higher the STC rating, the better the barriers ability to control found a transmission.
Transmission loss TL
The difference, in decibels, between the sound power incident on the barrier and a source room and the sound power radiated into a receiving room on the opposite side of the barrier. The transmission lost varies with the frequency being tested.
What are sound’s three basic qualities?
Frequency, velocity, and power.
What is the velocity of sound in air at sea level?
1130 ft./s
What is frequency
The number of cycles completed per second, measured in hertz.
What is the formula relating frequency and velocity?
F = C / W
Power, in acoustics, is…
The quality of acoustical energy is measured in watts.
What is the formula for sound intensity?
I = P / 4πr²
What is the inverse square law?
Sounds intensity is inversely proportional to the square of the distance to its source
Range of human hearing in decibels
0 dB is the threshold of hearing
… 130 dB is the threshold of pain
To compute the combined decibel level of multiple sources of sound producing identical decibel sound intensities use the formula:
Total IL = IL (of source) + 10log(#of sources)*33.8
Rules of thumb for combo sound intensity levels of two sounds…
(Hint: combined decibels)
When difference is.. 0-1dB add 3dB
When difference is.. 2-3dB add 2dB
When difference is.. 4-8dB add 1dB
When difference is.. 9dB or more add 0dB
Name some common subjective changes in decibel with their effect
1 dB - almost imperceptible
5 dB - noticeable
10 dB - twice/half as loud
20 dB - 4 times/one fourth as loud
Transmission of sound is primarily retarded by 1.) most important thing 2.) second thing
1.) mass of the barrier, 2.) stiffness
What is the formula for noise reduction?
NR = TL + 10log(A/S)
NR is noise reduction
TL is transmission loss
A is total acoustical absorption of receiving room
S is area of barrier/component bt rooms
If you know the value of transmission loss for individual materials, what is the formula for calculating coefficient of transmission?
t = 10 to the negative -(TL/10)
A conference room and an office I separated by a common wall 13 feet long and 9 feet high with an STC rating of 54. The total absorption of the office has been calculated to be 220 Sabins. What is the total noise reduction from the conference room to the office?
57 dB
What is the combines transmission loss of a wall 9 feet high and 15 feet long with the 3-ft by 7-ft door in it? Assume the transmission loss of the wall is 54 dB and at the door, with full perimeter seals, is 29 dB.
Total wall area = 135
Total door area = 21
Total partition area = 114
t = 10 to the negative -(TL/10)
t of partition = 10 to the -5.4
t of door = 10 to the -2.9
TL = 10log(total area/sum of coefficient of transmission times area of component)
TL = 10log(135/((10 to the -5.4)(114) + (10 to the -2.9)(21)))
= 10log5012
= 37 dB
What are some representative noise criteria for different spaces?
Concert halls, opera houses, recording studios… 15 to 20 dB
Bedrooms, apartments, hospitals… 20 to 35 dB
Private offices, small conference rooms… 30 to 35 dB
Large offices, retail stores, restaurants… 35 to 40 dB
Lobbyists, drafting rooms, laboratory work spaces… 40 to 45 dB
Kitchens, computer rooms, light maintenance shops… 45 to 55 dB
What is the difference between NC and PNC?
The difference between noise criteria and preferred noise criteria: it takes into account background noise containing low-frequency and high-frequency sounds, making the sound pressure levels lower than the NC curves on the chart
In general, transmission loss through a barrier tends to _____ with the frequency of sound.
Increase
A wall with .1% open area from cracks, holes, etc., will have a maximum transition loss of _____… A wall with 1% open area will have a maximum transmission loss of about _____…
30 dB… 20 dB…
A hairline crack will decrease a partitions transmission lost by about _____. A 1 in.² opening in at 100 square-foot Gypsum board partition can transmit _____.
6 dB… As much sound of the entire partition…
Some affects of barrier STC unhearing effect on hearing are…
25 STC… Normal speech can clearly be heard through barrier.
30 STC… Loud speech can be heard and understood fairly well; normal speech can be heard but barely understood.
35 STC… Loud speech is not intelligible but can be heard.
42 to 45 STC loud speech can only be faintly heard; normal speech cannot be heard.
46 to 50 STC… Loud speech is not audible; loud sounds of other than speech can only be heard faintly, if at all.
Sound intensity level decreases about _____ for each doubling of distance from the source in free space.
6 dB
Generally, a material with the color efficient below blank is considered reflective, and one with the coefficient above blank is considered sound absorbing.
0.2
Express any formula how the total absorption of a material is dependent on its coefficient of absorption and the area of the material.
A = Sa
What is the formula but describes an increase of sound absorption within a space. What is the formula for the noise reduction that happens once you increase the sound of sorption?
NR = 10log(A_1/A_2)
A room 15’ x 20’ with 9 foot ceiling has a carpeted floor with 44 ounce carpet on pad (a = 0.40), gypsum board walls, and a gypsum board feeling (a = 0.05. What would be the noise reduction achieved by directly attaching acoustical tile with a given NRC of 0.70 to the ceiling?
3.4 dB
The average absorption coefficient of a room should be at least what? But not above what (partly because of economical justification)?
0.20 dB… 0.50 dB…
Each doubling of the amount of absorption in a room results in a noise reduction of only __ dB
3
Each doubling of the absorption in the room reduces reverberation time by _____.
One half
Although absorptive materials can be placed anywhere, what is the best place for sound absorption in a large room? And what is the best place for sound absorption in the small room?
For large rooms… The ceiling. For small rooms… The walls.
Reverberation time is…
… The time it takes the sound level to decrease 60 dB after the source has stopped producing the sound.
What is the formula for reverberation time?
T = 0.05(V/A)
V is room volume
A is total acoustical absorption
In general a shorter reverberation time is best for what size space? And a longer reverberation time is best for what size space?
Shorter times for smaller spaces, longer times for larger spaces.
Name some recommended reverberation times.
Auditoriums speech and music… 1.5 to 1.8 seconds
Broadcast studios, speech only… 0.4 -0.6 seconds
Churches… 1.4-3.4 seconds
Elementary classrooms… 0.6-0.8 seconds
Lecture/conference room… 0.9 to 1.1 seconds
Movie theaters… 0.8 to 1.2 seconds offices, small rooms for speech… 0.3 to 0.6 seconds
Opera house… 1.5 to 1.8 seconds
Symphony concert halls… 1.6 to 2.1 seconds
Theaters, small dramatic… 0.9 to 1.4 seconds
What do helmholtz resonators do?
They absorb low-frequency sound while reflecting mid and high frequency sound
What is another name for the Helmholtz resonator?
A cavity resonator… Panel resonators also do the trick
What is the difference between articulation class and the articulation index?
The articulation class gives a rating of system component performance and does not account for masking sound. The articulation Index measures the performance of all the elements of a particular configuration working together.
PAI rating ranges from 0.00 to1.00 which is privacy and which is absolutely no privacy?
0.0 is complete privacy and 1.0 is where all individual spoken words can be understood. Confidential privacy is at or below 0.05. Normal speech privacy is between 0.05 and 0.20. Above 0.20 AI speech becomes readily understood. There is no privacy above 0.30.
What is the signal-to-noise ratio?
The balance between speech sound and background noise
What is impact noise? What quantifies it? A higher number means what?
Impacts noise is the sound resulting from direct contact of an object with a sound barrier, usually a floor and ceiling assembly. It is quantified by the impact insulation class (IIC) number, a single number rating of a floor ceiling impact on sound performance. A higher number means less sound interruption.
What is mechanical noise? What are a few good ways to control it?
Similar to impacts noise, mechanical noise occurs when a vibrating device is in continuous direct contact with the structure.
- Mount it on springs or pads.
- Connect ducts pipes and equipment with flexible connectors
- Noise producing equipment should be located away from quiet spaces that’s pretty obvious
In sound and acoustics what is the difference between reflection, diffusion, and diffraction?
Reflection is the return of soundwaves from a surface. If the surface is greater or equal to four times the wavelength of the sound striking it, the angle of incidence will equal the angle of reflection.
Diffusion is the random distribution of sound from a surface. It occurs when the surface to mention equals the wavelength of the sound of striking out.
Diffraction is the bending of sound waves around an object or through an opening. Diffraction explains why sounds can be heard around corners.
The higher the frequency in hertz the _____ the wavelength.
Smaller
Lighting in watts is to illuminance on a surface as sound power (watts, dB) is to ____ _____.
Sound pressure (dB)
