1. ACOUSTICS Stein Reynolds Chapter 17 The Fundamentals of
Architectural Acoustics

2. General Concepts Sound- desired and designed for
Noise- unwanted and designed out
We live and work in environments of increasing levels of noise-

3. 3 Common elements of all acoustic systems
Source
Can be made to be louder or quieter
Transmission path
Can be designed to transmit more or less sound
Receiver
Listener’s reception may also be altered

4. Sound Is simply an audible signal
Is a physical wave or a mechanical vibration or a series of pressure variation is an elastic medium
Airborne sound - medium is air
Structure born sound - building materials

5. Humans hear sounds in the 20-20,000 Hz range
Most sensitive to sounds in the Hz range
Many animals can hear and make sounds in ranges higher than this- some lower
Sounds lower than 20Hz are usually sensed as vibrations

7. Sound Intensity
Sound waves carry energy that can be used to do work- like move an eardrum, sonic boom
The amount of energy transported per second by a sound wave is the power of the wave- measured in j/s or watts
The eardrum is moved by air pressure
We are more interested in this pressure as force density and pressure level than in sound intensity as a power density.

8. Sound Intensity
Sound waves generated in free space, spread out and pass through surfaces of increasingly large areas
Sound Intensity (I) is sound power (P) passing perpendiculary through area (A)
I=P/A measured in W/m2

10. Sound Intensity
Sound reaches a listener’s ear it is interpreted as loud or soft, depending on the intensity of the wave.
Doubling intensity does not double loudness.

11. Sound Intensities and Decibels
Decibels are unitless values
Its greatest value is to compare sound intensities- typically by computing and comparing the ratio of the intensities.
The ear responds to sound pressure not intensity. BUT-Sound Pressure Level (SPL) is numerically equal to Intensity Levels (IL)
Pressure levels as a ratio of sound pressure to a base level, is expressed in decibels

14. Sound Pressure Levels (SPL)
SPL is the end result
It is the resultant sound or noise that in an enclosed space resulting from a source in that space as affected by the characteristics of the space and the position of the listener.

15. Sound Intensity 1-3dBs in barely detectable Example - 2 stereo systems
90dB and 93dB
∆ 3dB= 2x intensity
Perceived - 70dB is perceived as twice as loud as 60dB
80dB is perceived as twice as loud as 70dB

16. Sound Pressure and Sound Pressure Level (SPL)
Pressure levels as a ratio of sound pressure to a base level, is expressed in decibels
Corresponds to the threshold of hearing
dB pain limit occurs at all frequencies
0dB threshold occurs only at the 1000Hz level
Ear in most sensitive in the Hz level at which point the threshold is -5dB

17. Sound Masking
2 or more separate sources of sound perceived together it is difficult to clearly perceive one due to the presence of the other
Defined by the number of decibels by which the threshold of audible sound is raised by the presence of the other sound
Low frequency will more effectively mask a high frequency than the reverse
Can be used as a noise control technique

18. Directivity
High frequency direct line travel, reflective surfaces give clues to listener as to origin of sound- low frequencies are hard to pin point

19. Noise Criteria 2 issues concerning noise Psychological /practical
Noise levels that cause annoyance and disturbance of sleep, relaxation, work and other daily activities
Physiological
Physical impacts of noise such as hearing loss

20. Noise Criteria Psychological /Practical-annoyance
1. Proportional to loudness
2. Greater at high frequencies
3. Greater at intermittent levels than constant
4. Greater for pure tone than broad band noise
5. Greater for moving rather than fixed sources
6. Much greater for information bearing noise
- speech interference levels (SIL)-office/commercial design

21. Noise Criterion Curves
Contours represent the maximum continuous background noise that will be considered acceptable in the environment specified

23. Room Criteria Curves
Evaluate the acceptable mechanical system background noise

25. Sound in Enclosed Space
Sound generated in an enclosed space radiates out - strikes large surfaces and room boundaries where it is reflected, transmitted and or absorbed-sound intensity is attenuated by distance-
Greater reflection causes intermittent sounds to mix in room- possibly less intelligible speech, better music mix
Room can be noisy (reverberation) or
Dead (absorption)

27. Design Considerations
2 things happen with the reflection of sound in a room:
1- the noise level (volume) is greater than it would be in an open field
2- there is a delay factor- after the sound source has stopped some sound will persist in the space- reverberation

28. reverberation and echos
reverberation is similar to an echo but there are significant differences-
Echo- discreet reflection of a sound- usually delayed 1/10th of a second or more
an entire word can be return intact
canyon wall
Reverberation- continuous reflection over a short time span
organ note dying out slowly in a cathedral

29. Reverberation Reverberation-slow fading of sound in an enclosed space
Reverberation Time- the amount of elapsed time before complete silence after a 60dB sound has stopped
As space becomes larger TR tends to increase
absorption increases TR decreases
different room functions require different TR

30. Reverberation Speech typically requires short TR
Music typically requires longer TR

31. Absorptive treatment in a room will affect the reverberant noise level within that room but have little effect on the noise level in the adjoining spaces
Absorbent material can also be: carpets, drapery, sprayed on acoustic plaster

32. Absorption Recommendations
Absorption techniques are good:
1- to change room reverberation techniques
2- In spaces with distributed noise sources such as offices, schools, restaurants, and machine shops
3- In spaces with hard surfaces and little absorbent content
4- where listeners are in the reverberant field

33. Absorptive Materials Acoustic Tile
standard sizes, various colors, finishes, some fire rated,
mineral fiber or faced fiberglass
NRC absorption rating;
mineral fiber tiles
0.95 fiberglass

34. Noise Reduction 3 Components 1- Reduce noise generation at source
-and installation proper equipment selection
2- Reduce noise transmission point to point
- proper selection and installation of construction material
3- Reduce noise at the receiver
- meet noise criteria level through appropriate acoustical treatment
and possibly-Combination of all

35. Specific noise control
A specific, concentrated source of noise should be controlled by isolation
(mech room enclosure)

36. Impact Noise
Erratic noise caused by footfalls, dropped objects vibrations of mechanical equipment
Impact Isolation Class (IIC)
Code required for adjacent dwellings
Improved with carpet, resilient tile floor, suspended ceilings, concrete slab floated on compressed glass fiberboard laid on the structural floor

37. Sound Transmission and Isolation
Other than controlling sound within a space, it is important control the transmission of sound between 2 spaces
Transmission Loss (TL)- the amount of noise reduced as the sound travels through the wall. Ratio expressed in dBs of acoustic energy reradiated on receiving room side of wall.

38. Sound Transmission and Isolation
TL-lab controlled value for wall’s material ability to act as acoustic barrier
Noise Reduction (NR)- accounts for the TL and the receiving room’s acoustic character.
(reverbs and absorption)

39. Sound Transmission and Isolation
Sound Transmission Class (STC) is often used in place of the TL rating
STC- the index of a partition’s resistance to the passage of sound
The STC of a wall can be compromised when it only extends up to a light suspended ceiling, it is not sealed well at penetrations or if the barrier is penetrated by a common air duct

40. Sound Transmission and Isolation
Doubling the walls mass is one method of increasing TL but other methods are more viable such as
- staggered studs and resilient channels

41. Sound Isolation Descriptors
Hearing conditions in a room can be altered by changing either the barrier characteristics and or the background sound level
- barrier effectiveness expressed in STC
- ambient noise level is frequency dependent it is approximated by NC (HVAC)
The perceived isolation value of a barrier may be enhanced by raising the background , masking, noise level in the receiving room

42. Overall Approach Isolate specific concentrated noise generators
Design for room Noise Criteria
Control Absorption and Reverberation in room
Control the transmission of sound between 2 spaces
Certain rooms’ design require a more detailed control of reverberation times, location of reflections and listener positions (direct and reflected sound paths) - auditoriums, music halls, open office landscapes- this can be manually “roughed in” but should be modeled for a more complete analysis and design