ACOUSTICS Stein Reynolds Chapter 17 The Fundamentals of Architectural Acoustics

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

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

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

Humans hear sounds in the 20-20,000 Hz range Most sensitive to sounds in the 125-6000Hz range Many animals can hear and make sounds in ranges higher than this- some lower Sounds lower than 20Hz are usually sensed as vibrations

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.

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

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.

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

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.

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

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 120-130dB pain limit occurs at all frequencies 0dB threshold occurs only at the 1000Hz level Ear in most sensitive in the 3000-4000Hz level at which point the threshold is -5dB

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

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

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

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

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

Room Criteria Curves Evaluate the acceptable mechanical system background noise

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)

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

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

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

Reverberation Speech typically requires short TR Music typically requires longer TR

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

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

Absorptive Materials Acoustic Tile standard sizes, various colors, finishes, some fire rated, mineral fiber or faced fiberglass NRC absorption rating; 0.45- 0.75 mineral fiber tiles 0.95 fiberglass

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

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

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

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.

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)

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

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

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

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