PAUL HAY Capital Projects
Acoustics
Author: Paul Hay
e-mail: paul.hay@phcjam.com
profile: www.linkedin.com/in/phcjam
1.0 INTRODUCTION TO ACOUSTICS
1.1_ Sound is an electro-magnetic wave consisting of a series of pressure variations, or vibrations, in an elastic medium.
1.2 Noise is unwanted sound.
1.3 Hertz is the frequency of sound wave cycles per second
1.4 The human voice has a frequency range of 100 to 7,500 Hz.
1.5 Wave-lengths are longer for lower frequencies:
λ = C/f 1.1
where,
λ = Wave-length, m
C = Velocity of sound, m/s
f = Frequency of sound, Hz
1.6 The velocity of sound in air is 344 m/s.
1.7 Sound originates at a source and takes a path to a receiver.
1.8 The Inverse-Square Law states that the intensity of an electromagnetic wave is inversely proportional to the square of the distance of path from the source.
2.0 HUMAN HEARING
2.1 Sound audible to humans is within the frequency range 20 - 20,000 Hz.
2.2 Audible frequencies have wavelengths from 17 - 15,200 mm long.
2.3 The threshold of hearing occurs at a sound level of 10-20 W/m2.
2.4 Decibel is the intensity level (IL) equivalent to 10 times the logarithm of a measured sound level (I) over the threshold of hearing (Io):
IL = 10log(I/Io) 2.1
2.5 The human ear is less responsive to frequencies at the extremes of the audible range at low sound levels, but this improves as the volume is increased:
2.6 The apparent loudness of sound is also exponential:
2.6.1 A-weighed decibels [db(A)] only measures audible sound with the frequency responsiveness of the human ear at the respective sound level;
2.6.2 0 db(A) is the threshold of hearing;
2.6.3 10 db(A) is an apparent loudness twice that of the threshold of hearing;
2.6.4 60 db(A) is loudness of the human voice; and
2.6.5 130 db(A) is the threshold of pain.
3.0 ROOM ACOUSTICS
3.1 The acoustics within a room depends on interior noise control; reverberation and room shape.
3.2 Reverberation is the repetition of sound at reduced levels of loudness after sound has ceased from the source.
3.3 Reverberation time (Tr) is the time taken for the sound to reduce to -60db:
Tr = 0.16 V/3A 3.1
where,
Tr = Reverberation time, sec.
A = Total absorption at the respective frequency, m2
3A = αavg x S
αavg = S1 α1 + S2 α 2 + ............. + Snαn
S1 +S2 + .........................Sn
S = Total Surface Area, m2
αn = Coefficient of absorption at the nth surface
3.4 The units of total absorption are also called Sabins, in honour of W. C. Sabine, a pioneer in architectural acoustics.
3.5 The coefficient of absorption (α) is the ratio of intensity levels of an absorbed sound and an incident sound:
α = Ia/Ii 3.2
where,
Ia = Absorbed wave intensity, W/m2
Ii = Incident wave intensity, W/m2
3.6 Rooms with Alive@ sound have highly reflective surfaces [i.e. αavg < 0.2].
3.7 Rooms with highly absorptive surfaces are Adead@ [i.e. αavg > 0.4].
3.8 Auditoriums and Quiet Rooms require treatment of absorptive material be placed on all surfaces, but other spaces generally require only ceiling treatment.
3.9 The optimum reverberation time for speech can be determined by the relationship:
Tr = 0.30 logV/10 = 0.35 sec. 3.3
3.10 Typical volumes for drama and speech are 2.3 - 4.2 m3/seat.
3.11 Concert Halls adjudged excellent for music have reverberation times of 1.6<Tr<1.8.
3.12 Typical volumes for concerts are 6.2 - 11.0 m3/seat.
3.13 Recommended proportions for Music Rooms are 3:4:5.
3.14 Undesirable qualities can result from poor room design:
3.14.1 An echo is a sound reflected at sufficient volume to be heard with a delay greater than 50 ms;
3.14.2 Flutter is a buzzing or clicking sound comprised of repeated echoes between non-absorbing parallel surfaces;
3.14.3 Standing Waves are like flutter but accentuate specific frequencies that have wave-lengths twice as long as the distance between the parallel surfaces.
3.14.4 Standing Waves are only important in rooms that are small relative to the wave-length of the frequency [i.e. <9.1 m for music, and <4.6 m for speech.
3.14.5 Focusing is primarily reflection from concave surfaces that converge at one area, so round buildings should be avoided.
3.14.6 Creep is the reflection of sound along a concave surface from a source at one end to a receiver at the other.
4.0 NOISE CONTROL
4.1 Noise control is exercised by (a) reflecting and absorbing sound within a room, (b) intrusion and attenuation of sound from sources outside the room, and (c) reducing equipment and airflow noises.
4.2 Sound waves are reflected by objects larger than their wave-length.
4.3 Noise Reduction Coefficient (NRC) is an area-weighed average of sound absorption coefficients at frequencies 250, 500, 1000, and 2000 Hz.
4.4 NRC is increased by greater material thickness, composition, and resilience of the installation.
4.5 Effective reduction of outside noise involves keeping sound from going around, under and through a room=s walls, roof and floor.
4.6 Sound waves go around objects smaller than their wave-length.
4.7 Sound Transmission Coefficient (STC) is the measure of sound absorbed in transmission through a building element:
4.7.1 STC measurements are restricted to airborne sound at frequencies of 125 - 4,000 Hz and is therefore relevant to evaluation of speech privacy;
4.7.2 STC does not measure impact noise, low frequency noise (eg. traffic & VAC sources) or amplified music.
4.8 Impact (Structural borne) Isolation Class (IIC) should be equal or greater than STC ratings.
4.9 An architect analyzes and solves a noise control problem in a four stage process:
4.9.1 Establish Permissible Noise Criteria (PNC) for background noise in the respective occupancy;
4.9.2 Identify and measure intensity level of noise;
4.9.3 Calculate the Noise Reduction (NR):
NR = IL of noise source - PNC 4.1
4.9.4 Specify building element with transmission loss (TL) curves that exceed the required NR curve.
4.10 NR requirements can be reduced by using heavier construction and locating noisy space next to rooms having high PNC.
4.11 Machinery (eg. compressors) which vibrates due to rapidly moving parts should be isolated from the building=s structure.
4.12 Rapid air movement in ducts are also sources of noise:
4.12.1 Noise is reduced by creating smooth transitions from one duct to the next; and
4.12.2 Noise is reduced by increasing the cross-sectional area of the duct and reducing the air-velocity.
5.0 SOUND REINFORCEMENT
5.1 Sound Reinforcement Systems augment sound that would otherwise be inadequate:
5.1.1 Systems are required in spaces having volumes greater than 1,400 m3; but
5.1.2 These systems cannot achieve complete correction of poor room acoustics.
5.2 Sound Systems comprise (a) input from sources like microphones, (b) amplifiers and controls (such as equalizers), and (c) output from loudspeakers.
5.3 Loudspeakers should give the impression that the sound is originating from the source:
5.3.1 Time delay is necessary because electrical input travels at the speed of light while sound is slower;
5.3.2 Equalization may need to be applied to create a frequency response comparable to the original sound.
5.4 Distributed loudspeaker systems comprise a number of low-level loudspeakers located overhead which may be used as public address systems, where directional realism is not essential.
5.5 The sound system operator must be able to hear the sound as the audience.
FURTHER READING
Mechanical and Electrical Equipment for Buildings, 8th edition, Benjamin Stein, John S. Reynolds, John Wiley & Sons Inc., USA, 1992
Ramsey/Sleeper Architectural Graphic Standards, AIA, Robert T. Packard (ed), John Wiley & Sons Inc., USA, 1981
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