The Acoustic Environment
10.15 Classifying Sound Fields
Free Fields. A sound field is said to be a free field if it is uniform, free of boundaries, and is undisturbed Figure 10-7. Effect of thermal gradients in a room.
Temperature
Reflective path heater on
Space heater
+
−
Reflective path heater off
a EA EI
---=
10 10 W
0.1 W
---log = 20dB
100 10× –dB⁄20 = 10% reflected LP
The Acoustic Environment 179 by other sources of sound. In practice, it is a field in
which the effects of the boundaries are negligible over the region of interest. The flow of sound energy is in one direction only. Anechoic chambers and well-above-the-ground outdoors are free fields. The direct sound level from a sound source in a free field is labeled LD.
Diffuse (Reverberant) Fields. A diffuse or rever-berant sound field is one in which the time average of the mean square sound pressure is everywhere the same and the flow of energy in all directions is equally probable. This requires an enclosed space with essentially no acoustic absorption. The rever-berant sound level is labeled LR.
Semireverberant Fields. A semireverberant field is one in which sound energy is both reflected and absorbed. The flow of energy is in more than one direction. Much of the energy is truly from a diffused field; however, there are components of the field that have a definable direction of propagation from the noise source. The semireverberant field is the one encountered in the majority of architectural acoustic environments. The early reflections, i.e., under 50ms after LD, are labeled LRE.
Pressure Fields. A pressure field is one in which the instantaneous pressure is everywhere uniform.
There is no direction of propagation. The pressure field exists primarily in cavities, commonly called couplers, where the maximum dimension of the cavity is less than 1⁄6 of the wavelength of the sound.
Because of the ease of repeatability, this type of measurement is used by the National Bureau of Standards, NBS, when they calibrate microphones.
At low frequencies the pressure field can be large, i.e., big enough for a listener to sit in.
Ambient Noise Field. The ambient noise field is comprised of those sound sources not contributing to the desired LD, (i.e., active sources). The ambient noise level is labeled LN.
Outdoor Acoustics. If, for example, the ambient noise level measured 70 dBA (a not unreasonable reading outdoors) and the most SPL you could generate at 4ft was 110 dB LP how far could you reach before your signal was submerged in noise?
The problem actually is more complicated than this outdoors, but this serves as an illustration of how to begin.
We have now touched on the most important basics of the acoustics environment outdoors.
Before going indoors, let us apply some of this knowledge to a series of ancient outdoor problems.
A simple rule of thumb dictates that when a change of +10dB occurs, the higher level will be subjec-tively judged as approximately twice as loud as the level 10dB below it. While the computation of loud-ness is more complex than this, the rule is useful for midrange sounds. Using such a rule, we could examine a sound source radiating hemispherically due to the presence of the surface of the earth.
Fig. 10-8 shows sound in an open field with no wind. The sound at 100ft is one-half as loud as that at 30 ft, although the amplitude of the vibration of the air particles is roughly one-third. Similarly, the sound at 30ft is one-half as loud as the sound at 10 ft. Because the sound is outdoors, atmospheric effects, ambient noise, etc., cause difficulty for the talker and listener. The ancients learned to place a back wall behind the talker, and many Native Amer-ican council sites were at the foot of a stone cliff so the talker could address more of the tribe at one time. Fig. 10-9 illustrates how a reflecting structure can double the loudness as compared to the totally open space. The weather and some noise still inter-fere with listening.
Fig. 10-10 illustrates the absorptive effect of an audience on the sound traveling to the farthest Figure 10-8. Sound in an open field with no wind.
110LP–70LP = 40dB
20 x
4
---log = 40dB x = 4 10× 40 20⁄
400ft
=
4 8 8 4
Arbitrary loudness units 100 50 0 50 100 Noise
Noise
Noise
Distance–ft
180 Chapter 10
listener. Fig. 10-11 shows the right way and the wrong way to arrange a sound source on a hill. In Fig. 10-11A the loudness of the sound at the rear of the audience is enhanced by sloping the seating upward. In addition, the noise from sources on the ground is reduced. Fig. 10-11B is a poor way to listen outdoors. The sound at the rear is one-half as loud as it is at the rear in Fig. 10-11A.
While the Bible doesn’t say which way Jesus addressed the multitudes, we can deduce from the acoustical clues present in the Bible text that the multitude arranged themselves above him because:
1. He addressed groups as large as 5000. This required a very favorable position relative to the audience and a very low ambient noise level.
2. Upon departing from such sessions, He could often step into a boat in the lake, suggesting He was at the bottom of a hill or mountain.
We can further surmise that the reason Jesus led these multitudes into the countryside was to avoid the higher noise levels present even in small country villages.
The Greeks built their amphitheaters to take advantage of these acoustical facts:
1. They provided a back reflector for the performer.
2. They increased the talker’s acoustic output by building megaphones into the special face masks they held in front of their faces to portray various emotions.
3. They sloped the audiences upward and around the talker at an included angle of approximately 120° realizing, as many modern designers do not seem to, that man does not talk out of the back of his head.
4. They defocused the reflective “slapback” by changing the radius at the edges of the seating area.
Figure 10-9. Sound from an orchestra enclosure in an open field with no wind.
Figure 10-10. Sound from an orchestra enclosure with an audience.
0 50 100 Distance–ft
16 8
Noise Noise
Arbitrary loudness units
0 50 100 Distance–ft
16 4
Noise Noise
Arbitrary loudness units Figure 10-11. Sound sources and audiences on a hill.
16 8
Noise
Arbitrary loudness units
A. Correct way
B. Wrong way
16 4
Noise
Arbitrary loudness units A. Correct way
The Acoustic Environment 181 Because there were no aircraft, cars,
motorcy-cles, air conditioners, etc., the ambient noise levels were relatively low, and large audiences were able to enjoy the performances. They had discovered absorption and used jars partially filled with ashes (as tuned Helmholtz resonators) to reduce the return echo of the curved stepped seats back to the performers. It remained only for some unnamed innovative genius to provide walls and a roof to have the first auditorium, “a place to hear,”
Fig. 10-12. No enhancement of sound is provided in Fig. 10-12 because there is no reverberation in a room whose walls are highly sound absorbent.
Sometimes acoustic progress was backward. For example, the Romans, when adopting Christianity, took over the ancient echo ridden pagan temples and had to convert the spoken service into a chanted or sung service pitched to the predominant room modes of these large, hard structures. Today, churches still often have serious acoustical short-comings and require a very carefully designed sound system in order to allow the normally spoken word to be understood.
It is also of real interest to note that in large halls and arenas the correct place for the loudspeaker system is most often where the roof should have gone if the building had been designed specifically for hearing. A loudspeaker is therefore usually an electroacoustic replacement for a natural reflecting surface that has not been provided.
10.16 The Acoustic Environment Indoors The moment we enclose the sound source, we greatly complicate the transmission of its output.
We have considered one extreme when we put the sound source in a well-elevated position and observed the sound being totally absorbed by the
“space” around it. Now, let us go to the opposite
extreme and imagine an enclosed space that is completely reflective. The sound source would put out sound energy, and none of it would be absorbed.
If we continued to put energy into the enclosure long enough, we could theoretically arrive at a pres-sure that would be explosive. Human speech power is quite small. It has been stated by Harvey Fletcher in his book Speech and Hearing in Communication that it would take “…500 people talking continu-ously for one year to produce enough energy to heat a cup of tea.” Measured at 39.37in (3.28ft), a typical male talker generates 67.2dB-SPL, or 34 microwatts (µW) of power, and a typical female talker generates 64.2 dB-SPL, or 18µW. From a shout at this distance (3.28ft) to a whisper, the dB LP ranges from 86dB to 26dB, or a dynamic range of about 60 dB. Not only does the produced sound energy tend to remain in the enclosure (dying out slowly), but it tends to travel about in the process.
Let us now examine the essential parameters of a typical room to see what does happen. First, an enclosed space has an internal volume (V), usually measured in cubic feet. Second, it has a total boundary surface area (S), measured in square feet (ft2) (floor, ceiling, two side walls, and two end walls). Next, each of the many individual surface areas has an absorption coefficient. The average absorption coefficient (a) for all the surfaces together is found by
(10-15) where,
s1,2,...n are the individual boundary surface areas in ft2, are the individual absorption coefficients of the individual boundary surface areas,
S is the total boundary surface area in ft2. The reflected energy is 1 – .
Table 10-4 gives typical absorption coefficients for common materials. These coefficients are used to calculate the absorption of boundary surfaces (walls, floors, ceilings, etc).
Table 10-5 gives typical absorption units in sabins rather than percentage figures. Sabins are either in per-unit figures or in units per length.
Finally, the room will possess a reverberation time, RT60. This is the time in seconds that it will take a steady-state sound, once its input power is terminated, to attenuate 60dB. For the sake of illustration, assume a room with the following characteristics:
V = 500,000ft3, S = 42,500ft2, Figure 10-12. Means of eliminating noise and weather
while preserving outdoor conditions.
182 Chapter 10
Table 10-4. Sound Absorption Coefficients of General Building Materials and Furnishings
Materials Coefficients
125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz
Acoustical plaster (“Zonolite”)
½ in. thick trowel application 0.31 0.32 0.52 0.81 0.88 0.84
1 in. thick trowel application 0.25 0.45 0.78 0.92 0.89 0.87
Acoustile, surface glazed and perforated structural clay tile, perforate
surface backed with 4 in. glass fiber blanket of 1 lb/ft2 density 0.26 0.57 0.63 0.96 0.44 0.56
Air (Sabins per 1000 ft3) 2.3 7.2
Brick, unglazed 0.03 0.03 0.03 0.04 0.05 0.07
Brick, unglazed, painted 0.01 0.01 0.02 0.02 0.02 0.03
Carpet, heavy
on concrete 0.02 0.06 0.14 0.37 0.60 0.65
on 40 oz hairfelt or foam rubber with impermeable latex backing 0.08 0.24 0.57 0.69 0.71 0.73 on 40 oz hairfelt or foam rubber
40 oz hairfelt or foam rubber 0.08 0.27 0.39 0.34 0.48 0.63
Concrete block
coarse 0.36 0.44 0.31 0.29 0.39 0.25
painted 0.10 0.05 0.06 0.07 0.09 0.08
Fabrics
light velour, 10 oz/yd2, hung straight in contact with wall 0.03 0.04 0.11 0.17 0.24 0.35 medium velour, 10 oz/yd2, draped to half area 0.07 0.31 0.49 0.75 0.70 0.60 heavy velour, 18 oz/s yd2 draped to half area 0.14 0.35 0.55 0.72 0.70 0.65 Fiberboards, ½ in. normal soft, mounted against solid backing
unpainted 0.05 0.10 0.15 0.25 0.30 0.3
some painted 0.05 0.10 0.10 0.10 0.10 0.15
Fiberboards, ½ in. normal soft, mounted over 1 in. air space
unpainted 0.30 0.15 0.10
some painted 0.30 0.15 0.10
Fiberglass insulation blankets
AF100, 1 in., mounting # 4 0.07 0.23 0.42 0.77 0.73 0.70
AF100, 2 in., mounting # 4 0.19 0.51 0.79 0.92 0.82 0.78
AF530, 1 in., mounting # 4 0.09 0.25 0.60 0.81 0.75 0.74
AF530, 2 in., mounting # 4 0.20 0.56 0.89 0.93 0.84 0.80
AF530, 4 in., mounting # 4 0.39 0.91 0.99 0.98 0.93 0.88
Flexboard, 3⁄16 in. unperforated cement asbestos board mounted over
2 in. air space 0.18 0.11 0.09 0.07 0.03 0.03
Floors
concrete or terrazzo 0.01 0.01 0.015 0.02 0.02 0.02
linoleum, asphalt, rubber, or cork tile on concrete 0.02 0.03 0.03 0.03 0.03 0.02
wood 0.15 0.11 0.10 0.07 0.06 0.07
wood parquet in asphalt on concrete 0.04 0.04 0.07 0.06 0.06 0.07
Geoacoustic, 13½ in. × 13½ in., 2 in. thick cellular glass tile installed
32 in. o.c. per unit 0.13 0.74 2.35 2.53 2.03 1.73
Glass
large panes of heavy plate glass 0.18 0.06 0.04 0.03 0.02 0.02
ordinary window glass 0.35 0.25 0.18 0.12 0.07 0.04
Gypsum board, ½ in. nailed to 2 in. × 4 in., 16 in. o.c. 0.29 0.10 0.05 0.04 0.07 0.09 Hardboard panel, 1⁄8 in., 1 lb/ft2 with bituminous roofing felt stuck to
back, mounted over 2 in. air space 0.90 0.45 0.25 0.15 0.10 0.10
Marble or glazed tile 0.01 0.01 0.01 0.01 0.02 0.02
Masonite, ½ in., mounted over 1 in. air space 0.12 0.28 0.19 0.18 0.19 0.15
The Acoustic Environment 183
Mineral or glass wool blanket, 1 in., 5-15 lb/ft2 density mounted against solid backing
covered with open weave fabric 0.15 0.35 0.70 0.85 0.90 0.90
covered with 5% perforated hardboard 0.10 0.35 0.85 0.85 0.35 0.15
covered with 10% perforated or 20% slotted hardboard 0.15 0.30 0.75 0.85 0.75 0.40 Mineral or glass wool blanket, 2 in., 5-15 lb/ft2 density mounted over
1 in. air space
covered with open weave fabric 0.35 0.70 0.90 0.90 0.95 0.90
covered with 10% perforated or 20% slotted hardboard 0.40 0.80 0.90 0.85 0.75 Openings
stage, depending on furnishings 0.25–0.75
deep balcony, upholstered seats 0.50–1.00
grills, ventilating 0.15–0.50
Plaster, gypsum or lime
smooth finish, on tile or brick 0.013 0.015 0.02 0.03 0.04 0.05
rough finish on lath 0.02 0.03 0.04 0.05 0.04 0.03
smooth finish on lath 0.02 0.02 0.03 0.04 0.04 0.03
Plywood panels
2 in., glued to 2½ in. thick plaster wall on metal lath 0.05 0.05 0.02
¼ in., mounted over 3 in. air space, with 1 in. glassfiber batts right
behind the panel 0.60 0.30 0.10 0.09 0.09 0.09
3⁄8 in. 0.28 0.22 0.17 0.09 0.10 0.11
Rockwool blanket, 2 in. thick batt (Semi-Thik)
mounted against solid backing 0.34 0.52 0.94 0.83 0.81 0.69
mounted over 1 in. air space 0.36 0.62 0.99 0.92 0.92 0.86
mounted over 2 in. air space 0.31 0.70 0.99 0.98 0.92 0.84
Rockwool blanket, 2 in. thick batt (Semi-Thik),covered with 3⁄16 in.
thick perforated cement-asbestos board (Transite), 11% open area
mounted against solid backing 0.23 0.53 0.99 0.91 0.62 0.84
mounted over 1 in. air space 0.39 0.77 0.99 0.83 0.58 0.50
mounted over 2 in. air space 0.39 0.67 0.99 0.92 0.58 0.48
Rockwall blanket, 4 in. thick batt (Full-Thik)
mounted against solid backing 0.28 0.59 0.88 0.88 0.88 0.72
mounted over 1 in. air space 0.41 0.81 0.99 0.99 0.92 0.83
mounted over 2 in. air space 0.52 0.89 0.99 0.98 0.94 0.86
Rockwool blanket, 4 in. thick batt (Full-Thik), covered with 3⁄16 in.
thick perforated cement-asbestos board (Transite), 11% open area
mounted against solid backing 0.50 0.88 0.99 0.75 0.56 0.45
mounted over 1 in. air space 0.44 0.88 0.99 0.88 0.70 0.30
mounted over 2 in. air space 0.62 0.89 0.99 0.92 0.70 0.58
Roofing felt, bituminous, two layers, 0.8 lb/ft2, mounted over 10 in.
air space 0.50 0.30 0.20 0.10 0.10 0.10
Spincoustic blanket
1 in., mounted against solid backing 0.13 0.38 0.79 0.92 0.83 0.76
2 in., mounted against solid backing 0.45 0.77 0.99 0.99 0.91 0.78
Spincoustic blanket, 2 in., covered with 3⁄16 in. perforated cement-
asbestos board (Transite), 11% open area 0.25 0.80 0.99 0.93 0.72 0.58
Sprayed “Limpet” asbestos
3⁄4 in., 1 coat, unpainted on solid backing 0.08 0.19 0.70 0.89 0.95 0.85
1 in., 1 coat, unpainted on solid backing 0.30 0.42 0.74 0.96 0.95 0.96
Table 10-4. (cont.) Sound Absorption Coefficients of General Building Materials and Furnishings
Materials Coefficients
125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz