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Technology Report

June, 2003

Siemens Industry, Inc. Page 1 of 12

Confronting the Sound Issue

Attaining Acceptable Ventilation Related Sound in Laboratory

Rooms

Since laboratory work requires diligence and

concentration, the room occupants shouldn't be

distracted by excessive ventilation system sound.

However, attaining an acceptable laboratory room

sound level is especially challenging due to the high

ventilation airflows required. Yet many designers

feel uncomfortable dealing with the perplexing

factors of equipment sound ratings and their

potential effect on room sound. This Technology

Report cuts through much of the theoretical and

confusing aspects of this subject and provides you

with a straightforward way to establish and specify

sound ratings for laboratory ventilation system

components.

Getting a Handle on Decibels

Equipment sound ratings and room sound levels are

always given in decibels (dB). Therefore, what must

you know regarding dB's? The higher the dB, the

louder the sound is. Also, the dB scale is not linear.

In other words, a sound at 80 dB is not twice as loud

as one at 40 dB. Rather, for each 8 to 10 increase in

dB a sound seems to become about twice as loud.

The Frequency Component

Aside from loudness, sounds can range from low to

high in frequency or pitch. The range of human

hearing extends from about 16 Hz up to 20,000 Hz

(16 to 20,000 cycles per second) and ventilation

sounds usually lie within the range of 45 Hz to

11,200 Hz. However, ventilation sounds are

comprised of many individual tonal (frequency)

components. Therefore, to completely define a given

ventilation system related sound would require

stating the dB level and frequency of each individual

tonal component of the sound. Since there are over

11,000 potential tonal components to any given

ventilation related sound, this approach could

involve loads of data, and is therefore not a practical

approach for defining a sound.

The Octave Bands

A more practical approach divides the ventilation

related sound spectrum (45 to 11,200 Hz) into eight

continuous sections or bands called octave bands.

Rather than having to know the dB level for over

11,000 individual frequencies of a sound, you only

need to be concerned with the dB level at each

octave band. That's where the familiar 63 Hz, 125

Hz, 250 Hz, etc. frequency levels come from—they

are the mid or center frequencies of each of the

eight octave bands and are indicated in Table 1.

Note that the

se standard octave bands are

numbered from 1 to 8 to make it easier to refer to a

specific band.

Table 1. Standard Octave Band Frequencies.

Octave

Band

Number

Lowest

Frequency

Mid

Frequency

Hz*

Highest

Frequency

1 45 63 90

2 90 125 180

3 180 250 355

4 355 500 710

5 710 1000 1400

6 1400 2000 2800

7 2800 4000 5600

8 5600 8000 11200

* Note that the Mid or center frequency is not at

the exact mathematical mid-point of an octave

band.

Sound Curves

When a given sound's dB level at each octave band

is plotted on a graph, it becomes a sound curve that

helps to visually depict the sound's profile. In 1957

the familiar Noise Criterion (NC) curves (Figure 1)

Document No. 1

49-979

Summary of content (12 pages)

PAGE 1
Technology Report June, 2003 Confronting the Sound Issue Attaining Acceptable Ventilation Related Sound in Laboratory Rooms Since laboratory work requires diligence and concentration, the room occupants shouldn't be distracted by excessive ventilation system sound. However, attaining an acceptable laboratory room sound level is especially challenging due to the high ventilation airflows required.
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were developed to quantify various loudness levels of typical ventilation related sound in rooms. The NC curves extend over the eight standard octave bands and only represent ventilation related sound. They do not include other sounds that might also occur in rooms such as speech, work related sounds, office machines, etc.
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Figure 1. Noise Criterion (NC) Sound Curves. NC Sound Curves Figure 1 shows the familiar NC sound curves that you have probably seen before. Sound pressure level (loudness) is represented on the vertical axis as 0 to 90 dB. Frequency is on the horizontal axis and is represented by the standard octave band center or mid frequencies. The very bottom curve shows the approximate threshold of human hearing. Note that low frequency sounds must be at a considerably higher dB level in order to be heard.
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and expectations in this regard. Since laboratory clients probably won't know much about the technicalities of sound, they aren't likely to provide specific room NC sound level requirements. However, client normally expect new labs to be quieter than existing labs. If there are existing laboratory rooms in a client's facility it is advisable to get a feel for the room sound levels.
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Table 2. Acceptable Laboratory Sound Levels at Various Octave Bands. Laboratory Type 31 Hz 63 Hz 125 Hz 250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz A. Teaching lab with extensive speech capability required. 60 to 70 55 to 65 50 to 60 45 to 55 40 to 50 35 to 45 30 to 40 25 to 35 B. Research lab with appreciable speech capability required. 65 to 75 60 to 70 55 to 65 50 to 60 45 to 55 40 to 50 35 to 45 30 to 40 C. Research lab with minimal speech capability required.
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Room Ventilation - Component Sound Analysis Item 1 2 3 4 5 6 7 8 9 10 Description 2 3 125 Hz 250 Hz Octave Bands 4 5 500 Hz 1000 Hz 6 7 2000 Hz 4000 Hz Allowable Room Sound Level Attenuation Resultant Attenuation Resultant Attenuation Resultant Attenuation Resultant Attenuation Resultant Attenuation Resultant Attenuation Resultant Attenuation Resultant Attenuation Resultant Figure 2. Room Ventilation System - Component Sound Levels Analysis Chart.
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ALLOWABLE TERMINAL DISCHARGE SOUND ITEM 7 DUCT DIVISION ATTENUATION ITEM 6 DUCT ATTENUATION ITEM 5 DUCT END REFLECTION AT SUPPLY AIR DIFFUSER INLET SUPPLY AIR TERMINAL ITEM 4 ALLOWABLE AIR TERMINAL SOUND AT DIFFUSERS ITEM 1 SUPPLY AIR SOUND LEVEL ITEM 3 MULTIPLE SOUND SOURCES ITEM 2 SPACE EFFECT SOUND ATTENUATION Figure 3. Example Laboratory Room Supply System Arrangement.
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combined sound level that is 3 dB greater than their individual sound levels. To determine the allowable individual sound level for the diffusers, you must subtract 3 dB from the combined sound level. Table 4 gives the resulting sound levels for multiple sound sources. Table 4. Combining Multiple Sound Sources. Difference between the highest and lowest dB in a specific octave band. Add this to the highest dB to obtain the resultant dB.
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and IAQ concerns. Therefore, in this example assume that a 9-foot long section of 12-inch diameter unlined flex duct will be used between the supply air terminal outlet and the diffuser inlets. The attenuation levels for unlined flex duct are listed in Table 7. Table 7. Unlined Flex Duct Attenuation Levels for Up to 10-Foot Lengths. Dia.
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Example Analysis of a Room General Exhaust and a VAV Fume Hood  Room Size: 12 ft W × 25 ft L × 9 ft H (2,592 cubic feet)  Fume Hood Exhaust: 1200 cfm maximum  Room General Exhaust: 400 cfm max. with a 6inch unlined round duct to general exhaust terminal.  Six feet exists between the general exhaust grill and where the sound level would be measured. Figure 5 illustrates this arrangement.
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Allowable Room Sound Level It is typically accepted that the room sound level in the immediate area of a fume hood should not exceed the NC 55 sound curve.
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2. Use multiple large laminar flow room supply air diffusers. This lessens the airflow per diffuser and lowers the individual diffuser sound. Ensure that the airflow volume is equally divided among the diffusers. 3. Keep all diffuser throttling dampers fully open. If necessary, add throttling or balancing dampers upstream and closer to the supply air terminal. 4. Use a sound attenuator or lined flexible duct between the supply air terminal and the diffusers.