Sound Advice Presented by Randy Zimmerman
Download
Report
Transcript Sound Advice Presented by Randy Zimmerman
Sound Advice
Presented by Randy Zimmerman
Introduction
Good acoustical design
– Comfortable and productive environments
Systems
– Comfort vs. energy efficiency
Proximity to occupants
– Air terminal units
– Air outlets
Designers must understand acoustical ratings in
order to write good specifications
2
What You Will Learn
Sound power vs. sound pressure
Sound power determination
End reflection correction
Sound criteria
Determining NC ratings for catalog data
Specifying in terms of NC
Specifying maximum allowable sound power levels
Sound paths
Mock-up room testing
3
The Sound Room
Products are tested in a qualified
reverberant chamber (per AHRI
220)
Reverberant chambers are used for
quiet products
– Low absorption
– Low background
The reverberant field eliminates all
directionality from a sound source
Sound levels within the reverberant
field are equal at all points
4
The Comparison Method
Determine the sound power (Lw) by comparison to a
known reference sound source (RSS)
Measure the sound pressure (Lp) of the RSS in order
to determine the room attenuation
Lp = Lw – room attenuation
Lw = Lp + room attenuation
If we know that the RSS creates Lw = 80 dB in the first
octave band (63 Hz), but we read only Lp = 70 dB, we
know that we have 10 dB of room attenuation in that
octave band
Room attenuation is constant
All sound meters measure sound pressure (Lp)
5
The Test Procedure
Set-up any ductwork and equipment to be tested
Remove any unnecessary material from the test
chamber
Turn off equipment and close test chamber doors
Take background sound pressure level
Switch RSS on
Take RSS sound pressure level
Switch RSS off and set test conditions
Record sound pressure levels at various conditions by
changing flow rates, pressures, etc
6
The Decibel (dB)
Because of the great differences in energy (or
pressure) available, the log of the actual
value is used
Reference power is 10-12 watts
Reference pressure is 0.0002 microbars
dB is measured vs. frequency
An infinite number of frequencies, so they are
averaged into bands, typically called ‘Octave Bands’
7
Octave Bands
Octave bands are centered about
increasingly wider frequency ranges,
starting with 63 cycles/second (Hz)
Each band doubles in frequency
Bands are traditionally numbered,
in our industry, as shown
Octave Band Designations
Center Frequency
Band Designation
63
1
125
2
250
3
500
4
1000
5
2000
6
4000
7
8000
8
8
Octave Bands
Fan-powered products usually create their highest
sound levels in octave band 2 (125 Hz), but sometimes
octave band 3 (250 Hz)
Grilles, registers and diffusers create their highest
sound levels in octave bands 4 (500 Hz), 5 (1000 Hz) or
6 (2000 Hz)
Octave bands 4-6 are known as the speech
interference bands
It’s industry convention to report sound data for
octave bands 2-7 only
Sound room size and design can cause problems with
readings in octave bands 1 and 8
9
Decibel Addition Example
To add two decibel values:
80 dB
+ 74 dB
10
Decibel Addition Example
To add two decibel values:
80 dB
+ 74 dB
154 dB (Incorrect)
11
Decibel Addition Example
Correction To Be Added To
Higher Value (dB)
3
To add two decibel values:
80 dB
- 74 dB
= 6 dB
2.5
2
1.5
Difference in Values: 6 dB
1
From Chart: Add 1.0 dB
to higher Value
0.5
80 dB
+ 1 dB
0
0
2
4
6
8
10
Difference In Decibels Between Two
Values Being Added (dB)
81 dB
(Correct)
12
Good To Know
Any sound source 10 dB lower than background
level will not be heard
Add 3 dB (or 3 NC) to double a sound source
– Two NC40 terminal units over an office would
probably create an NC43 sound level
– Two NC20 diffusers in a room would create a worst
case sound level of NC23 (if they are close together)
– Don’t try to add-up dissimilar products in this manner
13
Sound Power Changes
Equation for sound power changes = 10logn
1 Fan on
1 Fan on
1 Fan on
vs. 2 Fans on
vs. 4 Fans on
vs. 10 Fans on
n=2
n=4
n=10
Add 3 dB
Add 6 dB
Add 10 dB
1 Fan on
vs. 100 Fans on n=100 Add 20 dB
50 Fans on
vs. 100 Fans on n=2
Add 3 dB
14
Proximity To Sound Sources
Would you really expect to hear 100 fans running at
the same time?
Properly selected diffusers shouldn’t be heard from
more than 10 feet away
Although there may be multiple diffusers in a space,
it’s unlikely that more than one or two are within 10
feet of an occupant
We would only expect to be able to hear a 10 foot
section of continuous linear diffuser from any single
location
15
For High Frequencies
1 dB
not noticeable
3 dB
just perceptible
5 dB
noticeable
10 dB
twice as loud
20 dB
four times as loud
16
For Low Frequencies
3 dB
noticeable
5 dB
twice as loud
10 dB
four times as loud
17
What We Hear
Our ears can be fooled by frequency
– Both tones sound equally loud
65 dB
40 dB
63 HZ
1000 HZ
A Difference of 25 dB
18
Acoustic Quality
Not too quiet
Not too loud
Not too
annoying
Not to be felt
Don’t destroy acoustic privacy
Avoid hearing damage
Don’t interfere with speech
No rumble, no hiss
No identifiable machinery sounds
No time modulation
No noticeable wall vibration
19
OCTAVE BAND LEVEL _ dB RE 0.0002 MICROBAR
NC Curves
80
70
NC-70
60
NC-60
50
NC-50
40
NC-40
30
20
NC-30
APPROXIMATE
THRESHOLD
OF HUMAN
HEARING
NC-20
10
63 125 250 500 1K 2K 4K 8K
MID - FREQUENCY, HZ
20
Typical NC Levels
Conference Rooms < NC30
Private offices < NC35
Open offices = NC40
Hallways, utility rooms, rest rooms < NC45
NC should match purpose of room
Difficult to achieve less than NC30
Select diffusers for NC20-25 (or less)
21
Sound Power Vs. Sound Pressure
Sound power (Lw) cannot be measured directly
Sound pressure (Lp) is measured with a very fast
pressure transducer (i.e. a microphone)
Calculate sound power (Lw) by correcting sound
pressure (Lp) readings in a reverberant chamber to a
known power source
– Reference Sound Source (RSS)
22
Reference Sound Source
Correction device for a
reverb room is the RSS (per
AHRI 250)
– Calibrated in an anechoic
chamber to simulate a free
field condition
– Used in a reverberant field,
so there is a known error
called the “Environmental
Effect”
23
In a Reverb Room
Sound power (Lw) is calculated from measured
sound pressure (Lp) and corrected for background
– Unless product sound is 10 dB above background
RSS is used to “calibrate” the room
Data is recorded per octave band (or 1/3 octave
band if pure tones are anticipated), for each
operating condition
24
Catalog Data
Sound pressure data is collected by a
frequency analyzer that samples microphones
via a multiplexer
Data is collected and sound power recorded
Spreadsheets are used to check the linearity
of data sets
Catalog data is prepared from actual sound
power data sheets using accepted regression
techniques
25
Diffuser Testing
Current test standard for
diffusers
– ASHRAE 70-2006
No significant changes in
many years
26
Terminal Unit Testing
Current test standard for
terminal units
– ASHRAE 130-2008
ASHRAE 130 is currently
under review
– SPC 130
– It will be updated to include
more products including
exhaust boxes
27
Sound Tests
Discharge sound, VAV terminals
– Unit mounted outside room
– Discharging into reverb room
Radiated sound, VAV terminals
– Unit mounted inside room
– Discharging outside reverb room
– All ductwork lagged to prevent
‘breakout’
Diffuser supply/return sound
– Unit mounted flush to inside the
reverb room wall
28
Performance Rating
Current rating standard for
terminal units
– AHRI 880-2011 (effective
Jan 1, 2012)
– Increases discharge sound
levels due to end reflection
– This affects all published
data and selection software
– The boxes will still sound
the same, but now the
acoustical consultants will
be happier
29
Sound Path Determination
Current standard for
estimating sound levels in
rooms
– AHRI 885-2008
– Provides sound path data
from ASHRAE research
– Attenuation factors for duct
lining, ceiling tiles, room
volume, elbows, flex duct,
etc
30
Industry Standardization
AHRI 885-2008 contains
Appendix E
– Recommends standard
attenuations to be used by
all manufacturers for
catalog data
– First presented in ARI 88598
– Makes comparing catalog
NC levels much less risky
31
AHRI 885-2008 Catalog Assumptions
Radiated Sound
Environmental Effect
Ceiling / Space Effect
Total dB Attenuation
Discharge Sound
Environmental Effect
Duct Lining
End Reflection
Flex Duct
Space Effect
Total dB Attenuation
2
2
16
18
2
2
3
9
6
5
25
3
1
18
19
Octave Band
4
5
0
0
20
26
20
26
6
0
31
31
7
0
36
36
3
1
6
5
10
6
28
Octave Band
4
5
0
0
12 25
2
0
18 20
7
8
30 53
6
0
29
0
21
9
59
7
0
18
0
12
10
40
mineral fiber tile
5/8 in thick
20 lb/ ft3 density
5 ft, 1 in fiberglass lining
8 in flex duct to diffuser
2500 ft3 room volume
5 ft from source
The following dB adjustments are used for the calculation of NC above 300 CFM
300 - 700 CFM
Over 700 CFM
2
2
4
Octave Band
3
4
5
6
1
1
-2
-5
3
2
-2
-7
7
-1
-1
32
Certified Performance Data
AHRI Program
– Directory of Certified
Product Performance
– www.ahrinet.org
– Random samples subjected
to annual third party lab
testing
– Verifies that performance is
within established test
tolerances
– Failures result in penalties
– Voluntary program
33
The dBA Scale
Used for outdoor noise evaluation
Also used for hearing conservation measurements
Basis of most non-terminal sound ratings
34
NC Specifying
Specifying and unqualified NC value is an
‘open’ specification
Specifying an NC with specific path
attenuation elements could result in
acceptable sound quality
It is far preferable to set maximum allowable
sound power levels than to specify NC
35
Example
Octave Band Level_ dB RE 0.0002 Microbar
80
NC rating given is NC-30
since this is highest point
tangent to an NC curve
70
NC-70
Sound Power
60
NC-60
Sound Power less 10 db
in each band
NC-50
50
40
NC-40
30
20
NC-30
Approximate
threshold
of human
hearing
NC-20
10
63 125 250 500 1K
2K
MID - Frequency, HZ
4K
8K
36
Example
90
NC rating given is NC-45
since this is highest point
tangent to an NC curve
NC-70
80
70
NC-60
60
Octave Band Level
dB RE 0.0002 Microbar
NC-50
50
40
NC-40
30
NC-30
20
NC-20
Approximate threshold
10
of human hearing
63
125
250
500
1K
2K
4K
MID - FREQUENCY, HZ
8K
37
Example
90
80
Both noise spectrums would be
rated NC-35, However, they would
subjectively be very different!
Typical grille noise
at a distance of 10FT
(high-frequency)
Typical fan noise from
adjacent mechanical
room (low-frequency)
Octave Band Level_ dB RE 0.0002 Microbar
NC-70
Approximate threshold
of human hearing
70
NC-60
60
50
NC-50
40
NC-40
30
NC-30
20
NC-20
10
Mid - Frequency, HZ
38
NC vs. RC
NC rates speech interference
and puts limits on loudness
NC gives no protection for low frequency
fan noise problems
NC stops at 63 Hz octave band
RC includes the 31.5 Hz and 16 Hz octave band
RC rates speech interference and defines
key elements of acoustical quality
39
Room Criteria
(RC) Curves
90
High probability that noise
induced vibration levels in
light wall and ceiling structures
will be noticeable. Rattling
of lightweight light fixtures,
doors and windows should
be anticipated.
Region B
Moderate probability that
noise-induced vibration will be
noticeable In lightweight light
fixtures, doors and windows.
Octave Band Sound Press. Level, dB
Region A
80
A
70
B
60
50
40
30
C
RC
50
45
40
35
20
Threshold
of audibility
10
ADAPTED FROM 2009 ASHRAE FUNDAMENTALS HANDBOOK - ATLANTA, GA
30
25
Octave Band Center Frequency, HZ
40
Two Parts of RC
Example – RC 40 N
The number is the speech interference level
The letter tells you speech quality
–
–
–
–
(N) = neutral spectrum
(R) = too much rumble
(H) = too much hiss
(V) = too much wall vibration
41
RC Number Calculation
Average of level of the noise in the
octave bands most important to speech
– 500Hz Octave band = 46 dB
– 1000Hz Octave band = 40 dB
– 2000 Hz Octave band = 34 dB
– RC = (46+40+34) / 3 = 40 dB
42
RC Letter Determination
Plot room sound pressure on RC chart
Determine rumble roof
– 5 dB greater then low frequency
Determine hiss roof
– 3 dB greater then high frequency
R - room sound pressure crosses rumble roof
H - room sound pressure crosses hiss roof
V - room sound pressure goes into vibration zone
N - room sound pressure does not cross
43
Rumbly
Spectrum (R)
80
Octave Band Sound Press. Level, dB
Measured data is outside the
reference region by >5 dB,
below the 500 Hz octave band,
therefore the noise is likely
to be interpreted as “rumbly”
90
PSIL=(38+35+29) / 3 = 34
70
60
50
40
30
20
10
RC-34(R)
Octave Band Center Frequency, HZ
44
Rumbly & Induced
Vibration (RV)
90
A
80
Even though the PSIL
Is only 33 dB, the
noise spectrum
falls within regions
A & B indicating a
high probability of
noise-induced
vibration in lights,
ceilings, air diffusers
and return air grilles
Octave Band Sound Press. Level, dB
70
B
60
50
40
30
20
10
PSIL= (38+32+29) / 3 = 33
RC-33(RV)
Octave Band Center Frequency, HZ
45
Neutral
Spectrum (N)
Measured data must
not lie outside the
reference region by
>3 dB, above the 1000 Hz
octave band
80
70
Octave Band Sound Press. Level, dB
Measured data must
not lie outside the
reference region by
>5 dB, below the 500 Hz
octave band
90
60
50
40
C
30
20
10
PSIL=(38+35+29) / 3 = 34
RC-34(N)
Octave Band Center Frequency, HZ
46
Hissy
Spectrum (H)
90
80
70
Octave Band Sound Press. Level, dB
Measured data is
outside the reference
region by >3 dB, above
the 1000 Hz octave band,
therefore the noise
is likely to be
interpreted as “hissy”
60
50
40
30
20
10
C
PSIL = (35+36+34) / 3 = 35
RC-35(H)
Octave Band Center Frequency, HZ
47
Who Uses RC?
NC remains the best way to make product
selections
RC is preferred as an analysis tool
Acoustical consultants will typically report
whether or not equipment meets NC spec but
will describe the resulting sound spectrum in
terms of RC
You should continue to see catalog
application data in terms of NC
48
Terminal Unit Installations
Sound characteristics
Optimal installation
Attenuators
Liners
49
Sound Characteristics
Radiated sound is primary issue with fan-powered
terminals
Discharge sound is primary issue with non-fan
terminals
Fan-powered sound is typically set in 2nd (125 Hz)
and 3rd (250 Hz) octave bands
– Long sound waves
– Harder to attenuate
Discharge sound is easily attenuated with lined
ductwork and flex duct
50
Ideal Terminal Unit Installation
>3D
Lined Sheet Metal Plenum
(Max velocity 1,000 FPM)
Max velocity 2,000 FPM
D
Maximize Height
Above Ceiling
VAV
UNIT
Flexible Connectors
For Fan-powered Units
Ceiling
4' Min.
Lined Flexible Ducts
To Diffusers
51
Attenuators
Single duct
– Equivalent to lined ductwork
Dual duct
– Provides a mixing area for unit, but not much
sound attenuation
Fan powered
– Lined elbow or “boot” may provide 2dB
attenuation by removing line of sight to motor
– Carefully engineered attenuators can provide
additional sound reductions
52
Liners
Different liners in single ducts do not affect discharge
sound much
– Unit is too short for the air to interact with liner
1" liner does not significantly
decrease sound compared to ½"
Foil faced liners add 6-8 dB
Fiber free adds 4-6 dB
Double wall is variable
– Kettle drum effect increases sound, but it is directional
53
Flex Duct
Don’t forget about flex duct
5' of flex can reduce mid frequencies
by 20 dB or more
Flex is better than lined duct or attenuators
in reducing low frequencies
You can have too much of a good thing
54
Diffuser Tests - ASHRAE Conditions
Measured Air Flow
10 equivalent Diameters, min
Pressure
Discharge Velocity
Sound
55
Inlets: 3 Equivalent Diameters - Ideal
~1 NC add to catalog data
Measured Air Flow
Flex Duct, 1 radius bend
3 equivalent Diameters
Pressure
Discharge Velocity
Sound
56
Inlets: Long 90 at Diffuser
~3 NC add to catalog data
Measured Air Flow
Flex Duct
Pressure
Discharge Velocity
Sound
57
Inlets: Hard 90 at Diffuser
~5 NC add to catalog data
Flex Duct
Measured Air Flow
Discharge Velocity
Sound
Pressure
58
Inlet ‘Kinked’
~7-9 NC add to catalog data
Flex Duct
Measured Air Flow
2 equivalent Diameters
Pressure
Discharge Velocity
Sound
59
Summary of Results
Minimum add for flex duct = 1 NC
Worst case add, ‘Kinked’ = 7-9 NC
Air distribution pattern can be greatly effected
– Plaque / Perforated shows most effect
– Multi-Cone / Louvered shows least effect
Results were not the same for all diffuser types
Don’t forget that catalog NC’s are based on
typical offices (-10 dB across all bands)
60
Some Diffuser Solutions
Locate balancing dampers at branch takeoff
Keep flexible duct bends as gentle as possible
– Flex duct is a great attenuator of upstream noise
sources
Keep duct velocities as low as possible
– But over-sizing can result in higher thermal loss
61
Additional Resources
Noise and Vibration Control
for HVAC Systems
– Mark Schaffer, 2005
ASHRAE Fundamentals
– Chapter 8, 2009 Edition
ASHRAE HVAC Applications
– Chapter 48, 2011 Edition
62
Summary
NC remains the preferred sound specification
RC is often used after-the-fact
Specified max sound power levels are safest
Lining materials affect sound levels
Careful selection, design and installation are
required to avoid problems
63