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Sound Power, Sound Pressure, and OctaveBands Explained

39 SERIES ENGINEERING GUIDELINES

Sound pressure is dependent on theacoustic environment. The factors involvedinclude the effects of nearby reflectingsurfaces, receiver distance, type of space,the amount and location of absorption inthe space, the location in the space, thepresence of barriers, and the intrusion of ambient sounds.

Sound Power and Sound Pressure arealso different in that Sound Power is ameasure of total energy per unit time

emitted by the source in all directions.Sound pressure is a measure of the pressureat the receivers location.

Typically, manufacturers provideequipment sound power data, in decibels(dB) per octave band. The quietest soundwe might measure, such as a whisper about10 feet away, represents 0.000000000001,or 10 -12 , watt. The loudest regularlymeasured noise, the space shuttle ontakeoff, is 100,000,000, or 10 8, watts. Toavoid dealing with all these zeros,engineers use a logarithmic convention,labeled Lw, to express the watts in adecibel (dB) scale:

Lw = 10*log(W/W ref ), where W ref =10 -12 watt

So, the shuttle rates at 200 dB, thewhisper is 0 dB, and a large AHU with 1watt of acoustic power rates at 120 dB.Two of these AHUs would double thesound energy, and with logarithmicaddition, the total sound power would be

123 dB.To understand the relationship between

sound power and sound pressure , we canlook at two analogies common to HVACapplications:

Electric Heater To represent power, anumber expressed in BTUs or kilowatts isused to rate the heat release of theequipment. Just as the number of BTUs

indicates the amount of heat released by apiece of equipment, the sound powerindicates the power radiated from a piece of equipment.

Light bulb A 100 watt light bulb has asource electrical power level of 100 watts.We can use the power rating as a relativeindicator, but what we are really interestedin is the brightness, or lumens, produced bythe bulbs. And that depends on many

things such as the size of the room, thereflectivity of the walls, and how far awaywe are from the light source.

Similarly, with building HVAC noisewe are concerned about the sound pressure sensed by the occupants ears. Althoughwe use the dB scale for both power andpressure, they are very differentparameters. Sound pressure is the

Sound

Power

Defined

What is

Sound

Pressure ?

Why use

Octave

Bands?

A-weighting

NC curves

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Sound Power, Sound Pressure, and Octave Bands Explained (page 2 of 3)

phenomenon we sense; it is also the physicalproperty that we can measure with microphones.The sound pressure resulting from a given AHU-generated sound power depends on:

Distance from the AHU to the room The size of the room The absorptive properties of interior furnishings Attenuating elements such as silencers, duct

liner, duct branches, elbows, etc.

Propagating sound creates pressure fluctuationsin the air, which vibrate the microphone diaphragmand cause it to make a voltage output proportional tothe pressure. We use an RMS (Root Mean Square)value of the pressure fluctuations to obtain alogarithmic measure, expressed as Lp.

Sound energy can occur over a broad frequencyrange, and the human ear is sensitive from about 20Hertz (Hz) to 20,000 Hz. We can measure theoverall level, the sum across all these frequencies,but it is usually helpful to break this down intofrequency bands. As in music, an octave band convention was chosen. Each progressive band hasdouble the bandwidth of the previous. The centerfrequencies assigned for the bands for the full rangeof human hearing are: 31.5, 63, 125, 250, 500, 1000,2000, 4000, 8000, and 16000 Hz. Usually, wedisregard the last band.

We can show the range of human hearing in aplot of sound pressure against octave bandfrequencies.

RANGE OF HUMAN HEARING

At the highest levels, noise above 120 dB cancause instant hearing damage. The lowest levels

represent the threshold of hearing, and it is evidentthat at lower levels our ears are less receptive to thelowest frequencies.

With this in mind, a sound measurement scheme A-weighting is sometimes used to filtermicrophone sound pressure readings to reflecthuman sensitivity, and give an overall soundpressure readings, in dB(A).

A-WEIGHTING CURVE

This suggests that noise at 125 Hz would sound15 dB quieter than 1000 Hz noise at the same soundpressure level, to most listeners.

Again considering the analogy relating soundpower to BTUs and sound pressure to degrees, weare fully aware that temperature is only one measureof comfort. We use the temperature-humidity-index,THI, as a measure of humidity and temperature;further, ventilation flow patterns and on/off cyclingalso affect our comfort.

Similarly, our perception of noise depends on acombination of sound level, spectral content, andmodulation. Sometimes the overall level is just tooloud, but generally, if the noise has a balancedcontour with energy in all bands, we dont find it tooobjectionable. Unfortunately, the noise from theHVAC system usually has certain components thatare emphasized. These are often defined as:

*Rumble when the 31.5, 63 and 125 Hz bandsare especially loud. The effect of this is to inducefatigue. The 6-oclock phenomenon is thesensorial relief we sometimes experience when the

HVAC system shuts off for the day.*Hiss when the 2000, 4000, and 8000 Hzbands are emphasized. Typically caused byundersized diffusers. This sound is rather irritating.

*Tonality when a particular octave band ismore than 5 dB above its adjacent bands, oftencaused by the blade pass frequency of the fan in theAHU. Most people find this particularly annoying.

S o u n

d P r e s s u r e

( d B )

Octave Band Frequency (Hz)

120

100

80

60

40

20

0

-20

3 1

. 5

6 3

1 2 5

2 5 0

5 0 0

1 0 0 0

2 0 0 0

4 0 0 0

8 0 0 0

-30

-25

-20

-15

-10

-5

0

5

6 3 1 2 5

2 5 0

5 0 0

1 0 0 0

2 0 0 0

4 0 0 0

8 0 0 0

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Sound Power, Sound Pressure, and Octave Bands Explained (page 3 of 3)

With regard to the frequency-dependentnature of hearing response, Noise Criterion(NC) curves were developed to plot octaveband sound pressure level data measured inrooms.

On the NC curves to the right, fourdifferent noise spectra are shown. Three havethe same overall dB(A) level. The green andblack spectra have similar overall soundpressure levels to the blue one, but do not havea nice balanced character. The NC rating,which is determined by looking for the highesttangent point on the background curves, is agood indicator that the blue spectrum is muchmore desirable.

NC ratings must be used cautiously. Note

that the red spectrum has the lowest dB(A)and NC rating, but would actually beperceived as the most annoying, because of the prominence of a discrete tone.

Of course each piece of HVAC equipmentcan contribute to the overall room sound invarious ways, and to make predictions is acomplicated process. But it is important tounderstand the acoustical goals and concernsof building design engineers, and what effectour equipment can have.

For more information on our 39M Air Handlers, contact your local Carrierre resentative, call 1-800-CARRIER, or visit our website, htt ://www.carrier.com/

2004 Carrier Corporation, Syracuse, NY Printed in U.S.A. GUD-04-002

Spectrum dB(A) NC-rating Quality / PerceptionBlue 44 35 Balanced - Pleasant

Green 44 40 Rumbly - FatiguingBlack 44 41 Hissy - Irritating Red 39 35 Tonal Annoying