Metering Sound in HVAC

[TomFitz]

I work in a position as a construction inspector (new construction) for the government. Last week I encountered a problem of louder than normal HVAC supply/return registers in several offices. To confirm my suspicions that this noise was above acceptable standards, I purchased a sound meter.
The readings received within these spaces ranged from 68 to 75db. My meter was set at "low range" (35-90db), "A" frequency weighting, for general sound level measurement, and "slow" for normal measurements.
Using these same settings,. one of our people checked a fire alarm and his reading was 83db..
My question:
How is it, that the HVAC noise had such high readings, and yet, it was not near as offensive to the ears as the fire alarm, and this alarm, reading only 10db above the HVAC, was extremely offensive to the ears?

This fact led my supervisor to question my readings. (which I double-checked)

Thank you for any rational explanation that I may give these people..

Tom Fitz

 

[MikeyP]

75 dBA sounds pretty loud to me (above the "first action level" of the Noise at Work Regs in the UK. When was the meter last calibrated? Is it a type 0, 1 or 2 meter (if you bought it in an electrical shop it is probably nowhere near accurate enough)? Did you test it with a field calibrator prior to the measurement?

Bear in mind that a fire alarm is usually a highly tonal noise where as HVAC noise is broadband atonal noise. The ear reacts to these sounds in different ways and the brain perceives different levels of loudness even if the energy content of the 2 sounds is the same.

M

 

[TomFitz]

I purchased it through Omega Instruments (see site below)
(

http://www.omega.com/ppt/pptsc.asp?ref=HHSL&Nav=teml12)

here's the info:
Omega Sound Level Meter MODEL#HHSL-1
Standard Applied: IEC 651 Type 2

Portability and Simplicity

AC and DC Output

A and C Frequency Weighting Characteristics

Slow and Fast Time Frequency Weighting Characteristics

Maximum Hold Function

Range: Lo: 35 to 90 dB, Hi: 75 to 130 dB

Under and Over Range Indication

Resolution: 0.1 dB

Accuracy: ±1.5 dB (ref 94 dB@1kHz)

Dynamic Range: 55 dB

Frequency Range: 31.5 to 8000 Hz

Valox Housing to Withstand Accidental Drops

Frequency Weighting: A, C

Time Weighting: Slow & Fast

Impedance: 600 Ohms

Maximum Hold: Decay <1 dB/3min

Electret condenser microphone

It's not the very cheapest,.. but not the very best either.

 

[Hatch]

HVAC sound levels are typically specified in terms of NC, PNC, NCB or other criteria. They basically allow for more low frequency sound and are determined by measuring the octave band sound levels and plotting the levels against the appropriate criteria curve.

Having said all of that, 75 dBA would be higher than normal for an office, even worse for a conference room. A normal person would not be very productive in that environment. Then again.... we are talking about a Government office...


Fire alarms are only meant to be used during an emergency. I would want it as loud as possilbe since people have a tendency to keep on working during a fire alarm.

C. Hugh

 

[Karinathan]

I do agree with MikeyP. Probably most of the energy of HVAC noise you've measured fall into low frequency range where human ears are less sensitive. The better way to observe the noise character is to use the sound analyser which will give you the sound level per each individual frequency range.

 

[Karinathan]

Try this website.. maybe it's useful for you..

http://www.hvac-toolbox.com/35.html

 

[TomFitz]

Thank you everyone for your responses... and thanks Karinathan for that link to &quot;HVAC Toolbox&quot;... there is a wealth of information there. Since delving into this subject of sound metering,. I've come to realize what a vast field acoustics can be.
However, for HVAC installations, is there any standard type meter that should be used used by QAR's (quality assurance representatives) to assure that the sound level meets required standards?
If so,.. could someone point me in a general direction or recommend a particular meter?
BTW... I'm purchasing this myself since the government is not providing one.. so go easy on me!

This is not a requirement for my job,. but in spite of what some people think of federal employees,.. some of us do want to give the tax payers quality work!

Thanks again for any assistance..

Tom Fitz

 

[Rob45]

Tom:
Two important points:
First, the decibel measurement system is on a log scale, so 10 dB difference is a huge difference'
Second, as mentioned above a fire alarm is highly tonal, and is intended to be annoying, while HVAC noise is most likely a very broad-band noise akin to white noise.

I'd suggest you get a basic text on noise and acoustics, and that you spend a few hours familiarizing yourself with the basic concepts before you spend a great deal more time measuring sound levels.

Best regards,
Robert

 

[TomFitz]

Thanks Rob for your response... And yes, I do intend to spend time familiarizing myself with noise and acoustics when time permits..
Unfortunately, with 17 buildings going up in one project,..several others in 2 other projects, a lack of personnel to handle the QA duties, and volumes of specs and drawings to read when I do get home,.. it is difficult and frustrating. I only wish there were 48 hours in the day! :-(
For the time being, I was hoping to accurately measure the HVAC noise levels with a meter that would be used in this particular area.. Judging from responses I have received,.. I am under the impression that the sound meter I bought(specs listed above)is not suitable.

TomFitz

 

[Karinathan]

There are 2 standard for sound level meter (SLM) depend on where are you living. For the European, The IEC Publication 651 is used while in America the standard is ANSI 1.4. They are completely alike except for one important point.. The IEC standard requires sensitivity calibration in a reference direction specified by manufacture while the ANSI standard prescribes the random incidence sensitivity.. I don't want to go deeper in the detail though..I think you could find these infos from the searching engine if you are interested.. anyway,.. followings are the guideline references that may relate to your work which you,again, can find out further on your own..

IEC179 : Precision Sound Level Meters (1965 and 1973)

ISO R 3745-1977 : Determination of sound power levels of noise sources- Precision methods for anechic and semi-anechoic rooms (Also 3744)

ISO3741-1975 : Determination of sound power levels of noise sources- Precision methods for broadband sources in reverberation rooms

 

[okdb]

Hello TomFitz,

First, the results you got appear awfully high to me. The noise limits used in Norway for HVAC is 40dBA.

Second, you got many answers which are a bit to complicated for your application. Your meter is good enough. Measure the sound level in the middel of the room or close to the place the person is sitting, depending what seems most relevant to you. Wait till the level is stable, like a variation within 1dB, and that's it. Just compare with the value I gave you, 40dBA. Above this, especially above 45dBA, you'll get complain from the users, garanteed.

It's a bit groove measurements, it's just a survey, but it'll tell all you need to know: whether there's a problem or not.

OkdB

 

[MikeyP]

Your meter is Type 2 which should be OK for most things, but check the standards listed by Karinathan for your particular applications. Type 1 would be better, but the standard may not require it.

M

 

[TomFitz]

Thanks &quot;OKDB&quot;... that's basically what I needed to know...
This area of Acoustical Engineering is away over my head and for the present, just needed to know that I was going in the correct direction and have the correct meter.

And yes indeed,.. the rooms are very noisy. But I was taking my readings from directly beneath the registers..
Steping back to the middle of the room,.. it has reduced the dBA approx. 6 to 8 dBA.. Still, it is very loud.

According to the &quot;NEBB&quot;(National Environmental Balancing Bureau)here in the states, I should be looking for 35 to 50 dBA in general office space.

Thanks again to everyone for your input..
The more I learn,... the more I find out how much I don't know! (and the more I need to study)..

TomFitz