A-Weight vs C-Weight Questions

[leeave96]

I need to know the basic differences between A vs C weighting when measuring sound. I am measuring sound for an outdoor event and in my searching I find a lot of stuff for A-Weight, but little on C-Weight.

When measuring sound with the above weighting, when would one use slow vs fast settings?

When is it desirable to switch from A to C? Again the event is an outdoor music concert. Any advice would be appreciated.

Thanks!
Bill

 

[SomptingGuy]

They are supposed to be based on how the human ear hears things at different levels. The A filter is for low levels (20-55dB), the B filter for medium levels (55-85dB) and the C for high levels (more than 85dB).

In practice though, the B and C filters don't correlate with subjective response, so most people use A for everything.

Industrial (in EU at least) noise levels are generally based on A-weighted levels.

I would probably use "slow" if wanting a reasonably avereraged reading.

- Steve

 

[MotorVib]

Form Motors world we only do A weighting for our sound testing and that is the only thinging that most people would like to see.

Ftome what Steve has Posted I would thing that A weighting would cover your needs.

Chris

 

[Tmoose]

Is this to assess for potential hearing damage, or public nuisance?

 

[GregLocock]

Good question, in fact C weightng might be appropriate for a rock concert as it at least lets some low frequency through. Note that A is attenuating 100 Hz by 20 dB, and 20 Hz by 40 dB

As to fast vs slow - fast seems to correlate better for most noises I am familiar with.

To be honest if you really want to do it properly you probably want to be using a more statistical approach like Leq



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[leeave96]

"Is this to assess for potential hearing damage, or public nuisance?"

The sound measurements are to find out the decibel levels, compare them to known db levels and finally try to figure out some upper limit so as to not have a public nuisance with future outdoor concerts.

Thanks!
Bill

 

[SomptingGuy]

This publication makes good background reading. More detailed than the evergreen "Measuring Sound" by B&K

http://www.nonoise.org/library/envnoise/index.htm

Depending on where you are holding these concerts, you may find there is already legislation in place. Worth a hunt.

- Steve

 

[Rob45]

I have known people to use 'C'-weighting and 'A'-weighting together, and to use the differences in levels as an indicator of the low-frequency content of the sound.
I.e., no difference = little or no low-freq content'
large difference = lots of low-freq content.

Believe it or not, this was the method used by a German automotive supplier firm to analyze exhaust tailpipe noise as recently as the late '90s.

- R

 

[GregLocock]

Yes, that was common before spectrum analysers were widely available. Say the 1970s!



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[hydtools]

I often compared A- and C-weighted levels to get a general feel for content while making field measurements. Yes, early 1970's. Used C-weighted for recording and subsequent analysis of sound.

All regulations are written around A-weighted levels. And usually slow response. A-weighted appoximates human ear response, perception of sound.

Ted

 

[DanEE]

The sound measurements are to find out the decibel levels, compare them to known db levels and finally try to figure out some upper limit so as to not have a public nuisance with future outdoor concerts.

ConsistentAs hydtools post, you would want to measure it consistent with the applicable ordinance in question..

And if the ordinance in question is written to be implementable, it would include a section on sound measurement standards as the ordinance in my county is written below.....

Sec. 16-54. Sound measurement standards.
Standards, instrumentation, and measurement procedures to be used in the measurement of sound as provided for in this article are as follows:
(1) Sound level measurement shall be made with a sound level meter using the "A" weighting scale set on "slow response," except for measurements of impact sound, set on "slow response. Impact sound shall be measured using the fast response of the sound level meter.
(2) The entire sound measurement system shall be serviced and operated as recommended by the manufacturer. Any officer authorized to enforce this ordinance shall be trained in the use of sound level measurement and the operation of sound level meters.
(3) Except in the case where noises are emanating from within multi-family structures and the complainant is a resident of the same multi-family structure, noise measurements shall be taken at the corner of the primary structure of the complainant nearest the noise source but when this location is not practical noise measurements shall be made at the boundary of the public or private right-of-way which adjoins the complainants' property.
(4) In the case of noises emanating from within a multi-family structure and where a complainant is a resident of the same multi-family structure, noise measurements shall be made in the unit of complainant at a height of at least four feet above the floor and at a point approximately equidistant from all walls.
(5) No individual other than the officer operating the meter shall be within ten feet of the meter during the sample period.
(6) The meter shall be calibrated immediately before use and recalibrated after use.
(7) The officer operating the meter shall face the noise source and record the meter's instantaneous response observed at consecutive ten second periods. The sound meter operator may cease taking readings as soon as the readings already taken show a violation of this article. Five readings above the allowed decibel limits as specified in subsection 16-53(b) attributed to the sound source or sources shall constitute prima facie evidence of a violation of the maximum permissible sound levels set in this article.
(Ord. of 6-7-2005, art. IV)


and there will be exceptions.. check the ordinance in your area of interest.. especially if what you are planning is a "permitted" event..


Sec. 16-55. Exceptions.
The following are exempt from the provisions of this article:
(1) Construction operations from 7:00 a.m. to 9:00 p.m. on weekdays and 8:00 a.m. to 9:00 p.m. on weekends for which any required building permits have been issued, provided all equipment is operated according to the manufacturers' specifications and with all standard equipment, manufacturers' mufflers and noise reducing equipment are in use and in proper operating condition.
(2) Noises of safety signals, warning devices, emergency pressure relief valves and church bells or chimes.
(3) Noises resulting from any authorized emergency vehicle when responding to any emergency call or acting in time of emergency.
(4) Nonamplified noises at street activity (such as fairs or parades) where the participants have a permit, if a permit is required by law for such activity.
(5) Agricultural or forestry activities from bona fide farming and forestry operations when equipment or machinery is operated in accordance with manufacturer's specifications and with all standard noise reducing equipment in place and in good working order.
(6) All noises coming from the normal and legal operation of properly equipped aircraft (not including scale model aircraft).
(7) All noises coming from normal operation of motor vehicles properly equipped with a standard mufflers, in good working order and in constant operation, including noises coming from compression release engine or transmission breaking systems when used in emergency circumstances.
(8) Noise from lawful fireworks and noisemakers on holidays.
(9) Lawn mowers, chainsaws and other landscaping equipment used between 7:00 a.m. and 9:00 p.m. when operated with all the manufacturers' standard noise reducing mufflers in use and in proper operating condition.
(10) Nonamplified crowd noise at regularly scheduled athletic events at county parks, and public or private schools.
(11) Nonamplified speeches made from a fixed location in nonresidential property.
(12) Trains and associated rolling stock.
(13) Activities by government personnel in the performance of their duties, including without limitation the armed forces and law enforcement personnel.
(14) Sound or noise emanating from construction or repair work and regulated activities of utilities regulated by the North Carolina Utilities Commission.
(15) Blasting, but only as permitted by state law and county regulations.
(16) Commercial race tracks, as permitted by state law and county regulations.
(17) Discharge of firearms, but only as permitted by state law and county regulations.
(18) Refuse and recycling collection vehicles when operated between the hours of 6:00 a.m. and 9:00 p.m. according to the manufacturer's specifications and with all standard noise reducing equipment in place and in proper condition.
(Ord. of 6-7-2005, art. V)

 

[Rob45]

DanEE:
"Exceptions" Section 16-55, item no. 7, should refer to engine and transmission "brakes," not "breaks," unless of course it is the noise from the failure of these devices which you wish to except...

 

[DanEE]

Rob45..,
The incorrect wording may well be in the final adopted copy, or it may be that

http://www.municode.com/resources/OnlineLibrary.asp

never picked up the final copy.

This is a private publisher of Codes of Ordinances, Land Use Codes and Charters for local governments. Caveat emptor.

In any event, I have no responsibility for it.

regards

 

[kmmnvh]

Even in the chance that noise regs exist they most likely do not include the C-weighting specification. This has been my experience. If this is the case I would still recommend either taking C-weighting measurements or calculating them. The reason being is that it is posssible that a noise source could generate high levels of low frequency noise while producing less mid and upper frequency noise. Excessive low frequency noise can be sensed as airborne vibrations, vibrating windows in nearby residential homes. So it's possible you can meet applicable A-wieghted noise regs but still disturb neighboring residences. A C-weighting target of 70dBC should be appropriate to avoid possible complaints. I guess it depends who's on the bill Snoop Dog or Cindy Lauper!