Sound Level Surveys for OSHA / DNL City Noise Ordinance Compliance



Environmental noise is a form of acoustic pollution which has the potential to disrupt human communication, create annoyance, damage hearing and is regulated by the federal OSHA (Occupational Safety and Health Administration) Noise Standard 29 CFR 1910.95 along with other state agencies. In some cases it can also interfere with sensitive equipment such as that used in the nanotechnology and semiconductor industry.

It is also regulated in many residential areas by municipalities such as Fort Worth to provide comfort and peace to residents.

So sound studies are generally divided into OSHA / worker protection dose exposure to hearing scenarios and “nuisance” noise for residential such as DNL (Day / Night Level) studies where the tolerable levels at night are invariably lower than during the day. The City of Grapevine for instance, has a night level limit of 52 dBA.
Based on experience, it is not only the sound level, but how distinct the noise is and what it “represents”. Sounds that are repetitive, impulsive, or have a negative connotation / association with them are less tolerated by residential sectors. For instance, I have done a number of exterior studies of indoor gun ranges and what you find is that the sounds emanating from those facilities do not raise the sound level very much; sometimes less than 1 dBA on average.
However, because it is a distinctive, semi-intermittent noise that can alter the mood of someone trying to relax on their patio / porch, etc. the actual sound level energy is not the predominant factor. There is a term in real estate called “psychologically impacted properties” and that term could apply to the above scenario.
What is unknown is if the noise sources at night will be distinct enough to be distinguished by future residents, OR if it will fall into the “white noise” of the ambient neighborhood? At X distance, can you hear a noise that seems out of place with the environment? Tolerance for noise is driven by context, expectation and choice. People will gladly pay good money to go to a concert or NASCAR event that arguably damages their hearing in both the short and / or long term, but will complain about a squeaky windmill, barking dog or train horn that comes by at 3:30 AM for 5 – 15 seconds.
And this is why ScanTech is often very detailed about not just the levels, but also the nature of sound sources in reports and surveys depending on the scenario.
Sound Survey Noise Level Map OSHA

Noise & Sound Level Survey Map for OSHA Compliance (numbers shown are in dBA)

If you are receiving complaints from workers, or have been contacted by OSHA regarding a potential excessive noise issue, ScanTech can evaluate the environment for continuous (data-logging over time) and impulsive noise on the dB(A) and dB(C) scales to determine compliance. We can also fit workers with a continuous dosimeter to track sound pressure levels as they move into different work areas at different times of day. (meeting ANSI & IEC Type II standards)

8 Hour Dosimetry Extrusion Area with 90 dB OSHA Line

8 Hour Dosimetry of Extrusion Area with 90 dB OSHA Sound Limit Line

Our sound level meters are able to average noise at slow, (1 second per sample) fast (0.125 seconds per sample which corresponds to the response time of the human auditory system) or custom sampling rates as fast as 0.035 seconds which are used for impulsive (sudden, short) noise measurements. Impulse sounds are generally defined as acoustic events lasting less than 1 second with a repeat interval of greater than 1 second.

The data logging function also enables the measurement of reverberation which is the measurement of time it takes for the sound level to drop 60 dB or a factor of 1 million, and characterizes how “reflective” a room is to acoustic vibrations.

Sound Level Decay Graph

Sound Level Decay Graph over Time

Because sound pressure levels are measured on a logarithmic scale, (power ratios of 10) an increase in decibels beyond a certain point means that the energy impacted on the ear can quickly pass a threshold where damage can occur. As shown in the chart below, even an increase of 5 dB can greatly reduce the duration before hearing loss can occur.

Sound Levels dB scale Physical Effects

Sound Levels in dB vs. Physical Effects

Note that sound levels above 70 dB begin to interfere with voice communication, while 75 dB and above are generally characterized as annoying. (but dependent on frequency as shown below)

The general effects can be studied in the response table below:

Hearing Damage from Different Noise Sound Levels dB decibels

Hearing Damage from Different Noise Levels

Fortunately, sound pressure levels also decrease with the square of the distance, so moving twice as far away means that the power level decrease by a factor of 4. Bear in mind that 3 decibels (dB) reflects a change in power level by a factor of 2, with +3 dB being twice as loud and -3 dB being half as loud.

Sound Level Decibels Decrease With Distance

Sound Level Decrease with Distance Example

Also, the human ear does not have a “flat” frequency response – so frequencies at the extreme edges of hearing perception (from 20 Hz to 20,000 Hz) are not perceived to be as loud as those centered around 1 KHz – 3 KHz which is in the range of normal human speech.

Perceived Human Hearing Decibels vs. Frequency

Perceived Human Hearing vs. Frequency

Therefore, different measuring scales on sound meters are used to distinguish between “human” hearing (dBA) and “machine” hearing (dBC) which has a flatter frequency response.

Sound Weighting of dBA dBB and dBC Curves at different frequencies decibels

Sound Weighting of dBA, dBB and dBC Curves at different frequencies

In general, interference with speech communication occurs when intrusive noise exceeds about 60 dB (Federal Interagency Committee On Noise 1992). Indoor speech interference can be expressed as a percentage of sentence intelligibility among two people speaking in relaxed conversation approximately 3 feet apart in a typical living room or bedroom (U.S. Environmental Protection Agency 1972).

The percentage of sentence intelligibility is a non-linear function of the (steady) indoor background A-weighted sound level. Such a curve-fit yields 100 percent sentence intelligibility for background levels below 57 dB and yields less than 10 percent intelligibility for background levels above 73 dB. The function is especially sensitive to changes in sound level between 65 dB and 75 dB.

As an example of the sensitivity, a 1 dB increase in background sound level from 70 dB to 71 dB yields a 14 percent decrease in sentence intelligibility. The sensitivity of speech interference to noise at 65 dB and above is consistent with the criterion of DNL 65 dB generally taken from the Schultz curve. This is consistent with the observation that speech interference is the primary cause of annoyance.

Typically, homes in the United States provide 15 dB of sound attenuation with windows open and 25 dB with windows closed and air conditioning operating. Commercial buildings offer anywhere from 30 – 45 dB of sound attenuation depending on construction and geometry with respect to noise point sources.

Some guidelines on judging sleep interference. The EPA identified an indoor DNL (Day-Night Average Sound Level)  of 45 dB as necessary to protect against sleep interference (U.S. Environmental Protection Agency 1978). Assuming a very conservative structural noise insulation of 20 dB for typical dwelling units, this corresponds to an outdoor day-night average sound level of 65 dB to minimize sleep interference.

ScanTech Technical Consulting also has experience in doing “on-paper” studies for environmental impact and Environmental Survey Assessments (ESA) including the DNL influence of major roadways, railways, airports, etc. including the CEQR noise level standards of New York City. There are DNL ordinances in cities like Fort Worth and Grapevine (Section 55 Part A 3) which is shown. Dallas does not give numbers, but situational guidelines for noise in the following link:

Dallas Noise & Sounds Codes

Grapevine Section 55 Part A 3 DNL Ordinance Example:

NOISE REGULATION: This ordinance shall apply to all sound originating within the limits of the City of Grapevine, Texas. It applies 24 hours of everyday of the week and year around. This ordinance does not apply to any moving vehicle or aircraft nor does it apply to sound emitted from any emergency warning device. This ordinance does not apply to the following activities as long as they are conducted in daytime hours as a normal function of a permitted, conditional or special use and the equipment is maintained in proper working condition:

· Lawn and yard maintenance

a. General Provisions

1. A person may not conduct a use that creates a sound which exceeds the sound levels established in Table 1 or that exceeds the background sound levels by 5 dBA., whichever is greater.

2. A sound level meter that meets the standards of ANSI with Type 2 or greater precision must be used to determine whether the level of sound violates those established in this section. The instrument must be maintained in good working order. A calibration check should be made prior to and following any noise investigation.

3. The sound levels must be measured at the bounding line between properties at a level five feet above ground level.

4. A sound level meter shall be used to determine the Leq in dBA over an 8 minute time period.

5. The sound receiving property with the most stringent requirements will apply.

6. Traffic, aircraft, and other background sounds are not to be considered in measuring sound levels except when the background sound level is being determined.

7. All outdoor speakers in non-residentially zoned properties shall face away from contiguous residential zoning districts.

DNL (Day Night Level) Noise / Sound Level Ordinance

Sound & Noise Level Survey Reference Table


Corrections for Character of Sound Applied to Table 1. values are as follows:

· If Sound is Impulsive in Character – (hammering, popping, exploding, etc.) Subtract 5 dB from Maximum Permissible Sound Levels

Note that for the purpose of this ordinance, an impulsive sound shall exist when the sound changes at rate greater than 10 dB per second. The sound level meter should use Fast Response for this evaluation.

· If Sound is Periodic in Character – (hum, buzz, screech, etc.) Subtract 5 dB from Maximum Permissible Sound Levels

Note that for the purpose of this ordinance, a pure tone shall exist if the one third octave band sound pressure level with the tone exceeds the arithmetic average of the sound pressure levels of the two contiguous one-third octave bands by 5 dB for center frequencies of 500 Hz and above and by 8 dB for center frequencies between 160 and 400 Hz and by 15 dB for center frequencies less than or equal to 125 Hz. A one-third octave band spectrum analyzer instrument will be required to make these evaluations.



Sound & Noise Level Survey Aerial Readings Map with Descriptive Annotations

Sound & Noise Level Survey on an Aerial Readings Map with Descriptive Annotations


Speed of Sound Variations

                                                    Speed of Sound Variations by Altitude / Temperature



Sound & Noise Decibel Level Comparison Chart

OSHA Extended Shifts Decibel Limits

Hearing Test Website

TWA Sound Dose Calculator for OSHA NIOSCH ACGIH

REW Software – Room EQ 1/3 Octave

Sound & Noise Distance Inverse Square Law Converter


OSHA Compliance Tools (including lookup of violations)

2 Responses

  1. A. J. Steinberg says:

    I know you no longer offer residential services, but can you point me in the direction of a local (Dallas) company that measures sound decibels for a noise complaint…any help will be greatly appreciated…thanks!

  2. EMF Testing Indoor Air Quality Consultant says:

    Mr. Steinberg,

    There are very few companies in this region who even offer noise / decibel / sound level measurement and consultation for commercial and industrial clients, so I have no direct referrals to offer.

    You might try talking either to some civil engineering firms and / or whomever does noise level enforcement for your municipality.

    JAG of ScanTech

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