Noise Dosimeter FAQ's

Will the infrared communications port of the db-4000EZ dosimeter interfere with other equipment or be affected by other equipment?

Unless communications are initiated in the db-4000EZ's COMM mode, the infrared port is inactive. The COMM mode may not be activated while the instrument is in the RUN mode collecting data. The infrared communications between the Metrosonics db-4000EZ dosimeter and ms-4000EZ Metrosoft Software has an error correcting format pack, which only allows communication by and between the db-4000EZ dosimeter and Metrosoft. In addition, the db-4000EZ dosimeter and Metrosoft mst be set to a specific request mode in order for this communication to occur.

What is Exchange Rate or (Doubling Rate)?

Exchange rate refers to how the sound energy is averaged over time. Using the decibel scales, every time the sound energy doubles, the measured level increases by 3dB. This is the 3dB exchange rate that most of the world uses. For every increase of 3dB in the time-weighted average the measured DOSE would double. For OSHA and MSHA measurements, the Exchange Rate is 5. Some organizations such as OSHA in the U.S. have argued that the human ear self compensates for changing noise levels and they felt that the 3dB exchange rate should be changed to more closely match the response of the human ear. OSHA currently uses a 5dB exchange rate, which would mean that the reported DOSE would double with every 5dB increase in the time-weighted average. The exchange rate affects the integrated reading LAVG, DOSE, and TWA but does not affect the instantaneous sound level.

What is LAVG?

LAVG is simply the average sound level measured over the run time. This becomes a bit confusing when thresholds are used. Any sound below the threshold is not included in this average. Remember that sound is measured in logarithmic scale of decibels therefore the average cannot be computed by simply adding the levels and dividing by the number of samples. When averaging decibels, short durations of high levels can significantly contribute to the average level.

Example: Assume the threshold is set to 80dB and the exchange rate is 5dB (the settings of OSHA’s Hearing Conservation Amendment). Consider taking a one-hour noise measurement in an office where the A-weighted sound level was typically between 50dB and 70dB. If the sound level never exceeded the 80dB threshold during the one hour period, then the LAVG would not indicate any reading at all. If 80dB was exceeded for only a few seconds due to a telephone ringing near the instrument, then only those seconds will contribute to the LAVG resulting in a level perhaps around 40dB (notably lower than the actual levels in the environment).

What is Criterion Level?

The criterion level is used in the Dose calculation; If the dosimeter is exposed to a decibel level equal to the criterion level for 8 hours the result will be 100% dose. The criterion level is typically set by a regulating agency such as OSHA and usually not applicable for community noise monitoring.

Example: OSHA mandates the criterion level (maximum allowable accumulated noise exposure) to be 90dB for 8 hours.  For an 8 hour sample, an average level (LAVG) of 90dB will result in 100% dose.

For the OSHA HEARING CONSERVATION AMENDMENT, the \Action Level\ is 85dB for 8 hours. This would result in a 50% dose reading. Note that the criterion level has not changed. [If the criterion level is changed to 85dB then an 8 hour average of 85dB would result in 100% dose.]

What is Dose?

Related to the criterion level, a dose reading of 100% is the maximum allowable exposure to accumulated noise. For OSHA, 100% dose occurs for an average sound level of 90dB over an 8-hour period (or any equivalent exposure). By using a TWA reading rather than the average sound level, the time period is no longer explicitly needed. A TWA of 90dB is the equivalent of 100% dose. The dose will double (halve) every time the TWA increases (decreases) by the exchange rate.

Example: OSHA uses an exchange rate of 5dB. Suppose the TWA is 100dB. The dose would double for each 5dB increase over the criterion level of 90dB. The resulting dose is therefore 400%. If the TWA was instead equal to 80dB then the dose would halve for each 5dB below the criterion level. The resulting does would be 25%.

When Taking noise samples less then the full workday, dose is an easy number to work with because it is linear with respect to time.

Example: If a 0.5 hour sample results in 9% dose and the workday is 7.5 hours long, then the dose for the full workday would be computed as (7.5 hours / 0.5 hours) *9%=135% dose. This is computed making the assumption that the sampled noise will continue at the same levels for the full 7.5 hour workday.

What is LEQ?

The true equivalent sound level measured over the run time.  The term LEQ is functionally the same as LAVG except that it is only used when the exchange rate is set to 3dB and the threshold is set to none.

What is MAX LEVEL?

The MAX LEVEL is the highest sampled sound level during the instrument’s run time allowing for the RESPONSE (fast or slow) and WEIGHTING (A or C). It is the highest value that is included in Average or Dose calculations.

What is PEAK LEVEL?

PEAK LEVEL is the absolute highest pressure wave that is detected by the microphone. Unlike the MAX LEVEL, PEAK is detected independent of dosimeter settings for RESPONSE or WEIGHTING. The PEAK circuitry responds in 50 micro seconds usually with Linear Weighting (depends on dosimeter model). Test this by simply blowing across the microphone. You will notice that the peak reading may be 120dB or greater. When taking a long term noise sample (such as a typical 8 hour workday sample for OSHA compliance), the peak level is often very high. Because brushing the microphone over a shirt collar or accidentally bumping it can cause such a high reading, the user must be careful of placing too much emphasis on the reading

What is RESPONSE (FAST, SLOW)?

The response determines how quickly the unit responds to fluctuating noise. Fast is a time constant of 125 milliseconds.  Slow is a time constant of 1 second.

Example: Typically, noise is not constant. If you were to try to read the sound level without a response time, the readings would fluctuate so much that determining the actual level would be extremely difficult. Using a response of slow or fast simply smoothes the noise fluctuation and makes the sound level easier to work with.  While the terms slow and fast have very specific meanings (time constraints), they work very much as you would expect. The fast response would result in a more fluctuating sound level reading then would the slow response. The OSHA regulations require slow response.

What is SEL?

The sound exposure level averages the sampled sound over a second period. Assuming the sampled run time to be greater than one second, SEL is the equivalent one second noise that would be equal in energy to the noise that was sampled. SEL is typically measured using a 3dB exchange rate without a threshold. (SEL is not used by OSHA).

Example: Suppose you wanted to measure in a location next to railroad tracks which also happend to be in the takeoff path of an airport. A train passes by taking 10 minutes with an average sound level of 82dB. A jet passes overhead taking 45 seconds with an average sound level of 96dB. Which of these events results in more sound energy? You can answer the question by comparing their SEL readings, which compress each event into an equivalent on second occurrence. (SEL for the train = 109.7dB, SEL for the jet=112.5dB).

What is THRESHOLD (CUT OFF)?

The threshold affects the LAVG, TWA, and DOSE measurements. All sound below the Threshold is considered non-existing noise for the averaging and integrating functions. The threshold does not affect measurements in the sound level mode.

OSHA uses two different thresholds. The original Occupational Noise Exposure Standard (1971) uses a 90dB threshold and called for engineering controls to reduce the noise levels if the eight-hour TWA was greater then 90dB. The Hearing Conservation Amendment (1983) uses an 80dB threshold and calls for a hearing conservation program to be put in place if the eight-hour TWA exceeds 85dB (50% dose). The Hearing Conservation Amendment is the more stringent of the two rulings and is what most US industrial users are concerned with.

Example: With a 80dB threshold, suppose you placed a 79dB calibrator on the unit for a period of time. Because all of the noise is below the threshold, there would be no average (you can think of it as an average of 0dB). If the calibrator were 80dB instead, then the average would be 80dB. On histogram printouts, typically 1-minute (or other specified increment) averages are printed. Because real noise fluctuates, it is quite possible to have an average level below the threshold. This also applies for the overall LAVG.

What is TWA?

The time-weighted average always averages the sampled sound over an 8-hour period. TWA starts at zero and grows. The TWA is less than the LAVG if the run time is less than eight hours, and grows higher than LAVG after eight hours. TWA represents a constant sound level lasting eight hours that would result in the equivalent sound energy as the noise that was sampled.

Example: Think of TWA as having a large 8-hour container that stores sound energy. If you run a dosimeter for 2-hours, your LAVG is the average level for those 2-hours, consider this a smaller 2 hour container filled with sound energy. For TWA, take the smaller 2-hour container and pour that energy into a larger 8-hour TWA container. The TWA level will be lower. Again, TWA is ALWAYS based on the 8-hour container.

When measuring using OSHA’s guidelines, TWA is the proper number to report provided that the full work shift was measured.

Example: If the work shift is 6.5 hours long, then measure for the entire 6.5 hours. TWA is the correct level to report to OSHA. It does not have to be modified.

What is UPPER LIMIT?

This is a feature available on many of the Quest dosimeters. The user can select a certain dB level. The dosimeter will then record the amount of time that the sound level was at or greater than the preset level. This time is then recorded as UL TIME.

What is WEIGHTING?

Weighting is the filtering of sound before averaging. A, B, C and LINEAR are the standard weighting networks available. These are frequency filters that cover the frequency range of human hearing (20 Hz to 20 KHz).

A weighting is the most commonly used filter in both industrial noise applications (OSHA) and community noise regulations. A weighted measurements are often reported as dBA. The A weighted filter attempts to make the dosimeter respond the way the human ear hears. It attenuates the frequency below several hundred hertz as well as the high frequencies about six thousand hertz.

B weighting is similar to A but with less attenuation. The B weighting is very seldom, if ever used.

C weighting provides a fairly flat frequency response with only slight attenuation of the very high and low frequencies. C weighting is intended to represent how the ear perceives sound at high decibel levels and is often used as a flat response when LINEAR is not available. C weighted measurements are often reported as dBC.

LINEAR is thought of as having a flat frequency response curve over the entire measurement frequency range. LINEAR is most commonly found on upper model sound level meters and is typically used when performing octave band filter analysis. LINEAR weighting is being replace by Z which is defined a flat from 20 Hz to 20 KHz.

What is the difference between Lavg and TWA?

Lavg is the average sound level over the run time of your sample. If you ran the meter for 30 minutes, Lavg is the average sound level over that 30-minute period. TWA always assumes an 8-hour run time. So if you ran the meter for 30 minutes, TWA would assume 7 hours 30 minutes of silence and give the 8 hour average.

I am unable to establish communication between my instrument and the computer. What should I do?

Re-Check the following and then try communication again:

  • Verify that the instrument is physically turned on

  • Verify that the RS-232 cable is plugged into the correct communication port

  • Verify that the RS-232 cable is plugged into the communications port and not the printer port

  • Verify that the communication port you are using is not already assigned to another device

  • If using Metrosoft, check the "Configure Metrosoft Screen" and verify that it is properly configured for your computer

When monitoring noise, if I have my criterion set at 90 dB for eight hours, and my shift will go longer than eight hours, do I need to change my criterion level?

No, the criterion level determines the maximum exposure level for a person. For US Federal OSHA standards, this level is eight hours.

Why does my noise dosimeter show multiple interruptions when it should have been continuously logging?

Test interruptions can occur due to the following:

  • The microphone was disconnected

  • The microphone cable is damaged and non-functional

  • Logging was manually stopped and then resumed

When calibrating my noise dosimeter, why does it read 40 to 140dB and then go back to 102 dB?

The instrument is displaying its dynamic range (40 to 140 dB) followed by the calibration point (102 dB).
 

How long do the instruments retain their memory after a low battery shutdown?

30 days.

Should the batteries be removed if the instrument will not be used for a long period of time?

Yes, remove the batteries if the unit will not be used. When installing fresh batteries remember to reset the real time clock and programming information.

Is it okay to swap microphones between dosimeters?

Yes, provided certain actions are taken each time this is done. Microphones are manufactured and tested to performance standards that are applicable to the microphone alone. Likewise the dosimeter electronics are manufactured and tested to performance standards that are applicable to the dosimeter electronics alone. The performance standards of each of these two components of the noise dosimeter are specified tightly enough that any microphone and dosimeter combination that passes their respective independent tests is assured to meet the overall performance standards applicable to the noise dosimeter inclusive of the microphone.

However, variances in sensitivity from one microphone to another must be addressed each time a microphone is swapped from one dosimeter to another. It is considered best practice to perform a field calibration using an acoustical calibrator before each use of a noise dosimeter regardless of whether or not the microphone is swapped. The variances in sensitivity from one microphone to another are automatically resolved when you perform the field calibration.

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