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MEASURING SOUND

dB, Hz and A weighting is all German to Me :)

The first question might be .. why would I WANT to measure sound? Well, the best answer I can give you is that .... you don't really have to! There are NO requirement, but..

In the industrial-commercial world we are becoming more and more sensitive to the environmental conditions that we expose our employees to. Governments have now introduced "noise" standards, and progressive companies know that creating a great working environment is not only good management but good for their bottom line. Happy and healthy employees are more productive.

So.. measuring sound gives us a yard stick to see if changes that we are making are helping. Which changes are the most cost effective? What is the year over year progress? Am I going in the right direction? .

Have I got you convinced? ... measuring is the yardstick of progress... it is the only true way to know if life is getting better in the back workshop!

For the casual woodworker, knowing what the noise level is in your workshop is helpful in selecting hearing protection: Earmuffs for instance are measured in the amount of "noise reduction" they can accommodate... therefore you need:

calculating the current noise level (what this section is all about)

- noise reduction rating of hearing protection

= acceptable noise level for your application
(depends on legal jurisdiction, length of time of exposure, your tolerance etc)

So first, what is sound?

To measure sound it is important to first understand that it is made up of three components:

1. a range of frequencies from low to high, (think of your favourite opera singer and your husband in the shower... )
2. relative intensities (what we think of as "loudness", or the position of my radio volume control as compared to my teenage son's)
3. rate of onset and decay of the combination of sounds of different frequencies (fluctuation of sound )

1. Frequency ( Hertz Unit of Measurement ):

The number of pressure variations per second is called the frequency of sound and is measured in Hertz (Hz).

Think about the speed of vibration of the skin on a typical drum. High notes cause the skin to vibrate very quickly (tweeter stereo speakers) and low notes make the drum skin vibrate very slowly (large base stereo speakers) . The speed of movement is measured in Hertz.

 

1 & 2 The Decibel:

The technical definition of a decibel is a little beyond my limited engineering education, but interpret it as a measure of sound loudness or loudness level. Loudness is a function of both frequency (quantity, as discussed above) and intensity (power).

.. so if you think about it in the context of wave action intensity would refer to how big the wave was (how loud), and frequency would refer to how many waves hit you in one unit of time.. so if you're a surfer you may want big waves and lots of them over a short period of time to create the ultimate surfing challenge.. that would equate to a large decibel rating It quantifies two sound characteristics with one number ... does this make sense??? I hope so.. but I'm not a surfer, only a woodworker :)

Human Speech:

Normal human speech communications rely on frequencies between 100 and 5000 Hz although the spectrum of good human hearing allows comprehension between 20 and 20,000 Hz .

This represents a very wide mixture of frequencies. The ear is most sensitive to frequencies between 1000 and 4000 Hz with sensitivity falling off at the low and very high end of this scale.

Audiometers are used to measure a humans ability to hear. They have developed an "average hearing threshold" consistent with the hearing ability of a young adult with disease-free ears. An individual with a positive threshold shift is one that is experiencing hearing loss, those with a negative threshold shift have above average hearing ability.

Sound pressure levels or the "frequency" of sound represent only part of the picture. The human perception of loudness of a sound depends on this frequency. Higher frequency sounds are heard much better than low frequency ones of equal intensity.

Further, an individual's loudness interpretation and experience are subjective, depending on the harshness or intrusiveness of the noise (i.e. bagpipes compared with a flute) which are factors not reflected by sound level measuring instrument.

 

So what does the A-weighting mean?

As suggested earlier, the human ear does not hear different frequencies of sound equally. Sound at 1 kHz (a high frequency) sounds louder than the same sound at say 500 Hz or 4 kHz (a lower frequency).

Sound levels measured by a traditional Noise Meter normally have a flat frequency response. i.e. they assign the same level of intensity across all frequency ranges.

Thus, industry has attempted to find a way to measure sound mechanically, in such a way that it represent more accurately, the sound measurements as the human ear would perceive them.

A traditional noise meter reading uses an "A-weighting filter",to modifies this recording to reflect variations in frequency. It is one mathematical model used to adjusting mechanical sound measurements to more truly reflect what the human ear actually perceives. The resulting sound is said to be A-weighted.

It is an attempt to more truly quantify how the human ear hears and provide more accurate industrial "at Work noise exposure measurements." It is important that the A-weighting filter in the Sound Level Meter (Noise Meter) is implemented correctly.

Aside from basic "hearing loss" , speech interference is considered to be the next largest occupational safety issue associated with noise. In fact it has its own acronym; Speech Interference Level ( SIL ) . A-weighted sound level measurement, is convenient and perceived to be a fairly accurate index of speech interference and thus used extensively in occupational applications.

And what does C-weighting mean?

Although The A-weighting filter closely follows the human ear at normal levels of sound, it is not accurate at high levels of sound. The ear behaves differently at different levels. The C-Weighting filter is more accurate at these higher levels.

The response of this filter is shown below compared with an A-Weighting filter and the rarely used B-Weighting. "A", "B", and "C" Weighting curves

The C-Weighting filter is not used for Noise Exposure assessments. However, it is used in the selection of hearing protectors.

What are the advantages of an integrating noise meter?

Noise Meters usually have Fast and Slow response times for measurements. The response determines how quickly the unit responds to fluctuating noise. The Fast measurement has a time constant of 125 milliseconds whilst the slow is around 1 second. The Slow response is used for Noise-at-Work exposure measurements. If the noise changes many times over a longer period, say 10 minutes, then even on the slow measurement setting, the reading would vary a lot and it would be difficult to take a interpret.

The Integrated Noise Meter overcomes this problem by keeping and displaying a running average of the Sound Pressure Level. Therefore, an Integrating Noise Meter is invaluable in measuring levels where the sound varies considerably with time.

 

Is the peak measurement important?

Just as the Integrated Noise Meter is useful for slowly varying sound levels, one needs to be able to measure noise that is very fast, for example from a riveting machine.

The fast setting on Noise Meters cannot react fast enough to these sounds and will give a wrong reading.

The Peak measurement has to be made with a time constant of 50 microseconds. It is important to use a Noise Meter with this facility when trying to assess these types of sounds. Other Noise meters will read low and give you a false sense of safety.

Selecting the right meter for the information and application of interest is then quite important.

What are the precautions for obtaining accurate readings?

The following precautions will lead to accurate measurements.

Keep the Noise Meter away from your body, to avoid reflections.

• Reflections can cause false peaks. So avoid taking measurements near walls, or large objects.
• Air blowing across the front face of the microphone can cause measurement errors. If there is any risk of this, make sure you use a wind shield.

Caluculating the right hearing protection.. 

This is a little long winded, but lets remember what our original objective was. We were trying to figure out what ear protection would be appropriate to a particular work environment so lets now do an example:

Example:

1. The environmental noise as measured at the ear is 92 dBA. via the appropriate meter as discussed above

2. The NNR (noise reduction rating) is 25 decibels (dB) of the hearing protection (this number is typically written on the ear muff box, or ask for the NRR for the particular type of hearing protection you are thinking of buying.

3. Therefore, the level of noise entering the ear is approximately equal to 92 - 25 = 67 dBA. .. below the industrial requirement for most jurisdictions.

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