3. Frequency (Or Pitch) Usually Measured in Cycles Per Second, Or Hertz (Hz)

CHAPTER ONE

1.0.  INTRODUCTION

1.1.  WHAT IS SOUND?

Sound is a form of energy, just like electricity and light. Sound is made when air molecules vibrate and move in a pattern called waves, or sound waves. Sound occurs when we clap our hands, or when we slam the car door shut. The action produces sound waves, which travel to the ears and then to the brain. Sound is a disturbance, or wave, which moves through a physical medium (such as air, water or metal) from a source to cause the sensation of hearing in animals. The source can be a vibrating solid body such as the string of a guitar or the membrane of a drum, but it can also be a vibrating gaseous medium, such as air in a whistle. The medium may be either a fluid or a solid. Sound is an energy that is transmitted by pressure waves in air or other materials and is converted by the ear to the sensation of hearing.

1.2.  SOUND WAVES

Sound is caused by small areas of high and low pressure propagating outward from the source. Sound waves are longitudinal waves originating from a source and conveyed by a medium. A sound wave is characterized by its velocity, frequency, wavelength and amplitude. The frequency is the number of waves per unit time while the velocity is the product of the wavelength and the frequency.

This simplest kind of pressure wave is called a sine wave. Interesting things to measure for a sine wave:

1.  amplitude (or loudness, size of pressure differences) usually measured in decibels (dB)

2.  wavelength

3.  frequency (or pitch) usually measured in cycles per second, or Hertz (Hz)

FIG. 1

Frequency and amplitude are independent of each other. Two sine waves may have the same frequency and different amplitudes, and vice versa.

FIG. 2

Wavelength is the converse of frequency: the shorter the wavelength, the higher the frequency; the longer the wavelength, the lower the frequency.

1.3.  SOUND AND NOISE

Physically, there is no distinction between sound and noise: sound is a sensory perception evoked by physiological processes in the auditory brain. The complex pattern of sound waves is perceptually classified as “Gestalts” and is labelled as noise, music, speech, etc. Consequently, it is not possible to define noise exclusively on the basis of the physical parameters of sound. Instead, it is common practice to define noise simply as unwanted sound. However, in some situations noise may adversely affect health in the form of acoustical energy.

CHAPTER TWO

2.0.  WHY MEASURE SOUND?

Noise is unwanted sound. However this can be subjective: some sounds are considered noise by some but not by others, e.g. certain music, church bells, sounds of playing children, birds, wind, sea, etc. Noise can also be any un-meaningful or unintended sound.

Measuring noise levels and workers' noise exposures is the most important part of a workplace hearing conservation and noise control program. It helps identify work locations where there are noise problems, employees who may be affected, and where additional noise measurements need to be made.

Because noise can be quite subjective, we need a reliable way of quantifying it. To be able to do this we need a reliable piece of equipment that can measure the noise (Sound).

Sound power levels must also be specified in instructions and sales literature for equipment conforming to the EU Machinery Safety Directive.

Sound power is measured in watts or picowatts, and sound power levels are traditionally given in decibels (dB re 1pW), where 0 dB corresponds to 1 picowatt. But frequently these days sound power level is given in bels (1 bel = 10 decibels) rather than decibels to avoid confusion with sound pressure levels. So a machine with a sound power level of 9.8 B (9.8 bels) is a machine with a sound power level of 98 dB.

CHAPTER THREE

3.0.  INSTRUMENTS USED TO MEASURE SOUND

3.1.  SOUND LEVEL METER

A noise level or sound level is usually a sound pressure level, a measure of the small pressure fluctuations in the air superimposed on the normal atmospheric pressure. Noise levels produced by a machine or a piece of equipment can be easily measured with a sound level meter. The meter shows the sound pressure level at the measurement position. The sound level depends on how far away the meter is from the machine, and on the measuring environment. For example, is the machine outdoors, in a large room or in a small room, and does the room contain soft furnishings or are the walls hard and bare? This sound level is important because it relates to the loudness of the sound and to the potential damaging effect on hearing.

The Sound Level Meter consists of a microphone, electronic circuits and a readout display. The microphone detects the small air pressure variations associated with sound and changes them into electrical signals. These signals are then processed by the electronic circuitry of the instrument. The readout displays the sound level in decibels. The Sound Level Meter takes the sound pressure level at one instant in a particular location.

FIG. 3. Sound Level Meter

To take measurements, the Sound Level Meter is held at arm's length. With most Sound Level Meters it does not matter exactly how the microphone is pointed at the sound source. The Sound Level Meter must be calibrated before and after each use. The manual also gives the calibration procedure.

With most Sound Level Meters, the readings can be taken on either SLOW or FAST response. The response rate is the time period over which the instrument averages the sound level before displaying it on the readout. Workplace noise level measurements should be taken on SLOW response.

Sound level meters used should meet required standard, for example, the American National Standards Institute (ANSI) Standard S1.4-1971 (R1976) or S1.4-1983, "Specifications for Sound Level Meters." These ANSI standards set performance and accuracy tolerances according to three levels of precision: Types 0, 1, and 2. Type 0 is used in laboratories, Type 1 is used for precision measurements in the field, and Type 2 is used for general-purpose measurements.

A Type 2 Sound Level Meter is sufficiently accurate for industrial field evaluations. The more accurate and much more expensive Type 1 Sound Level Meters are primarily used in engineering, laboratory and research work. Any Sound Level Meter that is less accurate than a Type 2 should not be used for workplace noise measurement.

An A-weighting filter is generally built into all SLMs and can be switched ON or OFF. Some Type 2 SLMs provide measurements only in dB (A), meaning that the A-weighting filter is ON permanently.

A standard Sound Level Meter takes only instantaneous noise measurements. This is sufficient in workplaces with continuous sound levels. But in workplaces with impulse, intermittent or variable sound levels, the Sound Level Meter makes it difficult to determine a person's average exposure to noise over a work shift. One solution in such workplaces is a noise dosimeter.

More sophisticated meters can store measurements taken over many days for further analysis. This is quite common for environmental noise measurements. A typical example is road traffic noise, when different road conditions and vehicle densities vary over time.

3.2.  INTEGRATED SOUND LEVEL METER

The integrating sound level meter is similar to the dosimeter. It determines equivalent sound levels over a measurement period. The major difference is that an integrating sound level meter does not provide personal exposures because it is hand-held like the sound level meter, and not worn.

The integrating sound level meter determines equivalent sound levels at a particular location. It yields a single reading of a given noise, even if the actual sound level of the noise changes continually. It uses a pre-programmed exchange rate, with a time constant that is equivalent to the SLOW setting on the sound level meter.

FIG.4 An Integrating-Averaging Sound Level Meter Complying With IEC 61672:

3.3.  NOISE DOSIMETER

A noise dosimeter is a small, light device that clips to a person's belt with a small microphone that fastens to the person's collar, close to an ear. The dosimeter stores the noise level information and carries out an averaging process. It is useful in industry where noise usually varies in duration and intensity, and where the person changes locations.

FIG. 5 NOISE DOSIMETER

A noise dosimeter requires the following settings:

(a) Criterion Level: exposure limit for 8 hours per day five days per week. Criterion level is 90 dB (A) for many jurisdictions, 85 dB (A) for some and 87 dB (A) for Canadian federal jurisdictions.

(b) Exchange rate: 3 dB or 5 dB as specified in the noise regulation.

(c) Threshold: noise level limit below which the dosimeter does not accumulate noise dose data.

Wearing the dosimeter over a complete work shift gives the average noise exposure or noise dose for that person. This is usually expressed as a percentage of the maximum permitted exposure. If a person has received a noise dose of 100% over a work shift, this means that the average noise exposure is at the maximum permitted. For example, with a criterion level of 90 dB (A) and an exchange rate of 3 dB (A), an eight-hour exposure to 90 dB (A) gives a 100% dose. A four-hour exposure to 93 dB (A) is also a 100% dose, whereas an eight-hour exposure to 93 dB (A) is a noise dose of 200%.

Dosimeters also give an equivalent sound or noise level. This is the average exposure level for noise over the time dosimeter was on. It has the same total sound energy as the actual, variable sound levels to which a person is exposed over the same time period. Scientific evidence suggests that hearing loss is affected by the total noise energy exposure. If a person is exposed over an eight-hour work shift to varying noise levels, it is possible to calculate an equivalent sound level which would equal the same total sound energy exposure. This would have the same effect on the person's hearing as the variable exposure actually received.

FIG. 6

In the figure above, the shaded area under the line that shows how the sound level changes over time (the "curve") represents the total sound exposure over eight hours.

3.4.  OCTAVE BAND ANALYZERS

Octave-band analyzers are sound level meters that can be used to:

·  Help determine the adequacy of various types of frequency-dependant noise controls.

·  Select hearing protectors because they can measure the amount of attenuation (how much a sound is weakened) offered by the protectors in the octave bands responsible for most of the sound energy in a given situation.

·  Divide noise into its frequency components

§  Some sound level meters may have an octave or one-third octave band filter attached or integrated into the instrument. Usually a Type 1 (precision) sound level meter is used for octave and one-third octave analysis.

§  The filters are used to analyze the frequency content of noise. They are also valuable for the calibration of audiometers and to determine the adequacy of various types of noise control.

The special signature of any given noise can be obtained by taking sound level meter readings at each of the centre frequency bands. The results may indicate octave-bands that contain the majority of the total sound power being radiated.

3.5 SPECTRUM ANALYZERS

Full Function Stand Alone Audio Spectrum Analyzers Measure Sound Level. The main function of this analyzer is to produce a display of the frequency contents of an input signal. The analyzer can be described as a frequency-selective, peak-responding voltmeter calibrated to display the RMS value of a sine wave. The oscilloscope plots the amplitude in the time domain whereas the spectrum analyzer plots the amplitude in the frequency domain. The analyzer is a wide band, very sensitive receiver. It works on the principle of "super-heterodyne receiver" to convert higher frequencies up to several 10s of GHz to measurable quantities. The received frequency spectrum is slowly swept through a range of pre-selected frequencies, converting the selected frequency to a measurable DC level and displaying it.

A picture of a typical spectrum analyzer is as shown in the next page (FIG. 7).

FIG. 7.

This analyzer is useful in studying interference and in troubleshooting radio equipment. It is a super heterodyne receiver with special filters, attenuators, amplifiers and display.

A typical block diagram of a spectrum analyzer is as shown in the Figure below.

FIG.8.
It has an input attenuator, followed by the input filter. The RF signal is fed into the mixer along with the swept local oscillator signal. The sweep generator also controls the display so that the horizontal sweep of the display is synchronized to the sweep of the local oscillator. The bandwidth filter determines the basic resolution of the spectrum analyzer. After the bandwidth filter, the signal is fed to the logarithmic amplifier. This allows a greater range of signal amplitude to be displayed on the screen of the spectrum analyzer. The signal is then detected, cleaned up by the video filter and applied to the display circuitry.

3.5.1. APPLICATIONS

a) Device Frequency Response Measurements

Measuring the amplitude response (typically measured in DBM) against frequency of device. The device may be anything from a broadband amplifier to a narrow band filter.

b) Microware Tower Monitoring

Measuring the transmitted power and received power of a Microware tower.

c) Interference Measurements

It can be used to verify identify and interferences. Any such interfering signals need to be minimized before going ahead with the site work. Interference can be created by a number of different sources, such as telecom microwave towers, TV stations, or airport guidance systems.

d) Other measurements could be in the areas of Return-loss measurement, satellite antenna alignment, spurious signals measurement, Harmonic measurements and Inter-modulation measurements.