THE IMPORTANCE OF ACOUSTICS

Sound is the vibratory phenomenon that, starting from an initial perturbation, propagates in the form of a periodic variation of pressure by a fluid medium. If to this phenomenon is added the subjective nuance that a sound causes when being picked up by the human ear, generating a feeling of discomfort or pain, it will be called noise.

Today, our society is surrounded by noise, as both at home and at work we often hear noises, coming from ventilation or heating systems, to which we hardly pay attention, as they do not have remarkable characteristics. These noises never stop and have no tone, but if all of a sudden the fan stops or starts buzzing, the change could get our attention or even bother us. Our sense recognizes information in the sounds we hear. The information we don’t need or don’t want becomes noise.

Numerous official bodies, including the WHO, have unanimously stated that noise has very detrimental effects on health. The list of possible consequences of noise pollution is long: hearing problems, communication interference, sleep disturbance, stress, aggression, irritability, fatigue, headaches, etc ...

Noise is a problem of great economic importance in modern society. Decreased school or professional performance, work or traffic accidents, certain antisocial behaviors, the tendency to leave cities, the loss of value of real estate and a long list of others are some of the consequences. The combination of all the factors described above has made many cities inhospitable, severely deteriorating their levels of communication and coexistence patterns.

For economic reasons, the industry is making a considerable effort to develop quiet products and try to achieve controllable conditions in offices and factories in terms of noise. However, it is not always possible to establish explicit relationships between noise and its effects on humans. Some of these effects are of such importance that it leads to companies and industries investing, annually, considerable amounts of money to achieve environments with a certain acoustic comfort.

Noise can be emitted from a point source (it is an ideal, omnidirectional emitter) where any noisy source at a certain distance will behave as a point source; a linear source which are point sources grouped and close to each other that generate a wavefront of cylindrical geometry, as could be considered wheeled traffic and, finally, a flat source which does not increase the area on which it is distributed the acoustic power.

The sound level decreases as the distance from the focus increases, in addition if there are obstacles in the path of sound, part of it is absorbed, part is reflected and part is transmitted. The amount that is absorbed, reflected, and transmitted depends on the acoustic characteristics of the object, its size, and the wavelength of the sound.

The most important factors that affect the propagation of noise are:

• Font type (point or linear)
• Distance from the source
• Atmospheric absorption
• Wind
• Temperature and temperature gradient
• Obstacles, such as barriers and buildings
• Land absorption
• Reflections
• Humidity
• Precipitation

These factors must be taken into account to obtain a representative result either in the measurement or in the calculation.

The main sources of noise in our environment are:

• Road, rail and air traffic
• Industrial and recreational activities
• The neighborhood

Among the different sources of noise, we can classify them into:

• Continuous Noise : Continuous noise is produced by machinery that operates in the same way without interruption, for example, fans, pumps, process equipment, is a constant noise that does not vary over time. To have a first view of the sound level, it is sufficient to measure for a few minutes with a manual equipment and to make a deeper analysis the sound spectrum will have to be measured.
• Intermittent Noise t When machinery operates in cycles, or when isolated vehicles or airplanes pass, or paddle matches, or the noise is not continuous, as there are changes in sound level over time, this is intermittent noise. To measure it, longer-term measurements will have to be taken to take both the noise when the source is running and when it is not, and to determine the running time of the source to finally determine the overall sound level.

Once the type of noise has been determined and the different measures are taken, it must be said that there are sound sources that despite having not very high sound levels, can continue to be annoying, so the regulations go wield that these sounds should be penalized, adding to the result of the measured sound an increase of dB by the annoyance it causes. These penalties that can be applied to the sources are produced by:

• Low Frequency Noise : Low frequency noise has significant acoustic energy, regulations state that low frequency levels between 20 and 160Hz should be analyzed. This type of noise is typical in large diesel engines of trains, ships and power plants, among others. Low-frequency noise is more annoying than might be expected with a measurement of the A-weighted sound pressure level. The difference between the A-weighted and C-weighted sound level may indicate whether or not there is a noise problem. low frequency. To calculate the audibility of low frequency components in noise, the spectrum is measured and compared to the auditory threshold where in the case of low frequency components the sound pressure level initially measured will be penalized. The infrasounds have a spectrum with significant components below 20 Hz. We perceive it not as a sound but rather as a pressure. The evaluation of infrasound is still experimental and is not currently reflected in international standards.
• Tones in Noise : Annoying tones can be generated in two ways: frequently machines with rotating parts such as motors, gearboxes, fans and pumps, create tones. Repeated imbalances or impacts cause vibrations that, transmitted through surfaces in the air, can be heard as tones. Pulsed flows of liquids or gases that occur due to combustion processes or flow restrictions can also generate tones. Tones can be identified subjectively, by listening to them, or objectively by frequency analysis. Audibility is then calculated by comparing the tone level with the level of the surrounding spectral components.
• Impulsive Noise: The noise of impacts or explosions such as blows to the ball playing paddle, a jack, stamping machine or gun, is called impulsive noise. It is short and abrupt, and its startling effect causes more discomfort than expected from a simple measurement of the sound pressure level. To quantify the noise impulse, the difference between a fast-response and a slow-response parameter can be used. The repetition rate of the pulses (number of pulses per second, minute, hour or day) must also be documented.

Therefore to characterize a sound, a sound level meter will be used.

What is a sound level meter? It is an instrument (usually hand-held) designed to measure sound levels in a standard way. It responds to sound in much the same way as the human ear and provides objective and reproducible measurements of sound pressure levels.

Why is it used? Sound level meters are used to measure and manage noise from various sources, including industrial plants, road and rail traffic, and construction works. With the addition of typical urban situations, such as sporting events, outdoor concerts, amusement parks, and residential and commercial neighbors, you have many different sound sources, each with different characteristics that pose specific problems for professionals who evaluate.

How it works? A sound level meter includes a microphone, a preamplifier, signal processing and a screen. The microphone converts the sound signal into an equivalent electrical signal. The most suitable type of microphone for sound level meters is the condenser microphone, which combines precision with stability and reliability. The electrical signal produced by the microphone is at a very low level, so it is made stronger by a preamplifier before it is processed by the main processor. Signaled processing includes the application of frequency and time weights to the signal specified by international standards such as IEC. 61672 - 1, to which the sonometers conform.

Time weighting. The time weight specifies how the sound level meter reacts to changes in sound pressure. It is an exponential average of the fluctuating signal, providing an easy-to-read value. The analyzer applies fast, slow, and driven time weights, which are the required weights according to most international and national standards and guidelines. Environmental assessment standards usually specify which weighting should be used.

Image: Brüel & Kjaer Sound

A-weighting - dBA / dB (A)

The weighting adjusts a signal so that it resembles the response of the human ear to mid-range levels. It is based on the sound curve equal to 40 dB. Noise parameter symbols often include the letter "A" (for example, LAeq) to indicate that the frequency measurement has been included in the measurement.

A weighting is required for almost all environmental and occupational noise measures and is specified in international and national standards and guidelines. Weighted filters cover the entire full audio range, from 10 Hz to 20 kHz.

Today, the A weighting network is the most widely used frequency weighting. The C-weighting does not correlate well with subjective tests, as the contours of the equal sonority were based on experiments using pure tones, and the most common sounds are not pure tones, but very complex signals formed by many different tones.

Frequency analysis

When more detailed information about a complex sound is required, the frequency range can be divided into sections or bands. This is done with electronic or digital filters, which reject all sound with frequencies outside the selected band. Typically, these bands have a bandwidth of one octave or one-third of an octave.

An octave is a frequency band where the highest frequency is twice the lowest frequency. For example, an octave filter with a center frequency of 1 kHz supports frequencies between 707 and 1414 Hz, but rejects the rest. (The name octave comes from the fact that an octave covers eight notes of the diatonic musical scale). A third octave covers an interval in which the highest frequency is 1.26 times the lowest frequency.

Image: Brüel & Kjaer Sound

The process of dividing a complex sound is called frequency analysis and the results are presented in a graph called a spectrogram. After the signal has been weighed and / or divided into frequency bands, the resulting signal is amplified and the Root Mean Square (RMS) value is determined in an RMS detector. RMS is a special type of mathematical mean value. It is important in sound measurements, as the RMS value is directly related to the amount of sound energy being measured.

Verification / Calibration

Verification / calibration is an adjustment of your sound level meter to measure and display correct values. The sensitivity of the transducer, as well as the response of the electronic circuit, may vary slightly over time or may be affected by environmental conditions such as temperature and humidity. While you are unlikely to ever experience a significant drift or change in sensitivity with the sound level meter, it is still good practice to periodically check the calibration of your sound level meter, usually before and after each set of measurements. This is best done by placing a portable acoustic calibrator directly over the microphone. This will provide a defined sound pressure level to which the sound level meter can be adjusted.

In addition to checking calibration before and after measurements, many regulations and standards governing sound level measurements also require the sound level meter to be checked in a laboratory once every 12 months.

International standards

International standards are important either because they are used directly or because they provide inspiration or reference for national standards. There are two main international bodies dedicated to standardization. The International Organization for Standardization (ISO) mainly deals with methodology to ensure that procedures are defined to allow for comparison of results. The International Electrotechnical Commission (IEC) deals with instrumentation to ensure that instruments are compatible and can be exchanged without significant loss of accuracy or data.

From the different measurements that are carried out, a series of parameters can be obtained that allow the calculation of the absorption and / or acoustic insulation to air noise, impacts and / or the levels of immission that are perceived inside the homes, offices, hospitals in order to acoustically characterize the space under study and thus establish the best corrective measures in case they are necessary in order to comply with the acoustics and vibration regulations that affect us.

These tasks are demanding and, given the scope and importance of noise pollution, an appropriate level of understanding on these issues is required, not only of professionals working in this field, but also of citizens and their representatives. For this reason it is important to understand what noise pollution is and how it is possible to improve or solve problems that affect us due to noise with the best specialist professionals to address these issues.