Sound Measurements Overview
Conducting accurate sound measurements requires a deep understanding of the sound definition and factors affecting its properties such as speed or propagation. In addition, basic knowledge of sound measurement quantities such as SPL or LEQ helps to learn more about measurement techniques and their applications.
Sound Meaurement Definition
Sound Measurement definition in physics refers to the quantification of the properties of sound waves, including their frequency in Hertz, and amplitude level in Decibels. Measurements are crucial for understanding and controlling sound in various applications, from acoustics and audio engineering to medical diagnostics and environmental science.
What is the role of sound measurements?
Sound Measurements play a crucial role in understanding and controlling sound in a variety of applications ranging from acoustics and audio engineering to environmental science and medical diagnostics. By quantifying sound properties and considering factors that influence accuracy, sound measurements allow for effective noise control, assessment of environmental noise, protection of human hearing, and advancement in scientific research and engineering applications.
What factors can influence the accuracy of sound measurements?
Factors that can influence the accuracy of sound measurements include the medium through which sound travels (air, water, solids), wind, humidity, temperature, air pressure, and vibrations. For example, wind can cause significant measurement errors, especially in outdoor sound level measurements. Similarly, changes in humidity and temperature can affect how sound propagates, potentially altering measurements. Vibrations can introduce extraneous noise, particularly when measuring low-level sounds or when high precision is required. Modern sound level meters can detect and account for these factors to improve the accuracy of measurements.
Wind can cause significant measurement errors, especially in outdoor sound level measurements. Wind noise is induced by the turbulence it creates as it flows over the microphone, which can lead to the overestimation of the actual noise level. To mitigate this issue, windshields are typically used over the microphone during outdoor measurements.
Humidity can have an effect on the propagation of sound. In general, sound travels further in humid air than in dry air. This is because humid air is less dense than dry air and thus provides less resistance to sound waves. The effect of humidity particularly affects the microphone’s membrane which is why professional noise monitors use internal heating systems to evaporate any moisture from the microphone.
The speed of sound varies with temperature. In the air, sound travels faster in warmer temperatures than in cooler ones. Temperature gradients can cause sound to refract, or bend, which can result in the perceived sound level being different from the actual sound level at a given location. Usually, noise monitors can operate in conditions from -10 °C to + 50 °C. To extend the temperature range to -30°C and + 60 °C noise monitors use heating and cooling systems.
At higher pressures, the air molecules are closer together, which can increase the speed of sound. Higher pressure levels can increase the intensity of a sound wave, making the sound louder to the listener. This is because the greater the air pressure, the more air molecules there are to vibrate and transmit the sound wave. Changes in air pressure can cause sound waves to refract, or change direction. This is due to the variations in air density that come with changes in air pressure. For instance, the sound will bend towards areas of lower air pressure.
Vibrations can interfere with sound measurement by introducing extraneous noise. This is particularly a problem when measuring low-level sounds or when high precision is required. Modern sound level meters detect vibration interfering with results with built-in accelerometers. It helps to exclude results that were affected by vibrations.
How to measure the speed of sound?
One common method of sound speed measurement in the laboratory is the resonance method. By using the principle of resonance and observing the standing wave patterns, it is possible to determine the wavelength of the sound in the tube. Since the frequency is known (set by the function generator), it is possible to calculate the speed of sound (speed = frequency * wavelength). Thus, the Resonance Tube provides a practical and hands-on approach to investigating the propagation of sound waves and measuring the speed of sound.
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Sound Meaurements Quantities
Sound Measurements Quantities rate sounds in a way that aligns with how the human ear perceives sound, which can be especially useful in contexts like noise control, sound design, and health and safety assessments. In the field of acoustics, the IEC 61672-1 standard defines sound measurement indicators to quantify and rate sounds in decibels:
- Time-averaged sound level or equivalent continuous sound level (LAeq) is the logarithm to the base 10 of the ratio of the frequency-weighted sound pressure over a time interval to the squared reference value of 20 micro Pascals. The LEQ is the most commonly used sound measurement quantity in acoustics because it is directly to sound energy.
- Peak sound level (Lpeak) is the logarithms to the base 10 of the ratio of the squared greatest sound pressure (positive or negative) during a time interval to the reference value of 20 micro Pascal
- Sound pressure level (Lp), is the logarithm to the base 10 of the ratio of the time-mean-square of sound pressure to the square of the reference value 20 micro Pascals. The SPL measures decibels with an A-weighting scale (dBA) to measure the human ear’s response to different sound pressure levels.
- Time-weighted sound level (LAF or LAS) is the logarithm to the base 10 of the ratio of the running time average of the time-weighted square of a frequency-weighted sound-pressure signal to the square of the reference value 20 micro Pascals. For time-weighted sound levels measurements symbols are LAF, LAS, LCF, and weightings A or C and time weightings Fast (F) and Slow (S). Time-weighted sound level is usually used for noise surveys and acoustic background measurements.
- Sound exposure level (LAE) is the logarithm to the base 10 of the ratio of A-weighted sound exposure over time intervals (EA, T) to the reference value of sound exposure. The LAE is equal to the sum of LAeq over a time interval and 10 logarithms of the ratio of the time interval length to 1s reference time. For this reason, LAE is often defined as LAeq normalized to 1s. LAE is used in environmental noise measurements (vehicle passages: cars, trains, aircraft)
- LEX daily noise exposure level – in the case of occupational noise, the LEQ is measured in 8 hours of a working day and such result is referred to as the daily noise exposure level (LEX). The daily noise exposure levels measured can be also presented as the % of the daily limit, and such representation of the result is called a noise dose.
LEQ Sound Energy
In practice, sound measurements use sound energy quantities such as Equivalent Continuous Sound Level (LEQ) as the main indicator of decibels. The reason is the direct relation of LEQ sound energy to human hearing risk damage. The LEQ is average, taken over time providing a single decibel value that represents the same amount of sound energy as the varying levels of noise experienced during that period. This is why it is often used in environments where people are exposed to varying levels of noise over time, such as workplaces or in the study of environmental noise pollution.
Peak Sound Pressure Level
Peak sound measurements are used in various fields, including acoustics, audio engineering, and occupational health. In occupational settings with a risk of hearing damage due to high noise levels, sound level measurements often use Peak C (maximum value of the ‘C’-frequency weighted instantaneous noise pressure).Peak sound pressure may occur in a very short period of time (ie. a couple of seconds) and may not be reflected by Leq, which is an average of longer time intervals. This is why the measurement of Peak is particularly important for monitoring and managing noise exposure in workplaces to safeguard workers’ hearing health.
Sound Measurements use a decibels scale (dB). Decibel is a logarithmic unit that reflects the ratio of a sound’s pressure level relative to a reference value. The decibel reference value is set at the quietest sound the average human ear can hear: 20 µPa (micro pascals). This is designated as 0 dB, marking the threshold of human hearing. Depending on the technique and application noise measurements are performed on a scale up to 130 dB (63.2456 Pa) – for example in the environment. Measurements of peak sound levels up to 140 dB (200 Pa), are performed in noisy workplaces, as this level can lead to immediate damage and potentially permanent hearing loss.