The noise of household fans: how do we measure it?

The noise of household fans: how do we measure it?

Noise is an inevitable companion of any device that contains a motor and moving parts. It particularly draws our attention where we seek calm and relaxation – in our homes and apartments. An extractor fan operates at a minimal noise level, which is indicated in the device's user manual and sometimes on the packaging. In our next blog post, we will show how to interpret the sound level of an extractor fan when we conduct our own sound level measurement and how to avoid misunderstandings arising from the results.

Noise is a collection of foreign sounds that adversely affect the human body. Noise levels can be measured with appropriate tools – sound level meters. Such devices, thanks to built-in microphones, allow for the recording of sound pressure vibrations, converting them into electrical signals measured in volts, and displaying the final data in decibels (dB) and acoustic decibels (dBA).

Unfortunately, a sound level meter is quite rare in households. But almost everyone has a smartphone that can also be used as a noise level meter. For this, it is enough to install a special smartphone application, of which there is quite a wide selection. By measuring the noise level of the ventilation system with a smartphone, we obtain the data in decibels. Then, by comparing the obtained data with the information indicated on the device's packaging and in the user manual, we may experience significant discrepancies. But what is the reason for this?

How can you measure the noise level of a fan at home?

1. Different decibels

But what exactly is an acoustic decibel? The sound level meter measures the sound pressure level at a specific point in space. So, does it measure the noise level? Not quite. Loudness and the noise level perceived by humans are subjective and depend on the characteristics of human hearing perception. For them, loudness depends not only on the intensity of the sound but also on the frequency of the sound vibrations, the spectral composition, the timbre, the spatial positioning, the duration of the sound effect, and even the time of day (sounds that are not disturbing during the day are perceived as unfavorable noise at night). For example, the human ear reacts with different sensitivity to sounds of different frequencies: it is more sensitive to mid-frequency (1000-3000 Hz) vibrations, and as the frequency of the vibrations decreases or increases, sensitivity decreases. A particularly sharp decrease in sensitivity is observed in the lowest and highest frequency ranges. Therefore, sounds of the same intensity but different frequencies are perceived as sounds of different loudness. In order for the results of objective noise measurements to come closer to human subjective perception, the sound pressure level is corrected at different frequencies. Such adjustments are internationally standardized and are called A-weighting. Thus, weighted sound pressure levels are measured in dBA, that is, A-weighted decibels.

We mentioned decibels (dB) and A-weighted decibels (dBA) above. Smartphone applications typically provide data in dB, while manufacturers of ventilation systems indicate them in dBA in the user instructions and on the packaging, which is significantly lower than the former. 

Thus, noise indicators expressed in dB and dBA for the same intensity can differ significantly from each other. In order to correctly assess the noise level of a working fan and determine how well it corresponds to the data indicated in the user instructions and on the packaging, the results expressed in decibels obtained with a smartphone or sound level meter must be converted to acoustic decibels with A-weighting.

2.  Different noise measurement conditions

If we have a sound level meter app installed on our smartphone, measuring the noise level of the ventilation system seems straightforward: we open the app, bring the phone closer to the noise source, and the data immediately appears on the screen. Most of us probably see it this way. And this is a big mistake! 

Manufacturers of ventilation systems do not use smartphones to measure noise levels. Existing regulations require them to test their products according to international ISO standards. This is done so that all technical parameters reported by different manufacturers are obtained under the same conditions, which eliminates the influence of subjective factors and allows the consumer to compare the characteristics of different devices of the same type. The conditions for testing the noise levels of equipment are also regulated by international standards.

During such tests, measurements are taken in a certified laboratory, in a special, echo-free space that is soundproofed, excluding background noise and sound waves reflected from various objects. A certified sound level meter is used for the measurement. This device measures the noise level at the inlet and outlet of the fan, as well as at a certain distance from the fan – at the null point (right at the fan), at distances of 1 and 3 meters, and sometimes at distances of 10 meters or more. The final data is corrected (A-weighted) and then incorporated into the data sheets regarding the noise level performance of the equipment.

Why do noise meter apps show different results?

Meanwhile, smartphones and traditional sound level meters, which anyone can use, measure the noise of the ventilation system without considering the nuances described above, so these results cannot be considered objective. Generally, there is always some background noise in every room, as well as sounds that are repeatedly reflected from walls, ceilings, and various objects. As a result, sound level meters respond to the overall sound fluctuation, and we will not get the noise level of the device, but rather the measure of the noise in that location. The distance from the device at the time of measurement is also important, as sound intensity varies at different distances from the object. If we measure the noise level of a fan at a distance of 20 cm and then compare the obtained data with the parameters specified on the data sheet or packaging at standard distances of 1 and 3 m, we will be quite surprised and likely disappointed, because the measured values will certainly exceed the values provided by the manufacturer. Moreover, the difference between the values will be even greater if we do not convert the read values from decibels to A-weighted decibels.

If sound level meters and smartphones with the appropriate smartphone apps do not provide correct results for a specific technical device, is there any point in using them? These devices and applications were roughly created as tools for measuring noise levels at a specific point in space. Nevertheless, they can still be used to measure the noise levels of different products. For example, to compare the total noise level of two or more devices under the same conditions (here and now). In this case, the sound level meter and the smartphone clearly show which device is quieter or louder. This is a very useful feature when selecting and testing any noisy equipment. However, it is not advisable to compare the measurement results of sound level meters with laboratory measurements.

dB or dBA? – The secrets of acoustic decibels

What is what? 

Sound power refers to the amount of energy transmitted by a sound source per unit of time. This is measured in watts (W). Since the power of a sound source can vary greatly, a logarithmic scale of sound power levels is commonly used for simplicity, and the values are recorded in decibels (dB) and acoustic decibels (dBA). The sound power level does not depend on the location of the equipment, environmental conditions, or the distance from the measurement point.

Sound intensity characterizes the amount of energy transmitted by a sound wave per unit area per unit time, perpendicular to the direction of wave propagation. This is measured in watts per square meter (W/m2).

Sound pressure: Sound vibrations are fluctuations in air pressure at different frequencies, which are perceived as sound by human hearing organs. Sound waves create a certain pressure at every point in space, measured in pascals (Pa). The value of sound pressure depends on the acoustic properties of the environment, the sound reflection capability of the room, and the distance from the sound source. Since the range of sound pressure is also very wide (from 2×10-5 Pa hearing threshold to 20 Pa), logarithmic scales of sound pressure levels are used in calculations, and the values are recorded in dB and dBA units.

Loudness is the subjective value of the hearing sensation, which depends on the loudness and its frequency. At a constant frequency, loudness increases with the increase in intensity. At a constant intensity, sounds in the frequency range of 700-6000 Hz have the highest loudness. A zero loudness level corresponds to a sound pressure of 2×10-5 Pa and a sound intensity of 10-12 W/m2 at a frequency of 1 kHz.