Here we need a bit of introduction to the guide.

What is sound?

Most will describe sound as incredible music, or associate it with something soothing, like the whirl of ocean waves. However, sound can also be unpleasant and disturbing and then be identified as noise. No matter, there is really no difference between a pleasant sound and noise – it is all pressure changes in the air. Described in the terms of physics as sound is when the molecules in the air start to oscillate.

The air molecules nearest a sound source are agitated which cause them to vibrate. The air molecules then bump into the neighbouring molecules, which start to vibrate and bump the next molecules along, etc. The variations in pressure then propagate in a wave form through the air. The wave spreads to the human ear, where the eardrum begins to oscillate with the same wave motion as the molecules. Our brains convert the eardrum’s movement and let us hear sound...or noise.

Indoor environmental quality (IEQ) and sound

Good indoor environmental quality (IEQ) is dependent on a number of parameters to be absolutely finetuned and to be perceived as comfortable, productive and healthy. Sound, or acoustics, is one of these factors. In fact, studies show that there is a link between noise elimination and increased productivity where each 1 dB of noise reduced in academic environments, increase productivity by 0.7%. Sound is also one of the key factors that affects people’s health and well-being. Estimates indicate that millions of quality-of-life years are lost due to a harmful soundscape, as much as 20,000 years are proposed to be lost to tinnitus, 60,000 to heart disease and 900,000 to sleeping disorders. This in the Western world alone.

With this in mind and considering heating, ventilation and cooling (HVAC), it must be obvious that creating a good acoustic environment is important, and that it is vital to make sure noise from fans and air flows, machine vibrations or cross-over talk in duct systems are eliminated.


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Sound in more detail

As said early in this guide acoustics is generated by air molecules starting to oscillate in response to vibrations. When the molecules collide with each other, they concentrate (compression) and spread out (rarefaction). The compressions and rarefactions create wave motions which propagate in all directions from the acoustic source.

Now, the distance between two waves, for instance compression to compression, is a wavelength (λ). Frequency (f), sometimes referred to as pitch, is the number of times per second a sound pressure wave is repeated. Frequency is calculated as the speed of the sound (c) divided by the wavelength (equation 1). To exemplify, (c) in air is 340 m/s and the wavelength may have been measured to 10 m, then the frequency will be 34 Hz. This is possible for a human ear to “hear” as it can detect sounds from 20 to 20 000 Hz.


Bild ljudvågor, frekvens.

The scientific study of sound is called acoustics

As this technical guide will tackle the matter of sound from a nearly scientific angle, it will from here on be mentioned as acoustics which is the physical term for sound. Acoustics is described as the physic that deals with the study of mechanical waves in gasses, liquids, and solids including vibration, sound, ultrasound, and infrasound. Meaning, acoustics is all about how sound is produced, transmitted, controlled, and received, and the effects of this process.

There is no sound in space, but more so in an indoor space

It was made clear that sound is the result of molecules starting to vibrate and bounce into each other. With that in mind and knowing that space have no air particles, it is quite obvious that there is no sound in space.

However, an indoor space here on earth can be a place filled with several kinds of sounds, audible and inaudible, thanks to the air molecules. Inaudible sound is called infrasound when it is of lower pitch, and ultrasound when it is a high pitch sounds. The audible sounds can be divided into direct sound that propagates from source to ear without bouncing against the indoor interior. Reflecting sound is then the sound that bounces off walls, ceilings, floors and interiors before being heard. Vibrating and monotonous sounds are often transmitted by rigid matters such as a building structure.


A reference case focusing on sound

Inaudible sound, also called infrasound

The inaudible low frequency noise, infrasound, can originate from various sources, HVAC included. While the low-frequency noise might be impossible to recognize, the intensity and pervasiveness of the noise can greatly affect people negatively. It disturbs our well-being and feeling of comfort and is therefore normally categorized as unwanted noise.

At Swegon we know a lot about infrasound even though it is a fairly untouched field of research. Learn more in our expert’s blog post or the recommended webinar below.


Our expert's blog: can inaudible noise be harmful?

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Reflecting sound – bouncing off the interior

Sound is more often than not influenced by the interiors of a room or space. In indoor environments are the most obvious reflecting surfaces walls, the ceiling and floor, but also furniture, windows and door openings affect the sound pressure. The placement of the acoustic source can for instance be of great importance as sound sources against walls or in corners increase the noise level in a space.

It is possible to lower the noise level in a room by introducing materials like floor mats, curtains and cushioned furniture. How much, depends on the sound absorption area which is calculated as the surface' area multiplied by the absorption factor of each surface. The latter is the ratio of reverberated sound to incident sound. Measure with xxx and stated in yyy.

The sound of modern times

The human ear has been familiar with the natural and modulated sounds for so many years. The sound of water in a stream, wind in the treetops, birdsong in the morning has been around for ages and it has for long been the only known sounds besides communication between individuals.

The most recent decades have provided the world with numerous artificial environments, filled with all various equipment and machinery producing sounds that are the complete opposite of what the nature offers – monotonous, sometimes more discernible or intrusive.


Learn more about how to eliminate monotonous noise