Explanations

Molecules push against the walls of a box by bouncing off

What exactly is pressure?

Sound is something that all of us have heard about. What we perceive as sound is really variations in the air pressure on the eardrum. These variations propagate deeper into the ear as vibrations and then into the brain as electrical impulses. Pressure is therefore central to acousticians and everyone else working with sound.

In this post, I want to tell you about where air pressure comes from. This also tells us something about how the nature of air pressure limits how good it is possible for ears and microphones to become.

arXiv proceedings, final

Publishing conference proceedings through arXiv

In 2010, 2016, and 2019, I was responsible for publishing the proceedings of the Scandinavian Symposium on Physical Acoustics. SSPA is a small conference organised every year at Geilo, Norway. It typically has around 50 participants. Most come from Norway, but many also come from other European countries. After the symposium, participants who held a presentation are encouraged to submit articles to the symposium’s proceedings. Typically, these proceedings end up with 5–10 articles.

In 2016 and 2019, I chose to publish the proceedings through the repository arXiv.org. This can be a very good approach, because many researchers know and follow arXiv. In addition, academic search engines such as Google Scholar indexes arXiv’s articles. However, there is not that much information available on how to use arXiv to publish proceedings. arXiv does encourage using its repository for conference proceedings and has some relevant help pages available. Unfortunately, these pages pooled together do not give you a clear procedure or tell you all that you should know. Therefore, I will show you the overall procedure that I have followed twice, and discuss some possible pitfalls of publishing proceedings through arXiv.

dB

Acoustic quantities, part 4: Quantities in noise regulations

In this series so far, we have looked at how we use decibels to put numbers on the loudness of sounds (Part 1), how we can compensate for humans hearing some sound frequencies better than others (Part 2), and how we can determine decibel numbers for sounds that vary significantly in time (Part 3). We can consider what we have looked at so far as being building blocks. With these blocks, we can construct the acoustic quantities used in noise regulations throughout the world.

In this fourth and final part, we will look more closely at these more advanced quantities. In Norway, for example, noise regulations use such quantities to define concepts such as red and yellow noise zones. (I will use Norwegian noise regulations as an example throughout this post; these are the ones that I have particular experience with. However, noise regulations in many other countries will be similar.) …

dB

Acoustic quantities, part 3: Time variation

In this series, we first looked at what sound pressure levels and decibels are. Then, we looked at how we can calculate sound pressure levels in a way that takes human hearing into account. So far, though, we have only considered steady and unchanging sounds. These come from e.g. ventilation systems or machines that run steadily. But what about sounds that change in time, e.g. sounds from passing cars or aircraft, or from explosions and other bangs? Fortunately, acoustic quantities and techniques exist that allow us to describe and compare these sounds as well. The most important are Slow- and Fast-weighting, sound exposure level, and equivalent level. These are the topics of this part of the series. …

dB

Acoustic quantities, part 2: Frequency weighting

In the previous part of this series, we looked at what decibels are. To put it simply, we can measure a sound, find a representative sound pressure for it, put this into a logarithmic formula, and voilà – we have a sound pressure level in decibels. However,  humans cannot hear every sound equally well. The basic calculation of sound pressure level does not take this into account. This means that there are sounds we can hear hardly or not at all, that have the same physical sound pressure level as sounds that we hear well. Therefore, a number of techniques, such as A-weighting and C-weighting, have been developed to let us calculate sound pressure levels that fit our hearing better.

In this part, we will discuss how sound consists of different frequencies, how we do not hear these frequencies equally well, and how we can take this into account when calculating sound pressure levels. …

dB

Acoustic quantities, part 1: What are decibels?

Sound is, simply put, weak but rapid fluctuations in air pressure: The air becomes a tiny bit denser, a tiny bit thinner, denser, thinner, and so forth. These fluctuations start at sound sources, for example loudspeakers, and spread out like waves. At the sound wave’s peak, the air is at its densest, while at the wave’s trough, the air is at its thinnest.

When these sound waves hit our ears, our auditory system translates them into something that we can perceive consciously — and thus, we hear that the sound is there. Still, it is difficult to describe, compare, and process these subjective experiences. For example, would you and I agree that this sound is stronger than that sound? And if so, how much stronger is it?

To make sound into something that we can measure, describe, compare, and handle, many different acoustic quantities have been introduced. We use these to make sound into something that we can discuss in a more concrete and objective manner. These quantities affect us all, not least because noise regulations use them to describe how much sound e.g. airports, roads, and concerts are allowed to make. In this article series, we will therefore go through the most important acoustic quantities. In this first part, we begin by discussing what decibels are. This is a fundamental quantity used everywhere where sound is described quantitatively. …