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Why your Noise Consultant ignores you when you say "my lawnmower is 80 dB"

  • Writer: Daniel Goodhand
    Daniel Goodhand
  • Sep 17, 2022
  • 7 min read

Updated: Sep 23, 2022

TLDR

There are different kinds of sound levels which isn't obvious from just quoting a decibel value. if you are talking about sound pressure level you must also give a distance. It's also important how the sound pressure level was measured. 80 dB by itself doesn't mean a lot.


What is sound?

Most text books on acoustics invariably begin with a chapter called "What is sound?". Well, maybe not those exact words - sometimes they use more fancy works like: "Fundamentals of Vibration" which starts off Kinsler's "Fundamentals of Acoustics" or "The nature and behaviour of sound" which Bob Peters begins his textbook "Acoustics and Noise Control". But to save you the reading, it basically boils downs to

  1. things vibrate

  2. those vibration travel through a medium (air) and

  3. our inner ears convert those vibrations into small electrical signals that our brains 'hear'

*The above animation is wrong by the way. Probably deliberately so because it's easier to visualise waves going up and down, but really they go back and forth.


This is what some text books have to say about it (use the left and right arrow keys to see more text):

  • 1.1 What is sound? What is the physical nature of sound? At first we can state that the generation, propagation and perception of sound is connected with mechanical vibrations or oscillations. In some cases we can convince ourselves immediately of this fact, for instance, by touching our larynx when speaking or singing. Likewise, the vibrations of noise producing machines can often be felt with the hand, if the vibration stops no sound is heard. The vibration of the strings Of a musical instrument can be seen with the naked eye, and in ancient times it was observed that the perceived Pitch Of a tone is related to the length of the string and hence to the number of oscillations per second or as we say nowadays: on the frequency of the vibration. However, in most cases these vibrations are so weak that it is impossible to see or feel them immediately. This is true, for instance, when sound penetrates a Wall; ln this case the vibrations can only be observed by means of special measuring devices. Many sounds have a ‘tonal' quality, that is, a certain pitch can be ascribed to them. Such sounds form the basic elements of music. Besides them, there are other sounds which although having a more general character such as ‘bright' or ‘muffled', do not have a distinct pitch. Imagine, as an example, a bang or the noise of an air stream. Such types of sounds can also be related to vibrations as we shall see later on. Let us now consider the generation of sound by a vibrating body, for instance, by the corpus of a stringed musical instrument, the membrane of a loudspeaker or by some part of a machine in operation. In Figure 1.1 an element of its surface is sketched as a solid line. When it moves from the left side to the right as shown in the upper part of the figure, it cannot displace all the air in front of it but it will press some of it together. When moving in reverse direction the body will suck in some air, again not by moving the whole column of air but by expanding some of it (see middle figure). Now any density change of the air is associated with a change in air pressure. Hence the compressed air tends to transfer the pressure increase to the neighbouring air volume. Likewise, a decompressed air volume exerts under pressure to its vicinity. Generally, all pressure disturbances induced by the body's movement will travel into the resting air. Finally, we assume the surface of the body to move back and forth or in other words, to oscillate.
    Acoustics An Introduction
  • 1 Fundamentals of sound waves and hearing Sound can be visualised physically as a wave motion, which is transmitted through a whole range of elastic media. It is called a sound wave. On the other hand, it is also a sensation subjectively perceived by the ear, which is stimulated by the sound wave. This is referred to as auditory sensation, a phenomenon which is the subject of advanced research and comes under the general heading of psychophysiology. 1.1 Sound waves A sound wave is transmitted through a medium that has both inertia and elasticity. The space in which sound waves travel is called the sound field. In a sound field the medium particles exhibit a repetitive movement backwards and forwards about their original position. Since a particle in the medium causes a neighbouring particle to be displaced by the repetitive movement produces a wave motion, i.e. vibration that is transmitted from particle to Particle successively in the medium. The direction of the particle's movement is the same as that of the transmission path of the sound wave. Therefore, it is called a longitudinal wave. As shown in Figure 1.1, the medium particles are crowded together at a certain point, producing a high pressure while at a neighbouring point they are dispersed resulting in a reduced pressure. Two such points of condensation and rarefaction exist alternately in the wave motion. Thus, at a fixed point, the dense and rare parts of the wave arrive alternately and the pressure consequently repeatedly rises and falls. This pressure fluctuation is called sound pressure P and the velocity of motion of the particles of the medium is called the particle velocity v. The number of fluctuations in 1 s is called the frequency generally expressed by f, the unit of which is the hertz (Hz). The distance that a sound travels in 1 s is called the sound speed and generally denoted by c (m/s). If we let ). represent the wavelength, then
    Env. and Arch. Acoustics
  • CHAPTER 1 "A SOUND BASIS'' 1.1 INTRODUCTION Although it is not necessary to be a qualified mechanic to drive a car, most people find it easier to learn to drive - and thereafter to make the most of their motoring if they have at least a general idea of what goes on ~under the bonnet''. Similarly, the environmental engineer will find that a basic knowledge of the fundamental theory of sound is of great help in appreciating the problems of noise in heating, ventilating and air-conditioning systems. Any introduction to the subject must inevitably include some material which may seem elementary and some which may appear to be of mainly academic interest. Nevertheless, ·the fundamental principles have a great bearing on practical acoustic problems and a knowledge of fundamentals can help the engineer to avoid the more common acoustic pitfalls. It is the purpose of this chapter to provide the background to the physics of sound as a basis for the consideration of specific installation problems in the remainder of the book. 1.2 WHAT IS SOUND? 1.2.0 The Question Subjectively, sound is something we hear as a result of vibration in the air. These vibrations yield pressure fluctuations which can be measured on a sound level meter. Vibrations are more usually associated with mechanical devices such as a mass vibrating up and down on a spring. Air also has mass and like a spring it has stiffness - that is it resists compression as it does in the pneumatic tyres of a car. As sound travels through the air, this air is locally compressed and expanded. The air tries to regain its normal equilibrium state; a region of compressed air will try to expand and in so doing acts on the adjacent region compressing it in turn and so the sound wave spreads and is propagated. Figure 1.1 shows a representation of a sound wave generated by a piston reciprocating in a tube. As the piston moves forward into the tube it compresses the air in front of it and this compression travels down the tube as a pressure wave - sound. On retraction the piston creates a rarefaction and this then follows the compression down the tube. They travel at the “speed of sound”. There are two mechanisms for generating sound that will be considered.
    Noise Control in Building Ser.
  • 1.1 A qualitative picture of wave motion Acoustics is the science of sound, and sound is a wave motion. In a wave a change or disturbance in some physical property of a medium is transmitted through that medium. For example when a sound occurs in air the sound wave causes the particles in the air to move to and fro (i.e. to Vibrate), and because the particles are elastically connected (air being an elastic medium) this vibration is transmitted through the air. The vibrating layer of air contain energy and so another feature of all is that they contain energy. The essential features of medium which is able to transmit sound waves are that it must possess elasticity and inertia (mass); sound wave can travel through solids, liquids and gases but not through a vacuum. In any real medium there will always be some friction processes at work so that some of the energy of the vibrating particles of the medium will be lost to the sound wave and turned into heat, a process known as sound absorption. The two simplest types of sound waves are spherical waves and plane waves (see Figure 1.1), and it helps to understand them if we consider the analogy of waves on the surface of water. If we drop a small object such as a stone into water we see circular ripples travelling outwards. The invisible spherical sound waves in air are their three-dimensional counterparts. If the stretch of water is linear, e.g. a canal, and the stone is replaced by a long plank of wood we would see plane ripples move along the water surface. The wavefront represents the leading edge of the wave, i.e. it tells us how far the wave has travelled and the rays, always perpendicular to the wavefronts, indicate the direction in which the wave is travelling. These two forms of wave are idealized models of wave propagation and are useful because waves for sound sources can often approximate to one of these models. Sound from a. loudspeaker tends to radiate equally in all directions at low frequencies (i.e. like spherical waves) but be much more directional (i.e. more like plane waves) at high frequencies. Plane waves travelling in one direction only are the simplest form of waves and can be used to explain frequency and wavelength. In a sound wave in air, as a result of the to and fro motion, sometimes the air particles are bunched together, causing a very slight increase in pressure in the atmospheric pressure (a compression) and sometimes causing them to be spaced further apart, causing a very slight reduction in pressure (a rarefaction). This is shown in Figure 1.2 where compressions and rarefactions from the vibrations of a tuning fork are shown travelling in one dimension (down a tube or pipe for example). These very small fluctuations in pressure in the tube constitute the sound pressure caused by the passage of the sound wave down the tube. The disturbance caused by the sound waves could be described in terms of the vibrations of the air particles, either as a displacement, as a velocity or as an acceleration, and these alternatives will be discussed in more detail in Chapter 7 on vibration. However, since these movements cannot be seen, and since our human ears and our microphones respond to the changes in
    Acoustics and Noise Control
  • Fundamentals of Sound Sound can be considered as wave motion in air or oth.er elastic media. In this case, sound acts as a stimulus. Sound can also be considered as an excitation of the hearing mechanism that results in its perception. In this case, sound is a sensation. This duality of sound is familiar to those interested in audio and music. The type of problem at hand dictates our approach. If we are interested in the physical disturbance in the air in a room, it is a problem of physics. If we are interested in how that disturbance is perceived by a person listening in the room, psychoacoustical methods must be used. Because this book addresses acoustics in relation to people, both aspects of sound will be considered. Sound can be characterized by objective phenomena. For example, frequency is an objective property of sound; it specifies the number of waveform repetitions per unit of time (usually l second). Frequency can be readily measured on an oscilloscope or a frequency counter. On the other hand, sound can be characterized subjectively. For example, pitch is a subjective property of sound. Perceptually, the ear hears different pitches for soft and loud 100-Hz tones. As intensity increases, the pitch of a low-frequency tone goes down, while the pitch of a high-frequency tone goes up. Harvey Fletcher found that playing pure tones of 168 and 318 Hz at a modest level produces a very discordant sound. At a high intensity, however, the ear hears the pure tones in the 150- to 300-Hz octave relationship as a pleasant sound. We cannot equate frequency and pitch, but they are analogous. Another duality exists between intensity and loudness. Similarly, the relationship between waveform (or spectrum) and perceived quality (or timbre) is not linear. A complex waveform can be described in terms of a fundamental and a series of harmonics of various amplitudes and phases. But perception of timbre is complicated by the frequency-pitch interactions in the human hearing mechanism as well as other factors. The interaction between the physical properties of sound, and our perception of them, Poses delicate and complex and issues. It is this complexity in audio and acoustics that creates such interesting problems. On one hand, the design of a loudspeaker or a concert hall should be a straightforward and objective engineering process. But in practice, that objective expertise must be carefully tempered with purely subjective wisdom. As has often been pointed out, loudspeakers are not designed to play sine waves into calibrated microphones placed in anechoic chambers. Instead, they are designed to play music in our listening rooms. In other words, the study of audio and acoustics involves both art and science. To learn the complexities of audio and acoustics, we begin with the science, keeping in mind that our ears will ultimately determine the success or failure of our projects.
    Master Handbook of Acoustics
  • Some facts on sound waves, sources and hearing In principle, any complex sound field can be considered as a superposition of numerous simple sound waves, e.g. plane waves. This is especially true ofthe very involved sound fields which we have to deal with in room acoustics. So it is useful to describe first the properties of a simple plane or a spherical sound wave or, more basically, the general features of sound propagation. We can, however, restrict our attention to sound propagation in gases, because in room acoustics we are only concerned with air as the medium. In this chapter we assume the sound propagation to be free of losses and ignore the effect of any obstacles such as walls, i.e. we suppose the medium to be unbounded in all directions. Furthermore, we assume our medium to be homogeneous and at rest. In this case the velocity of sound is constant with reference to space and time. For air, its magnitude is c = (331.4 + 0.6 θ) m/s where θ is the temperature in degrees centigrade. In large halls, variations of temperature and hence of the sound velocity with time and position cannot be entirely avoided. Likewise, because of temperature differences and air conditioning, the air is not completely at rest, and so our assumptions are not fully realised. But the effects which are caused by these inhomogeneities are so small that they can be neglected. 1.1 Basic relations, the wave equation In any sound wave, the particles of the medium undergo vibrations about their mean positions. Therefore, a wave can be described completely by indicating the instantaneous displacements of these particles. It is more customary, however, to consider the velocity of particle displacement as a basic acoustical quantity rather than the displacement itself.
    Room Acoustics

Noisy neighbour, sound levels, gardens, noise, soundscape
B&K Environmental Noise Measurement

What is Noise?

Noise and sound often get used interchangeably but they aren't the same thing. The one sentence definition is: noise is sound we don't want to hear.


For some reason, I think acousticians are fixated on illustrating noise with lawnmowers. Or maybe it's just me - but the image of a lawnmower in an old B&K publication always stuck in my head.




What isn't a "dB"?

It gets a bit more complicated when trying to define things in terms of a quantity. The problem is that we feel we know what sound and noise is - mainly because it is part of their everyday lives - we then try to treat decibels (abbreviated dB) in the same way as other things like cm or kg or mph. And this is where people trip up. Decibels are not a measure of anything. It's not a quantity like pints are a quantity of milk*.


* at least in the UK where we use a confusing mix of SI units and imperial units


What is a "dB"?

The 'd' stands for 'deci' as in one tenth of something and the 'B' stands for 'Bel' and they get smashed together as 'decibel' or dB. Bel is named in honour of Alexandra Graham Bell - hence the B is capitalised as a proper noun. So there are ten dBs to a B or to say it the other way round: one dB is a tenth of a B.


A 'Bel' (never used without the deci), is the common logarithm of the ratio of two numbers. That's one number divided by another number and then log whatever the result is. You would then times that by ten to get a decibel. You can do it on your calculator at home like this -> -> ->


The reason we use logarithms is because they compress a very wide range of ratios into something a little more manageable. Logarithms changes multipliers into additions. Add 10 dB to a something and it is the same as multiplying by 10. An increase in 20 dB is an increase by 100 time and an increase by 30 dB is an increase by 1000 times. So 80 dB is a ratio of 1 to 800 million!


And in all that explanation I have not made one mention of sound or noise. That's because decibels are not exclusive to sound. They can be used as a measure of many things - electronics, optics, video and imaging, to name a few. All of which need a wide range of quantities compressed into a range of numbers that are a little more manageable than using millions and billions.


So how does sound pressure come into it?

Firstly, the actual units for sound pressure are 'pascals' - the same unit we use for any normal sort of pressure - like water pressure or atmospheric pressure. But, for sound pressure we normally mean the root-mean-square (rms) of the pressure. This is because sound is where the pressure is constantly changing - its the changing in pressure that we are hearing - see the picture to the left.


The sound pressure level (note the extra word 'level' which is what dBs are used for) is the rms sound pressure divided by 0.00002 pascals, then squared, and then you do the 10 x log thing on it. That's a bit of a mouthful. It's easier to write as an equation:

Where Lp is the sound pressure level;

Prms is the rms sound pressure; and

Pref is 0.00002 pascals


That's quite technical for an easy read blog so you can just forget the equation, But, an easy way to put it is that the sound pressure level in decibel describes the sound you hear relative to the quietest thing we can hear. So 80 dB is 800 million times more than the quietest thing we can hear.*


And yes, you can technically get negative decibels - and that's not sound being sucked away - it's just sound pressures less than 0.00002 pascals.


* This is not quite correct but close enough to get the gist of it


Obviously we mean sound pressure!

If we are talking about decibels in the context of sound levels then isn't it obvious we mean it as the ratio of sound pressures and not whatever they use for optics? But hang on. It's still not as simple as that!


We also use decibels to describe sound power levels which is measured in watts and not pascals!* Sound power is how much power is in the sound being generated. You might be familiar with audio amplifiers delivering power in watts**.


But even if we were sure we were talking about sound pressures, we still need to know how far away you are. Think about it. A lawnmower a mile away can't be heard. But if you put your ear right up to the motor, it might be a lot more than 80 dB. So if we are talking about sound pressures we must also know the distance at which it was measured. E.g. 80 dB at 10 m. If we are talking about sound powers then there is no distance involved because we are talking about a power. The fact that we use decibels to describe both caused all sorts of confusion.


But even if we do say 80 dB at 10 m, that's still not the end of the story! Is that the instantaneous sound level? Is it the maximum sound level? Is it the average over some time period? Each of those has a distinct unit which in itself is not straightforward to define. And we use decibels for all of them! Usually we use some other letters to tell us exactly what we are referring to - such as LAeq - which is sort of the average - also not a straightforward definition.


*There is also a much less frequently referred to 'sound intensity level' which is the flow of sound power in a direction.

**Technically this is not a measure of sound power because there are losses as the signal gets converted to sound.


I measured the sound pressure level with my phone


Or maybe you bought a cheap sound level meter from Amazon. Is it good enough? I think it's more than likely that your phone app or cheap sound level meter is getting all the calculations correct... maybe. But the main issue is likely the microphone and the placement of the microphone in the phone. The sensitivity of the microphones is probably unknown to the app and very unlikely to be consistent across time and frequencies. Also, how you are holding the phone? What hard surfaces are near by? What reflections are coming off you? These are all things that are going to come into play.


Maybe you'll get a decent measurement. But, more often than not the acoustic consultant will nod politely as you tell them about it and then they'll do their own measurements.


The specification sheet for my lawnmower has the sound level

We tend to take what's written in specification sheets more seriously. After all, if they have reported a number on a document with the equipment and it's wrong then surely that's on them. Well maybe. I don't what the penalty for getting it wrong is. I've never heard of a case where it was discovered the reported sound level was wrong which led to some kind of punishment.


But even so, we often run into all the same definition problems. Sometimes it's not stated whether it's a sound power level or a sound pressure level. Sometimes the sound pressure level doesn't give a distance. More often than not it's in there for noise at work reasons and it is given at the operators position - which is no help if that's inside a cab or we don't even know where in relation to the machinery. The operating conditions are almost never defined.


But at least it was done by someone suitably qualified following a standardised methodology - right? Well sometimes. Sometimes when you dig a little, no one knows who did the measurements or how or when they were done. Then you ask yourself was it some gap year student standing next to the lawnmower with his phone?


Specification sheets are hit and miss. There are times I have had to resort to using sound level measurements of something similar that I know is reliable and disregard the specification sheet altogether.


I want to know more?

Sound is equally fascinating and confusing if you are not used to the concepts. You don't need to be an expert to understand it but it's not a topic that gets a lot of exposure. So it's not very well understood by most people.


Our cars have speedometers to tell us how fast we are going, we have kitchen scales to measure weights, jugs to measure volume, and we know how to use a ruler to measure the length of something. Why shouldn't you be able to point you phone at your lawnmower to get an accurate measurement? And when 10 kg + 10 Kg = 20 kg, why shouldn't 10 dB + 10 dB = 20 dB (it actually equals 13 dB). Well there you go. It feels like it should follow all the normal rules but it doesn't


If you want to find out more then one of these books is a good place to start (click on the picture for a link to where you can buy them). If you want guidance on measuring sound then this publications provides some best practice. You also can't go wrong with one of these courses.



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