Session 4: Sound level measurement

Objectives: To protect the public against excessive noise exposure regulations about noise emissions were enacted by the European Union

(e.g. Environmental Noise Directive - Directive 2002/49/EC, END). The implementation of the directives in the member states includes physical measurable quantities such as the sound pressure level to describe the emissions.

In this session the pupils will understand

  1. how to quantify the volume of sounds
  2. the reason for the usage of a logarithmic scale
  3. what noise is.

The main question of this session is how sounds and noise can be measured. Therefore we need knowledge about the sound pressure level and the usage of the dB scale.

Maximum duration

90 minutes or even more if you do the excursion mentioned in 3.

Material

Sound pressure levels

fig 6 Sound pressure level over pressure to demonstrate the logarithmic scaling of the dB scale

Introduction/Starters

What makes noise in your environment? Make a list of annoying sounds.

Let the pupils do an assessment of different sound samples (Chat, whisper, music, ventilator, traffic noise). What is loud / quiet / annoying / pleasant? Make a list with the collected information.

Be quiet, what do you hear?

Hint: Adjust the volume of the sounds to their actual sound pressure level (SPL) prior to the presentation if possible. Beware of the fact that the sound pressure level decays with distance to the sound source, in this case the loudspeakers. To adjust the volume you will need information about the SPL of a sound and the distance between measurement point and sound source. Measure A-weighted SPL in dB(A).

Main activities

1) What would you measure to describe the volume of a sound or noise? How can we describe (characterize) the pressure fluctuations in an appropriate way?

And how would you do it?

Explanation: A sound wave is a fluctuation of pressure (physical unit is Pascal Pa), so physically we measure pressure values to describe the volume of sounds. To measure the sound pressure level (SPL) a microphone can be used. As in session 3 the sound energy is transmitted from the source to the membrane of the microphone in the form of a wave. This pressure fluctuation causes the vibration of the membrane which can be measured electrically. The principles are the same as in a loudspeaker, where an electrical signal causes the vibration of the membrane, but the other way around.

2) Use one set of weights per group of pupils. One should take one weight on the palm of the hand (not too light, for example 50g or 100g) and close his/her eyes. A second pupil should try to place a second lighter weight on top of the first one, may be placing a piece of paper between the weights to avoid any noise. Can you feel with closed eyes the additional weight? When does it become heavier? Note the values “original weight” and “additional weight” in case of feeling the additional weight.

Explanation: When placing 10% of the original weight on top of it, one should feel the additional weight. Below this threshold you may hear it or feel the pressure during placement. But it is very difficult to feel an additional weight of 1g when holding 50g in your hand. When you calculate (original mass + additional mass)/original mass you should get something around 1.1. This means you need 10 % of the original weight to register a change in weight. This is not a linear behaviour and the same principle can be found in hearing. In fact, it is a logarithmic (with base 10) behaviour which means you register an increase in volume when the pressure is increased by 10 to 12%.

The sound pressure that a human ear can detect varies from 2*10-5 Pa (20µPa) at the Threshold of human hearing to 100 Pa at the Threshold of pain. It is remarkable that our ears can distinguish such a huge range of pressure. For making out the difference between the sound of a mosquito and normal speech on one side and recognizing loud sounds like a starting airplane on the other side the sound pressure is not perceived in a linear way by the human ear.

To take this effect into account the sound pressure level (SPL) was introduced and with it a scale from 0 dB to 130 dB is available. In this scale 1 dB is an audible volume increase. The SPL is a measure of the effective sound pressure p relative to a reference value p0. This is needed because of the use of the logarithm in the definition of the SPL

SPL = 10*log(p2/p02) = 20*log(p/p0)

with p0 as reference value (2*10-5 Pa) which was defined according to the threshold of audibility at 1 kHz. This value has no physical unit but is marked as sound pressure level by adding dB (decibel). Show a plot of SPL versus pressure to illustrate the logarithmic scale (fig. 6).

3) Organize an excursion in your area to measure sound levels of daily sound sources. Let the pupils take notes, put them together and use this in 5.

Hint: It is good to note the distance between sound level meter and source, the SPL, the kind of sound or noise (car and barking of a dog for example), the feeling of the pupils about the source and its sound and the duration of the sound (like “the whole time”, “3 barks”, “one car in city speed driving by in 10 seconds”)

4) Play the sound samples of different sound sources from the introduction again. Let the pupils measure it with sound level meters. They should take notes on cards about the SPL, the distance to the loudspeaker and the kind of sound. Use them in 5 to discuss different aspects of sound and noise.

5) Show examples of different sources and their sound pressure level. If you made an excursion with the pupils, use their notes, too. Discuss the dB-scale. What kind of description is connected to the SPL? What cannot be explained with the SPL?

Hint: Prepare some cards (size of a postcard) and write the name of the source and its sound pressure level on the card. The pupils can do the same with the measured sources during the excursion. Put them together in a chart, low sound pressure levels at the bottom and high sound pressure levels on top. The pupils can also make a poster for the classroom from the postcards.

Explanation: Discuss the scale and the placing of the sources in the chart. The pupils should get an impression and a feeling for the dB scale. If possible, discuss the influence of distance to a source on the SPL.

Ask them:

a) Can you identify loud or quiet sounds with the sound level meter? What sound pressure level do you measure in a quiet room without talking and moving around?

Explanation: Loud and quiet sound can be distinguished by comparing the numbers on the display. A smaller number is less loud than a larger number. Even in a very quiet room you can measure a sound pressure level higher than zero dB (noise coming from outdoor, pupils moving around and so on). It is possible to measure less than zero dB as the reference for calculating the sound pressure level is 2*105 Pa. Therefore you have less than zero dB if you measure a sound pressure below this reference.

b) Can you identify annoying or pleasant sounds with the sound level meter? Why not? Discuss this point!

Explanation: This is not possible by measuring the sound pressure level because annoyance and pleasure are connected to judging. Because everybody uses his or her knowledge and experiences to identify annoying sounds the judging is a very individual process and not only connected to the pressure level. It is not an exactly measurable physical quantity.

c) What is noise? Is it connected to annoyance? Which kind of sound is annoying and when does it become noise?

Discuss which sound pressure levels are disturbing, which causes damage (are unhealthy) and which destroys our ears immediately. Use the chart from 5 to discuss what the differences between sound and noise are.

Explanation: With a sound level meter showing the SPL it is not possible to get information about the frequencies of the sound and this is a second reason why annoyance cannot be measured with a simple sound pressure level. An annoying and/or unwanted sound is defined as noise (see if you can find more than this definition).

d) Do this experiment outdoor far away from walls to show the effect. Let the pupils measure the SPL in distances of 1m, 2m and 4m to the speakers while playing a continuous sound sample (random noise for example). Let them discover that a doubling of the distance to the sound source causes a 6 dB(A) decrease in SPL.

Explanation: The small loudspeaker can assumed to be a small source radiating sound in every direction the same way. This leads to a spherical sound wave. As analogy you can bring the picture of throwing a stone into a lake to the pupils mind. The stone causes a circular wave on the water surface travelling away from the central point. While travelling away from the central point, the radius of the wave expands. The same happens with a spherical wave in air. If you double the distance to the source, let us say from 1m to 2 m, the surface of the sphere increases from S1 = 4πr2 = 12.57 m2 to S2 = 50.27 m2. With this the SPL will be attenuated by ∆L = 10 log (S2/S1) = 6 dB(A) because the radiated energy is uniformly distributed on the small and the bigger sphere. Keep in mind that reflections influence the spherical wave radiation.

Lessons learned

In this session the pupils should experience the quantity of sound levels in daily life. They should have built a connection from the physically measureable pressure to the dB scale and realise that the distance to the source plays an important role.