Reflection and absorption of sound (Item No.: P )

Transcription

1 Teacher's/Lecturer's Sheet Reflection and absorption of sound (Item No.: P ) Curricular Relevance Area of Expertise: Physics Education Level: Age Topic: Acoustics Subtopic: Generation, propagation and perception of sound Experiment: Reflection and absorption of sound Difficulty Preparation Time Execution Time Recommended Group Size Easy 10 Minutes 10 Minutes 1 Student Additional Requirements: Experiment Variations: Microphone Pair of earphones Sheet of paper Piece of adhesive tape, approx. 5 cm PC Keywords: Task and equipment Information about teachers Additional information In acoustics, a distinction is made between sound insulation (sound reflection) and sound damping (sound absorption). Sound insulation is about the transmission of sound through a wall. Sound is optimally reflected on massive walls. In room acoustics, sound damping is important for the prevention of reverberation. In this case, the sound is absorbed (damped) by porous materials, e.g. wadding, felt, wood fibre materials, or glass and rock wool. For optimum solutions, often both methods are combined. The students work out the fundamental principles of sound insulation and sound damping based on several experiments. Notes concerning the execution of the experiments 1. For the experiments, a sequence of 8 sound pulses will be emitted. A sound pulse can be recorded at the push of a button. Figure 7 shows a measurement example of experiment 1. The sequence of the 8 sound pulses is programmed so that an individual pulse consists of a brief sinusoidal oscillation with a maximum relative amplitude of approximately 1 ms. The time difference between two pulses is approximately 100 ms so that usually only one pulse will be visible in the diagram. 2. For the evaluation of the experiments, the maximum of a pulse must be measured. The sound volume of the earphone and the sensitivity of the microphone have a very strong effect on the magnitude of the recorded signals, i.e. on the measured values of the relative amplitude. The values that are given in the tables are part of an example measurement. The actual measurements that are performed by the students can vary from these values to a considerable extent. The relationships between the measurements and the resulting conclusions, however, are the same. 3. In experiment 1, the sound pulse is considerably reduced when it passes through the CD case. This is why the ambient noise should be as low as possible during the measurement of this pulse. Otherwise, the variations of the baseline could impair the measurement of the signal. 4. In experiment 2, the students examine the sound damping effect of various different wall set-ups based on reflection measurements. Figure 8 shows a measurement example concerning the reflection on the CD case. The first pulse is the direct signal from the earphone, while the other three pulses originate from reflections (echoes) at the ends of the tube. Since the microphone is not positioned directly in front of the earphone but slightly behind it and also with a lateral offset, the first pulse will not be measured under the same conditions as the other pulses. Its maximum amplitude changes depending on the position of the microphone. Often, it is smaller than the echo. This is not important for the execution of the experiment, but it may lead to discussions. This is why the students worksheet describes a very precise arrangement of the earphone and microphone with which the first and second pulse of the reflection on the CD case are approximately identical.

2 Teacher's/Lecturer's Sheet 5. The "wall set-ups" in experiment 2 differ from each other in so far as felt is placed in front of a reflecting wall (in this case the CD case) in different shapes. A sound pulse is transmitted and reflected on this wall. A small echo indicates a good sound damping effect of the wall. The relative amplitudes of the first echoes (the second recorded pulse in Fig. 8) of the various different "wall" types will be compared. This means that the experiment conditions must be identical, i.e. the earphone, microphone, and glass tube should not be offset with regard to each other. A potential way of checking as to whether the set-up is unchanged is to measure the first pulse (direct signal from the earphone). It should be approximately identical in all of the partial experiments. Fig. 7: Time course of pulse Fig. 8: Pulse with three echos Note All of the measured values that are stated originate from an example measurement. The values that are measured by the student may vary from these values to a considerable extent (particularly the first pulse in Fig. 8). The relationships between the measured values, however, are similar and, therefore, also the conclusions. Reasons for deviating measurement values: Even the slightest variations concerning the distance between the earphone and microphone can lead to considerable changes of the measured value. Other earphones or microphones than the ones that were used for the current experiment may have a different level of sensitivity and, as a result, they may generate different pulse heights. The measurement of the maximum is a quick, but also very simple, method for determining the pulse height. It was chosen because we wanted to focus on a qualitative comparison. Under stable geometrical conditions, the values are sufficiently reproducible.

3 Reflection and absorption of sound (Item No.: P ) Task and equipment Task How can sound be reduced? In our everyday lives, we encounter different types of noise. It is often produced by others, which is why it disturbs us. Sometimes, however, it is the sounds that we produce ourselves that reverberate loudly in a room. This is why there are two ways of reducing sound: sound insulation (sound reflection) and sound damping (sound absorption). Sound insulation prevents sound from being transmitted through a wall so that the neighbours will not be disturbed. This is basically achieved by a reflection of the sound at the wall. The absorption into the wall can be neglected for sound insulation. This is in contrast to sound damping: An empty room with highly reflecting walls, floor, and ceiling is very loud. This is why for good room acoustics, sound-absorbing objects (furniture, carpets) or wall panels are installed. In concert halls or recording studios, sound damping is optimised by an arrangement of porous damping materials on or in front of the walls, for example. Sound insulation and sound damping will be examined in various different experiments. 1. Measure the sound transmission through solid walls made of paper or cardboard. 2. Measure the reflection of a sound pulse on a solid wall and examine the effect of felt arrangements in front of the wall. Equipment Position No. Material Order No. Quantity 1 Felt sheet, 100 x 100 mm Metal angle bracket for glass tube o.d. = 44 mm Glass tube, d(outside) = 44 mm, l = 340 mm Beaker, low form, plastic, 100 ml Silk thread, l = 200 m Software "Measure Acoustics", single user license CD case as a reflection shield Additional material Microphone 1 Pair of earphones 1 PC 1 Sheet of paper 1 Piece of adhesive tape, approximately 5 cm 1

4 Set-up and procedure Set-up Connect the earphones and microphone correctly to the computer. Place the glass tube on the two metal brackets (see Fig. 3). Place the beaker upside down on the table and fasten one of the earphones to the beaker with the aid of some adhesive tape (height: approximately the centre axis of the glass tube, see Fig. 3). Fig.1: Experiment 1: earphone and microphone (distance approx. 2 mm) Fig. 2: Thin wall of the CD case between the earphone and microphone. Open the audio settings of the PC. Set the sound output volume to maximum and change the balance so that the sound output takes place only via the earphone that has been fastened to the beaker. Procedure Experiment 1: Sound transmission ttrougt walls - Position the microphone in front of the earphone at a distance of approximately 2 mm (Fig. 1). Adjust a distance that is just about enough for inserting the thin cardboard of the CD case as a "wall" between the microphone and earphone for a second measurement (Fig. 2). This experiment, however, will be performed without this wall. Start the software "measure Acoustics". Open the experiment "3.6 Reflection and absorption of sound". Help 1: Open the experiment overview (Menu "File" "Open experiment" or select "Open experiment" on the menu bar). Open the folder "3 Applications in the field of medicine, music, and everyday life" and select the experiment "3.6 Reflection and absorption of sound". Start the playback in the diagram "Spectrum of the signal at the audio output (loudspeaker or headphones)". After 2 seconds, a sequence of eight sounds will be played. Help 2: Select "Play tones" in the diagram window "Spectrum... (loudspeaker or headphones)". Start the playback again (see Help 2) but this time freeze the time course of the sound recording so that one of the eight sound pulses is clearly visible in the diagram window "Time function... (microphone)". Help 3: Select "Activate/freeze diagram" in the diagram window "Time function of the signal at the audio input (microphone)". Adapt the frozen diagram section so that you can clearly see the recorded sound signal. If necessary, repeat the process until you have successfully recorded a sound signal. Help 4: Select "Fit in curve". Use also the magnifying glass "Zoom" and click in order to drag a rectangle around the area that you would like to zoom. Read the maximum of the relative amplitude and note down the value in Table 1 in the report. Help 5: In the diagram window "Time function... (microphone)", use the crosshair "Mark" in order to determine the y-value (here: relative amplitude) at the location of the crosshair by reading the value off the status bar at the bottom of the screen. Readjust the standard section for further measurements. Help 6: Select "Standard diagram section". Repeat the experiment with the following changes concerning the set-up and measure the maximum of the relative amplitude each time: Sheet of paper between the earphone and microphone Thin wall (CD case) between the earphone and microphone (Fig. 2) Experiment 2: Reflection of sound on various different wall surfaces

5 Place the earphone and microphone directly in front of the same opening of the glass tube (Fig. 3). Push the earphone approximately 2 mm into the glass tube and ensure that it points directly in the direction of the tube. The microphone should be positioned as horizontally as possible directly at the end of the tube (Fig. 3). As a result, the earphone signal and the echo will have approximately the same magnitude. Seal the other end of the tube by placing the CD case in front of the opening. The wall should be perpendicular and it should seal the opening of the tube well (Fig. 3). Fig. 3: Reflection of sound on the CD case Note: Hold the glass tube firmly in place when you change the set-up in-between the measurement series in order to keep the glass tube, earphone, and microphone correctly aligned so that all of the measurement series are comparable. For verification: During all of the measurements for experiment 2, the relative amplitudes of the first pulse should be approximately identical. Start the output of the sound pulses (see Help 2) and freeze the time course (see Help 3). Adapt the diagram section (see Help 4). Describe the recorded time course (under "Evaluation - Question 2"). Read the maximum of the relative amplitude of the first and second pulse (see Help 5) and note down the values in Table 2. Readjust the standard section (Help 6). Repeat the measurements with the following changes to the reflecting end of the tube: Place a felt sheet in front of the CD case (Fig. 4) Place a felt sheet on the inner wall at the end of the glass tube; seal it with the CD case (Fig. 5). Fig. 4: Reflection of sound on a flat felt sheet Fig. 5: Reflection of sound on the CD case with a felt roll Place a felt funnel (Fig. 6) in the glass tube directly in front of the sealing CD case so that the sound "flows" into the funnel. If possible, the small end of the funnel should be closed.

6 Fig. 6: Funnel made of felt

7 Report: Reflection and absorption of sound Result - Table 1 (3 Punkte) Experiment 1: Sound transmission through walls Table 1: Maximum relative amplitude of the pulse Without a wall Sheet of paper CD case Pulse 20.5% 1 8.1% 1 2.2% 1 Result - Table 2 (8 Punkte) Experiment 2: Reflection of sound on various different wall surfaces Table 2: Maximum relative amplitudes of the pulses CD case CD case with CD case and a felt roll CD case and a funnel-shaped f alone a felt sheet inside the tube inside the tube First pulse 12.2% % % % Second pulse 11.8% 1 6.7% 1 2.7% 1 1%

8 Evaluation - Question 1 (10 Punkte) Compare the maximum relative amplitudes of the pulses with a sheet of paper and a CD case to the value without a "wall" (experiment 1). Which of the "walls" provided the highest sound insulation? Evaluation - Question 2 (10 Punkte) Compare the recorded time courses of experiment 1 and 2. What is the meaning of the pulses that can be seen additionally in experiment 2?

9 Evaluation - Question 3 (10 Punkte) In experiment 2, compare the results of the two smooth walls, i.e. the CD case alone and the CD case with a felt sheet, and explain your observations. Evaluation - Question 4 (10 Punkte) In experiment 2, compare the results of the open felt roll and the closed felt roll (funnel) inside the tube. Explain your observations.