A sound engineer places acoustic foam panels on the walls of a recording room, as shown in Fig. 1. A loudspeaker sends sound waves towards the wall. The foa...
Question 1 Report
A sound engineer places acoustic foam panels on the walls of a recording room, as shown in Fig. 1. A loudspeaker sends sound waves towards the wall. The foam has many small air spaces. These make the sound less likely to reflect strongly back into the room. The engineer compares recordings made with the panels fitted and without them. Electrical energy in the amplifier circuit is transferred to the loudspeaker, which produces sound and some thermal energy.
(a) What type of wave is sound? [1]
(b) Which energy transfer takes place from the loudspeaker to the air? [1]
(c) Describe how the foam panels reduce reflected sound. [2]
A wind turbine is shown in Fig. 1. Moving air turns the blades, which turn a generator. During a 10 s interval, the wind transfers 90 000 J of kinetic energy to the blades. The generator delivers 54 000 J of electrical energy to a nearby circuit. Some energy is dissipated as sound and heating in the bearings. The turbine is switched off when wind velocity becomes dangerously high, even though the rate of energy transfer could be large.
(a) Calculate the efficiency of the turbine-generator system. [2]
(b) Calculate the electrical power delivered in the 10 s interval. [2]
(c) Give one wasted energy transfer from the turbine. [1]
Answer Details
Acoustic foam panels
- Sound is a longitudinal wave. The vibrations of particles are parallel to the direction in which the wave travels. [1]
- Energy is transferred from the loudspeaker to the air by sound waves, or mechanically by vibrating the air. [1]
- Sound enters the foam's small air spaces and is absorbed. Consequently, less sound is reflected back into the room. [2]
Wind turbine
- \[\text{efficiency}=\frac{54\,000}{90\,000}\times100=60\%\]Efficiency = 60%. [2]
- \[P=\frac{E}{t}=\frac{54\,000\text{ J}}{10\text{ s}}=5400\text{ W}\]Electrical power = 5400 W. [2]
- An unwanted transfer is energy transferred by sound or to thermal energy stores in the surroundings, for example through heating in the bearings. [1]
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