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Broadcasting and acoustics: board exam questions with solutions

On the ECE exam under Electronic Systems and Technologies (EST). Try each question before you open its solution, then practise with fresh numbers below: 2 computed question types on this topic.

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Questions

1. What is the sound intensity level of a sound with intensity 1 × 10⁻⁶ W/m² (reference 10⁻¹² W/m²)?

  1. 6 dB
  2. 120 dB
  3. 60 dB
  4. 30 dB
Show the answer and solution

Answer: C. 60 dB

L = 10 log(10⁻⁶/10⁻¹²) = 10 log 10⁶ = 60 dB.

2. Two uncorrelated machines each produce 70 dB at a listening point. What is the combined sound level?

  1. 140 dB
  2. 73 dB
  3. 76 dB
  4. 70 dB
Show the answer and solution

Answer: B. 73 dB

Add intensities: L = 10 log(2 × 10⁷) = 70 + 10 log 2 = 73.0 dB.

3. A small source produces 90 dB at 1 m in a free field. What level is expected at 4 m?

  1. 78 dB
  2. 84 dB
  3. 66 dB
  4. 22.5 dB
Show the answer and solution

Answer: A. 78 dB

Inverse-square law: L2 = L1 − 20 log(r2/r1) = 90 − 20 log 4 = 78.0 dB. (84 dB results from using 10 log.)

4. A 500 m³ room has total absorption of 80 m² sabins. What is its reverberation time by Sabine's formula (RT60 = 0.161V/A)?

  1. 0.10 s
  2. 2.48 s
  3. 0.62 s
  4. 1.01 s
Show the answer and solution

Answer: D. 1.01 s

RT60 = 0.161 × 500/80 = 1.01 s.

5. What is the sound pressure level of a 2 Pa rms sound pressure (reference 20 µPa)?

  1. 50 dB
  2. 200 dB
  3. 100 dB
  4. 80 dB
Show the answer and solution

Answer: C. 100 dB

SPL = 20 log(2/(20 × 10⁻⁶)) = 20 log 10⁵ = 100 dB.

6. Taking the speed of sound in air at 20 °C as about 343 m/s, what is the wavelength of a 1 kHz tone?

  1. 3.43 m
  2. 0.343 m
  3. 0.034 m
  4. 0.686 m
Show the answer and solution

Answer: B. 0.343 m

λ = v/f = 343/1000 = 0.343 m. (v = 331.3√(1 + 20/273.15) = 343.2 m/s.)

7. What is the standard peak frequency deviation (100% modulation) of a VHF FM broadcast station?

  1. ±75 kHz
  2. ±25 kHz
  3. ±5 kHz
  4. ±200 kHz
Show the answer and solution

Answer: A. ±75 kHz

Standard VHF FM broadcasting (88–108 MHz) defines 100% modulation as ±75 kHz deviation; ±25 kHz was the analog TV aural standard and 200 kHz is the channel spacing used in the Americas.

8. A 400 m³ hall has RT60 = 2.0 s. Using Sabine's formula (RT60 = 0.161V/A), how much absorption must be added to bring RT60 down to 1.2 s?

  1. 53.7 m² sabins
  2. 32.2 m² sabins
  3. 80.5 m² sabins
  4. 21.5 m² sabins
Show the answer and solution

Answer: D. 21.5 m² sabins

A1 = 0.161 × 400/2.0 = 32.2; A2 = 0.161 × 400/1.2 = 53.67; added = 21.5 m² sabins.

Worked examples with computed numbers

These numbers are fixed on this page; in practice they change every time.

9. One machine produces 77 dB at a point. What level do 3 identical machines produce there?

  1. 81.8 dB
  2. 80 dB
  3. 86.5 dB
  4. 231 dB
Show the answer and solution

Answer: A. 81.8 dB

Equal incoherent sources add in power: L = 77 + 10 log 3 = 81.8 dB.

10. A small source measures 107 dB at 1 m in the open. What is the level at 4 m?

  1. 26.8 dB
  2. 95 dB
  3. 142 dB
  4. 101 dB
Show the answer and solution

Answer: B. 95 dB

A point source in free field drops 20 log(d₂/d₁) = 12 dB: 95 dB.

Other topics in the same subjects

Communications fundamentals (dB, noise) · AM, FM and PM · Digital communications · Transmission lines · Antennas and wave propagation · Microwave, fiber optics and satellite · Data communications and networks

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