EEEN 462 - MEASUREMENT IN DECIBELS TEST

This quiz tests your understanding of decibel calculations relevant to electrical and communication engineering. Select the best answer for each question.

1. A sound intensity increases from I to 2I. What is the increase in sound intensity level (SIL)?
a) 10 dB
b) 3 dB
c) 6 dB
d) 1 dB
2. If a sound intensity increases by a factor of 10, what is the increase in sound intensity level?
a) 3 dB
b) 10 dB
c) 20 dB
d) 100 dB
3. Two identical 80 dB sound sources are combined. What is the total sound intensity level?
a) 80 dB
b) 83 dB
c) 86 dB
d) 160 dB
4. How many 80 dB sources are needed to achieve a combined sound intensity level of 110 dB?
a) 10
b) 100
c) 1000
d) 10000
5. An electrical signal has a power ratio of 1000:1. What is this ratio in dB?
a) 30 dB
b) 20 dB
c) 10 dB
d) 3 dB
6. A voltage ratio of 10:1 corresponds to how many dB?
a) 10 dB
b) 20 dB
c) 30 dB
d) 40 dB
7. The threshold of human hearing is approximately 0 dB. What is the sound intensity level of normal breathing at 10 dB compared to the threshold?
a) 10 times more intense
b) 100 times more intense
c) 2 times more intense
d) 3 times more intense
8. If a sound has an intensity level of 60 dB, and another sound is 1000 times more intense, what is the intensity level of the second sound?
a) 63 dB
b) 70 dB
c) 90 dB
d) 120 dB
9. A communication system has a signal-to-noise ratio (SNR) of 40 dB. If the noise power is 1 mW, what is the signal power?
a) 10 mW
b) 100 mW
c) 1 W
d) 10 W
10. Which of the following sounds has a frequency of approximately 10 dB according to common sound level charts?
a) Thunderclap
b) Dishwasher
c) Normal breathing
d) Violin

Post-Test Answers & Explanations

Change in Intensity Change in Intensity Level
Multiply/divide by two Add/subtract 3 dB
Multiply/divide by ten Add/subtract 10 dB

Question 1: b) 3 dB

According to decibel rules of thumb, doubling the intensity (from I to 2I) increases the sound intensity level by 3 dB:cite[10]. This is a fundamental relationship in acoustic measurements.

Question 2: b) 10 dB

When sound intensity increases by a factor of 10, the sound intensity level increases by 10 dB:cite[10]. This logarithmic relationship is why decibels are useful for representing large ratios compactly.

Question 3: b) 83 dB

When two identical sounds combine, their intensities add. Doubling the intensity increases the sound level by 3 dB:cite[10]. Therefore, two 80 dB sources produce 83 dB, not 160 dB, because decibels are logarithmic units, not linear.

Question 4: c) 1000

To increase from 80 dB to 110 dB requires a 30 dB increase. Each 10 dB increase requires multiplying intensity by 10. Therefore, 30 dB requires 10 × 10 × 10 = 1000 times the intensity:cite[10]. Since intensities add, we need 1000 sources.

Question 5: a) 30 dB

For power ratios, dB = 10 × log₁₀(P₂/P₁). A ratio of 1000:1 gives 10 × log₁₀(1000) = 10 × 3 = 30 dB. This formula is essential for electrical engineering power calculations.

Question 6: b) 20 dB

For voltage ratios (assuming same impedance), dB = 20 × log₁₀(V₂/V₁). A ratio of 10:1 gives 20 × log₁₀(10) = 20 × 1 = 20 dB. Note the different multiplier (20) for voltage/pressure vs. power/intensity (10).

Question 7: a) 10 times more intense

A 10 dB increase corresponds to a 10-fold increase in intensity:cite[10]. Normal breathing at 10 dB is barely audible and is used as a reference for almost inaudible sounds:cite[1].

Question 8: c) 90 dB

If a sound is 1000 times more intense, this represents three factors of 10 (10 × 10 × 10). Each factor of 10 adds 10 dB, so 3 × 10 dB = 30 dB increase. Adding 30 dB to 60 dB gives 90 dB:cite[10].

Question 9: d) 10 W

SNR = 10 × log₁₀(Psignal/Pnoise) = 40 dB. Therefore, log₁₀(Psignal/Pnoise) = 4, so Psignal/Pnoise = 10⁴ = 10,000. With Pnoise = 1 mW, Psignal = 10,000 mW = 10 W.

Question 10: c) Normal breathing

According to common sound level charts, normal breathing produces sound at approximately 10 dB:cite[1]. This represents an almost inaudible sound level that serves as a good reference point for the lowest end of the audible range.