CTS Audio Systems and Acoustics — Questions and Answers
Question 1: A microphone with a cardioid polar pattern is most effective at rejecting sound from which direction relative to the front of the capsule?
- 45 degrees off-axis
- 90 degrees off-axis
- 180 degrees off-axis (rear) (Correct answer)
- All directions equally
Correct answer: 180 degrees off-axis (rear)
A cardioid microphone provides maximum rejection directly behind the capsule (180 degrees off-axis). Sensitivity falls off gradually as you move from front (maximum) to sides (–6 dB) to rear (maximum rejection), making it ideal for reducing feedback from monitors placed behind the mic.
Question 2: What does the term 'gain structure' refer to in an audio signal chain?
- The physical arrangement of amplifiers in a rack
- Setting appropriate signal levels at each stage to maximize headroom and minimize noise (Correct answer)
- The impedance matching between audio devices
- The number of gain stages before the power amplifier
Correct answer: Setting appropriate signal levels at each stage to maximize headroom and minimize noise
Gain structure is the discipline of setting each device in the signal chain to operate at an optimal level—high enough above the noise floor to maintain signal quality, but with enough headroom below clipping to handle peaks without distortion. Poor gain structure is the leading cause of hum, hiss, and distortion in AV systems.
Question 3: What is the primary purpose of a graphic equalizer in an installed sound system?
- To boost all frequencies uniformly for louder output
- To correct for room acoustic anomalies and speaker frequency response irregularities (Correct answer)
- To convert balanced signals to unbalanced signals
- To control the dynamic range of the audio signal
Correct answer: To correct for room acoustic anomalies and speaker frequency response irregularities
A graphic equalizer allows the AV technician to cut or boost specific frequency bands, compensating for room modes, reflective surfaces, speaker coloration, and other acoustic issues that cause uneven frequency response at the listener position. It is a key room-tuning tool, not a volume control.
Question 4: Which impedance is the standard load impedance for professional loudspeaker systems driven by a power amplifier?
- 600 ohms
- 150 ohms
- 8 ohms (or 4 ohms) (Correct answer)
- 75 ohms
Correct answer: 8 ohms (or 4 ohms)
Professional power amplifiers are designed to drive loudspeaker loads of 4 or 8 ohms. Connecting loads below the rated minimum can overheat or damage the amplifier. 600 ohms is the classic professional line-level impedance standard; 75 ohms is a video/RF coax standard.
Question 5: The 'critical distance' in room acoustics is defined as the point where:
- The reverberant field level equals the direct sound level (Correct answer)
- The room reaches its maximum SPL capacity
- Bass frequencies no longer propagate
- The speaker's near field transitions to its far field
Correct answer: The reverberant field level equals the direct sound level
Critical distance (Dc) is the distance from a sound source at which the direct sound pressure level equals the reverberant field level. Inside the critical distance, direct sound dominates and speech intelligibility is high. Beyond it, reverberant energy dominates, degrading clarity—a key concept for loudspeaker placement in CTS design.
Question 6: A 70-volt distributed speaker system is preferred over a low-impedance system for which installation scenario?
- A home theater with a single pair of high-quality loudspeakers
- A small conference room needing precise stereo imaging
- A large facility requiring many speakers spread across long cable runs (Correct answer)
- A recording studio control room
Correct answer: A large facility requiring many speakers spread across long cable runs
70-volt (constant-voltage) systems allow many loudspeakers to be connected in parallel over long cable runs without significant power loss, because the high voltage keeps current (and thus resistive loss) low. Each speaker uses a transformer tap to draw its rated wattage. This is the standard approach for paging systems, retail stores, airports, and other large distributed audio installations.
CTS Audio Systems and Acoustics
A microphone with a cardioid polar pattern is most effective at rejecting sound from which direction relative to the front of the capsule?