Microphones in Telecommunication

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By the end of this topic, you should be able to:

Introduction:

Microphones play a central role in telecommunications, where the primary goal is to capture intelligible, clean, and consistently understandable speech under a wide range of real-world conditions.
Unlike in music or film production—where tonal richness or spatial realism may be desired—the telecommunications sector prioritizes:

• Speech clarity
• Noise robustness
• Minimal coloration
• Stable level independent of head movement
• Easy integration into wearable or embedded devices

Because it is often necessary to transmit speech over narrow-band or bandwidth-limited channels, the signal must be free from reverberation, background noise, and room coloration. Therefore, dry recording conditions, with minimal reflections, are generally preferred

Audio Recording Techniques in Telecommunications:

Telecommunication systems rely on microphone types and placements optimized for speech capture in close proximity, often under noisy and unpredictable conditions. Below are the major categories.

Near-Field Microphones (e.g., Headsets)

A near-field microphone is positioned just a few centimeters from the mouth.
Advantages:

• Strong direct-to-reverberant ratio
• High signal-to-noise ratio (SNR)
• Less variation with head movement
• Ideal for VoIP headsets, gaming headsets, call-center applications

Technical note: Proximity effect may boost low frequencies in directional microphones, but in telecommunication devices, DSP equalization often compensates for this.

Microphone Worn Directly in Front of the Mouth:

Seen in:

• Broadcast headsets
• Sports communication
• Aviation headsets
  The microphone element is usually directional (often cardioid or hypercardioid) to suppress environmental noise.

Advantages:

• Highly consistent level
• Good rejection of ambient noise
• Minimal influence of room acoustics

Lavalier Microphones:

A lavalier microphone is a miniature microphone clipped onto a shirt collar or lapel.

Features:

• Omnidirectional or cardioid
• Captures speech while allowing freedom of movement
• Often used in broadcasting, conferencing, wearable devices

Limitations:

• Further from mouth → more ambient noise
• Clothing friction noise
• Coloration due to chest reflections

Telecommunication systems often use DSP to counteract these issues.

Miniature Body-Worn Microphones:

These extremely small microphones are:

• Integrated into clothing
• Hidden for consumer electronics (e.g., earbuds)
• Used in hearing aids and AR/VR systems

Modern units are usually MEMS microphones, valued for:

• Tight tolerances
• Low power consumption
• Built-in digital output options
• Good robustness and consistency

Surface Microphones:

A surface microphone (boundary microphone) is mounted on a table or solid surface.

Useful in conference rooms:

• Reduces comb-filtering caused by desk reflections
• Widens pickup area for multiple talkers
• Works well with teleconferencing DSP algorithms

DSP Algorithms in Telecommunications:

Telecommunications relies heavily on Digital Signal Processing (DSP) to optimize speech capture:
✔ Noise Reduction:
Suppresses stationary and non-stationary noise using:

• Spectral subtraction
• Statistical model-based estimators
• Deep-learning speech enhancement

✔ Dereverberation:
Reduces late reverberation that reduces intelligibility.

✔ Echo Cancellation:
Important in speakerphones.

✔ Source Separation:
Used in multi-microphone devices to isolate individual speakers in meetings.

Critical Distance / Reverberation Radius:

A key concept in microphone placement—especially when aiming for clean speech capture—is the critical distance (or reverberation radius).

Definition
The distance from the sound source at which the direct sound level equals the diffuse (reverberant) sound level.

Mathematically:

Or in intensity:

Implication:

• Inside the critical distance → direct sound dominates → ideal for speech and telecommunication capture.
• Outside the critical distance → reverberation dominates → speech becomes less intelligible.

Telecommunications microphones are therefore kept well inside the critical distance, usually within 5–15 cm.

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