When a microphone picks up sound, it is amplified by mixers, peripherals, power amplifiers, and speakers. The sound re-enters the microphone through direct radiation or sound reflections, causing the entire amplification system to generate positive feedback. This leads to self-oscillation of the electro-acoustic signal, resulting in a loud whistle from the speakers. This phenomenon is known as microphone feedback.
What Are the Harms of Microphone Feedback?
- It disrupts the atmosphere of the sound environment, making speakers or singers flustered and leaving the audience disappointed or even annoyed.
- It can significantly impact the high-frequency units of power amplifiers or speakers, potentially causing overload and damage.
- Feedback limits the gain and output power of the entire amplification system, preventing the volume from being increased further.
How Can We Avoid Feedback Whistles?
- Avoid placing the microphone in the radiation area of the speakers (or at least ensure it is not facing the speakers).
- Select the appropriate microphone for the situation. For environments with high background noise, choose a close-range pickup microphone (low sensitivity, cardioid or super-cardioid with frequency attenuation).
- Decorate the sound environment with suitable sound-absorbing materials, especially near the microphone, to minimize sound reflections.
- Ensure equipment is firmly connected to avoid poor soldering or unstable connections that could lead to resistance increases, attenuation, or voltage instability.
- Adjust equipment settings uniformly to prevent devices from operating at critical thresholds, which can cause signal instability or oscillation.
- Add feedback suppressors or frequency shifters to suppress and eliminate feedback whistles.
Whether in large stadiums or home karaoke setups, feedback is inevitable in systems with microphones and sound amplification. Microphone feedback occurs when the sound field of the speakers feeds back into the microphone. If the feedback coefficient exceeds 1, self-oscillation will occur. Even if the feedback coefficient does not exceed 1, as long as it approaches 1, the following problems can arise:
Issues Caused by Feedback
- The delayed effect of amplified sound entering the microphone produces a comb filtering effect, especially when the feedback coefficient is close to 1, narrowing the perceived sound field.
- Delayed feedback from the speaker sound field generates a series of echo delays, intensifying comb filtering and causing distorted reverb tails, known as "rigid noise distortion."
Effective Methods to Mitigate Feedback
Microphone Adjustment
Positioning the microphone further from the speaker or closer to the audience improves sound transmission gain. If possible, place the microphone in the back radiation direction of the speaker. If the microphone is handheld, suspend the speakers out of reach to minimize feedback risks.
Frequency Shifting
This method shifts the microphone signal's frequency by 2–8 Hz, preventing the speaker's sound field from feeding back to the original spectrum. While frequency shifting does not entirely eliminate feedback, it increases system gain by approximately 6 dB.
Phase Modulation
Introducing phase modulation disrupts the positive feedback loop required for self-oscillation. Experimentation shows optimal system stability when the phase deviation is within 140° and the modulation frequency is below 4.5 Hz to avoid perceptible distortion.
Delay
Delaying the feedback signal can effectively block feedback. For instance, in sports arenas, referee microphones often record signals before playback to prevent self-oscillation, even at high volumes.
Frequency Equalization
Using a room equalizer, frequency response peaks can be leveled to match average levels, increasing gain by at least 6 dB. Adjust each band systematically to achieve the desired balance while minimizing distortion.
Overall, preventing feedback and increasing system gain requires a combination of techniques such as proper microphone placement, frequency shifting, phase modulation, delay, and frequency equalization.
