When using a microphone for live sound reinforcement, one of the most common and frustrating issues is feedback or "howling." This occurs when the sound from the speakers is picked up again by the microphone, creating a loop that amplifies itself, resulting in an unpleasant high-pitched noise. While this phenomenon is often referred to as positive feedback, it’s not always easy to eliminate, even with modern technology.
Sound engineers have tried numerous methods and devices over the years to tackle this issue, but many of them come with limitations. Based on my own experience, here are some techniques that can be used, depending on the situation:
1. **Feedback Suppressors**: These devices detect frequencies that exceed a certain threshold and automatically reduce them to prevent howling. They work reasonably well in fixed setups like conferences, but they struggle during dynamic performances where the singer moves around. If the suppressor misidentifies a normal frequency as feedback, it could accidentally mute parts of the vocal, ruining the performance.
2. **Frequency Shifters**: These tools shift the microphone signal slightly up or down in frequency to avoid feedback. While effective in controlled environments, they’re not ideal for live shows because the pitch change can make the voice sound unnatural and confusing to the audience.
3. **Automatic Mixing Consoles**: These systems use noise gates to turn microphones on and off based on whether there's a signal. They're great for conference settings or language-based programs, but less so for music performances, where sudden silences due to low volume can disrupt the flow.
4. **Noise Gates**: A basic form of control, these gates only allow the microphone to pass through when the sound level exceeds a set threshold. They are commonly used for drum mics, as they only activate when the drum is hit, preventing feedback between the speaker and the mic. However, they aren’t suitable for vocal performances where the voice is soft or inconsistent.
5. **Compressors**: These help manage dynamics by reducing the volume of loud signals. While useful for controlling sudden peaks, they don’t address howling that occurs below the threshold, which can still cause problems.
6. **Graphic Equalizers**: These allow you to cut specific frequency bands that tend to cause feedback. However, since feedback frequencies can vary and aren’t always consistent, it's hard to predict exactly which ones to target. Overuse can lead to distorted sound, making them better suited for lower-demand environments.
7. **Parametric Equalizers**: These offer more precision than graphic EQs, allowing you to fine-tune specific frequencies. By identifying the exact frequency causing feedback and adjusting its bandwidth, you can reduce the problem without affecting the overall sound quality. This makes them ideal for a wide range of applications.
In practice, I often combine a parametric EQ with a compressor on each microphone channel, along with a noise gate on the drum channels. The setup requires investment, but it offers the best balance between sound quality and feedback control. It’s also important to test with the actual microphones used during the performance, not just a test mic, since real-world conditions can vary significantly.
Chest mics (lavalier mics) are particularly prone to feedback due to their omnidirectional pickup pattern and placement near the body. Since the mouth is often far from the mic, the signal can be too strong, increasing the chance of feedback. To mitigate this, you can slightly reduce the gain at the mic or mixer. For stage performances, it's better to use condenser or dynamic lavaliers rather than electret mics, which may introduce distortion and are more sensitive to feedback.
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