Last Updated on March 5, 2024 by Muisc Pro Editorial Team
Microphone feedback is a familiar and unwelcome phenomenon at live events, characterized by a high-pitched screeching sound that can make both performers and audiences cringe. This feedback occurs when sound from one or more speakers in a sound system is picked up by an open microphone, amplified, and then sent back through the speakers again. This cycle can repeat multiple times, boosting certain frequencies each time, leading to the discomforting noise we all want to avoid. The main culprit behind this is the interaction between the microphone and speaker characteristics, alongside the unique acoustic properties of the room, which can amplify specific frequencies, making feedback more likely.
The root cause of microphone feedback lies in the technicalities of how microphones and sound systems operate in real environments. No microphone or speaker has a completely flat response; they all have tendencies to boost certain frequencies. For instance, an omnidirectional microphone designed to enhance clarity might have a peak at 6kHz. If your speaker system is also boosting those higher frequencies, the likelihood of mic feedback increases when the amplified sound is picked up by the microphone and re-amplified. This problem is exacerbated by the room’s acoustics, where dimensions can create ‘room modes’ that emphasize certain frequencies, making feedback more probable when these frequencies align with the boosts in microphone and speaker responses.
Strategies to Prevent Microphone Feedback
Knowing Your Equipment
To combat microphone feedback effectively, it’s crucial to start with a solid understanding of your equipment, particularly the microphones. Sound engineers often prefer directional microphones over omnidirectional microphones for their ability to focus on the source and reduce the risk of capturing sound from speakers. Recognizing the directional characteristics of microphones can be a significant first step in preventing feedback. Directional microphones, such as cardioid, hypercardioid, and supercardioid, are designed to pick up sound primarily from one direction. This design reduces the risk of capturing and amplifying sounds from speakers that are properly positioned relative to the microphone. However, it’s essential to note that hypercardioid and supercardioid microphones can be more sensitive to sounds coming from behind them, which could pose a challenge in certain stage setups, especially those using open wedge monitors.
Placement and Management of Speaker and Microphone
Sound engineers recommend the strategic placement of speakers and microphones to minimize the risk of feedback. Additionally, the judicious use of muting for unused open microphones can feedback stop by minimizing the number of open microphones at any given time. The aim is to position speakers in such a way that the sound they produce is less likely to be picked up by the microphones. This often means placing speakers in front of microphones and ensuring that performers are behind the main speaker system. In scenarios where the speaker system is small and needs to be placed behind the microphone for better performer audibility, the risk of feedback increases, underscoring the importance of strategic equipment setup.
One effective strategy to combat feedback is the use of in-ear monitors to replace traditional wedge monitors, which can significantly reduce the sound pressure level on stage and the likelihood of feedback .Furthermore, adjusting the sound system with a graphic equalizer to notch out the offending frequency can prevent specific frequencies from reaching the feedback threshold. Additionally, adhering to the inverse distance law, which states that doubling the distance between a sound source and its receiver reduces the sound level by 6dB, can significantly reduce feedback potential. Spacing microphones further apart not only decreases the likelihood of comb filtering but also ensures a cleaner, more linear sound. This principle of physics can be leveraged to improve overall sound quality and reduce feedback.
Microphone Handling and Technique
The way a microphone is handled can greatly influence its susceptibility to feedback. Omni-directional microphones, with their flatter frequency response, are generally less prone to feedback compared to cardioid microphones. However, altering the natural pickup pattern of a cardioid microphone by covering it, such as holding it around the ball, can inadvertently increase its feedback potential, mimicking an omni-directional response. Similarly, distancing a microphone from the vocalist may seem like a solution but can actually worsen feedback issues. This is because increasing the distance requires a boost in gain to maintain volume levels, thereby elevating the risk of feedback. Educating performers on proper microphone techniques is crucial in mitigating these feedback triggers and maintaining optimal sound quality.
The Physics of Sound on Stage
Feedback is further complicated by the physical properties of sound. All microphones on stage pick up sound from every source, not just the intended one. The distance between sound sources and microphones introduces a delay in sound reception, with every foot adding approximately one millisecond of delay. This delay can cause comb filtering when multiple microphones pick up the same sound source at different times, leading to a more complex and feedback-prone frequency response. The close proximity of microphones, such as two vocalists standing near each other, exacerbates this issue, highlighting the importance of strategic microphone placement on stage.
Utilizing Technology
Addressing the low-frequency build-up is essential, especially in smaller venues where room dimensions can amplify certain frequencies. The proximity effect, where a microphone boosts low-frequency sounds when the source is close, can overwhelm a system, especially in smaller, square rooms. Implementing high-pass filters on the mixing console or directly on microphones can effectively mitigate this low-frequency energy, decreasing the risk of feedback and allowing for a clearer, more balanced sound.
Mastering Microphone Feedback Control
Managing microphone feedback effectively is crucial for maintaining the clarity and quality of live sound. This involves understanding the root causes of feedback and implementing strategies to minimize its occurrence. By addressing feedback proactively, you can ensure a smoother, more enjoyable audio experience for both performers and audiences.
Identifying and Eliminating Problem Frequencies
A common indicator of potential feedback issues is when you notice you’re cutting the same frequency across multiple channels on your EQ. This situation suggests it’s time to “ring out” the room, a process aimed at identifying and attenuating problem frequencies that are prone to causing microphone feedback. While hiring a professional to tune your room is an option, there are also DIY approaches that can be quite effective. Many digital consoles are equipped with a Real-Time Analyzer (RTA) and graphic or parametric EQs on the output, allowing you to analyze and adjust the audio response of your room.
The process begins with playing pink noise through your speakers, which has an even distribution of frequencies. By placing a measurement microphone at the listener’s position and observing the RTA, you can identify frequency spikes indicating resonance or buildup in the room. Attenuating these specific frequencies on your EQ can significantly reduce the risk of feedback. However, it’s important to note that attempting to fill in gaps in the frequency response curve by adding EQ will not be effective due to the room’s geometry and acoustics.
Automated Feedback Suppression Solutions
For those overwhelmed by the manual process of feedback management, some mixers offer automated solutions. These systems can automatically tune the room and apply feedback suppression filters with minimal user intervention. The simplicity of such systems, often requiring just a push of a button to identify and eliminate feedback frequencies, makes them an appealing option for managing live sound environments. This technology represents a significant advancement in live sound management, providing an efficient and user-friendly approach to handling microphone feedback.
Practical Steps for Feedback Reduction
After identifying problem frequencies, the next step involves testing your system’s feedback threshold. With the pink noise turned off and all microphones open, gradually increase the main fader until feedback occurs. The problematic frequency will become apparent on the RTA, allowing you to adjust your output EQ accordingly. For those without an RTA-equipped console, smartphone apps can provide a makeshift solution. In some cases, intentionally inducing feedback before an event and using the mixer’s frequency recognition capabilities can preemptively address feedback issues, ensuring a smoother performance.
Conclusion
In conclusion, mastering the art of feedback control is a critical aspect of live sound engineering, ensuring that performances are delivered with the highest audio quality and without the interruption of microphone feedback. By understanding the underlying causes of feedback, applying strategic adjustments to equipment setup, and utilizing both manual techniques and automated technologies, sound engineers can effectively minimize and eliminate feedback. This not only enhances the audience’s listening experience but also supports performers in delivering their best work. Whether you’re a seasoned professional or a newcomer to live sound, the journey towards mastering feedback control is an ongoing process of learning and adaptation, ultimately leading to more successful and enjoyable live events.