11.5 Feedback and echo control
9 min read•august 19, 2024
Feedback and echo control are crucial aspects of architectural acoustics, impacting sound quality and in various spaces. Understanding their causes and implementing effective control methods can significantly enhance the acoustic performance of rooms and sound systems.
Techniques for managing feedback and echo include strategic placement of microphones and speakers, using directional microphones, and applying sound absorption and . These approaches, combined with electronic solutions like equalizers and feedback suppressors, help create optimal acoustic environments for different applications.
Feedback in sound systems
- Feedback is a common issue in sound systems that occurs when the output signal from loudspeakers is picked up by microphones and re-amplified, creating a self-sustaining loop
- Understanding the causes, frequency response, , and is crucial for designing and operating sound systems in architectural spaces
Causes of feedback
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- Occurs when the sound from loudspeakers is captured by microphones and re-amplified, creating a closed loop
- Insufficient isolation between microphones and loudspeakers can lead to feedback
- High gain levels in the sound system amplify the feedback signal, making it more pronounced
- Reflective surfaces in the room can direct sound from loudspeakers back to the microphones, increasing the likelihood of feedback
Frequency response and feedback
- Feedback tends to occur at specific frequencies where the sound system has peaks in its frequency response
- These peaks are determined by the combined frequency response of the microphones, loudspeakers, and the room acoustics
- Equalizing the sound system to reduce peaks in the frequency response can help mitigate feedback
- Using microphones and loudspeakers with flat frequency responses can also minimize the risk of feedback
Gain before feedback
- Gain before feedback (GBF) is a measure of how much gain can be applied to a sound system before feedback occurs
- Higher GBF values indicate a more stable sound system that is less prone to feedback
- GBF is influenced by factors such as microphone and loudspeaker placement, directivity, and the acoustic properties of the room
- Optimizing these factors can increase the GBF and allow for higher sound reinforcement levels without feedback
Feedback threshold
- The feedback threshold is the maximum gain level at which a sound system can operate without experiencing feedback
- It is determined by the GBF and the specific characteristics of the sound system and room
- Operating a sound system below the feedback threshold ensures stable performance without unwanted feedback
- Monitoring the sound system levels and adjusting them as needed can help maintain operation below the feedback threshold
Ringing and feedback
- is a sustained, tonal sound that occurs just before the onset of full-blown feedback
- It is caused by the sound system approaching the feedback threshold at specific frequencies
- Ringing can serve as a warning sign that the system is close to feedback and adjustments should be made
- Equalizing the sound system to reduce the gain at the ringing frequencies can help prevent feedback from occurring
Controlling acoustic feedback
- Controlling acoustic feedback is essential for achieving clear, intelligible sound reinforcement in architectural spaces
- Techniques such as microphone and loudspeaker placement, using directional microphones, graphic equalizers, and automatic feedback suppressors can effectively mitigate feedback
Microphone placement techniques
- Placing microphones closer to the sound sources (e.g., talkers or instruments) reduces the amount of sound from loudspeakers that is picked up, minimizing feedback
- Positioning microphones away from reflective surfaces prevents sound reflections from entering the microphones and causing feedback
- Using the minimum number of microphones necessary for adequate coverage helps reduce the overall feedback potential in the system
- Adjusting microphone angles and orientations can minimize the pickup of unwanted sound from loudspeakers
Loudspeaker placement techniques
- Placing loudspeakers closer to the audience and farther from microphones reduces the amount of sound that can be picked up by the microphones, lowering the risk of feedback
- Aiming loudspeakers away from microphones and reflective surfaces minimizes the direct and reflected sound entering the microphones
- Using multiple, distributed loudspeakers instead of a single, centralized loudspeaker system can provide more even coverage while reducing the overall sound level at any given location, which helps prevent feedback
Directional microphones for feedback control
- Directional microphones (e.g., cardioid, supercardioid, or hypercardioid) are more sensitive to sound coming from the front and less sensitive to sound from the sides and rear
- Using directional microphones helps reject sound from nearby loudspeakers, reducing the potential for feedback
- Selecting the appropriate microphone polar pattern based on the specific application and room acoustics can optimize feedback control
- Proper microphone placement and orientation are still important when using directional microphones to maximize their effectiveness in controlling feedback
Graphic equalizers for feedback control
- Graphic equalizers allow for precise adjustment of the sound system's frequency response
- Identifying and reducing the gain at frequencies prone to feedback (i.e., ringing frequencies) using a graphic equalizer can help prevent feedback from occurring
- Equalizing the sound system to achieve a flatter overall frequency response reduces the likelihood of feedback at specific frequencies
- Using a real-time analyzer (RTA) in conjunction with a graphic equalizer can help identify problem frequencies and guide the equalization process
Automatic feedback suppressors
- Automatic feedback suppressors are digital signal processing devices that continuously monitor the sound system for potential feedback
- When feedback is detected, the feedback suppressor automatically applies narrow notch filters to reduce the gain at the offending frequencies
- Feedback suppressors can react quickly to feedback and provide an additional layer of protection against feedback in sound systems
- While feedback suppressors are effective, they should be used in conjunction with proper microphone and loudspeaker placement and equalization techniques for optimal results
Echo in acoustic spaces
- Echo is a distinct, delayed repetition of a sound caused by reflections from surfaces in a room
- Understanding the causes, perception, and acceptable levels of echo is important for designing and managing the acoustics of architectural spaces
Causes of echo
- Echo occurs when a arrives at the listener's position more than 50-100 milliseconds after the direct sound
- Large, reflective surfaces (e.g., walls, ceilings, or floors) in a room can cause echo by reflecting sound back to the listener with sufficient delay and level
- Concave surfaces can focus sound reflections, leading to stronger and more noticeable echoes
- The distance between the sound source, reflective surface, and listener determines the and influences the
Echo vs reverberation
- While both echo and reverberation involve sound reflections in a room, they are distinct phenomena
- Reverberation is the overall sound field created by the accumulation of many reflections in a room, resulting in a prolonged decay of sound
- Echo, on the other hand, is a distinct, delayed repetition of a sound that is clearly distinguishable from the direct sound
- Reverberation is generally desirable in many acoustic spaces, as it enhances the richness and fullness of sound, while echo is often considered a defect that should be minimized
Flutter echo in small rooms
- is a rapid, repeating echo that occurs in small rooms with parallel, reflective surfaces
- It is caused by sound reflecting back and forth between two surfaces, creating a "ping-pong" effect
- Flutter echo is most noticeable at mid and high frequencies and can be highly distracting and detrimental to speech intelligibility
- Treating one or both of the parallel surfaces with sound-absorbing materials or diffusers can help mitigate flutter echo
Calculating echo delay time
- The echo delay time is the time difference between the arrival of the direct sound and the echo at the listener's position
- It can be calculated using the formula: , where is the echo delay time, is the distance traveled by the reflected sound, and is the speed of sound (approximately 343 m/s at room temperature)
- For example, if the reflected sound travels an additional 20 meters compared to the direct sound, the echo delay time would be: seconds or 117 milliseconds
- Understanding echo delay times can help in determining the placement of sound-absorbing materials or the design of room geometry to control echo
Perception of echo
- The human auditory system can perceive echo when the delay between the direct sound and the reflected sound is greater than the echo threshold (typically 50-100 milliseconds, depending on the type of sound and the listener's hearing ability)
- The perception of echo is influenced by factors such as the level and spectrum of the reflected sound, the type of sound source (e.g., speech or music), and the listener's expectations and familiarity with the acoustic space
- Echoes can be more noticeable and distracting for transient sounds (e.g., speech or percussive instruments) compared to continuous sounds (e.g., sustained musical notes)
- The presence of echo can reduce speech intelligibility, as the delayed reflections can mask or interfere with the direct sound
Acceptable echo levels
- The acceptable level of echo in a room depends on the intended use of the space and the type of sound source
- For speech-oriented spaces (e.g., classrooms, lecture halls, or conference rooms), the goal is to minimize echo to ensure high speech intelligibility
- In these spaces, the strength of the echo should be at least 10 dB below the level of the direct sound to be considered acceptable
- For music performance spaces, a certain amount of echo can be desirable to enhance the richness and spaciousness of the sound, but excessive echo can still be detrimental to and ensemble precision
- In general, the acceptable echo level should be evaluated based on the specific requirements and preferences of the users of the acoustic space
Controlling echo in rooms
- Controlling echo in rooms is crucial for achieving good speech intelligibility, musical clarity, and overall acoustic comfort
- Techniques such as sound absorption, diffusion, and considering the room shape and can effectively mitigate echo in architectural spaces
Sound absorption for echo control
- Sound-absorbing materials can be used to reduce the strength of reflections and control echo in a room
- (e.g., acoustic foam, fiberglass, or mineral wool) are effective at absorbing mid and high-frequency sound, which is particularly important for controlling flutter echo
- (e.g., perforated panels or Helmholtz resonators) can be tuned to absorb specific low-frequency ranges, helping to control low-frequency echo and
- Placing sound-absorbing materials on the surfaces responsible for the most problematic reflections (e.g., rear walls or ceiling) can significantly reduce echo and improve the overall acoustic quality of the space
Diffusion for echo control
- are designed to scatter sound reflections in various directions, reducing the coherence and strength of the reflected sound
- By breaking up strong, focused reflections, diffusers can help control echo and improve the spatial uniformity of the sound field
- Diffusers are particularly useful in spaces where a certain amount of sound reflection is desired for acoustic support or spaciousness, but the echo needs to be mitigated
- Combining diffusers with sound-absorbing materials can provide a balanced acoustic treatment that controls echo while maintaining a suitable level of reverberation
Critical distance and echo
- The critical distance is the distance from a sound source at which the direct sound level equals the reverberant sound level in a room
- Beyond the critical distance, the reverberant sound dominates, and the risk of echo increases
- Designing rooms to have a shorter critical distance (e.g., by increasing the amount of sound absorption) can help control echo and improve speech intelligibility
- In larger spaces where the critical distance is unavoidably long, using multiple, distributed sound sources (e.g., loudspeakers) can help reduce the distance between the listeners and the sound sources, minimizing the impact of echo
Room shape and echo
- The shape of a room can significantly influence the presence and severity of echo
- Parallel walls, floor, and ceiling can create strong, focused reflections that lead to flutter echo and other echo-related problems
- Concave surfaces (e.g., domes or curved walls) can focus sound reflections, causing strong echoes and uneven sound distribution
- Designing rooms with non-parallel, irregular, or sloped surfaces can help break up and scatter sound reflections, reducing the risk of echo
- Incorporating sound-absorbing materials or diffusers into the room design can further control echo and improve the overall acoustic quality of the space
Electronic echo cancellation
- In situations where physical acoustic treatment is not feasible or sufficient, can be used to reduce echo in audio systems
- Echo cancellation algorithms work by estimating the echo path between a loudspeaker and a microphone and generating an inverted copy of the expected echo signal
- The inverted echo signal is then subtracted from the microphone input, effectively canceling out the echo
- Echo cancellation is commonly used in teleconferencing, video conferencing, and other communication systems where echo can be a significant problem
- While electronic echo cancellation can be effective, it is not a substitute for proper room acoustic design and treatment, and it may introduce other artifacts or limitations in the audio signal
Key Terms to Review (29)
Acceptable echo levels: Acceptable echo levels refer to the intensity and duration of echoes in a space that do not disrupt the primary sound being produced. These levels are crucial in determining how sound is perceived within an environment, influencing clarity and intelligibility of speech or music. Achieving acceptable echo levels often involves managing sound reflections to create a balance that enhances the listening experience without overwhelming it with distracting reverberations.
Acoustic absorption: Acoustic absorption refers to the process by which sound energy is absorbed by materials, converting it into a small amount of heat rather than reflecting it back into a space. This is crucial in managing sound quality within environments, as it helps reduce reverberation and control feedback, which can otherwise lead to echoes and distortion in audio systems.
ANSI S1.26: ANSI S1.26 is a standard developed by the American National Standards Institute that focuses on the measurement of sound levels in various environments, particularly for assessing noise levels in relation to feedback and echo control. This standard establishes methodologies for measuring sound pressure levels and offers guidelines for achieving optimal acoustic conditions in spaces where sound reinforcement systems are used, ensuring that feedback and echo do not adversely impact audio clarity and performance.
ASTM E336: ASTM E336 is a standard test method developed by ASTM International for measuring the sound insulation of building elements. This standard provides procedures for evaluating airborne sound insulation performance, which is critical in feedback and echo control and can be applied in real-world architectural acoustics scenarios.
Bass trapping: Bass trapping refers to the acoustic treatment used to control low-frequency sound waves in a space, specifically to reduce the buildup of bass frequencies that can create muddiness and distort sound clarity. This is crucial in environments like recording studios or performance spaces where feedback and echo can hinder sound quality. By strategically placing bass traps in corners and along walls, the low frequencies are absorbed, improving the overall acoustic environment.
Ceiling treatments: Ceiling treatments refer to various architectural modifications and enhancements applied to ceilings in order to improve the acoustic properties of a space. These treatments can include the use of sound-absorbing materials, the design of ceiling shapes, or the installation of sound-diffusing elements, all aimed at controlling sound reflections and reverberations. By optimizing the ceiling design, these treatments play a crucial role in managing feedback and echo, ensuring clarity in speech and sound within an environment.
Clarity: Clarity refers to the intelligibility and perceptibility of sound in a given environment, particularly focusing on how well speech or musical notes are understood by the listener. It is an essential aspect of acoustic design, impacting the listener's experience by ensuring that sounds are distinct and easily recognizable, which is crucial for effective communication and enjoyment in various spaces.
Critical Distance: Critical distance is the distance from a sound source at which the direct sound level and the reverberant sound level are equal, creating a balance between clarity and richness in the sound field. This concept is essential for understanding how sound behaves in enclosed spaces, influencing design choices for acoustic quality in various environments.
Diffusion: Diffusion refers to the scattering of sound energy in various directions after it strikes a surface, which helps to create a more uniform sound field in a space. This phenomenon is crucial for improving room acoustics, as it minimizes the intensity of sound reflections and reduces the impact of echoes and standing waves, leading to better clarity and a more pleasant listening experience.
Echo delay time: Echo delay time is the interval between the original sound and the subsequent reflection that reaches the listener, contributing to the perception of echo in a space. This time duration plays a significant role in how sounds interact, affecting clarity and intelligibility in communication and musical performances. When echo delay time is too long, it can lead to confusion and diminished sound quality, emphasizing the need for effective feedback and echo control strategies.
Electronic Echo Cancellation: Electronic echo cancellation is a technology used to eliminate or reduce echo in audio communications, enhancing clarity and intelligibility. This process involves analyzing the incoming audio signal and subtracting any delayed versions of that signal, which helps to prevent feedback loops and improves the overall quality of sound transmission in various applications like teleconferencing and live sound reinforcement.
Feedback Threshold: Feedback threshold refers to the specific point at which sound generated by a loudspeaker is picked up by a microphone and re-amplified, creating a loop that can result in a loud, often undesirable sound known as feedback. Understanding this threshold is crucial in managing and controlling feedback and echo in audio systems, ensuring that sound reinforcement remains clear and intelligible without unwanted noise disruptions.
Flutter Echo: Flutter echo is a phenomenon that occurs in enclosed spaces when sound waves bounce back and forth between two parallel surfaces, creating a rapid series of reflections that can produce a distinct, repetitive echo effect. This effect can significantly influence the overall acoustic quality of a space, making it important to consider in the design and treatment of auditoriums, performance venues, and other environments where sound clarity is crucial.
Gain before feedback: Gain before feedback refers to the amount of amplification applied to an audio signal before it is sent through a feedback loop. This concept is crucial in managing how loud a sound system can be before it begins to produce unwanted feedback, which can lead to disruptive and harsh sounds. Properly adjusting gain ensures that the audio system can deliver clear sound without creating echoes or loops that degrade quality.
Impulse Response: Impulse response refers to the way a space reacts to a sound signal when a brief sound burst or 'impulse' is introduced. This reaction captures all the characteristics of how sound propagates in that environment, revealing details about absorption, reflection, and diffusion, and is crucial for understanding acoustic behavior in different settings.
Intelligibility: Intelligibility refers to the clarity and comprehensibility of speech or sound within a specific environment, significantly impacting how well individuals can understand spoken words. It is crucial in various acoustic settings, as it is influenced by factors such as background noise, reverberation, and the spatial arrangement of surfaces that affect sound propagation.
ISO 3382: ISO 3382 is an international standard that outlines methods for measuring the acoustic characteristics of rooms, specifically focusing on parameters such as reverberation time, early decay time, and clarity. This standard is vital in understanding how sound behaves in various environments and helps inform the design and evaluation of spaces for optimal acoustic performance.
Loudness Perception: Loudness perception refers to the subjective experience of the intensity of sound, influenced by various factors including sound frequency, duration, and the listener's hearing abilities. It connects to how we perceive music and speech, as well as the physiological aspects of hearing, the impact of background noise, and individual evaluations of sound environments.
Masking: Masking is the process by which the perception of one sound is affected by the presence of another sound, often making the first sound less audible. This phenomenon is important in various areas such as room acoustics, noise control, and audio engineering, as it can influence how sounds are experienced in a space, how noise levels are rated, and how sounds are processed by the human ear.
Octave Band Analysis: Octave band analysis is a method used to evaluate sound frequencies by dividing them into bands, each of which has a width of one octave. This technique allows for a detailed examination of sound characteristics and how they relate to human perception and various acoustic criteria. The division into octave bands is particularly useful in contexts like noise control, echo reduction, and ensuring effective sound isolation, where understanding the frequency content of noise is crucial for achieving desired acoustic performance.
Perception of Echo: The perception of echo refers to the auditory experience where a sound is heard after a delay, as it reflects off surfaces and returns to the listener's ears. This phenomenon is crucial in understanding how sound travels in spaces, affecting clarity and quality of audio, especially in environments like auditoriums or concert halls where sound reflections can influence communication and musical experiences.
Porous absorbers: Porous absorbers are materials that reduce sound energy through absorption, relying on their porous structure to trap air and convert sound waves into heat. These materials play a crucial role in improving acoustics by minimizing sound reflections and enhancing speech clarity, making them vital in various architectural applications.
Rayleigh Criterion: The Rayleigh Criterion is a formula that determines the minimum angular separation between two point sources of sound or light that can be resolved by a given receiver, such as an ear or a microphone. This concept is essential in understanding how feedback and echoes can occur in acoustical systems, as it influences the clarity and intelligibility of sound by dictating the limits of spatial resolution in auditory perception.
Resonant Absorbers: Resonant absorbers are specialized acoustic devices designed to reduce sound reflections by absorbing specific frequencies of sound waves through resonance. They work by utilizing the principle of resonance, where the natural frequency of the absorber matches the frequency of the sound wave, allowing for efficient energy absorption and minimizing undesirable sound effects in various environments.
Reverberation Time: Reverberation time is the duration it takes for sound to decay by 60 decibels in a space after the source of the sound has stopped. This measurement is crucial because it influences how sound behaves in a room, affecting clarity, intelligibility, and overall acoustic quality.
Ringing: Ringing refers to a sustained oscillation in sound levels, often perceived as an unwanted or distracting echo that can occur in acoustically lively environments. It is typically caused by the interaction of sound waves with the surfaces of a room, leading to a prolonged sound after the original sound source has stopped. Understanding ringing is crucial for controlling reverberation times and managing feedback in audio systems.
Room Modes: Room modes refer to the resonant frequencies that occur in a room due to the dimensions of the space, impacting how sound behaves within that environment. These modes are influenced by the speed of sound and the frequency and wavelength of sound waves, creating unique patterns of sound reinforcement and cancellation that are crucial for understanding acoustic quality.
Sound Diffusers: Sound diffusers are acoustic devices designed to scatter sound waves in different directions, reducing echoes and creating a more balanced sound field within a space. By breaking up and dispersing sound energy, they help to minimize reflections that can lead to feedback or unwanted echo, which is crucial for achieving clarity in sound environments like concert halls or recording studios.
Sound reflection: Sound reflection is the phenomenon where sound waves bounce off surfaces and travel in different directions after encountering an obstacle. This process plays a critical role in how we perceive sound in spaces, influencing clarity, volume, and the overall acoustic experience in environments like performance venues and everyday settings. The design of these spaces takes into account sound reflection to enhance both music and speech intelligibility, while managing unwanted feedback and echoes.