The Recording Environment part 1
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This document discusses sound waves and room acoustics. It explains that sound travels as longitudinal waves through air and other substances. When sound waves hit surfaces in an enclosed space, they are reflected and create reverberation over time. The time it takes for reverberation to decay by 60dB is known as the reverberation time or RT60, which provides an objective measurement of room acoustics. Room reflections are important for both the direct sound picked up by microphones and the diffuse room tone, which conveys information about the size and surfaces of the space.
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The Recording Environment part 1
- 1. The Recording Environment stuart.jones3@southwales.ac.uk www.stuartjones.org
- 2. Agenda Sound Waves Room Acoustics Sound Wave Reflections Absorption Coefficients The Sabine Equation Reverb Calculation Example Estimating the Reverberation Time Reverb Calculation Example 2 Correcting the Reverberation Time Absorbers Reverberation
- 3. Sound Waves Sound Propagation The Recording Environment
- 4. What is Sound? Sound is produced when an object (the source) vibrates and causes the air around it to move. The Recording Environment
- 5. Sound Propagation • Sound travels in air as a lengthwise or longitudinal wave. This sort of density wave is the way in which sound is transmitted through air, gases and liquids. • In solids we also find transverse waves. The Recording Environment
- 6. Sound Propagation Longitudinal Wave The Recording Environment Snell's Illustration of Sound Waves http://www.youtube.com/watch?v=OQPI5Ng-3vI&list=PL689066FDE3D631CE
- 7. Sound Propagation Transverse Wave Disturbance takes place perpendicular to the direction which it is moving The Recording Environment
- 8. Sound Propagation Transverse Wave Machine The Recording Environment http://www.youtube.com/watch?v=tihcRFWeZlQ&list=PL689066FDE3D631CE
- 9. Clip taken from - Sound Waves and their Sources (1933) The Recording Environment
- 11. Sound Waves Sound Wave Properties The Recording Environment
- 12. Sound Waves: Fundamental Mathematical Relationships The Recording Environment
- 13. Time Amplitude + 0 - t The rate at which the source oscillates is the frequency of the sound wave it produces, and is quoted in hertz (Hz) or cycles per second (cps). 1000 hertz is termed 1 kilohertz (1kHz) The Recording Environment
- 14. In air, the speed of sound is approximately 340 meters per second The frequency and wavelength of a sound wave are related very simply if the speed of the wave (usually denoted by the letter c) is known. c = fλ (speed of the wave = frequency x wavelength) or λ = c/f (wavelength = speed of the wave x frequency) (λ = The Greek letter lambda is often used to represent wavelength) The Recording Environment
- 15. time (ms) 2 4 6 8 10 12 14 16 18 20 22 24 ms 0.7 1.4 2.0 2.7 3.4 4.1 4.8 5.4 6.1 6.8 7.5 8.2 m distance (m) Relationship between elapsed time and traversed distance for the propagation of a sound wave in air The Recording Environment
- 16. Room Acoustics The Recording Environment
- 17. Influence of Acoustical Space on the Sound Event The Recording Environment
- 18. In enclosed performance spaces, a new phenomenon appears: reverberation, which is caused by sounds being repeatedly reflected from all surfaces and objects in the room. The Recording Environment
- 19. Room influences may be described in two ways: 1. Objectively through measurement of the sound events and their variation with time (room acoustics. 2. Subjectively through verbal description of the audible experience (aural acoustics) Both methods are necessary depending on the question at hand; either the objective or subjective one may assume the greater importance. The Recording Environment
- 21. Fundamentals of Aural Acoustics The Recording Environment
- 22. Fundamentals of Aural Acoustics 1. The Listenability of a Room - Generally describes its suitability for certain sound events.e.g Good listenability of a room for speech means that we hear speech well at every seat in the house without the need for reenforcement. 2. The Transparency of a Room - The ability to differentiate between simultaneously played instruments or instrument groups in spite of superimposed room reverberation. Transparency is a basic requirement when we hear complex musical structures. The Recording Environment
- 23. Fundamentals of Room Acoustics The Recording Environment
- 24. Sound propagation in an enclosed room (the rays show the propagation direction and intensity) The Recording Environment
- 25. Direct Sound First Reflections Reverberation Sound Level Time Direct Sound and diffuse sound (reverb build-up, early reflections and decay) in an enclosed space The Recording Environment
- 26. RT60 The term RT60 refers to the time it takes the reverb to decay by 60dB. RT is measured at the point at which the reverb decays to -60dB of its peak level. The Recording Environment
- 28. • Direct sound arrival is followed by reflections from room surfaces. • Overlapping reflections are heard as reverberation. • Direct-to-Reverberant ratio gives cues to size of room, type of room surfaces, and distance from source. The Recording Environment
- 29. Significance of Room Tone for Microphone Placement and the Listening Experience The Recording Environment
- 30. Significance of Room Tone for Microphone Placement and the Listening Experience • The microphone generally picks up both direct and diffuse sound. • While the direct sound is influenced little by the nature of the room, the diffuse room tone transmits information about the room size and the nature of the wall treatment. • The acoustical attributes of the room tone provide information about the cultural and social environment into which a musical performance has been placed. The Recording Environment
- 31. Significance of Room Tone for Microphone Placement and the Listening Experience • Thus church music requires the acoustics of a large church for which it generally is written; symphonic music is written for concert halls, chamber music for the small, private room in a castle or home. • Folk music needs the intimate atmosphere of a pub. • In pop music and other similar musical forms, we see the creation, through the use of artificial reverberation of new acoustical surroundings which really do not exist in real life. The Recording Environment
- 32. Sound Wave Reflections The Recording Environment
- 33. Sound Wave Reflections Creation of Sound Reflections The Recording Environment
- 34. Sound Wave Reflections Creation of Sound Reflections 1. Sound reflections within a room occur when sound reaches a boundary surface without too much absorption. 2. Dimensions and nature of the surface determine how the diffused sound is scattered. The Recording Environment
- 35. Sound Wave Reflections The Haas Effect The Recording Environment The Haas effect can be summarised as follows: • The ear will attend to the direction of the sound that arrives first and will not attend to the reflections providing they arrive within 30 ms of the first sound. • The reflections arriving before 30ms are fused into the perception of the first arrival. However, if they arrive after 30ms they will be perceived as echoes. Angus, J & Howard, D (2009) Acoustics and Psychoacoustics. UK, Elsevier.
- 36. Comb Filtering The Recording Environment
- 37. direct path Diagram showing an example of a comb filter created by the combining of two signals with the same amplitude, but with a time delay between them of just 1 ms. Comb Filtering A.C.M.E Mixing Desk The Recording Environment speaker
- 38. Mic Reflected Sound Sound Source Direct Sound Surface Resulting Frequency Response dB An affected waveform shows lots of sharp peaks and troughs that look not unlike the teeth of a comb The Recording Environment
- 39. Splayed Ceiling Design Ceiling reflections cause acoustic interference at the listeners position The Recording Environment
- 40. Splayed ceiling reduces unwanted reflections The Recording Environment
- 41. Standing Waves
- 42. Pressure Max Min Max Min Max Min Max. Boundary Maximum Minimum Pressure Reflection The formation of a standing wave by reflection at a boundary
- 43. Room Resonances, Natural Frequencies & Modes The Recording Environment
- 44. Image showing room resonances, natural frequencies & modes
- 45. Boundary Boundary F 2F 3F 11.3 Feet 100Hz 200Hz 300Hz Image showing room resonances, natural frequencies & modes 100Hz The Recording Environment The same effect will occur at frequencies that are multiples of 100Hz (200Hz, 300Hz, etc)
- 46. Node Anti Node The Recording Environment
- 47. The Recording Environment stuart.jones3@southwales.ac.u k www.stuartjones.org
Editor's Notes
- Sound is produced when an object (the source) vibrates and causes the air around it to move
- Consider the sphere shown here. It is a pulsating sphere which could be imagined as something like a squash ball, and it is pulsating regularly so that its size oscillates between being slightly larger than normal and then slightly smaller than normal. perpendicular - at an angle of 90° to a given line, plane, or surface: dormers and gables that extend perpendicular to the main roofline.
- Consider the sphere shown here. It is a pulsating sphere which could be imagined as something like a squash ball, and it is pulsating regularly so that its size oscillates between being slightly larger than normal and then slightly smaller than normal.
- Consider the sphere shown here. It is a pulsating sphere which could be imagined as something like a squash ball, and it is pulsating regularly so that its size oscillates between being slightly larger than normal and then slightly smaller than normal.
- The individual particles of air only oscillate to and fro, therefore, the sound waves does not transport particles but energy: sound energy.
- In enclosed performance spaces, a new phenomenon appears: reverberation, which is caused by sounds being repeatedly reflected from all surfaces and objects in the room.
- http://www.ted.com/talks/david_byrne_how_architecture_helped_music_evolve.html Here David Byrne talks subjectively through verbal description of the audible experience (aural acoustics)
- When sound waves from a sound source spread within an acoustical enclosure, they quickly meet up with walls, ceilings and floor and are reflected from there as sound reflections. These reflections meet up with these room boundaries again and again, resulting in an increase in the density of these reflections with time, as they arrive at a point within the room - at a microphone or listener.
- Since the sound wave lose energy with each reflection thought absorption, the intensity of the reflections, and with them the intensity of the reverb weaken rapidly following the end of the sound event. Reverberation time is the measure of that behaviour.
- The comb filter function is almost never intentional, but it is heard all the time in sound productions, where it can arise both acoustically and electrically. Acoustically, it typically occurs when the sound on its way from source to recipient takes in part a direct path and in part an indirect path via a single reflective surface. The reflection must be attenuated at least 10 dB and preferably 15 dB in order for it not to have an effect on the sound field at the recipient position. Electrically, the phenomenon arises when two microphones with a certain distance between them capture the same signal and the level from each microphone is of the same order of magnitude
- One significant effect of hard boundary surfaces is the formation of what are called standing waves. This figure shows what happens when a continuous sound, at one frequency, strikes a reflective boundary head-on. The reflective sound wave combines with subsequent incoming waves. Where the wave crests (maximum pressure) coincide, they combine and reinforce one another. The troughs (minimum pressure) also combine.......... Note from the previous slides how standing waves affect low frequencies more than high.
- Suppose a point source (shown here in the middle) emits a brief tone. The sound waves travel out in all directions, with those frequencies propagated to the sides eventually reaching the walls. Some energy is absorbed by the walls; most is reflected back. Reflected waves from each wall travel to the other wall, reflecting again. The process continues until the energy of the sound is completely dissipated by absorption in the air and walls. In this situation, standing waves will be formed if - and only if- the wavelength of the sound ‘fits’ the distance between the walls. Such standing waves are also called room resonances, natural frequencies or modes.
- Suppose a point source (shown here in the middle) emits a brief tone. The sound waves travel out in all directions, with those frequencies propagated to the sides eventually reaching the walls. Some energy is absorbed by the walls; most is reflected back. Reflected waves from each wall travel to the other wall, reflecting again. The process continues until the energy of the sound is completely dissipated by absorption in the air and walls. In this situation, standing waves will be formed if - and only if- the wavelength of the sound ‘fits’ the distance between the walls. Such standing waves are also called room resonances, natural frequencies or modes.
- You will have noted that the walls in the last figure where 11.3ft apart. This is the wavelength of a frequency of 100Hz. Therefore, successively reflected waves will reinforce each other, forming stationary nodal and anti nodal points in the room. The same effect will occur at frequencies that are multiples of 100Hz (200Hz, 300Hz, etc).
- Good acoustic design takes room resonances into account, and strives to minimise them through use of non-parallel walls and various types of absorptive treatments.