Approach to Sound System Design

1. Approach to Sound System Design • Sound: a little bit of Physics • SPL and sound propagation in free field • Room Acoustic: some useful definitions • Intelligibility • Sound System Design: some suggestions • Q & A

2. SOUND is produced by vibrating objects. These move air, “pushing” and “pulling” from its resting state These small fluctuations in air pressure travel away from the source at relatively high speed, gradually dying off as their energy is absorbed by the medium. What we call sound is simply the sensation produced by the ear when stimulated by these vibrations.

3. PROPERTIES OF SOUND A sound wave is a series of pressure changes moving through the air. • Amplitude(dB) is the difference between maximum and minimum pressure: defines the loudness Frequency(Hz) is the rate at which the pressure changes occur: defines the pitch and timbre of the sound • Wavelength(m) is the physical distance between two maxima( or minima): depends on the speed of sound in the medium and on the frequency: • V = l * f • [Velocity = Wavelength * Frequency]

4. Sound Speed(m/s) Refers to the speed of travel of the sound wave. This varies between mediums and is also dependant on temperature. In other materials, the speed of sound can vary quite substantially.

5. AUDIBLE RANGE The ear can hear sounds ranging from 20Hz to 20kHz. It is most sensitive to frequencies between 500Hz and 4000Hz, which corresponds almost exactly to the speech band.

6. MEASURABLE CHARACTERISTICS Just how can we measure a sound? Acoustic Power (Watts)Measures energy output by a source, that sound's ability to do work Intensity (W/m²)The amount of sound energy within a specific area normal to the direction of propagation Pressure (Pa)Measures fluctuations about the local atmospheric pressure. Use of root-mean-square (rms) rather that peak-to-peak measures.

7. Sound pressure level (SPL) or sound level Lp is a logarithmic measure of the rms pressure (force/area) of a particular sound relative to a reference sound source. It is usually measured in decibel (dB(SPL), dBSPL, or dBSPL). It can be useful to express sound pressure in this way when dealing with hearing, as the perceived loudness of a sound correlates roughly logarithmically to its sound pressure

9. Sound Power refers to the absolute power of a sound source (in Watts) whereas Sound Power Level refers to the magnitude of that power relative to a reference power (in dB). The sound pressure ( dB) of a given speaker can be easily calculated knowing the sensivity and the driving power (W) SPL= Sensivity + LW

10. Sound propagation in free field

11. Sound propagation in free field Walking away from a sound source, the perceived level of the sound decrease This is known as the standard inverse square lawfor point sources. Practically results in 6 dB reduction in relative intensity per doubling of distance. NOTE: 1 dB increase is barely audible 3 dB is a generally noticeable change 10 dB is considered as twice as loud

12. Mathematically looks like New level= Old level + 20xlog(old distance)- 20xlog(new distance) L*=L+20logD - 20logD*

13. Sound: Wind & Temperature Gradients

14. Sound and Barriers: a matter of wavelength

15. Room Acoustic Sound waves will propagate away from the source until they encounter one of the room's boundaries where, in general, some of the energy will be absorbed, some transmitted and the rest reflected back into the room.

16. Part of the sound emitted from the source will go directly to the listener, part will be absorbed, and reflected by walls. The indirect sound after several reflections from different surfaces becomes “ diffuse” creating a steady state field . In the created field the sound does not have directivity and the inverse square low doesn’t hold anymore. This is called reverberation

17. When the sound source is turned off, direct sound will stop and only the reverberant field will remain After some seconds even the reverberant field decays. The length of time taken for a sound to decay 60 dB after the source has ceased transmitting is defined as Reverberation time

18. Room Acoustic depends on: Primary: volume, shape,linear dimensions • Volume: defines if a sound reinforcement system is needed or not. Defines directly the reverberation time • Shape: flat, parallel walls, domes, defines echoes and reflections These are fixed and can be hardly changed Secondary: Walls, Ceiling, Materials, Furniture Treatment can be suggested to improve the room acoustic

19. Different materials reflect sound in different way:

20. Marble Carpet with foam base

21. Reverberation time, RT60, depends on room dimensions and absorption of the walls whereRT is the reverberation time in seconds,V is the volume of the room in cubic meters, is the average absorption coefficient of the room, andS is the total surface area of the room in square meters The reverberation time affects most of the acoustic features of the room.

22. In acoustic, rooms with smaller reverberation times are appropriate for speech, whereas spaces designed for music require longer reverberation times.

23. More complex equations was developed to take care of different environment

24. In every room coexist a Direct Sound and a Reverberant Field There is a point in which the Direct SPL and Reverberant SPL are equal. This point is at a distance , from the source, called CRITICAL DISTANCE Where Q is the directivity of the source Every point farther than the Dc from the source will hear just the Reverberant field. The inverse square law is no more valid

25. What is a “good sound? Fidelity. Is given by the frequency response. It depends on each item of the audio chain Loudness: must be sufficient to achieve the required effect. Is determined by the dynamic range of the sound system Intelligibility: is linked by the signal/noise ratio and the direct-to-reverberant sound ratio at listener’s ear.It depends directly on room acoustic Room Acoustic is as important as the sound system itself.

26. Audibility  Clarity ability to detect the structure of a sound ability to hear a sound This distinction is more important in speech than in music

27. Speech • Is made of consonant and vowels • Vowels range from 250-500hz, carry power • Consonants range from 1-4kHz, carry information Lose consonants = Lose intelligibility

28. Intelligibility Is not a physical quantity as Ampere, Volt, Watt Measure of the degree of understanding spoken language There are many index to express this degree, many way to measure, and predict it

29. Factors Affecting Intelligibility In on-to-one conversation there aren’t any problems of intelligibility Sound System Bandwidth and Frequency Response Room Reverberation Geometric Factors Signal-to-Noise Ratio Distortions Non Linear Factors

30. Bandwidth and Frequency Response Sound system have to guarantee a response from 100 to 10000 Hz. Limits are fixed by worse performance

31. Signal to Noise Ratio SPL must be adequate and heard comfortably (normal conversation 70-90 dB) Noise masks direct sound and lowers intelligibility Increasing S/N ratio increases intelligibility Intelligibility becomes independent from S/N for S/N>25 dB

32. Reverberation and Reflections Long RT60’s decrease intelligibility Late reflections (> 50 ms) smear and blur direct speech Early reflections ( < 35-50 ms) are perceived as reinforce

33. Distortion Clipping Are form of NOISE Intermodulation Acoustic distortion Specification of various items that compose the sound system have to be carefully studied

34. Measure and Predicting Intelligibility Design for speech intelligibility is as important as design for gain, SPL and coverage While it is quite easy to calculate SPL and RT60 there aren’t models to calculate Intelligibility degree taking care of all parameters There are more way and several index to express Intelligibility Degree Subject Based ( AI, %ALCONS) Quantitative ( STI, RaSTI)

35. Predicting %ALCONS ALCONS is an index expressing Intelligibility degree, in terms of lost consonants in the talker-listener path The simplest Peutz formula take care of Directivity, RT60, Room Volume, Number of Speakers, Distance Loudspeaker-Listener The modified Peutz formula includes also Direct SPL, Reverberant SPL, and Noise SPL

36. %ALCONS INDEX High Q’s and Large V’s improve %ALCONS Long D’s, long RT60’s lowers %ALCONS This formula fails when strong non-linear effect are present

37. STI and RaSTI These methods are fully independent of human being and are fully quantitative Take care of all factors affecting the intelligibility because measures the corruption of a speech based signal during the talker-to-listener path Varies from 0 = no intelligibility to 1= perfect intelligibility

38. STI and RaSTI main features: Replace speech with a high frequency noise (consonants-vowels) modulated in amplitude by a low frequency signal (phonems) Knowing the m(f) means predict intelligibility

39. Alcons and RaSTI are linked

40. Common Intelligibility Scale (CIS) There is a common scale to simplify to define the limits of acceptable intelligibility CIS=0.7 Standard CEI EN60849 states that CIS> 0.7 %ALcons=12 STI=0.5

41. Speech Intelligibility Optimisation: Practical Criteria Sound quality and intelligibility are not the same thing • Aim the loudspeaker to the listener: keep as much sound as possible off the walls and the ceiling • Provide a direct line between loudspeaker and listener • Ensure adequate bandwidth • Avoid frequency response anomalies (corner bass increment) • Minimize D where possible • Ensure S/N ratio>10dB • Avoid delays> 50ms ( inter speaker spacing< 15m) • Use high Q in reverberant environment • Minimize SPL variations • Improve RT and acoustic environment

42. A sound system is basically composed of • Electro-Acoustic components ( speakers, microphones detectors) • Electronic items (mixer, amplifier, digital processors, music/message sources) • Environment ( Room Acoustic, RT60) Any result is a mesh of these components, and the lower quality component will lower the performance of all the others together

43. A Sound Reinforcement system is a system for accurately amplifying, reproducing, and sometimes recording audio, so that persons not near the original source may experience the sound as if they were. PA system, controls to mix the signals coming from the various microphones or other input sources (such as Tuner, CD, MP3 and so on).

44. How to approach a study of a sound system It is advisable to begin your study of a sound system with the loudspeakers, after which the amplifier power and model can be defined, and finally the sound sources and appropriate connection system can be selected. Specifically, you need to 1) Establish the required system functions on the basis of the user’s needs. 2) Analyse the characteristics of the environment 3) Choose the loudspeakers on the basis of the nature and dimensions of the space, the type of message to be transmitted (speech/music), and the noise level of the environment. 4) Choose amplifiers that are suitable for driving the speakers selected and with a sufficient number of inputs for all the sounds sources. 5) Define the sound sources (microphones, tuners, cassette, players, etc.). 6) Evaluate the connection system for the speakers and establish cable sections.

45. The Sound System Design flow chart

46. Speaker Placement There are essentially two types of speaker system; A distributed system/multi point diffusion A centrally located system

47. Centrally Located System Minimize the Reverberant field but can result in long speaker/listener distances Need for Loudness Calculation Need for Coverage Calculation

48. 9 x H1315 Central Cluster

49. Distributed system Increase the Reverberant field, lower the speaker/listener distance

50. MQ60H: 1800 coverage, 97dB 3m away Small-medium size spaces DM61: 1200 coverage, 96dB 3m away IP55