ENSEA conference Loudspeaker driver Loudspeaker enclosure. Jeremie Huscenot January 8, 2000

Transcription

1 ENSEA conference Loudspeaker driver Loudspeaker enclosure Jeremie Huscenot January 8, 2000

2 What is sound? Air molecules The room contains a huge number of air molecules, but there is still a lot of empty space between the molecules in the room. The molecules are moving in random directions at an average speed of over 1000 miles per hour. Each molecule has about 5 billion collisions per second. The molecules are colliding with my eardrums. The collisions on one side of my eardrum are precisely balanced by collisions on the other side. The eustachian tube equalizes the pressure on either side of the eardrum as long as pressure varies slowly. My eardrums don't move, and I hear nothing. 2

3 What is sound? Pressure Sound is typically described as a small rapid variation in pressure. This is one part of the story, but there is another effect A variation of the average molecular velocity (propagation) 3

4 Why do we need a loudspeaker? A driver is made to set air in motion Several technologies available Electro-dynamic Electrostatic Piezoelectric Ribbon Distributed Mode Loudspeaker 4

5 Historic Beginning of the story Ernst W. Siemens was the first to describe the "dynamic" or moving-coil transducer, with a circular coil of wire in a magnetic field and supported so that it could move axially (not use for audible transmission) Alexander Graham Bell patented the first electrical loudspeaker. The modern design of moving-coil drivers was established by Oliver Lodge in

6 Historic Applications Vitaphone sound system for motion pictures used a new speaker developed at Bell Labs. In the 1930s, loudspeaker manufacturers began to combine two and three bandpasses worth of drivers in order to increase frequency response and sound pressure level. (Bell Labs) Multi-way loudspeaker In 1937, "The Shearer Horn System for Theatres (a two-way system) was introduced by Metro-Goldwyn-Mayer (the first film industry standard) At the 1939 New York World's Fair, a very large two-way public address system. 6

7 Historic Altec Lansing Altec Lansing Corp. was formed when Altec bought Lansing. Altec introduced their coaxial Duplex driver in put on the market the 2-way "Voice of the Theater" speaker system with improved horns and magnet drivers. 7

8 Historic Next evolutions Fundamentals were set Continuous developments in design and materials led to significant audible improvements The most notable improvements in modern speakers are cone materials introduction of higher temperature adhesives improved permanent magnet materials improved measurement techniques computer aided design finite element analysis. 8

9 Electro-dynamic loudspeaker driver A driver is composed of a moving mass a motor a suspension 9

10 Motor Is composed of the the pole the voice coil front and rear plates the magnet To set the motion Front plate Rear plate Air gap Voice coil Pole Magnet 10

11 Motor How? Permanent magnetic field: B AC signal in the voice coil: i Force perpendicular to B and I: F Main parameter: Bl factor Measure the strength of the motor (Magnetic field x length of the voice coil) Unit: Tesla-meter Non-linearity given by the geometry of the pole and the front piece (air gap), length of the voice coil, 11

12 Bl non-linearity Air gap geometry influence 12

13 Bl non-linearity Length of the voice coil influence 13

14 Membrane Piston mode Low limit given by the driver s resonance frequency High limit given by the radiation impedance 14

15 Membrane But the truth is out there: Resonance modes: The membrane is not infinitely rigid Directivity: all drivers become more directive with frequencies, according to the ratio λ (wavelength) / D (driver diameter) 15

16 Membrane non-linearity 16

17 Membrane directivity The directivity becomes significant when λ (wavelength) ½ membrane circumference λ 3,1416 x membrane radius. Examples: 380mm driver diameter: λ 0,60 m f 570 Hz. 20mm driver diameter λ 0,031 m f Hz. Tweeter directivity (with enclosure) 17

18 Suspension Is composed of Surround Spider The surround To center the membrane in the pole piece To damp the membrane s resonances The Spider Compliance of the system (1/rigidity) Non-linearity given by geometry and the material of the surround and the spider Spider 18

19 Transformers A driver is composed of two transformers, to convert an electrical signal to a acoustic signal 1st transformer: Electric Mechanic 2nd transformer: Mechanic Acoustics The motor connects the electrical and the mechanical worlds. The parameter is the strength of the motor: Bl The membrane connects the mechanical and the acoustic worlds. The parameter is the projected area of the driver diaphragm: S d 19

20 Thiele and Small parameters Almost all the parameter needed to describe a loudspeaker driver were set by others scientists. In the 60 s, Thiele and Small collected all these parameters and defined a complete methodology to deduce them from the impedance measurement. These parameters are called T&S parameters. To deduce all the T&S parameters, two impedance measurements with two specific conditions are needed. 20

21 Driver Impedance measurement Low frequencies: Impedance module = driver resistance at DC Phase is about 0 deg When frequency is increasing (below the resonance frequency) Impedance is increasing Phase angle is positive At f s = mechanical resonance frequency Impedance is maximal Phase equal zero f s : resonance peak Frequency response Impedance curve 21

22 Driver Impedance measurement Above the resonance frequency Impedance is decreasing Phase angle becomes negative High frequency Impedance is increasing effect of the voice coil Phase angle is positive f s : resonance peak Impedance curve 22

23 T&S parameters Small signal Small signal means linearity assumption between input and output R e - DC resistance of the voice coil, measured in ohms. L e - Voice coil inductance measured in millihenries (mh) (Frequency dependent, usually measured at 1 khz). Bl - The product of magnet field strength in the voice coil gap and the length of wire in the magnetic field, in tesla-metres (T m). 23

24 T&S parameters Small signal M ms - Mass of the diaphragm, including acoustic load, in kilograms. C ms - Compliance of the driver's suspension, in metres per newton (the reciprocal of its 'stiffness'). R ms - The mechanical resistance of a driver's suspension (ie, 'lossiness') in N s/m S d - Projected area of the driver diaphragm, in square metres. Mind that M ms = M md + air mass in front of the membrane 24

25 Useful T&S parameters F s Resonance frequency of the driver Q es Electrical Q of the driver at Fs Q ms Mechanical Q of the driver at Fs Q ts Total Q of the driver at Fs 25

26 Useful T&S parameters V as Volume of air (in cubic metres) which, when acted upon by a piston of area Sd, has the same compliance as the driver's suspension. To get Vas in litres, multiply the result of the equation below by Where ρ is the density of air (1.184 kg/m3 at 25 C), and c is the speed of sound (346.1 m/s at 25 C). 26

27 Electrical analogy Driver Back wave of the driver Front wave of the driver 27

28 Acoustic Short circuit Front and back waves must be separated, especially when the driver is omni-directional 28

29 First idea A baffle Easy to built Not really optimized for a real living room 29

30 Enclosure Closed box Back waves enclosed in a specific volume Driver Back volume Box Electrical analogy: volume capacitor High pass filter (1st order) Volume dimensions dependent 30

31 Enclosure Vented box Closed box architecture + a tube connects the inside of the box to the outside Driver Back volume Box Vent Bass Pipe Electrical analogy: volume capacitor tube induction coil High pass filter (2nd order) Volume and vent dimensions dependent 31

32 Closed box VS vented box Simulation with WinISD: Green (vented box) and yellow (closed box) Additional resonator 2 nd order versus 1 st order 32

33 A little bit of theory f c proportional to1/ (LC) From Loudspeaker and Headphones handbook by John Borwick 33

34 Electrical Analogy Bass reflex enclosure 34

35 Enclosure Passive radiator Instead of using a tube to create an resonator, another driver is used Driver Volume Box Passive driver /radiator Volume in series and passive driver acted as a resonator High pass filter (2nd order) Volume and passive radiator parameters dependent 35

36 Enclosure effect Vibrations Diffraction Standing waves 36

37 Vibrations Vibrations create noise Small vibrations on a large surface area B&W matrix 37

38 Diffraction Edges create secondary sound sources that interfere with the primary sound source (driver) Influence the directivity of the driver B&W enclosure 38

39 Standing waves Made of two waves (incident and reflected) Enclosure top view 39

40 Multi-way loudspeaker Objective: get a flat and broad frequency range (measured at 1 meter with a 1 Watt signal) Audible range 20Hz 20kHz impossible to get this frequency range with a single driver. Different drivers for different applications 40

41 Multi-way loudspeaker Different drivers for different applications 1 2.5cm 41

42 Different type of loudspeaker membrane Front plate Tweeter (Timphany) Woofer (Peerless) Mid-range Full-range (Visaton) Subwoofer (Alpine) 42

43 Electrical crossover 2-way system Used to separate the high frequencies form the bass frequencies Tweeter SPL frequency Enclosure f c Woofer 43

44 Electrical crossover Is ART It is the last step of the development of a loudspeaker. Could be seen as the loudspeaker conductor! At this step, the objective is to make a loudspeaker to sound good With the best loudspeakers ever made and a wrong electrical crossover, sound quality won t be good. An electrical crossover is made for a specific loudspeaker driver Electrical and Acoustic part need to be closely evaluated to get good performance 44

45 Analog crossover Between amplifier and loudspeaker Crossover in parallel with the driver, each driver is separately filtered (gives flexibility) Filter types: RLC network High Pass, Low Pass, Band-pass Alignment: Butterworth, Bessel, Around fc both drivers are playing at the same frequency and the same time Interferences 45

46 Digital crossover Between pre-amplifier and amplifier Almost everything is possible phase, frequency response, independently adjustable But it is expensive compare to analogue solution Other advantage: Less inter-modulation distortion in the amplifier. 46

47 A hifi loudspeaker Tweeter Mid-range Mid-range volume Woofers volumes Woofers Vents Cross-section of an hi-fi loudspeaker Model: Quartet Range: Genese Brand: Triangle 47

48

49 Bibliography web site In English Acoustic phenomena Loudspeaker design Linkwitz: A lot of thing Art Ludwig: Headphones: Headwise In French Loudspeaker driver: hp.htm A lot of thing: DIY websites Wikipedia 49

50 Bibliography Books Loudspeaker and Headphones handbook by John Borwick Loudspeaker design cookbook by Vance Dickason (Enceintes acoustiques et haut-parleurs) Testing loudspeakers by Joseph d'appolito (Le haut-parleur: manipulation et measures electro-acoustiques) 50