Fal Fal A g e o f t h e P h o t o n s A u d i o

Transcription

1 Fall 13 Aeternus Lux 2.2 Modular Speaker System Paul Kirby A complete listing of the design goals, realizations, and specifications of the Aeternus Lux Speaker System Age of the Photons Audio

2 Table of Contents FUNCTIONAL DESCRIPTION 4 DESIGN GOALS 5 VISUAL AESTHETICS 5 SIZE & SHAPE 6 SPL OUTPUT 7 BANDWIDTH 7 DRIVER SIZE AND SPACING 8 DIRECTIVITY 8 TECHNICAL DETAILS 10 BAFFLE STEP 10 CONSTRUCTION MATERIALS 10 DIFFRACTION AND SPEAKER SHAPE 11 DRIVER ANALYSIS & SELECTION 12 TWEETER ANALYSIS 12 TWEETER #1 DETAIL 13 TWEETER #2 DETAIL 13 TWEETER #3 DETAIL 14 TWEETER #4 DETAIL 14 TWEETER #5 DETAIL 15 WOOFER ANALYSIS 16 WOOFER #1 DETAIL 17 WOOFER #2 DETAIL 18 WOOFER #3 DETAIL 19 WOOFER #4 DETAIL 20 WOOFER #5 DETAIL 21 SUBWOOFER ANALYSIS 22 SUBWOOFER #1 DETAIL 22 SUBWOOFER #2 DETAIL 23 SUBWOOFER #3 DETAIL 24 SUBWOOFER #4 DETAIL 25 SUBWOOFER #5 DETAIL 26 DRIVER SELECTION 27 TWEETER SELECTION 27 WOOFER SELECTION 27 SUBWOOFER SELECTION 27 CROSSOVER DESIGN 28 INITIAL DESIGN 28 TESTING & TUNING 29 INITIAL PERFORMANCE AND TUNING 29 ENCLOSURE OPTIMIZATION 31 2

3 FINAL SYSTEM DOCUMENTATION 32 FINAL TESTING RESULTS 32 FREQUENCY RESPONSE 32 INTEGRATED RESPONSE 32 HARMONIC DISTORTION 32 MINIMUM PHASE RESPONSE 33 HORIZONTAL OFF- AXIS RESPONSE 33 VERTICAL OFF- AXIS RESPONSE 33 DIFFERENCE PLOT 34 STEP RESPONSE 34 INTEGRATED STEP RESPONSE 34 IMPULSE RESPONSE 35 WATERFALL PLOT 35 SUB RESPONSE 35 CROSS- OVER SCHEMATIC 36 AS- BUILT DRAFTING 37 BIBLIOGRAPHY 40 3

4 Functional Description The first step of any good loudspeaker design process is to choose at what point in the consumer chain you want your speakers to exist. 1 I decided to make a pseudo- step in the process, somewhat deriving from the 5 steps that Newell & Holland talked about in their book. The Aeternus Lux design will be somewhere in- between the mixing and consumer listening stage. My speakers will function as reference monitors that I can sound design with, but also need to have very low fatigue and ability to be enjoyed as simply listening loudspeakers. When sound designing, I can be listening to something for hours on end, placing sounds for linear media and trying numerous things out. If the monitors are too fatiguing at mid to high SPL, it is a very unpleasant process. The system will be forward listening, 2 or listening toward the consumer end of the product and will have low fatigue at a relatively high SPL. For size, I want these speakers to be as universal as possible so that I won t end up in a living situation where they are not reasonable to use. However, I don t want to compromise the low- end response by making them small. They need to be configurable in as many rooms as possible. Because the speakers are going to be used for pure enjoyment as well, I would like to have a good off axis response so they may be enjoyed from many different positions. This system is the initial part of a future 5.1 or 7.2 surround sound system. However, due to resources available, this initial design is only for the front left and front right speakers. The rest of the design will be available as soon as the resources are. 1 (Newell and Holland 2007) 2 (Moulton 2000) 4

5 Design Goals Visual Aesthetics My primary visual aesthetics goal for my speaker design is to make them look classy and versatile with a modern or contemporary décor. I was imagining a beautiful natural wood finish face with piano black sides. Different woods I was considering include a cherry or possibly even a birds- eye maple. Some examples: Figure 1 ( hifi- audio.com) Figure 2 ( Figure 3 ( 5

6 Size & Shape For the shape and cabinet design of my speakers I took inspiration from the Penaudio Chara- Charisma line speakers. (See figure 4) In most situations I want to have rectangular floor standing speakers with an amazing low- end response. However, as said in the functional description, they need to be as universal as possible. That s why a modular system works the best. There will be a smaller cabinet that houses my 2 way monitors that will then rest on top of a floor standing, side- firing subwoofer with the exact same width and depth. They will then act like a single floor standing, full range speaker but be able to be used separately as just desk monitors with satellite subwoofers that can be moved based on the room I m in. Figure 4 (Penaudio s Chara- Charisma modular 3- way system) 6

7 SPL Output As far as maximum SPL output is concerned, the only stipulation I have is to be able to mix at standardized film sound levels. The average SPL for film is 85dB with a 20db crest factor, and an extra 10dB of headroom for the LFE s (Low Frequency Effects channel). 3 With that condition, I would like the maximum SPL to be about 120dB before mechanical breakdown. I would like to be able to reach 105dB without much distortion. Bandwidth Song/Artist Noticed Didn t Like Couldn t Stand Ed is a Portal/Akron Family 36Hz 62Hz 118Hz Danse Caribe/Andrew Bird 37Hz 64Hz 94Hz Dope Crunk/Beats Antique 35Hz 68Hz 110Hz Get Innocuous/LCD Soundsystem 42Hz 62Hz 96Hz If You Want Me to Stay/Red Hot Chili Peppers 32Hz 62Hz 110Hz Average 36.4Hz 63.6Hz 105.6Hz In order to determine what low- end extension I want, I decided to listen to 5 different songs that I know very well on a full range system as I slowly increased a high- pass filter on the songs. I noted the frequencies at which I noticed the cut- off, didn t like the cut- off, and absolutely hated the cut- off. I want to find a general average of the point at which I didn t like the cut- off and make that my 3dB point for my low- end bandwidth. The cut off frequency goal I decided upon is 65Hz for the 2way cabinet, just about in- between where I noticed the cut of and didn t like it. The cut off frequency goal for the side firing subwoofers is 30 Hz, well below and detrimental frequency. See the table above. 3 (Holman 2010) 7

8 Driver Size and Spacing To accommodate the low- end response wanted of the 2way cabinets I would need decent sized woofers or mid- woofers. I am choosing to use woofers to get the desired low- end response, but minimize the directionality at mid- frequencies. Because the speakers are an M- T- M design, I will have to get the woofers as close together as possible to reduce vertical lobbing. This is a design issue that I will have to face with some creativity. The tweeter size I plan on using will be a 1.25 dome. This will allow better power handling at the lower frequencies; around the 2kHz crossover. However, I may choose to end up with a smaller tweeter in order to get the woofer spacing as close as possible. The subwoofers are 10 woofers, which is the best diameter for performance in the size of the cabinet that I will be using. As I have said earlier, the most important thing to do is to get the woofers as close together as possible for the MTM design to works its best. There are multiple ways of attempting this. One way is to countersink the tweeter so that the edge of the woofers can actually overlay on top of the tweeter. Another would be to put the tweeter off- center of the array so that to two drivers could be as close as possible. I ve decided to do a combination of both. If I could get the two woofer centers within 8 of one another, that would give me minimal lobbing up to approx. 1000Hz. (See Figure 5) Directivity The directivity goal of the Aeternus Lux system is to be as widely dispersed in the horizontal as possible at the cost of vertical lobing. The Midwoofer- Tweeter- Midwoofer (MTM) has good horizontal dispersion, however does not perform as well with vertical dispersion. Although that is not necessarily a bad thing, as THX standards call for tighter vertical dispersion. 8

9 Figure 5 9

10 Technical Details Baffle Step Figure 5 (Effects of a baffle with a width of 18")(Neal) The Aeternus Lux system will have a front baffle with a width of approximately 8 inches. To determine the frequency at which my speakers will begin to propagate into free space, I found the equation of f! =!,!"#!!, Where W! is the width of your baffle and f! is the 3dB down point when moving from halfspace to full space in frequency. 4 The f! frequency for this design will be at about 506Hz, which will require some mid- high and high frequency padding to maintain a flatter response. Construction Materials For the construction of the speaker cabinets, I plan to use a combination of hardwood, ¾ medium- density fiberboard (MDF) and ¾ ply wood. For the exterior, the speakers will have a cherry baffle and MDF sides. The MDF allows for ease of sanding and fine adjustments, which is very beneficial for a glossy paint finish. The MDF is also extremely heavy and will aid greatly in mechanical grounding. The 3/4" ply will be used internally for bracing, added support and reduction of resonant modes. 4 Baffle Step Diffraction, (Neal Jason M.) accessed January 13th,

11 Diffraction and Speaker Shape The Aeternus Lux system will be a rectangular cabinet, modular, floor standing speaker system. The rectangular cabinet, as can be seen in Olson s 5 study of cabinet shapes and diffraction results, does not have the best frequency response out of the possible shapes. However, as far as design priorities go I am willing to deal with the frequency variations due to the diffraction effect for ease of construction and aesthetic quality. The speakers will have rounded edges around the front baffle to smooth out the diffraction effect across the frequency spectrum and not have a sudden severe phasing effect at a specific frequency. 5 Henry F. Olson, Direct Radiator Loudspeaker Enclosures, AES Paper, October 27 th, 1950,

12 Driver Analysis & Selection This section will detail the process of elimination used to choose the final drivers implemented in the system. Tweeter Analysis For the system at hand, I plan to use a soft dome- tweeter with a minimum sensitivity of 90dBspl. The tweeter needs to have a resonant frequency of 1000Hz or less to allow for a lower crossover point. Some of the tweeters I have picked to analyze further are: Jantzen JA /8" Soft Dome Tweeter Tang Band S 1-1/8" Fabric Dome Tweeter Morel MDT Soft Dome Tweeter Morel ET /8" Soft Dome Tweeter Morel MDT30S- 4 1 Textile Dome Tweeter Name Sensitivity Fs Power Handling (RMS) Price Jantzen JA dB 700Hz 100 Watts $85.94 Tang Band S 94dB 900Hz 12 Watts $ Morel MDT 29 92dB 1000Hz 80 Watts $53.10 Morel ET dB 750Hz 120 Watts $ Morel MDT30S 92dB 1020Hz 200 Watts $51.00 (sale) 12

13 Tweeter #1 Detail Jantzen JA I found the Jantzen JA to be a very prime contender as my tweeter choice. It has an incredibly low Fs, couple with its natural first order roll off due to its internally built transmission line. It is also ferrofluid cooled to maintain a high power handling. The price also resonates well with me, sitting at a reasonable $85. Some of the drawbacks include the fact that the frequency response doesn t include an off- axis response, which is a major component of my design. I also didn t like how smoothed the response is. Tweeter #2 Detail Tang Band The tang band is a very nice tweeter. One of the things that really stood out to me is sensitivity coupled with the relatively low Fs. With 94dB sensitivity, there would be no problem get this speaker as loud as I need it to be. However, if you look at the impedance graph, the slope is rather gradual and it begins rising around the point where I want by crossover to begin. However, it has an incredibly flat frequency response but again, it doesn t have an off axis response which is a huge component of my design. 13

14 Tweeter #3 Detail Morel MDT29-4 The Morel MDT29-4 has an amazing off- axis response, one of the first things I noticed about this tweeter. Its ferrofluid cooled and has an amazing power handling of 80 watts, giving you a safety net for overpowering. The Fs is at the highest I would really want it, but the impedance spike is rather narrow. It also has a good sensitivity of 92dB, and the price is great at $53. Tweeter #4 Detail Morel Elite ET 448 The Morel Elite ET 448 is an amazing tweeter. The off- axis is again phenomenal, and the power handing capabilities of the driver is amazing. The sensitivity at 92dB is more than I need. Not only is the off- axis response of this driver fantastic, but it also has a very flat response. It is also ferrofluid cooled, which is one of the reasons the power handling is so amazing. Another greater feature is the small diameter of the faceplate, at only 2.83, which works great for an MTM design. The major setback is the price, coming in at $

15 Tweeter #5 Detail Morel MDT30S- 4 The Morel MDT30S- 4 is another greater tweeter from morel. Again, this tweeter has an incredible off axis response. The frequency response in general is absolutely amazing. Its power handling has beaten anything else I looked at with a RMS value of 200watts. That makes me feel very comfortable with this driver, and not worried about damaging it with a clipping amp as much. The sensitivity sits the same as the rest of the Morel s I ve looked at, 92dB. The price is also very good, only $51, which is on sale from its normal price of $72. The major downside of this tweeter is its somewhat high Fs at 1020Hz. 15

16 Woofer Analysis The system at hand will be using two matched 5-6 mid- woofers per speaker. The mid- woofers need to have a very low resonant frequency, good response into the low end, and a minimum sensitivity of 85dB. They also need to perform well in a vented box in order to get the low- end frequency response that I desire. Some of the woofers I have choose to analyze further are: Audax HM170C0 6.5" Carbon Fiber Woofer SB Acoustics SB17NRXC35-4, 6.5" Woofer ScanSpeak Discovery 15W/4434G 5.25 Midwoofer SEAS Prestige L15RLY/P (H1141) 5.5" Aluminum Cone Woofer Morel MW " Damped Polymer Composite cone Name Sensitivity Power Handling F3 Flat F3 Extended Price Audax HM170C0 90dB 60 Watts 55Hz 46Hz $ SB Acoustics SB17NRXC35 92dB 60 Watts 51Hz 43Hz $62.40 ScanSpeak Discovery 15W 90dB 120 Watts 92Hz 72Hz $70.70 Seas Prestige L15RLY 86dB 80 Watts 60Hz 45Hz $88.40 Morel MW dB 150 Watts 62Hz 40Hz $

17 Woofer #1 Detail Audax HM170C0 6.5 The Audax HM170C0 is an interesting driver. It has a carbon fiber Woven cone. Apparently it allows for a more dynamic bass response and helps create a natural first order roll off on the high end, making the crossover design that much easier. With a flat response bass reflex cabinet, I can get down to 55Hz with the drivers. They also have a fantastic sensitivity at 90dB. The main downside of this is the driver style (shape) and the size. A 6.5 driver will be hard to get 8 CTC (center to center) apart with an MTM design. 4 th order vented box Flat Response 4 th order vented box Extended response 17

18 Woofer #2 Detail SB Acoustics SB17NRXC35-4 I am astonished with the quality of this driver for the price that it sells for. It has an amazing frequency response and extremely high sensitivity, which is perfect for the high SPL that I want. The two downsides of this driver is the large size of the box necessary to have a flat 4 th order bass reflex cabinet and the fact that it would be hard to get a really close MTM design with that large of a driver. 4 th Order vented box Flat Response 4 th Order vented box Extended Response 18

19 Woofer #3 Detail ScanSpeak Discovery 15W/4434G The ScanSpeak Discovery is probably one of the poorest choices for this design. However, it is a great driver for the application it s meant for. The F3 point I got with the vented box with a flat tuned response was only 92Hz. That is not low enough to be a stand- alone driver. The size is perfect to get the MTM close together, but the response won t work for the design. 4 th order vented box- flat response 4 th order vented box extended response 19

20 Woofer #4 Detail SEAS Prestige L15RLY/P (H1141) The Seas Prestige L15RLY/p is an excellent driver. The low end response for the size, 5.5, is absolutely phenomenal. The driver also has really great frequency response. These drivers are known for their tight, clear sound, which is a very wanted trait. The breakup frequency, which is inherent in aluminum cone drivers, doesn t happen until very far past my desired crossover range. They also come at a very reasonable price. 4 th order vented box Flat Response 4 th order vented box Extended Response 20

21 Woofer #5 Detail Morel MW144-4 The main component that stands out on the Morel MW144 is its amazing power handling capabilities. It also has a relatively tight frequency response and a pretty low F3 for the size of the driver. However, after all of this its largest and most prominent con is the cost. It s hard to consider this as a feasible driver when you need to purchase four and have a reasonable budget. 4 th order vented box Flat Response 4 th order vented box Extended Response 21

22 Subwoofer Analysis Subwoofer #1 Detail SB Acoustics SB29SWNRX- S75 10" Woofer Sensitivity = 86dB F3 = 25Hz db = 99dB Price = $ th Order Vented Box Extended Response 4 th Order Vented Box Flat Response 22

23 Subwoofer #2 Detail Dayton Audio TIT280C- 4 10" Titanic Mk III Sensitivity = 85dB F3 = 42Hz db 2m = 104 Price = $ th Order Vented Box Extended Response 4 th Order Vented Box Flat Response 23

24 Subwoofer #3 Detail Peerless " Paper Cone SLS Sensitivity = 87.9dB F3 = 39Hz db 2m = 101dB Price = $ th Order Vented Box Extended Response 4 th Order Vented Box Flat Response 24

25 Subwoofer #4 Detail Vifa NE265W " Subwoofer Speaker Sensitivity = 84dB F3 = 38Hz db = 97dB Price = $ th Order Vented Box Extended Response 4 th Order Vented Box Flat Response 25

26 Subwoofer #5 Detail Dayton Audio SD315A " Sensitivity = 89.6dB F3 = 45Hz db = 103dB Price = $ th Order Vented Box Extended Response 4 th Order Vented Box Flat Response 26

27 Driver Selection Tweeter Selection Morel MDT30S- 4 The Morel MDT30S- 4 seemed to be the best choice for the Aeternus Lux Modular System. The sensitivity is 92dB, which is more than efficient enough for the SPL that I want. The off axis is also phenomenal, a must have for home theater and 5.1 systems. The price, which on sale is at $51.00, also sits very nicely in any budget and will make for painless repairs in the future. The only downside to the driver is the high Fs of 1020Hz which borders my maximum. However, this can be dealt with by some clever crossover design. Woofer Selection Seas Prestige L15RLY- P The SEAS Prestige L15RLY- P ended up being the best choice for the Aeternus Lux system. The sensitivity is not that high, however it will work for the required crest SPL. The F3 I was able to model in the 4 th order vented box was at about 60Hz, which is pretty low for the standalone speaker itself. The size of the drivers was also a huge selling point, being that I could maintain the original goal of getting my CTC for the two MTM woofers to be at about 8 to minimize lobing. They also sit very nicely in any budget, at the cost of about $88. Subwoofer Selection SB Acoustics SB29SWNRX- S75 10" The SB acoustics SB29SWNRX subwoofer seemed to be the ideal woofer for the enclosure I designed for the side- firing subwoofers. After modeling, these woofers gave me the lowest F3 at 25Hz, which is more than enough. This comes at the sacrifice of a lower SPL than some of the other options, but I am willing to rely on the equal loudness curve and cabin gain (room gain) to account for this loss in d decibels. 27

28 Crossover Design Initial Design For the crossover design, I made sure to implement a voicing that helps contribute to the low fatigue, mellow sound I desire for the Aeternus Lux system. The crossover will be third order on both ends with the F3 for the tweeter at about 3000Hz and the F3 for the woofer at 1500Hz. This will allow for a nice dip in the fatiguing frequencies. I also implemented an L- pad for the tweeter to match it to the woofer, and a baffle step compensation circuit for the woofer. 28

29 Testing & Tuning Initial Performance and Tuning The initial sweep of the mains (the MTM modules) showed that I had boosted low end and dropped highs. There were also some issues with my baffle step compensations circuit, as it seemed to be over compensating. Some serious action needed to be taken, and I started with the woofer. Initial Integrated response (Woofer is Red, Tweeter Yellow, Overall Purple) Noticing the large boost at around 100Hz, the first successful measure taken was to remove the baffle step correction circuit to see if the smoothed out the response. The result is as follows: Woofer (In Green) after the baffle step correction circuit had been removed 29

30 Now that the woofer was successfully tuned, I wanted to see what happened if I took the L- Pad off of the tweeter, as the high end was dipped quite a bit in the initial test. The results were as follows: Tweeter without L- Pad As you can see, taking the L- pad off evened out the frequency response until a very large boost at around 7kHz. Because the boost looked so much like a shelf, I decided to use a shelving filter to bring down that high- end sizzle. After much experimentation, the results were as follows: Tweeter with high- shelf as compared to with out 30

31 The high- shelf helped a lot to flatten out the high end response and bring it with in approximately +/- 2dB of the woofers response. However, if you notice there is quite serious wavering in the upper highs, starting at about 7kHz. After some contemplation, I realized that it seemed to be a diffraction issue, inherent with the inset tweeter and the raised edge around it. Instead of trying to sand this edge down to perfection and risk having a very strange looking baffle, it was suggested to try acoustically absorbent foam around the edge of the tweeter. After some trial and error, the results were as follows: Tweeter with black foam surround The foam surround help tremendously with the diffraction issues, and saved the aesthetic appeal of the final design with- out compromise. Enclosure Optimization One of the goals of the Aeternus Lux system is to have a very spacious sound stage, but not blurry necessarily. Without any stuffing inside the cabinet, the sound stage is massive, however very blurred as far as specificity of imaging. After much trial and error, the final stuffing I choose to use was a small amount of blue construction foam above the brace, and cover the back wall of the speaker in pink insulation. This tightened up the special imaging until almost laser- like, created a beautiful phantom center, and also let the speakers breathe enough to have incredible depth. 31

32 Final System Documentation Final Testing Results Frequency Response Integrated Response Purple overall, Yellow Woofer, Blue Tweeter, Green Port Harmonic Distortion 32

33 Minimum Phase Response Horizontal Off- Axis Response Purple 0, Yellow 15, Red 30, Blue 45, Green 60 Vertical Off- Axis Response Purple 0, Blue 15, Yellow 30, Green 45, Red 60 33

34 Difference Plot Step Response Integrated Step Response Woofer is yellow, Tweeter is blue 34

35 Impulse Response Waterfall Plot Sub Response 35

36 Cross- over Schematic 36

37 As- built Drafting 37

38 38

39 39

40 Bibliography Chini Hifi Audio. hifi- audio.com (accessed January 2013). Design Buzz. (accessed January 2013). Holman, Tomlinson. Sound for Film and Television. Focal Press, Moulton, David. Total Recording. Sherman Oaks: KIQ Productions, Neal, Jason M. Baffle Diffraction Step: An introduction. linespeakers.org/tech/bafflestep/index.html (accessed Jan 2013). Newell, Philip, and Keith Holland. Loudspeakers for Music Recording and Reproduction. 1st Edition. Burlington : Focal Press, Olson, Henrey F. "Direct Radiator Loudspeaker Enclosures." AES Paper, October 27, 1950: 34-38; Sound Gallery. (accessed January 2013). 40