TPA Speaker Design Paul Bristol FA /30/14

Transcription

1 1 TPA Speaker Design Paul Bristol FA /30/14

2 Table of Contents Functional Description... 5 Technical Goals... 7 Physical Design Goals Visual Aesthetics... 7 SPL... 8 Frequency Response... 8 Coloration and Time Response... 9 Driver Size and Spacing... 9 Baffle Step Predictions... 9 Diffraction and Speaker Shape... 9 Construction Materials Power Response and Directivity Driver Selection Woofers F3 in a.7 Q Box Sensitivity Breakup Price Seas Prestige L16RN- SL Hz dB khz $ SB Acoustics SB17MFC Hz db khz $ Pearless PBB Hz db khz $

3 3 Dayton RS Hz db khz $ Tang Band W Hz db khz $ Woofer 1: Seas Prestige L16RN- SL Woofer 2: SB Acoustics SB17MFC Woofer 3: Pearless PBB Cone Woofer 4: Dayton RS Woofer Woofer 5: Tang Band W Woofer Tweeters Tweeter 1: Eton 25SD- 1 1 Soft Dome Tweeter 2: FounTek NeoX 1.0 Ribbon Tweeter Tweeter 3: SB Acoustics SB26ADC- C Tweeter 4: Seas Prestige 27TBFC/G Tweeter 5: ScanSpeak Classic D2008/ Crossover Design and Testing Initial Design Testing Procedure and Standards Crossover Testing and Tuning Final Crossover Design Overall Loudspeaker Performance Frequency Response (Average of 10) Integrated frequency Response Harmonic Distortion Percentage Horizontal Off Axis Response Vertical Off Axis Response Minimum Phase of Tweeter and Woofer # Minimum Phase of Woofer #1 and Woofer #

4 Difference Between Left and Right Step Response Integrated Step Response Impulse Response Waterfall Plot Tweeter Frequency Response Harmonic Distortion Minimum Phase Step Response Woofer Frequency Response Harmonic Distortion Minimum Phase Plot Step Response Impulse Response Woofer Frequency Response Harmonic Distortion Minimum Phase Step Response Impulse Response Bibliography

5 Functional Description The speakers I am building are for my parent s living room. This room is used for listening to music, watching movies and relaxing. The space is a 12- foot x 20- foot room with many drapes and other absorptive materials on the walls. The room is a little over 150 feet from train tracks, which often introduces a low frequency rumble to the space for about 15 minutes at a time. These speakers will stand on the floor on either side of the TV. They will eventually be included in a surround system, but for now the system will include the new left and right pair along with an external sub that is already in the room. The speakers will be used for listening, and not mixing. For this reason, I would like to make them high enjoyment speakers. I would still like for the speakers to have good depth localization and image separation 1. I would also like spectral uniformity 1. This will allow many live concert DVDs that are watched in the room to sound pristine, All listening done on these speakers will be listening back 2 to recreate what has already happened. SPL output does not need to be extremely high. I would categorize it as normal home listening. It should be loud enough for listening to music and watching movies, but it does not have to be much louder than this. There are many tradeoffs but we will focus on three: SPL, Size of the cabinet, and Bass response 3. For my design, they are all equally important. SPL does not need to be extremely loud since they will be used for home listening. The size does not need to be compact, so it does not get much attention. The bass response does not need to go extremely low, since there will be an external subwoofer to cover this range. A graph of the trade offs can be seen in figure 1. 1 "AES Recommended Practice for Professional Audio." Standards and Information Documents, Print. Pg: Moulton, David. Total Recording. Sherman Oaks, California: KIQ Productions, Print. Pg: Murphy, John L.. Introduction to Loudspeaker Design. Andersonville: True Audio, Print. Pg:

6 SPL Size Bass Figure 1: Graph of Loudspeaker Trade offs Visually, the speakers will be a tall, thin rectangular box. This is required because the speakers need to fit close to the TV and blend in to the background. I would like to use a nice hardwood to build them. This way they can be stained to match the natural wood finish of the entertainment center in the room. They should be reasonably heavy, so the little kids that frequent the room wont knock them over. They will not be portable because once they are setup; they won t be moved, except when moving homes. I would like to add some type of protective covering so wandering children don t poke holes in the drivers. However, I would like to stay away from grills based on how they can affect the sound quality. 6

7 Technical Goals Physical Design Goals The box will be tall and slender. They will be floor standing, so they should be tall enough to reach sitting ear height. The weight should be reasonably heavy. Many small children frequently run wild through the room, so I want them to be heavy enough to not easily fall over. Personally, I can lift 80 pounds for a reasonable amount of time, so this will be my weight limit for the speakers. They should also be made of a robust material, for the same reason as the weight. They will also need to fit in the back of my car, so the height must be less than six feet. Visual Aesthetics I would like my speakers much like the Jamo E 855 speakers that can be seen in figure 2. I would like to keep the wood grain finish, but round of the edges. The front will be stained to match the wood of the room, and the sides and rear will be stained black to add contrast. Figure 2: Jamo E 855 (Source: types/floorstanding/?sku=e855) 7

8 SPL These speakers will be mainly used for watching movies and listening to music, so I would like to use the K20 standard for listening level. This is an RMS listening level of 83dB with a 20 db Crest factor 4. I monitored my listening levels for a day, which produced an average listening level of 79dB. The data can be seen in figure 3. This means that following the K20 standard will more than suffice for max SPL at the listening position. The listening position in this room is about 3 meters from the speakers. By the inverse square law, need to add 9 db to the total output gain to find our true max spl. This means that our max spl is =109 db at a distance of 1 meter. I plan to use a 100 W amplifier, which gives me a dbw value of 20. This means the speaker must have a sensitivity of at least 89 db at 1 meter. Morning Afternoon Evening Late Night Average Level Figure 3: SPL preference levels Frequency Response In order to determine my required a low frequency response, I listened to a bunch of different genera s of music. I added a high pass filter and moved it up the frequency spectrum. I took note of when the roll off was noticeable, acceptable, and unacceptable. The results of this experiment can be seen in figure 4. I would like my roll of to start between the noticeable average frequency and acceptable average frequency. This puts my desired low frequency roll off to start at 60 db. For my high frequency roll off, I would like it to reach 20 khz, but not any higher than that. I am not looking for an extremely hi- fidelity pair of speakers, so I do not feel extension beyond 20kHz is necessary. The overall frequency response, I would like to be slightly on the warm side. This way, listening on the speakers will always be smooth and not harsh. They are for high enjoyment and not mixing, so I do not need a completely flat frequency response. Pop Jazz Rock Hip- Hop Folk Orchestral Average 18 db/ Octave Noticable db/ Octave Acceptable db/ Octave Unacceptable Figure 4: Low Frequency preferences in Hz 4 Katz, Bob. "Level Practices." AES Journal, Pg. 8 8

9 Coloration and Time Response The low frequencies only need to be extended to 60 Hz. The box will be rather large and sealed, so I don t believe this be very difficult to do. With the sealed box, we will get a tight transient response. This will help with the clarity of the sound. I would also like a fast decay time. To help with external reflections, I will round the corners of the box and inset the drivers. Both of these will help prevent early reflections on the front baffle 5. Internally, I will use absorptive material to reduce reflections. The speakers will be mounted on the floor. This will help with mechanical grounding. There will also be very little between the speakers and listener, so boundary effects will not be a big deal. The speakers will also have wide dispersion. This makes the sound stage less precise, but covering the entire room is more important. Driver Size and Spacing I would like to use two 6 woofers and a dome tweeter for my design. I would like them to be as close as possible to help with the acoustical axis, and off axis lobing. Both drivers will be close to the top of the box to keep the tweeter close to sitting listening height. Baffle Step Predictions Based on the one woofer, one tweeter design, I will end up with a 6dB boost in the high frequencies. I will counter act this using a high shelf filter in my cross over. f3 = 4560 / WB where WB = width of the baffle in inches 6 Since my baffle is 10 wide, my f3 of my baffle is 456 Hz. Diffraction and Speaker Shape For easy construction, I will build a rectangular box. A diamond shaped box would allow for better diffraction, but I don t have the time to commit to this style box. I will also round the corners to lessen reflections off the corners of the baffle. To further reduce diffraction, I will inset both drivers. This will help with diffraction issues off of the driver edges 7. 5 Newell Pg Martin J. King, Simple Sizing of the Components in a Baffle Step Correction Circuit 2005, 7/23/05 Revision, wave.com/general/bsc_sizing.pdf, Accessed February 20th, George Short, North Creek Cabinet Handbook, Old Forge: North Creek Music Systems, 1992, 5 9

10 Construction Materials To help with resonances, I would like all of my walls to be thick and sturdy. The front baffle will be made of a beautiful hardwood and the inner most layer will be ½ MDF. Both layers are different densities, which means different resonances. This allows for very few standing waves to form 8. The sides will be made of ¾ plywood and the inner layer will be ½ MDF. This will also help breakup resonances in the box. There will be two braces made of ¾ plywood. The first will be at 1/3 of the height, and the second will be at 5/8 of the height. This creates three regions of different lengths. This will help breakup resonances at low frequencies 9. Power Response and Directivity The speakers should have a wide dispersion to cover the entire living room seating area. Since the room they will be used in has mostly absorptive materials, I am not overly worried about issues with reflections off the walls that can appear with a wide dispersion speaker. 8 Short Pg. 8 9 Short Pg

11 Driver Selection Woofers My ideal woofer has a frequency response of +/- 2 db from 60 Hz to 1000 Hz. The F3 should at or below 50 Hz to get me down to 60 Hz or lower. I would like a high breakup frequency so I have a range of crossover points to chose from. It also needs to be rated for a sealed box for obvious reasons. F3 in a.7 Q Box Sensitivity Breakup Price Seas Prestige L16RN- SL SB Acoustics SB17MFC35 Pearless PBB Dayton RS150-8 Tang Band W Hz 84dB 4 khz $ Hz 90 db 3 khz $ Hz 87 db 3.5 khz $ Hz 89 db 4 khz $ Hz 89 db 4 khz $

12 Woofer 1: Seas Prestige L16RN- SL 6 Specifications: Price: $ Sensitivity: 84 db Break up Start: 4000 Hz Weight: 3.85 lbs Notes: In a two cubic foot box provides an f3 of 55 Hz. This is well within the range that I am looking for in my woofer. It has a quite high breakup frequency, which gives me room to play with my crossover point. It also looks extremely nice. My only issue is the price is slightly higher than I d like since I am creating a 2.5 way loudspeaker. 1 2

13 Woofer 2: SB Acoustics SB17MFC Specifications: Price: $70.00 Sensitivity: 90 db Break up Start: 3000 Hz Weight: 5.00 lbs Notes: The driver has an extremely flat frequency response all the way up to 3kHz. It has a f3 of 80 Hz. It has a wider breakup as well. It it looks nice, and also has a slightly more reasonable price than woofer

14 Woofer 3: Pearless PBB Cone Specifications: Price: $60.00 Sensitivity: 87 db Break up Start: 3500 Hz Weight: 3.75 lbs Notes: In a two cubic foot box provides an f3 of 90 Hz. This is well within the range that I this is a little out of the range I would like to be in for my woofer. It has a quite high breakup frequency, which gives me room to play with my crossover point. It also looks extremely nice, but will be a pain to inset into the front baffle. The price is also well within my price range. 1 4

15 Woofer 4: Dayton RS Woofer Specifications: Price: $50.00 Sensitivity: 89 db Break up Start: 4000 Hz Weight: 3.00 lbs Notes: In a two cubic foot box provides an f3 of 100 Hz. This is a little outside of the range I would like my woofer to cover. It has an extremely flat frequency response from 100 Hz to 2 khz. It has a quite high breakup frequency, which gives me room to play with my crossover point. It also looks pretty slick. It is also extremely cost effective, so that helps as well. 1 5

16 Woofer 5: Tang Band W Woofer Specifications: Price: $95.00 Sensitivity: 89 db Break up Start: 4000 Hz Weight: 12 lbs Notes: In a two cubic foot box provides an f3 of 65 Hz, which is about perfect for my speakers. It has a frequency response of +/- 2 from 50 Hz to 3 khz. It has a quite high breakup frequency, which gives me room to play with my crossover point. It also looks pretty slick. It is near the top of my price range, but it gives me what I want. 1 6

17 Tweeters My ideal tweeter would be+/- 2 db from 1kHz to past 20 Khz. This is to make tuning easy. The F[s] should be low to give me space to move my cross over point around. It should also have extremely wide Dispersion. This is to increase ASW and listener enjoyment. My price range for the tweeter is $50- $100. F[S] Sensitivity Price Eton 25SD Hz 90 db $70.00 FounTek NeoX Hz 92 db $ SB Acoustics 680 Hz 92 db $90.00 SB26ADC- C000-4 ScanSpeak Classic D2008/ Hz 88 db $

18 Tweeter 1: Eton 25SD- 1 1 Soft Dome Price: $70.00 Sensitivity: 90 db F[s]: 1000 Hz Weight: 2.50 lbs Notes: I am looking for a tweeter that has very wide dispersion. This reaches only 2.5 db down at 10 khz. This is a very wide dispersion that will give me what I want. It is also extremely flat all the way to 20 khz. 1 8

19 Tweeter 2: FounTek NeoX 1.0 Ribbon Tweeter Price: $ Sensitivity: 92 db F[s]: 1500 Hz Weight: 2.50 lbs Notes: I chose a ribbon tweeter for my second option because they have an extremely wide dispersion pattern. They also look aesthetically pleasing and have a very flat response. They only issue with this driver is that is less cost effective than my other tweeter 1 9

20 Tweeter 3: SB Acoustics SB26ADC- C000-4 Price: $55.00 Sensitivity: 92 db F[s]: 680 Hz Weight: 1.70 lbs Notes: This tweeter is great. It is +/- 1dB from 700 Hz to 20 khz. It has good dispersion up to 30 degrees. Beyond that it starts to roll off quite fast at about 5 khz. It has an a quite low Fs, allowing for a wide range of crossover points. It is also well within my budget. 2 0

21 Tweeter 4: Seas Prestige 27TBFC/G Price: $50.00 Sensitivity: 91.5 db F[s]: 550 Hz Weight: 1.1 lbs Notes: This tweeter is alright. It is +/- 3dB from 1 khz to 20 khz, which is okay. It has good dispersion up to 30 degrees. Beyond that it starts to roll off quite fast at about 6 khz. It has a quite low F[s], allowing for a wide range of crossover points. It is also well within my budget. 2 1

22 Tweeter 5: ScanSpeak Classic D2008/8512 Price: $75.00 Sensitivity: 88 db F[s]: 800 Hz Weight: 1.20 lbs Notes: It is +/- 2dB from 2000 Hz to 20 khz. This is pretty good. It has good dispersion up to 60 degrees. Which is excellent! It has a quite low Fs, allowing for a wide range of crossover points. It also has a nice roll off starting around 2 khz and rolls of 6 db/octave. It is also within my budget. 2 2

23 Crossover Design and Testing Initial Design My initial Crossover was built as as a 3 rd order filter at 1200 Hz between the Tweeter and woofer one. To get the desired low end out of my second woofer, I added a 1 st order low pass at 125 Hz. The schematic can be seen below in Figure 3. The frequency response of the TPA with this crossover can be seen in Figure 4. Figure 3: Initial Crossover Schematic Figure 4: Initial frequency response. 2 3

24 Testing Procedure and Standards For Each Test the speaker was placed on an 18 cube with 4 square feet of fiberglass insolation on the floor in front of it. A Rational Acoustics RTA- 420 was placed at a distance of 1M (3 4 ). The microphone was pointed at the ceiling. The top of the capsule was at a height equal to center of the tweeter. A Crown CTS amplifier was used for all testing. IT was set to 4/8Ω mode, and the volume was turned all the way up. This amp was fed by a balanced output from a Motu 828 with a sample rate of 96kHz. The Microphone was run through the same interface with a gain of +30 db. Fuzz Measure was used to record all data. The sinusoidal sweep was played at a volume of - 24 db. All graphs in this report are an average of 3 sweeps unless otherwise noted. 2 4

25 Crossover Testing and Tuning The initial testing in Figure 4 showed that the tweeter was significantly louder. I fixed this with a 6 db pad on the tweeter circuit. A comparison of the pad to the initial test can be seen in Figure 5 with the new circuit in purple. Figure 5: Initial Testing vs. 6 db Tweeter Pad Next, the baffle step circuit was added to help with the 500-1,000 range. This circuit was placed upstream from the cross over for the woofers since these frequencies are mainly being reproduced by these drivers. The frequency response with this new circuit can be seen in Figure 6. Figure 6: Frequency response with Woofer Baffle Step Circuit The crossover has two major issues. It still has some issues in the Hz range and 6 khz above is very harsh. The second issue can be fixed with a high shelf. The results of this can be seen in Figure

26 Figure 7:Frequency Response after 4kHz High Shelf By adding this shelf we flattened out the highs so they no longer sound as harsh. This did make the midrange issue worse. The best way to fix this is to widen out the crossover region. The results of this can be seen in Figure 8. Figure 8: Frequency Response with Widened Crossover Region As seen in Figure 8, the crossover region has smoothed out since we widened the crossover region. 2 6

27 Final Crossover Design As you can see in figure 9, my crossover completely changed by the end of tuning. All components changed in value, and many new circuits were added. You can see the Baffle step circuit that was added to both the woofers and tweeter. There is also a pad on the tweeter to bring its amplitude down. The final thing that was added was the Zobel circuit on the woofer. This is to stabilize the impedance of the low drivers. Figure 9: Final Crossover Schematic 2 7

28 Final Loudspeaker Performance Overall Loudspeaker Performance Frequency Response (Average of 10) Integrated frequency Response Harmonic Distortion Percentage 2 8

29 Horizontal Off Axis Response Vertical Off Axis Response Minimum Phase of Tweeter and Woofer #1 2 9

30 Minimum Phase of Woofer #1 and Woofer #2 Difference Between Left and Right Step Response 3 0

31 Integrated Step Response Impulse Response 3 1

32 Waterfall Plot 3 2

33 Tweeter Frequency Response Harmonic Distortion Minimum Phase Step Response 3 3

34 3 4

35 Woofer 1 Frequency Response Harmonic Distortion Minimum Phase Plot 3 5

36 Step Response Impulse Response 3 6

37 Woofer 2 Frequency Response Harmonic Distortion Minimum Phase 3 7

38 Step Response Impulse Response 3 8

39 Bibliography "AES Recommended Practice for Professional Audio." Standards and Information Documents, Print. Dickason, Vance, The Loudspeaker Design Cookbook, 7th edition, Audio Amateur Press, Eargle, John The Loudspeaker Handbook, Kluwer Academic Publishers, 2003 Moulton, David. Total Recording. Sherman Oaks, California: KIQ Productions, Print. Murphy, John L.. Introduction to Loudspeaker Design. Andersonville: True Audio, Print. Newell, Philip Richard, and Keith Holland. Loudspeaker for Music Recording and Reproduction. Amsterdam: Elsevier/Focal Press, Print George Short, North Creek Cabinet Handbook, Old Forge: North Creek Music Systems, 1992, 3 9