Andrew Villa Transducer Theory: Loudspeaker Design. The Dark Passengers

Transcription

1 Andrew Villa Transducer Theory: Loudspeaker Design The Dark Passengers

2 Table of Content Functional Description... 3 Goal:... 3 Listening Purposes:... 3 Environment:... 4 Sound Quality:... 4 Prioritization:... 5 Technical Description... 5 Size/Weight/Portability:... 5 SPL Output:... 6 Frequency Response/Bandwidth:... 8 Coloration/Time Response:... 9 Cabinet Design:... 9 Mounting: Woofer Selection: Tweeter Selection: Crossover Design Type: Crossover Diagram: Drafting Overview: Cut Sheet: Testing & Tuning Initial Performance and Tuning: Enclosure Optimization: Final system documentation Final Testing Results: Frequency Response: Integrated Response: Harmonic Distortion: Minimum Phase: Horizontal Off- Axis Response: Vertical Off- Axis Response: Difference Plot: Step Response: Integrated Step Response: Impulse Response: Waterfall:

3 Individual Driver Tests: Woofer Frequency Response: Tweeter Frequency Response: Woofer Harmonic Distortion: Tweeter Harmonic Distortion: Woofer Minimum Phase: Tweeter Minimum Phase: Woofer Horizontal Off Axis: Tweeter Horizontal Off- Axis: Woofer Vertical Off- Axis: Tweeter Vertical Off- Axis: Woofer Step Response: Tweeter Step Response: Woofer Impulse Response: Tweeter Impulse Response: Bibliography Functional Description Goal: The goal is to create 2- way near field mixing monitor speaker that can be built within a budget of $350. Through building these speakers, a better understanding of how loudspeakers are constructed will be obtained. Considerations to make when designing loudspeakers will be learned, along with making clear choices on the design and learning how to achieve them. Listening Purposes: The use of these speakers will be to work on various sound design projects. This will help in composing music and mixing sound effects for multiple media formats such as video games, film, and commercials. I will be mixing sound effects and music to a point that is satisfactory for the audiences of video games and other forms of multimedia, therefore needing to be able to listen backward in time so that I can 3

4 hear the mistakes that I have made and go back to fix them. To achieve listening back, the speakers will need to have a flat frequency response in order to reproduce the majority of the frequency spectrum accurately 1. The unreality of the speaker sound is hi- fidelity, making them capable of accurately producing original sounds. This is excellent for creating and editing quality sound effects. To listen for enjoyment, I will use a different set of loudspeakers. Environment: The location that my speakers will be is a medium sized, carpeted and furnished room. This location will resemble that of potential audiences of my work, mixing in a similar location replicating what they will hear. The background noise in this location will be that similar to a family living room or a home office, which would be very minimal such as the hum of the computer, or cars faintly driving by. I want to put the 2- way loudspeakers on top of a mixing console or on a desk, which will result in a more flat and fast response 2. Since I am a semester from graduating, there will be a lot of moving in the near future. Due to this fact, I want my speakers to be very portable and capable of travel. The size of the speaker boxes will be small enough to fit in my car and on my desk, but big enough to have a well placed LF roll- off. Visual Aesthetics: The plan is to give the speakers a personal artistic look by having the outer enclosure made of MDF, painted in white with a red splatter design on the front baffle and two side walls, giving it a unique look that matches that of the show Dexter. Due to lack of skill in woodwork and limited funds, the popular rectangle shape will be the design for my enclosures. The rectangle shape staggers the resonances unlike the cube shape that would stack the resonances on top of each other 3. The weight of the speakers will be of a mass that isn t too heavy or uncomfortable to pick up, but will be heavy enough that the enclosure doesn t move from the cabinet resonance. The speakers will be of medium size. This makes it easy to fit on a desk and be easily transported, and will be large enough with produce low frequencies that my work will entail. With the materials of MDF and plywood to build the enclosures, the lifetime for these speakers should be 5-6 years. Sound Quality: The speakers will have qualities of robustness giving it stability of performance with usual listener movements and listening environments. The sound quality will match 1 Moulton, David. "Part IV About Recording Production and Studio Operations." In Total recording: the complete guide to audio production. Sherman Oaks, Calif.: KIQ Productions, Newell, Philip Richard, and K. R. Holland. "Form Follows Function." In Loudspeakers: for music recording and reproduction. Oxford: Focal, Murphy, John L.. "Enclosure Design and Construction." In Introduction to loudspeaker design. Escondido, CA: True Audio,

5 that of anchor loudspeakers and as a reference and reality check. They will be hi- fi with low levels of non- linear distortion and fast time response. They will be used for critical listening and uncovering faults in sound recordings 4. Prioritization: Chart Prioritization Cost LF Size SPL Between the levels of SPL, Size/Weight, Fidelity (LF), and cost, my focus will be equally on cost, size, and LF. The SPL will be my least prioritized. I chose this because the size and LF play a huge factor in my design choice. The cost is a personal choice but also correlates with how large of a design I can go. The emphasis on size makes my speakers more portable, easier to for use as mixing monitors that sit on the console. The importance on hi- fidelity creates a flatter frequency response and a more realistic sound for mixing. Technical Description Size/Weight/Portability: In order to build desk monitors for mixing purposes, the right dimensions have to be made. To get the low frequencies that my mixing will require, the ideal max size of the speakers will be dimensions of 15" x 10 1/2" x 8-7/8" 5. The size of the speakers will be adequate to the diversity of spaces that it will be used in. For example, I will presently be using them on a desk that is 24 x 24 x30. In the future I plan on using them on a more spaced out L- shaped desk of dimensions that are inches tall, about 29.5 inches wide, and around inches deep. Being used in a medium sized room of 12 ft x 16ft x 8 ft will make the speaker size ideal. The box is going to be small in size, so to get the desired bass response that I want, I will be making a vented enclosure. 4 AES recommended practice for professional audio Subjective evaluation of loudspeakers Reprint, New York: Audio Engineering Society, Inc., "Yamaha NS10s." Features and Specifications. (accessed January 26, 2014). 5

6 The weight of the cabinet should be around lbs., due to the size and thickness of materials. The amount of weight will be under my weight carrying capacity. This will help for its portability. To test the weight- to- portability ratio, I moved around my microwave that is the closest size/weight to my speakers, being of 40 lbs., which fit nicely on my desk when tilted vertically. This size will work well in long distance travels in the back of my car. The materials of the external ½ MDF and internal ½ plywood will make the speakers robust and be able to withstand extremities in the music/sfx that will be played through them. SPL Output: For the purpose of mixing music and sfx in multiple formats of media, using the THX average SPL of 85 db and 20 db of headroom would be ideal for my speaker design 6. Using those standards, I want my maximum SPL to be at 105 db and my minimum to be at 95 db, easily reaching that level without distortion or malfunction. Using NIOSHA guideline, my action level when I am mixing will be at a maximum of 85 db, giving me a total of 9 hours of exposure before hearing damage takes in 7. To find my preferred mixing levels I listened to multiple types of music and film/video game sound at different times of the day. Then recorded the results and calculated the averages that are shown below. 6 "Thx Technology." THXcom THX Reference Level Comments. (accessed January 26, 2014). 7 Plummer, Christopher. "SPL Cheat Sheet." Lecture, Transducer Theory from Michigan Tech, Houghton, January 24,

7 Based on these results, if I am not able to reach a max SPL of 106 db, then 95 db will be an acceptable minimum to my listening tastes, given that songs and sfx will have pikes in their audio files. For the loudspeaker sensitivity, given that I use the THX standard of 85 db, my listening habits will be at a distance of 1-3 meters from the speakers. Adding in the headroom of 20 db from the amp and taking into consideration of the inverse square law, there would be a 6 db loss at 2m, which is where I will be mixing, all equaling to a sensitivity of 92 db SPL 1w/1m. If that sensitivity level is not reachable, then I should be able to reach the average of db. The graph below explains the relationship of the inverse square law, based on my preferred mixing distance. 7

8 For the power of my speaker, I am generally looking for at a 100- watt amp. With a 100- watt amp I will be able to hear those high peaks in my music and sound effects, allowing for enough comfortable room to mix in. If 100- watt amp is not obtainable, I am willing to go in the range between watts. Here is a dbw graph that shows the amount of headroom I will have depending on the wattage of amp that is chosen. Frequency Response/Bandwidth: An ideal frequency response for mixing speakers is a flat frequency response. That is the type of voicing that is desired for my speakers. I would like to achieve the perceived low bass, since I am building a smaller pair of speakers that may not satisfy my LF needs. My bandwidth that I want to achieve would be a low frequency that can support something as low as a bassoon, which it s lowest note plays at a frequency of 40Hz. Because of my limited budget, I am willing to sacrifice some LF and go between Hz. The desired low frequency that I want to achieve is of 40 Hz. Based on a LF frequency experiment that I did, listening to multiple recordings and using a low pass filter on the audio tracks, I was able to find the average LF roll- off that is acceptable to my needs and budget. The minimum LF roll- off that I will accept is that of 65 Hz. Below are the results of using an 18 gain/slope to resemble that of a vented box. To be able to use my speakers efficiently, a high frequency roll- off of 20 khz would work very well. Genre of Music Noticeable Bass Loss Acceptable Bass Loss Unacceptable Bass Loss 8

9 Rap/Hip-Hop 50 Hz 60 Hz 70 Hz Punk/Hardcore 40 Hz 60 Hz 70 Hz Folk/Rock 55 Hz 70 Hz 80 Hz Funk/Disco 55 Hz 60 Hz 70 Hz Old Country 60 Hz 70 Hz 80 Hz Fight Scene 50 Hz 70 Hz 80 Hz Scary/Horror 55 Hz 65 Hz 75 Hz Racing Game 50 Hz 60 Hz 80 Hz Film Score 50 Hz 60 Hz 70 Hz Coloration/Time Response: My design is to be that of a smaller pair of loudspeakers, making my low frequency capabilities limited. To make up for this, I want to extend my low frequency response to get that bass that I was missing from my small box design. To do this I will be constructing a vented enclosure, which will result in a worse transient response but I am willing to trade that off for the extended low frequency response that vented boxes provide. In terms of coloration, the design of these speakers should avoid noticeable coloration of sound. It is desired for these speakers to have as flat of a frequency response as possible in order to play back sounds in their purest form. To achieve such coloration, I plan on my enclosure to have a dead internal reflection, otherwise, any reflections and resonances occurring within the box are likely to pass outside through the cone and combine with the directly radiating sound in such a manner that will result in undesirable coloration 8. To make this certain, I will be applying absorbent lining foams or fibrous materials in between the inside of the two loudspeaker enclosures, making them non- reflective as reasonably possible. Cabinet Design: 8 Philip Newell, and Keith Holland, Loudspeakers For Music Recording And Reproduction, (Burlington, MA: Elsevier Ltd., 2007), 86. 9

10 The design of my cabinet will ideally have no vibrations so that there is no coloration created in the overall sound output. In order to prevent the vibration of my cabinets, but based on my woodworking skills, I am going to build a cabinet shape that resembles model K, a rectangular shape. It has a reasonable flat response and will decrease the effects of corner diffraction and standing waves, more than the cube shape. The diagram shows the different shapes and their frequency responses 9. Internally, to reduce vibration and the effects of coloration from the cabinets, I plan on installing 1/8 vinyl in between the internal and external enclosures. Due to the small size, there will not be any bracing and the structure will be strong enough with just the ½ plywood, ½ MDF, and 1/8 vinyl that is place in between the two boards. Mounting: With experimenting between the types of power response, my preference would be that of a narrow dispersion so that the sound hits me directly. Since I am the only one who will generally be using these speakers, wide dispersion can be ruled out. Woofer Selection: In a low frequency driver I am looking for one that has a roll- off at a relatively low frequency and has a flat response with and a break- up in the high frequency range that is high enough to get a good cross- over point. The cost of the driver needs to be $50- $60. There was one woofer driver that I found that had a decent price and a quality that was satisfactory. 9 Philip Newell, and Keith Holland, Loudspeakers For Music Recording And Reproduction, (Burlington, MA: Elsevier Ltd., 2007),

11 Woofer Name Freq Range Power Handling f(3) Sensitivity Diameter Price DARS Paper Woofer 45 Hz - 8 khz 90 W 53Hz 89 db 7 $51.75 SB17NRXc35-8, Hz khz 60 W 46 Hz 89 db 6.5 $62.40 FounTeck FW Aluminum 40 Hz - 4kHz 45 W (rms) 47 Hz 87.3 db 6.5 $46.00 ScanSpeak 18W/8434G- 00 Discovery 45 Hz - 8 khz 170 W max 48 Hz 88.7 db 7 $67.90 SB17MFC35-8 6" Polypropylene Cone 35 Hz - 3 khz 60 W 45 Hz 88 db 6 $ The Dayton Audio RS Paper woofer I have modeled this woofer model and have It has an f3 of 53 Hz, which is close to the max LF roll- off that I will accept. Tuning the box at 55 Hz and with a size of.47 cu ft, it seems that the box size would be too small to give quality sound for a vented enclosure and would not be satisfying to my taste. Woofer Name Frequency Range Diameter Sensitivity Power Handling Price Dayton Audio RS khz 7 89 db 90 W $

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13 2. The SB Acoustics SB17NRXC35-8, 6.5" Woofer. I found a slightly more expensive woofer that is a lot better than the previous woofer. It has a wider frequency range, allowing lower frequencies to be produced. According to the model, if I increase the size of the enclosure and lower the box frequency, I can get an f3 of 46 Hz with a flat response. With the HF range going to 35 KHz, the crossover point could be lower in frequency. Woofer Name Freq. Range Diameter Sensitivity Power Handling Price SB17NRXc35-8, Hz khz db 60 W $

14 3. The FounTeck FW Aluminum Cone is the woofer I have decided to choose. With a cheaper price and smaller design, this woofer has an excellent resonating frequency and a decent sensitivity. The frequency Response is fairly smooth besides the peak that starts at 4 khz. The modeling of the woofer in WinSpeakerz showed me that with a box volume of.5 cu ft and a box frequency of 47 Hz, I could get an f3 of 47 Hz. I am willing to sacrifice a little box frequency in order to decrease my box size. Also, the design of this woofer is more aesthetically pleasing to the eye. Woofer Name Frequency Range Diameter Sensitivity Power Handling Price FounTeck FW Aluminum 40-4kHz db 45 W(rms) $

15 4. The Scanspeak 18W/8434G- 00 Discovery, 7" Midwoofer. This woofer has a really smooth frequency response from about 200 Hz - 1 khz. Out of the five woofers I compared, this was my least favorite. It is the most expensive and yet has the highest resonating frequency. The frequency response breaks up at around 1 khz, which is not good for the high frequency crossover. It has an f3 of about 47, but the box size is.75 cu ft, which I want to keep the size down for price and design. It does have a good sensitivity and frequency range, but I think the price compared to the specs, it doesn t match up with a cheaper woofer like the FounTeck. Woofer Name Freq. Range Diameter Sensitivity Power Handling Price Scanspeak 18W/8434G Hz - 8 khz 7 Discovery, 7" Midwoofer 88.7 db 170 W max 15 $67.90

16 Woofer Name 5. SB Acoustics SB17MFC35-8 6" Poly Cone Woofer The frequency range of this woofer is the widest out of all woofers and has the lowest f(3) making it capable of reaching nice low frequencies. The frequency response is pretty smooth from 200 Hz - 1 khz, but starts to break up at around 3 khz. It is the smallest woofer, which makes it good for my design, but is the second most expensive out of the 5 woofers. The f3 of the enclosure was 45 Hz, which is fairly well, but the box size was.80 cu ft, making it a little too large for my design. Freq. Range Diameter Sensitivity Power Handling Price SB17MFC35-8 6" Poly Cone Woofer 35 Hz - 3 khz 6 88 db 60 W $

17 Tweeter Selection: My desired tweeter is one that has a flat frequency response into the high frequency, to match that of my woofer. Having a high power handling will be beneficial with the sharp, fast peaks in the sound files due to action and sfx s. Having an f(s) that is relatively low will help with the placement of the crossover point. Tweeter Name Freq. response Diameter f(s) Sensitivity Power handling Price Dayton Audio RS28F- 4 1,200 Hz- 20 khz 1- ⅛ Hz 88 db 100 W $54.75 Morel MDT /8" Neodymium SB26ADC- C000-4 Aluminum Dome 1,500 Hz- 20Hz 1- ⅛ 780 Hz 89 db 120 W $ khz- 20 khz 1- ⅛ 680 Hz 90dB 120 W $39.00 Vifa NE25VTA 716 Hz- 25 khz Hz 87.6 db 160 W max $39.60 Vifa XT25TG ,500 Hz- 40 KHz Hz 91.1 db N/A $ Dayton Audio RS28F- 4 Although this tweeter is a bit pricey for my budget, it has an excellent f(s) allowing for a lower frequency crossover, and has a flat frequency response making it go well with any decent woofer. It also has an excellent amount of power handling and sensitivity and a wide range frequency response. The off- axis response is not too different from the on- axis response. The only downfall is the price. 17

18 2. Morel MDT /8" Neodymium Tweeter With a price that is about $20 cheaper, there are only a few things that are different from the previous tweeter. The higher f(s) makes the crossover frequency lower. This tweeter is still decent for its price because it has a very high power handling, able to take those sudden peaks of SPL and a sensitivity that is excellent as well. Tweeter Name Freq. Range Diameter Morel MDT /8" 1,500-20,000 Hz 1- ⅛ Neodymium Tweeter Sensitivity Power Handling f(s) 89 db 780 Hz $ W Price 18

19 Tweeter Name Freq. Range 3. SB26ADC- C000-4 Aluminum Dome Tweeter This has a very flat frequency response that is appealing and it stays within +/- 2 db of 90 db from 1 khz- 20 khz, making it perfect for its use. This tweeter exceeds the previous one, not just with frequency response but with price as well. The f(s) is a little bit higher, making the HF crossover point a little worse. Diameter SB26ADC- C khz- 20 khz 1- ⅛ Aluminum Dome Sensitivity Power Handling f(s) 90 db 120 W Price 680 Hz $ Vifa NE25VTA, 1" Aluminum Dome Tweeter, 4 ohm Aesthetically, this tweeter is the most unique looking out of all the compared 5. The frequency response of this tweeter is fairly flat from about 800 Hz - 5 khz and then starts to dip before the break- up. It is a decent tweeter, but is not the best. It is a little more expensive but has a lot wider frequency range and much higher power handling level. 19

20 Tweeter Name Freq. Range Diameter Sensitivity Power Handling f(s) Price NE25VTA, 1" 716 Hz- 25 khz db 160 W max 716 Hz $ Vifa XT25TG Dual Ring Radiator Tweeter This is excellent for its price in comparison with its very low f(s). It is appealing to me because it has a really flat frequency response and stays within 2 db of 90 db throughout the spectrum that it will be used in. This tweeter exceeds my needs and still manages to stay within my budget. It has the widest range of frequency, the highest sensitivity, and the lowest f(s). The only downfall is that the power handling is not given. In comparison of all the tweeters I am going to use this tweeter for my speaker design. Tweeter Name Freq. Range Diameter Sensitivity Power Handling f(s) Price Vifa XT25TG ,500 Hz- 40 KHz db N/A 530 Hz $

21 Crossover Design Type: My goal for my crossover is to create the flattest frequency response in such a way that it does not affect voicing. In order to allow my drivers to be used over almost all of their flat frequency response, I am using a 3 rd order crossover. As you can see in the chart/graph below, the crossover will be symmetrical having crossover points at both the woofer and tweeter at 2 khz and a - 3 db pad on the tweeter, resulting in a reasonably flat response resulting in +/- 1.5 db. 21

22 Driver Attenuation: Due to the fact of the difference in the sensitivities of my tweeter and woofer, I am going to integrate driver attenuation into the design of my crossover. With the woofer having a sensitivity of 87.3 db and my tweeter of 91 db, I will be creating a 3 db padding of attenuation for my tweeter in the crossover. I will also be adding a low pass filter at the crossover point of my woofer in compensation of the baffle step, which should be easy to fix. Crossover Diagram: Here is a diagram of a 3 rd order crossover that I will use that has a negative 18 db per octave. 22

23 Drafting Overview: 23

24 Cut Sheet: Testing & Tuning Initial Performance and Tuning: The first initial sweep of the sum of my drivers showed that I had a boosted high end and a peak between Hz, along with a sharp peak at 1kHz. The sharp peak at 1kHz was due to resonance in my box, so to fix that I add fiberglass for dampening. After this I started working on my woofer to see if I can get rid of the 2db peak between Hz. To fix this boost at Hz, I used a baffle step compensation, which was just an inductor in parallel with a resistor, all in series with the positive line. To figure out the values of my baffle step compensation, I used a baffle step compensation calculator online. With my baffle step at the end of my woofer circuit, my initial sweep of this was a bit too much, and brought down my mid range a lot. So my process of figuring out what to do was first, change the parts to larger and smaller values of the ones I already were using. This method did not work, so instead I tried 24

25 putting the baffle step compensation at the very beginning of the positive line on the tweeter. This method worked very well and resulted in a smoother mid range from 400-1kHz. The issue with my tweeter was that there was a huge peak at 9kHz. To compensate for this issue, I tried using an L pad system, which is two resistor, one in series with the positive line with the other connected to that, going in parallel to the negative line. To figure out the values of the resistors I used an L pad calculator online, but that did not work, so I had to do trial and error, making the values larger and smaller until I got values that worked well. After I was able to fix my tweeter issue, I ran a sweep of the sum and got a boost in at my crossover point which was suppose to be at 2kHz at the time. To fix this boost I decided to lower my woofer crossover point because my tweeter is able to go as low as 400 Hz. In order to change my crossover point without having to reconstruct my entire crossover, I had to use trial and error to see which specific parts of my crossover, change the point of crossing over for my woofer and tweeter. Once I found the right components, I then used trial and error again to figure out what values I need to use that will smooth out the crossover point. After some lengthy tedious work, I was able to smooth out that boost from my crossover point. After all of my issues were fixed, I did another full sweep of the sum and was very pleased with +/- 1.6 db from khz. Enclosure Optimization: The enclosure of my box is quite small, but during my initial testing I had a lot of resonance at 1kHz from the standing waves. I started out doing sweeps with no dampening material in the box, and slowly worked my way, adding more dampening material until the peak was gone and I was pleased with the sound. The final stuffing that I chose was quite a bit. The sidewall by the drivers have fiberglass on them and so did back of the sidewalls near the porthole has fiberglass as well. The fiberglass is not glue at the moment, because I may decide to take out some dampening material to see if I could increase the volume capabilities and depth. 25

26 Final system documentation Final Testing Results: Frequency Response: Integrated Response: Woofer: Yellow, Tweeter: Purple, Sum: Blue Harmonic Distortion: 26

27 Minimum Phase: Woofer: Yellow, Tweeter: purple Horizontal Off- Axis Response: Blue: 0, Green: 15, Yellow: 30, Purple: 45, Bottom Yellow: 60 Vertical Off- Axis Response: Green: 0, Red: 15, Blue: 30, Yellow: 45, Bottom Green: 60 27

28 Difference Plot: Step Response: Integrated Step Response: 28

29 Impulse Response: 29

30 Waterfall: 30

31 Individual Driver Tests: Woofer Frequency Response: Tweeter Frequency Response: Woofer Harmonic Distortion: 31

32 Tweeter Harmonic Distortion: Woofer Minimum Phase: Tweeter Minimum Phase: 32

33 Woofer Horizontal Off Axis: Tweeter Horizontal Off- Axis: Woofer Vertical Off- Axis: 33

34 Tweeter Vertical Off- Axis: Woofer Step Response: Tweeter Step Response: 34

35 Woofer Impulse Response: Tweeter Impulse Response: 35

36 Bibliography AES Standards and Practices. New York: Audio Engineering Society, Inc (Accessed January 17, 2014) Apple Inc. Soundtrack Pro 3 User Manual. ter=c%26section=6%26tasks=true (accessed January 18, 2014). Moulton, David. Total Recording: The Complete Guide to Audio Production and Engineering. Sherman Oaks, CA: KIQ Productions, Murphy, John L. In Introduction to Loudspeaker Design. Escondido, CA: True Audio, Newell, Philip Richard, and K. R. Holland. Loudspeakers: For Music Recording and Reproduction. Oxford: Focal, North Creek Music Systems. Cabinet Handbook. 2nd Edition. Old Forge, New York: North Creek Music Systems, Plummer, Christopher. FA 4740 Transducer Theory. Houghton, Michigan, Spring Michigan Technological University. "THX Technology." THX.com. THX Reference Level Comments. technology/thx- reference- level/ (accessed January 26, 2014). "Yamaha NS10s." Features and Specifications. (accessed January 26, 2014). 36