Loudspeaker Active Spectral Divider Design Project
Loudspeaker Active Spectral Divider Design Project
ECE 40020 Sound Reinforcement System Design Spring 2018 Loudspeaker Active Spectral Divider Design Project From left to right: AJ “Andrew Jackson, 7th President of the United States” Trent, Frank “The Tank” Kim, Oscar “Orthogonal Olive Oil” Otero, Joe “Trying to Entertain Himself” Coleman
ECE 40020 Sound Reinforcement System Design Spring 2018 2 Project Report Evaluation – Team ID: 02 Team Member 1.0 2.0 3.0 4.0 App Tech * TOTAL Tae Hyun Kim (Frank) Joe Coleman AJ Trent Oscar Otero Maximum Points Possible 10 30 10 10 20 20 100 * technical content, writing style, professionalism, clarity, completeness Instructor comments: _ _ _ _ _ _ _ _ _ _ _ _ _ _
ECE 40020 Sound Reinforcement System Design Spring 2018 3 TABLE OF CONTENTS Abstract i 1.0 Introduction 1 2.0 Design Narrative ? 3.0 Results ? 4.0 References ? Appendix A: Activity Logs ? Appendix B: Schematic (Analog) or Tuning Parameters (DSP) ? Appendix C: Frequency Response Measurement ? Appendix D: Subwoofer Design Documentation ? Abstract Your job is to design an active 3-way spectral divider (crossover network) for a “stock” 3-way (tri-amplified) labyrinth loudspeaker system. Both the loudspeaker system and class D amplifier modules (4 x 100W) with power supply will be provided. Your spectral divider can either be analog (op amp based) or digital (DSP based, implemented using a “stock” microcontroller development board).
Level controls should be provided for each loudspeaker output drive. Primary goals including optimizing frequency response (target ±3 dB over the range of 40 Hz to 16,000 Hz) and minimizing THD (total harmonic distortion). For bonus credit an (add-on) subwoofer can be designed and constructed, which will require a 4-way spectral divider.
ECE 40020 Sound Reinforcement System Design Spring 2018 4 1.0 Introduction Tae Hyun Kim (Frank): I’m a senior in electrical engineering and I am focusing my study in power electronics. However, I worked with audio engineering for hobby since I was young age. Before coming to Purdue, I made my own stereo system amplifier and speaker during my free time and I wanted to take ECE40020 for fun before graduation. However, I may not have same experience or knowledge with other people in this class but I am willing to learn more materials so I can “expand” my knowledge into wide area of sound engineering.
Joe Coleman: I’m currently a senior in Electric Engineering. Basically, I took this class as result of my interest in this subject, and wanted to explore more into sound and speaker design. With my education that I have received so far, I was very interested in seeing how I could apply my electrical engineering knowledge to this project. When we decided to make an analog crossover filter, versus a digital one, for this project, I was excited to be able to apply my circuitry knowledge to this course. Through this class, I hope to expand my knowledge on sound design, and use this to further my enjoyment of my hobby.
Potentially, one day, I hope to end up in some sort of audio consumer-product related field.
AJ Trent: I’m a current junior majoring in Acoustical Engineering and minoring in ECE and Sound for the Performing Arts, and outside of my academic connection to audio, I’m a musician and long time audiophile. ECE 40020 is likely the single most applicable class at Purdue to those interests, and so it was an obvious choice as one of my upper level ECE credits. This project has already served as a connecting link between some of my earlier coursework and has given me the opportunity to dive into the basics of the industry I plan to get into after graduation, audio electronics design.
Oscar Otero: I am currently a Junior in Acoustical Engineering.
Through my internships and personal studies, most of my breadth of knowledge pertains to architectural acoustics. However, I am interested in sound in all its applications, so I took this course so I could learn more about the electrical side of sound and audio. Through this project, I have learned much about the physics behind speaker enclosures, as well as much more about the circuitry behind filters for an analog system. While I am excited to increase my knowledge in the audio side of sound, I still hope to make a career in architectural acoustics. However, the knowledge obtained from this class will help me communicate with the audio engineers I will likely work with in the future.
ECE 40020 Sound Reinforcement System Design Spring 2018 5 2.0 Design Narrative Analog Crossover Design When the group first started our design process, we were given a stock 3-way labyrinth loudspeaker system. Our design process, however, differed greatly from the other three groups. Our system would be an analog system, which meant that we would have to design our crossover filters ourselves. This meant that the tuning process would be much more labor intensive and less opportunities for trial and error adjustment would be available. This obstacle was kept in mind throughout the planning and design process.
Before we began our actual design process, it was important to assess the stock loudspeaker system we were given. In our initial subjective listening tests of the stock loudspeaker system, we found that the system sounded too harsh in the higher frequencies, which we determined to be a downside of the hard dome tweeter installed in our cabinet. To remedy this and shoot for a less “tinny” and fatiguing sound we decided to swap the original tweeter out for a soft dome tweeter available in the lab, which we felt would help improve our sound from the start and better match our subjective preferences.
We also felt that the cabinet was lacking low end power and extension and decided to design and build an external subwoofer as well. Because we designed a passive crossover network, our process included less trial and error and relied more heavily on research and analysis of the speakers’ frequency responses and usable ranges. For example, based on the response curves provided, the woofer can produce frequencies down to about 70 Hz. To ensure a smooth crossover and avoid distortion in the woofer and port noise in the subwoofer, we settled on a crossover point of 80 Hz with a filter slope of 12 dB/octave.
A similar approach was used to select the crossover points between the woofer, midrange driver, and tweeter.
When choosing what crossover frequencies and gains for our analog filter, we first considered looking at the frequency response of our drivers. Our group noted a few characteristics of the frequency response of all three drivers that we prioritized when choosing our crossovers and dB gains. First, we noted the large peak in the midrange driver at around 500 Hz, so in order to counter this peak, we decided to have the crossover between the woofer and squawker at 800Hz. Second, we noted that the squawker rolled off at around 3 KHz, with as well as the tweeter beginning to level off. This frequency would be noted as the crossover frequency between the squawker and tweeter.
Third, our group noticed a dip in dB around 2 Khz for the squawker, so in order to counteract this dip, we would aim to increase the gain around that area. Finally, we desired to crossover with our designed subwoofer at 80 Hz. Once we choose our desired frequencies and gains, it was time to design how to implement this filter. Our choice of filter ended up being 2nd-order Butterworth filters for both our high-pass and low-pass cutoffs. For each driver, the signal would be passed through one or more 2nd-order Butterworth filters, and from there the signal would enter a TL074C operational amplifier.
These op amps would be placed in non-inverting configuration, with the resistor values of our op amp circuit determining the gain of the filtered signal. This gain would have the value of 1 plus the ratio of the output to grounded resistor. The output signal of the amplifier would be passed through a small resistor, where it then enters its specific driver. Along with our LPFs and HPFs, if a driver needed more than one filter, we would need a inverting op amp with
ECE 40020 Sound Reinforcement System Design Spring 2018 6 a gain inverted to contrast the filters’ gains before the signal passes through the crossover filters. This is due to Butterworth filters causing a 90° phase shift of the signal, but in opposite directions for the LPFs and HPFs. From our desired frequencies and gains, we were able to calculate the components needed for both our 2nd-order Butterworth HPFs and LPFs. Across our circuit, we would use 10nF capacitors for our filters, though our resistor values would vary. For our woofer, we had our LPF with a 20 kΩ resistor and a HPF with a 200 kΩ resistor, to reach our desired crossovers.
Our op amps, both inverting and non-inverting, would have an input resistor of 10 kΩ and output of 8200 Ω. For our squawker, we had our LPF with a 5820 Ω resistor and a HPF with a 20 kΩ resistor. Our op amps, both inverting and non-inverting, would have an input resistor of 10 kΩ and output of 5820 Ω. For our squawker, we would have a HPF with a 5820 Ω resistor, and our non-inverting amp would have an input resistor of 10 kΩ and output of 5820 Ω. Along with our crossover filters, the input signal would lead into voltage follower with a 100 kΩ resistor, which would be connected the each driver’s crossover filter.
Also the output signal would feed into a 100 Ω resistor.
The main difficulties that we found while tuning our system came with difficulties from working with an analog system. When we subjectively listened to the system, we felt the need to adjust the crossover filters to better tune the system. However, due to our analog system, adjusting crossover filters was a rather lengthy process, so we were unable to do so as much as we would have had we had more time. Also, making adjustments to the gain on the woofer was a lengthy, difficult process. When we came to problems with our lower midrange frequencies, solving the problem was difficult because it took much time that we did not have to adjust the filters.
As stated above, the woofer of the stock speaker system provided to us was not effectively covering the deep bass we hoped it would. Because of this finding, we decided to also design a subwoofer to give the lower frequencies the power we wanted. With the wood being cut by an outside source, our group was in charge of the dimensional design, assembly, and tuning of the subwoofer. Subwoofer Design Firstly, we had to decide what type of enclosure to use. After considering sealed, transmission line and bass reflex designs, we decided on bass reflex for its smaller size and efficiency compared to sealed boxes and its vastly simpler design compared to transmission line systems.
While a sealed box design may have met our targets for subwoofer power upwards of 50 Hz, it couldn’t even come close to the power of a bass reflex design down into the lower bass frequencies. Since we wanted the power to match that of the speaker system and not be too large, we felt that we could provide more power in lower frequencies using the bass reflex design. When selecting a driver, we wanted to be sure to have a driver that would be easier to tune, due to our analog system. We initially were looking at three drivers, the Dayton Audio SD315A-88 12” DVC, the Peerless SLS-P830945 5’¼” Paper Cone, and the Dayton Audio DCS165-4 6-½” Classic Subwoofer.
The SD315A-88 had the largest cone size, as well as the flattest low end of the three. Due to these facts, we felt that this driver would be the simplest to tune with the rest of our system.
ECE 40020 Sound Reinforcement System Design Spring 2018 7 The following are the woofer’s relevant small signal parameters. These parameters were taken into account throughout our design process: ● Qts: .33 ● Qes: .37 ● Fs,: 24.2 Hz ● Vas: 151.7 ● Pmax : 240 W, ● Vd: .366 l, ● Xmax: 7mm, ● Sd: 522.8 cm2 ● SPL: 89.6 @ 1W/1m, Now, since we chose to use a vented enclosure, we also had to choose what type of alignment our subwoofer would use. Our choices consisted of SBB4, QB3, and SC4. We chose to use the QB3 alignment. In addition to extending the 3dB point below that of the other alignments, QB3 controlled cone displacement much more effectively than the other two alignments.
In fact, both the SBB4 and SC4 alignments exceeded our driver’s maximum for cone displacement. By using design tools such as Speaker Box Lite, we were able to find minimum port diameters. Because we did not want the port to make the subwoofer too loud, we decided to use the minimum port diameter, which was 3”.
Our final dimensions were about 26.5”x16.25”x10”. These dimensions are almost exactly matched to our designs done through ajdesigner and other subwoofer design websites, which were 26.4”x16.3”x10.0”. The dimensions calculated using Speaker Box Lite were about the same as our calculations done on ajdesigner, which reinforced our confidence in our calculations. Confirmed through our final measurements and comparisons with our calculations, our subwoofer enclosure fulfills the “golden ratio” requirements. This means that the geometry of our box should optimize the sound it creates.
We also needed to find several characteristics of our subwoofer.
These included features such as box resonance, volume, and power frequency. By inputting specifications of our subwoofer into ajdesigner, we determined several parameters for our loudspeaker. The box volume came out to be 70.3 liters, the box resonance was 29.1 Hz, the -3 dB half power frequency was 33.48 Hz, and the port length was about 17.3 cm. These numbers were also confirmed by Speaker Box Lite. From the frequency response curve predicted by ajdesigner, it was expected that the subwoofer would begin rolling off around 50 Hz. From our measured response curve (see appendix D), the frequency began to drop around 63 Hz.
This was higher than expected, and could have resulted in less accurate crossover frequencies. However, this shouldn’t be much of a problem, as the general consensus of our subjective listening tests was that the system sounded a bit “muddy” (meaning that the bass was overpowering).
ECE 40020 Sound Reinforcement System Design Spring 2018 8 3.0 Results Before assessing our results, it should be noted that we used our own custom-built subwoofer during our testing phase and subjective listening tests. While a test subwoofer was provided for us, we felt that since we wanted our system to be a 4-element system, it would be better to test the system as one unit. Across the frequency spectrum, there was a relative balance in the frequency response of our system. However, there were certain frequencies that dropped, particularly drops in the midrange and 2kHz. Additionally, our system did not quite satisfy the target constraint of +/-3 dB from 40 Hz to 16,000 Hz.
While we were fairly close, problems with our crossover filters were most likely the leading causes of our issues in our frequency response. Because of the problems from our crossover filters, some drivers were powering frequencies outside of their specified range. For example, our crossover from midrange to the tweeter was at 3 kHz. However, the midrange driver is not capable of handling frequencies much above 2 kHz. For this reason, the crossover frequency between these two drivers should have been much lower, likely around 2 kHz or even 1.5 kHz. These issues with the crossover frequencies made certain frequencies erratic and “choppy”.
These problematic frequencies were made evident during our subjective listening tests.
During our subjective listening tests, the group listened to a wide variety of music on the system. With genres ranging from hip-hop to jazz to pop to hard rock to classical music, we wanted to ensure that our system sounded accurate by listening to tracks with which we were familiar. Songs used as reference tracks included: 1. “Treetop Flyer” by Stephen Stills 2. “Check the Rhime” by A Tribe Called Quest 3. “So It Goes” by Hi-Lo Jack 4. “Make a Change” by Durand Jones & the Indications 5. “O Fortuna” by Carl Orff 6. “Dat Dere” by Art Blakey and the Jazz Messengers 7. “Girlfriend” by NAO 8.
“Cut Your Hair” by Pavement 9. “Norwegian Wood” by Victor Wooten 10. “When You Sleep” by My Bloody Valentine 11. “The Way You Make Me Feel” by Michael Jackson 12. “Here Comes the Sun” by The Beatles Particularly in the low end, the texture of the sound created by our system is rather soft. Additionally, the added power from the subwoofer gives the system a warmth that it especially lacked before we implemented it into our system. Also, the brightness created by the original tweeter we had is gone, and the higher frequencies are well balanced with the rest of the frequency spectrum. The main problem we found through our subjective listening tests was a “peakiness” in the midrange frequencies.
Through our subjective listening tests, we also found problems with vocals for certain pop songs. Particularly in “The Way You Make Me Feel” by Michael Jackson and “Here Comes the Sun” by The Beatles, we found that the voice often felt “masked” by the other parts of the
ECE 40020 Sound Reinforcement System Design Spring 2018 9 song. With songs like these, the vocals should be sitting in front of the instruments, but the voice was often too soft and drowned by the rest of the sounds in the songs. Overall, the system performs fairly well. There is a relatively flat balance across the frequency spectrum, and the subwoofer certainly adds a significant warmth and bass “thump” to the system.
However, there were some problems with uneven response in the midrange/presence region and midbass frequency range. At some points in the frequency response (200 Hz being a great example), significant dips in driver response call for a boost or reduction in power. At other points, our crossover cut-offs could be shifted to better follow the tendencies of the Had we had more time, we would likely work more on our crossover filters and tuning the system.
ECE 40020 Sound Reinforcement System Design Spring 2018 10 4.0 References Raymond, J. (n.d.). AJ Design Software. Retrieved April 05, 2018, from https://www.ajdesigner.com/ Free subwoofer box calculator online. (n.d.). Retrieved April 05, 2018, from https://speakerboxlite.com/ (n.d). Parts Express. Retrieved April 05, 2018, from https://www.parts-express.com/resources- crossover-faqs/ "Dayton Audio SD315A-88 12" DVC Subwoofer" Product. (n.d.). Retrieved April 05, 2018, from https://www.parts-express.com/dayton-audio-sd315a-88-12-dvc-subwoofer--295- 488#lblProductDetails
ECE 40020 Sound Reinforcement System Design Spring 2018 11 Appendix A: Activity Logs Activity Log for: Tae Hyun Kim (Frank) Role: Activity Date Start Time End Time Time Spent Discussed about doing analog design over using DSP development board for the crossover.
Also briefly talked about planning for the designing analog crossover design. 03/01 09:00 10:30 1.5hr Researching about designs of crossover 03/06 09:00 10:30 1.5hr Spent time on a crossover filter design and meet with professor Meyer about some technical related questions. 03/10 12:30 13:30 1Hr Started working on a crossover filter design 03/14 12:30 15:30 3Hr Continued working on a crossover design 03/18 12:30 15:30 3Hr Reviewed materials for exam 2 03/28 20:00 3:00 7Hr Picked up the MDF enclosure 04/02 16:00 16:30 0.5Hr Finished designing crossover design 04/04 21:30 23:00 2.5Hr Finished testing the crossover design 04/05 1:00 2:00 2Hr
ECE 40020 Sound Reinforcement System Design Spring 2018 12
ECE 40020 Sound Reinforcement System Design Spring 2018 13 Activity Log for: Joe Coleman Role: Activity Date Start Time End Time Time Spent Discussed and researched enclosures for the subwoofer. Ended up deciding on a ported subwoofer enclosure. 03/19 8:00 PM 10:00 2 hr More subwoofer design discussion and planning 3/24 8:30 PM 10:00 1.5 hr Discussed analog crossover design 4/2 8:30 PM 9:30 1 hr Corrected cuts on subwoofer enclosure and planned construction of subwoofer. 4/3 10:15 AM 11:15 1 hr Bought materials needed for construction of subwoofer 4/3 2:30 PM 3;15 .75 hr Constructed subwoofer 4/3 6:15 AM 9:15 3 hr Helped design, construct, test, and refined final analog crossover filter.
4/4 6:00 PM 4:00 AM 10 hr
ECE 40020 Sound Reinforcement System Design Spring 2018 14 Activity Log for: AJ Trent Role: Activity Date Start Time End Time Time Spent Researched Subwoofer Drivers 03/08 14:30 15:30 1 hr Researched sub drivers/preliminary design 03/19 14:30 15:30 1 hr Worked on sub design/ordered parts 03/19 17:00 19:00 2 hr Discussed sub design, did some calculations and decided on more details of the design 03/24 20:30 22:00 1.5 hr Finalized and double checked calculations of volume (+dimensions), port size/length, and ordered a port from Parts Express.
03/27 21:00 23:00 2 hr Constructed Subwoofer 4/3 19:00 00:30 5.5 hr Tested System, Wrote Report, and finalized Crossovers 4/4 21:00 04:30 7.5 hr
ECE 40020 Sound Reinforcement System Design Spring 2018 15
ECE 40020 Sound Reinforcement System Design Spring 2018 16 Activity Log for: Oscar Otero Role: Activity Date Start Time End Time Time Spent Worked on sub design/ordered parts 03/19 17:00 19:00 2 hr Discussed sub design, did some calculations and decided on more details of the design 03/24 20:30 22:00 1.5 hr Finalized and double checked calculations of volume (+dimensions), port size/length, and ordered a port from Parts Express. 03/27 21:00 23:00 2 hr Corrected Subwoofer Cuts and Planned Construction 4/3 10:15 11:15 1 hr Constructed Subwoofer 4/3 19:00 00:30 5.5 hr Tested System, Wrote Report, and finalized Crossovers 4/4 21:00 4:30 7:30
ECE 40020 Sound Reinforcement System Design Spring 2018 17 Appendix B: (DSP) System Tuning Parameters
ECE 40020 Sound Reinforcement System Design Spring 2018 18 Include schematic (with optional PCB layout) for analog spectral divider design -or- system tuning parameters (screen shots or photos) for DSP design. Appendix C: Frequency Response Measurement
ECE 40020 Sound Reinforcement System Design Spring 2018 19 Include frequency photos of test setup and spectrum analyzer output here. Make sure that the “scale” settings are clearly visible.
Total Frequency Spectrum: Subwoofer: Woofer: Squawker:: Tweeter:
ECE 40020 Sound Reinforcement System Design Spring 2018 20 Photos of AJ Critically Listening
ECE 40020 Sound Reinforcement System Design Spring 2018 21 Appendix D: Subwoofer Design Documentation (optional – bonus credit)