David Nichols. FA Design Statement

Transcription

1 David Nichols. FA Design Statement

2 Application of Design - Near-field listening in small environments - Ability to reach 80 db RMS in a K20 setting at 4 meters - To minimize audible distortion - To achieve flat frequency response from 20 Hz through 20 KHz - To provide accurate stereo imaging and width - To be reasonably affordable not to exceed $800 for the entire system Limitations of Design - Subwoofer must be smaller than 29 wide by 27 tall by 25" deep - Stereo pair must be smaller than 12 wide by 20 tall by 15 deep - Speakers must function with a 2-way 50W per channel amplifier Driver Specification - Tweeter that can reach lower than 1 KHz and up to 20 KHz evenly - Woofer that can span 40 Hz through 1 KHz evenly - Subwoofer that can reach 20 Hz through 90 Hz evenly Driver Selection - Peerless XXLS 12 Subwoofer - Excellent low-frequency response in the given enclosure. Peerless XXLS 12 Manufacturer Frequency Response Plot 1 Winspeakerz plot of the XXLS in a 4 cu ft 23 Hz enclosure 1 Peerless XXLS 12 Subwoofer. 1

3 -Excursion limit allows maximum power of 248 W peak. With a 90.6 db/1 W sensitivity, this driver allows a peak output of nearly 114 db peak, or 94 db RMS. - HiVi M8a 8 Woofer -Smooth output from 80 Hz to 1 KHz HiVi M8a Manufacturer Frequency Response Plot 2 2 HiVi M8a Bass-Midrange. 2

4 Winspeakerz plot of the HiVi in a 1 cu ft 30 Hz enclosure -Excursion limit allows maximum power of 160 W peak. With an 88 db/ 1 W sensitivity, this driver allows for a peak output of 109 db peak, or 89 db RMS. - Eton 25SD-1 1 Soft-dome Tweeter - Flat frequency response from 800 Hz up to 20 KHz. Eton 25SD-1 Manufacturer Frequency Response Plot 3 - Max power handling of 100W peak with 90 db sensitivity allows for a peak output of 110 db, or 90 db RMS. 3 Eton 25SD-1. 3

5 Other Drivers Considered -Competition for the Peerless subwoofer included the Eminence LAB12 (Parts-Express, $ ). It exhibited very similar properties to the Peerless, both in frequency response and in T/S parameters. Ultimately my decision was made based on the success of the Peerless in existing systems such as Christopher Plummer s and Kris Thompson s, both owners satisfied by the results. Other drivers that were looked at included the Aurasound NS-12 and the Morel UW1258, but both were way beyond my budget, at $548 and $449 respectively. 5 -Competition for the HiVi M8a included the Eminence BETA-8A (Madisound, $44.99), the Fostex FW208N (Madisound, $174.20), and the Peerless SLS (Madisound, $52.75). While browsing through drivers, the Fostex were the first I noticed, and would perform the best in my design. However, at their price they would push me too far outside of my budget. The Peerless drivers exhibit a breakup mode roll-off of 10dB between 700 and 800 Hz, which is unacceptable in my design. The Eminence driver had a huge 87Ω impedance spike at its resonance of 65 Hz which concerned me. It had a higher breakup mode than the HiVi. Ultimately, though, the lower resonance of the HiVi (33Hz) plus the much more attractive finish made my choice clear. The lower bass response of the woofers will help with imaging of lowfrequency instruments, even with the subwoofer, 6 and excellent image clarity is a design goal for this system. -The tweeter hunt was a very interesting part of the driver selection process. I looked at tweeters first so that I could base my woofer choice on my tweeter. Initially I looked at things like the Vifa DX25TG09-04 (Madisound, $28.50) or the Scan-Speak DT608 (Madisound, $78.00) when I came across the Eton and noticed its low frequency output. After I discovered the Eton I began looking for similar-performing tweeters. The only ones that seemed to come close were the Vifa XT25TG30-04 ring radiator (Madisound, $36.95) which had very poor off-axis response, and the Scan-Speak Illuminator R3004 ring radiator (Madisound, $205.00) which was both way outside of my budget and also had poor off-axis response, though it could go down to 700 Hz. Ultimately, the uniqueness of the Eton at an amazing price was my deciding factor. Enclosure Design - A subwoofer enclosure that is 4 cubic feet and tuned to 23 Hz will provide flat response down to 25 Hz with an F3 at 23 Hz, according to Winspeakerz. - Using ¾ plywood, the external dimensions of 29 wide by 13 deep by 23.5 tall, with a 6 inner diameter long port will result in the necessary tuning. 7 - A speaker enclosure that is 1 cubic foot and tuned to 30 Hz will provide flat response down to 60 Hz with an F3 at 40 Hz, according to Winspeakerz. -Using ¾ plywood, the external dimensions of 10 wide by 13 deep by 20 tall, with two 1 inner diameter 3.97 long port will result in the necessary tuning. 4 Eminence Lab12 Generation II 12 Subwoofer Subwoofer Components. 6 Subkey, Cabrera, and Ferguson, Localization and Image Size Effects for Low Frequency Sound. 7 Port Size Calculations. 4

6 Proposed design for main speakers -Main enclosure features centered 8 Woofer as the distance between its edge and the enclosure edge is beyond the range of the driver. Driver is 2 from the bottom, which is still outside of the range of the driver; thus reflection effects are irrelevant. Tweeter is offset 1 from center to the outside of the baffle, resulting in 3 reflective frequencies but spread across a wide range. Two 1 diameter ports are centered on the baffle. Proposed design for subwoofer -The subwoofer enclosure features a multi-axis offset of the 12 woofer. The side port reduces any odd reflections from inside the box while allowing a straight pipe of 6 diameter. The side port orientation will enable the port to play into the room without being muffled into a wall or the floor. Crossover Considerations -Because the main enclosures naturally experience a 1st order roll-off starting at 50 Hz, the subwoofer will be able to use a 1st order roll off centered at 50 Hz. This will allow me to not need to construct a high pass filter for the main enclosures, saving a significant amount of money. -The 8 HiVi woofers experience a very sharp 10dB spike at 2 KHz. If the crossover is centered at 800 Hz, the lowest possible frequency for the tweeter, then the HiVi will require a 4th order crossover. With a 4th order crossover, 1600 Hz will be -24dB, making the 10 db spike at 2 KHz at most -14 db. The tweeter experiences a natural 3rd order crossover centered at 800 Hz. Adding 5

7 another 1 st order crossover at 800 Hz will create a matching 4 th order roll-off to match that of the HiVi. Subwoofer Plate Amp Considerations -Because the user amp is underpowered and because crossover components can be quite pricey, it is worth investing in a plate amp. One that produces a clean 250W will be more than ample for this application using a higher wattage amp usually means better electronics quality, but at a significant cost. Amplifier considerations include the Dayton SA W amp from Parts- Express for $139.98, the Bash 300S 300W amp from Parts-Express for $149.87, and the Keiga KG W amp from Madisound for $ The Bash amp has a Total Harmonic Distortion (THD) of 1% 8 and the Dayton has a similar rating 9. By comparison, the Keiga has a THD of.08%. 10 The consensus on internet forums such as audioholics.com and diyaudio.com is that the Keiga is the best sounding of the three. However, from what I have read on the above forums, I am not convinced that it is nearly $100 better. I believe the Dayton will be my primary choice. 8 Bash 300W Digital Subwoofer Amplifier. 9 Dayton SA W Subwoofer Amplifier. 10 Keiga KG5230 (300 Watt). 6