Design Statement

David Nichols
FA 4740
10-2-09
Design Statement
.
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 Plot1
Winspeakerz plot of the XXLS in a 4 cu ft 23 Hz enclosure
1
“Peerless XXLS 12” Subwoofer”.
1
-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 Plot2
2
“HiVi M8a Bass-Midrange”.
2
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 Plot3
- 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
Other Drivers Considered
-Competition for the Peerless subwoofer included the Eminence LAB12 (Parts-Express,
$156.974). 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 830667 (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 13.08” 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”.
“12” Subwoofer Components”.
6
Subkey, Cabrera, and Ferguson, “Localization and Image Size Effects for Low Frequency
Sound”.
7
“Port Size Calculations”.
5
4
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
another 1st order crossover at 800 Hz will create a matching 4th 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 SA240 240W amp from PartsExpress for $139.98, the Bash 300S 300W amp from Parts-Express for $149.87, and the Keiga
KG5230 300W amp from Madisound for $225.00. The Bash amp has a Total Harmonic
Distortion (THD) of 1%8 and the Dayton has a similar rating9. 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.”
“Dayton SA240 240W Subwoofer Amplifier.”
10
“Keiga KG5230 (300 Watt).”
9
6