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Bookshelf Speaker Design Proposal BOOKSHELF SPEAKER DESIGN PROPOSAL Charles Young FA 4740 spring 2009 These speakers will be used primarily to monitor CDs and music generated by music composing programs to accompany musical instrument practice. They will be located in a spare bedroom used as a music practice room. Space for speakers is limited in the room and thus the speakers will be small and may be mounted on a wall. Proposed design size is 8 inches thick with a baffle of 10 by 12 inches, thus fitting my bookshelves, with a volume of .55 cubic ft or 15 liters. Anticipated loudness or power handling capability is small, less than 30 watts RMS amplifier capacity. It is expected that the driver design shall consist of a 4 to 6 inch woofer, a tweeter and a crossover. Possible enhancements to the frequency response should include enhanced bass response with a ported enclosure and separate attenuation of mid-range and high frequency response, to accommodate personal taste. Half-wave resonances in this size box are 500, 600 and 750 Hz based on c=1000 ft/sec. Proposed frequency response should have 3 to 5 dB ripple over the range 60 to 12,000 Hz, with a fall-back narrower range of 200 to 5000 Hz. Some smooth variation may be included for personal listening taste. It is doubtful that the sound pressure level will exceed 90 dB at 1 meter. Some low frequency enhancement is expected since the speakers will be mounted near or close to the wall, Subjective qualities should include 1) clarity of individual musical instruments and 1 clear high frequencies for reproduction of cymbals and high partials on stringed instruments and 2) clear low frequencies for reproduction of string basses and bass drum sounds. These qualities should be reflected in a wide, flat frequency response and a transient response, which is non-oscillating; smooth for the woofer and single-spiked for the tweeter. An extended low end of the frequency response is achievable at a cost of enclosure size and increased cost. The fundamental frequency for the low E string on a string bass is 41 Hz, which is about the lowest musical note that a listener may encounter, but it is well known that humans perceive the musical note not by its fundamental frequency but by its partials (harmonics), and the partials have amplitudes as great or greater than the fundamental tone. Thus, even if a speaker cannot produce such a low note, the sensation of the sound from such a musical instrument can still be convincingly created. The cost of the speakers should be modest. I can appreciate that some builders will be very serious audiophiles who want to spend plenty of money to have the very best. My goal is to learn the design, building and evaluation techniques but I don’t feel I have to spend top dollar in the process. ADDITIONAL DESIGN CONSDERATIONS 2 It is desirable to avoid the step in frequency response due to transition from infinite baffle to sealed or ported box as the signal frequency is reduced. (AKA “baffle step” 1. Solutions to the baffle step effect are: 1: Incorporate a baffle filter in the crossover. 2: Place the speaker near the wall. The combination of the speaker and the wall approximates an infinite baffle. These speakers will be placed on or near a wall. Also, I want to roll off the woofer response enough to reduce audibility of cone breakup and furthermore to provide some attenuation of midrange to accommodate personal taste. A plot of woofer response shows undulations about 1500 Hz, followed by a roll off with increasing frequency and a bump at about 8 kHz. I want to roll off the tweeter response with lower frequencies to reduce audibility of tweeter resonance at about 800 Hz. It is desirable to use a Zobel filter to counteract the inductance of the woofer voice coil 2. The crossover design usually assumes that the woofer is pure resistance, ignoring the inductive reactance of the voice coil. The inductive reactance reduces the effectiveness of the crossover at higher frequencies. 1 Rod Elliot, Elliot Sound Products, http://sound.westhost.com/bafflestep.htm)., (Dec 2001), Accessed Feb 13, 2009). 2 John L. Murphy, “Neutralizing L (e) with a Zobel”, True Audio, http://www.trueaudio.com/st_zobel.htm, ND, Accessed Feb 13,2009.l 3 DESIGN CHOICES AND ALTERNATIVES Woofers and all other parts and their data sheets were found at the Parts Express website 3. The woofers were chosen from others costing approximately twenty dollars. Shielding of the driver magnets is not important since the speakers will be about 1 meter away from any CRT display. Woofer possible choices and specifications are: Dayton DC-160-8 6 ½ inch classic woofer specifications. Specifications: * Power handling: 50 watts RMS/75 watts max * Voice coil diameter: 1-3/8" * Le: 2.34 mH * Impedance: 8 ohms * DC resistance: 6.7 ohms * Frequency response: 30-4,000 Hz * Magnet weight: 15 oz. * Fs: 34 Hz * SPL: 88 dB 1W/1m * Vas: .87 cu. ft. * Qms: 3.47 * Qes: .36 * Qts: .33 Goldwood GW-206/4 Specifications: *Power handling: 90 watts RMS/180 watts max *Voice coil diameter: 1-1/2" *Le: .4 mH *Re: 3.7 ohms *Frequency range: 60-3,000 Hz *Fs: 60 Hz *SPL: 91 dB 2.83V/1m *Vas: .42 cu. ft. *Qms: 3.50 *Qes: .73 *Qts: .61 *Xmax: 3.5 mm Vifa TP16WJ-06-08 6-1/2" Woofer 8 Ohm (frequency response plot not available) Specifications: *Power handling: 70 watts RMS/100 watts max *VCdia: 1-1/4" *Le: 1.0 mH *Impedance: 8 ohms *Re: 5.6 ohms *Frequency response: 38-5,000 Hz *Fs: 38 Hz *SPL: 87 dB 1W/1m *Vas: 1.05 cu. ft. *Qms: 2.48 *Qes: 0.52 *Qts: 0.43 *Xmax: 4.0 mm 3---------------, http://www.parts-express.com/home Accessed Feb 13, 2009. 4 The frequency response plots are given below. Parts Express did not have frequency response plots for many of the drivers in this price range. Figure 1. Dayton DC-160-8 frequency response. 5 Figure 2. Frequency response for Goldwood GW-206/4 The source for all response plots is data sheets from Parts Express website. Of these drivers, I reject the Vifa because no plot is available, and I find the Dayton DC-160-8 preferable because its Fs is lower, and because of the lower magnitude of the cone breakup effects at about 5000 Hz, and because it has less drop in frequency response at about 1200 Hz. Tweeter. The tweeters were chosen from Parts Express website from among many others costing approximately twenty dollars. 6 Dayton DC28F-8 silk dome tweeter, resonance at about 600 Hz (shown in impedance plot on spec sheet), undulations in frequency response between 3000 and 15,000 similar to cone breakup in a woofer. The response plot is given below. Specifications: * Power handling: 50 watts RMS/75 watts max * Voice coil diameter: 1-1/8" * Impedance: 8 ohms * Re: 5.5 ohms * Frequency response (± 2 dB): 1,300-20,000 Hz * Fs: 637 Hz * SPL: 89 dB 1W/1m * Magnet weight: 6 oz. Figure 3. Frequency Response of Dayton DC28F-8 Silk dome tweeter. Tang Band 25-11666SJ 1 inch Neodymimum Tweeter. Specifications: *Power Handling: 50 watts RMS/80 watts max *Voice coil diameter: 1" *Impedance: 4 ohms *Re: 3.0 ohms *Frequency range: 2,000-22,000 Hz *Fs: 900 Hz *SPL: 93 dB 2.83V/1m 7 Figure 4. Tang Band 25-11666SJ 1 inch Neodymimum Tweeter. HiVi X1 1" Textile Dome Tweeter Specifications: *Power handling: 15 watts RMS/30 watts max *Impedance: 4 ohms: *Frequency range: 2,000- 20,000 Hz *Fs: 1,300 Hz *SPL: 92 dB 2.83V/1m *Dimensions: A: 4-9/16", B: 3-1/2", C: 1-1/8". No frequency plot available. The technical specifications for the Tang Band tweeter exceeds the Dayton: The bandwidth is greater, but the power handling capability is smaller, which is acceptable in a bookshelf speaker intended for low SPL. The high frequency response may not be important in this application, since the listener is a senior citizen with poor high frequency hearing. I will choose the Dayton tweeter because it has a price and convenience-in-ordering advantage because it is part of the BR-1 parts kit. 8 CROSSOVER DESIGN The proposed crossover design is shown below. The woofer crossover provides a second order low pass filter (L2 and C2) with a 3 dB down point at about 2300 Hz, and a Zobel impedance equalizer (R1 and C3) with a 3dB down point at about 2200 Hz. The tweeter crossover provides a high pass filter (the 6.2 mfd capacitor and 0.4 mH inductor) with a 3 dB down point of about 1000 Hz and an ~3dB attenuator (the 4 and 6.2 ohm resistors) to equalize the slightly higher tweeter sensitivity. The above computations were carried out with a quick analysis of RLC filters4. The response of the completed crossover will be measured with spectrum analysis software. Figure 5. Proposed crossover design. 5 FINAL CONSIDERATION OF PARTS CHOICE: 4-------------, “Okawa Electric Design, RLC Filter Design Tool, ” http://sim.okawa- denshi.jp/en/RLCtool.php Accessed February 13, 2009. 5-----------------, BR-1 2-Way Reference Monitor System, Dayton Audio, Dayton Loudspeaker Co., PO Box 52, Springboro, Ohio 45066- 1158 http://www.parts-express.com/pdf/300-640.pdf, page 5, Accessed February 13. 2009. 9 The parts are available as a single order item from Parts Express as a BR-1 2 Way Reference Monitor kit at a cost of $139 representing about a $25 savings compared to purchasing the parts separately.
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