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Junior Varsity
Yes, it's cliche, yes it's overdiscussed, but yes, I need new subs.
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<blockquote data-quote="Phil Graham" data-source="post: 33686" data-attributes="member: 430"><p>Re: Yes, it's cliche, yes it's overdiscussed, but yes, I need new subs.</p><p></p><p></p><p></p><p>Silas,</p><p></p><p>Both a 1w "sensitivity" and a "power rating" have so little meaning from virtually any manufacturer as to be essentially useless beyond the math review of setting up the spreadsheet.</p><p></p><p>Let's make a more rational view of this calculus: <ol> <li data-xf-list-type="ol">Forget "power" entirely</li> <li data-xf-list-type="ol">Forget about spec sheet "sensitivity"</li> <li data-xf-list-type="ol">You own a certain number of voltage sources (i.e. amp channels and/or bridged amps)</li> <li data-xf-list-type="ol">Each of those voltage sources can produce a specific +/- V_peak</li> <li data-xf-list-type="ol">If not unduly loaded, each of those voltage sources will not enter current limiting in normal use</li> <li data-xf-list-type="ol">You want to purchase/build designs that are "stout" enough to take the full peak to peak voltage swing of your sources</li> </ol><p>At this point, then, Hoffman's "iron law" will dictate the sensitivity of your cabinets. Namely, the sensitivity is proportional to the volume of the cabinet and inversely proportional to the <em>cube</em> of the cutoff frequency delta. You pay incredibly dearly for additional extension from a sensitivity perspective. The other way to think of this circumstance is that you decide on the box sensitivity by choosing the cutoff point.</p><p></p><p>Vented boxes aren't magic. If you know the driver in a particular box, and the tuning frequency, you can reliably calculate the driver output and excursion as well as the velocity of air in the port(s). In fact, other than a few tweaky points I won't dig into presently, WinISD Pro 0.57a does a good job of this for exactly $0.00. Everyone in audio owes the linearteam guys a beer, as their predictions closely match much more expensive programs, and reality.</p><p></p><p>WinISD asks for the "Power" input to the driver, but in reality that "Power" is equal to the RMS voltage input to produce the desired power at an impedance equal to R_dc of the driver voice coil.</p><p></p><p>So, for a "Power" input of 2000W into a driver with a 6ohm voice coil R_dc the RMS voltage is as follows:</p><p></p><p>V_rms = sqrt(P_in*R_dc) or V_rms = sqrt(2000*6) = 109.5V_rms</p><p></p><p>To calculate V_peak, multiply the V_rms by 1.414:</p><p></p><p>V_peak = 1.414(109.5) = 154.9V_peak or 2(154.9) = 310V_peak_to_peak</p><p></p><p>Thus, WinISD can easily be used to calculate the peak voltage input at which a driver exceeds Xmax (or Xlim). The transition from Xmax to Xlim is not a dramatic one, but rather a gradual one marked by the progressive, continual decrease in the driver's Bl product, amongst other things. The character of the driver in this transitional region will define much of how we think of that driver when "driven hard."</p><p></p><p>In similar fashion, once the Reynold's number in the port(s) goes dramatically north of 100,000 the port compression effects on the vented box response start getting very severe. Practically, for boxes with nearly square ports (not "slot" porting), this Reynold's number is hit with a port airspeed of a little under 10 meters/second.</p><p></p><p>Since WinISD naively calculates port airspeed by dividing the volume velocity of the air in the port by the port area, it cannot predict the effects of a slot port. But if the box port is square, or divided into square(ish) sections, the airspeed it predicts will be useful, and can help you determine if the box has enough port area. A rule of thumb is that the port area should be a <em>minimum</em> of 50% of the driver Sd.</p><p></p><p>So, start on your amp's spec sheet, at the amp terminals, and work backwards. You will find that the choices are no longer so broad.</p></blockquote><p></p>
[QUOTE="Phil Graham, post: 33686, member: 430"] Re: Yes, it's cliche, yes it's overdiscussed, but yes, I need new subs. Silas, Both a 1w "sensitivity" and a "power rating" have so little meaning from virtually any manufacturer as to be essentially useless beyond the math review of setting up the spreadsheet. Let's make a more rational view of this calculus:[LIST=1] [*]Forget "power" entirely [*]Forget about spec sheet "sensitivity" [*]You own a certain number of voltage sources (i.e. amp channels and/or bridged amps) [*]Each of those voltage sources can produce a specific +/- V_peak [*]If not unduly loaded, each of those voltage sources will not enter current limiting in normal use [*]You want to purchase/build designs that are "stout" enough to take the full peak to peak voltage swing of your sources [/LIST]At this point, then, Hoffman's "iron law" will dictate the sensitivity of your cabinets. Namely, the sensitivity is proportional to the volume of the cabinet and inversely proportional to the [I]cube[/I] of the cutoff frequency delta. You pay incredibly dearly for additional extension from a sensitivity perspective. The other way to think of this circumstance is that you decide on the box sensitivity by choosing the cutoff point. Vented boxes aren't magic. If you know the driver in a particular box, and the tuning frequency, you can reliably calculate the driver output and excursion as well as the velocity of air in the port(s). In fact, other than a few tweaky points I won't dig into presently, WinISD Pro 0.57a does a good job of this for exactly $0.00. Everyone in audio owes the linearteam guys a beer, as their predictions closely match much more expensive programs, and reality. WinISD asks for the "Power" input to the driver, but in reality that "Power" is equal to the RMS voltage input to produce the desired power at an impedance equal to R_dc of the driver voice coil. So, for a "Power" input of 2000W into a driver with a 6ohm voice coil R_dc the RMS voltage is as follows: V_rms = sqrt(P_in*R_dc) or V_rms = sqrt(2000*6) = 109.5V_rms To calculate V_peak, multiply the V_rms by 1.414: V_peak = 1.414(109.5) = 154.9V_peak or 2(154.9) = 310V_peak_to_peak Thus, WinISD can easily be used to calculate the peak voltage input at which a driver exceeds Xmax (or Xlim). The transition from Xmax to Xlim is not a dramatic one, but rather a gradual one marked by the progressive, continual decrease in the driver's Bl product, amongst other things. The character of the driver in this transitional region will define much of how we think of that driver when "driven hard." In similar fashion, once the Reynold's number in the port(s) goes dramatically north of 100,000 the port compression effects on the vented box response start getting very severe. Practically, for boxes with nearly square ports (not "slot" porting), this Reynold's number is hit with a port airspeed of a little under 10 meters/second. Since WinISD naively calculates port airspeed by dividing the volume velocity of the air in the port by the port area, it cannot predict the effects of a slot port. But if the box port is square, or divided into square(ish) sections, the airspeed it predicts will be useful, and can help you determine if the box has enough port area. A rule of thumb is that the port area should be a [I]minimum[/I] of 50% of the driver Sd. So, start on your amp's spec sheet, at the amp terminals, and work backwards. You will find that the choices are no longer so broad. [/QUOTE]
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Junior Varsity
Yes, it's cliche, yes it's overdiscussed, but yes, I need new subs.
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