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Junior Varsity
Subwoofer comparison SRX728 vs Vertec 4883
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<blockquote data-quote="Phil Graham" data-source="post: 59320" data-attributes="member: 430"><p>Re: Subwoofer comparison SRX728 vs Vertec 4883</p><p></p><p></p><p></p><p>Not only is this common, it was a requirement in some of the original alignments that were proposed by Small for vented boxes.</p><p></p><p></p><p></p><p>TJ,</p><p></p><p>It should be pointed out that Ivan's statement above is a good rule of thumb, but it is not strictly true. I'll attempt to explain for the classic bass reflex/vented box loudspeaker:</p><ol> <li data-xf-list-type="ol">A vented box has an excursion minima at the box tuning frequency, Fb. The air in the port provides the lion' share of the acoustic output, and the cone moves very little at this frequency.</li> <li data-xf-list-type="ol">About a half octave above the tuning frequency, the port no longer has any effect on output, and the driver will experience its maximum excursion.</li> <li data-xf-list-type="ol">Because the excursion near Fb is lower, and the port provides most of the output, a narrow boost near Fb does not incur a great penalty increase in driver excursion.</li> <li data-xf-list-type="ol">A boost 1/2 octave above Fb will typically result in the maximum increase in driver excursion, and is to be avoided.</li> <li data-xf-list-type="ol">The driver impedance is closest to purely resistive at Fb, so a boost here results in more power dissipation in the voice coil. Because the boost is narrow, however, the overall increase in voice coil heating is usually fairly minor.</li> <li data-xf-list-type="ol">Driving the air in the port harder at Fb to beef up the low end response is going to speed the onset of port nonlinearities. Therefore the effectiveness of the boost at Fb for increasing output depends on the port behaving well. Once the port starts behaving badly, the benefits of the boost are lost. This results in less output, but it is as much because of the port as the driver.</li> <li data-xf-list-type="ol">As the port behaves worse, it provides less excursion protection to the driver, and eventually this reduced excursion protection will result in the driver's strain. Typically, though, the port is making all kind of terrible noises before this happens, and therefore is the arbiter of maximum output.</li> </ol><p></p><p>So, in summary, a boost at Fb will give you more practical output, with little downside, until the port gives out. The more the boost the sooner this will happen, and the more dependent the maximum output becomes on the porting.</p><p></p><p>Basing maximum SPL discussion on nominal 1W sensitivity ratings is not useful, as the 1W values provide no insight into the things that limit maximum output. Also, the electrical input ==> acoustic power can be calculated directly from the Theile Small parameters, and there's no need to look at a datasheet for these values.</p><p></p><p>In a perfect world where ports and boxes behaved in a completely linear fashion at all drive levels, and drivers never did anything nasty with increasing excursion, a 12" driver is going to need approx 1.8x the linear excursion capability to match the output of an 18" driver. This is not impossible, but it is not likely for two drivers based on the same underlying motor geometry. Depending on how much output you truly need, this may not be a big deal.</p><p></p><p>---</p><p></p><p>A tapped horn style design has two excursion minima, and provides better excursion protection for the driver. Tapped horns are not immune from air nonlinearities as the output increases, but generally fair better in this regard than a vented box. Air velocities are globally lower than the typical ported box, and the maximum velocities are near the large mouth, and low near the small throat.</p></blockquote><p></p>
[QUOTE="Phil Graham, post: 59320, member: 430"] Re: Subwoofer comparison SRX728 vs Vertec 4883 Not only is this common, it was a requirement in some of the original alignments that were proposed by Small for vented boxes. TJ, It should be pointed out that Ivan's statement above is a good rule of thumb, but it is not strictly true. I'll attempt to explain for the classic bass reflex/vented box loudspeaker: [LIST=1] [*]A vented box has an excursion minima at the box tuning frequency, Fb. The air in the port provides the lion' share of the acoustic output, and the cone moves very little at this frequency. [*]About a half octave above the tuning frequency, the port no longer has any effect on output, and the driver will experience its maximum excursion. [*]Because the excursion near Fb is lower, and the port provides most of the output, a narrow boost near Fb does not incur a great penalty increase in driver excursion. [*]A boost 1/2 octave above Fb will typically result in the maximum increase in driver excursion, and is to be avoided. [*]The driver impedance is closest to purely resistive at Fb, so a boost here results in more power dissipation in the voice coil. Because the boost is narrow, however, the overall increase in voice coil heating is usually fairly minor. [*]Driving the air in the port harder at Fb to beef up the low end response is going to speed the onset of port nonlinearities. Therefore the effectiveness of the boost at Fb for increasing output depends on the port behaving well. Once the port starts behaving badly, the benefits of the boost are lost. This results in less output, but it is as much because of the port as the driver. [*]As the port behaves worse, it provides less excursion protection to the driver, and eventually this reduced excursion protection will result in the driver's strain. Typically, though, the port is making all kind of terrible noises before this happens, and therefore is the arbiter of maximum output. [/LIST] So, in summary, a boost at Fb will give you more practical output, with little downside, until the port gives out. The more the boost the sooner this will happen, and the more dependent the maximum output becomes on the porting. Basing maximum SPL discussion on nominal 1W sensitivity ratings is not useful, as the 1W values provide no insight into the things that limit maximum output. Also, the electrical input ==> acoustic power can be calculated directly from the Theile Small parameters, and there's no need to look at a datasheet for these values. In a perfect world where ports and boxes behaved in a completely linear fashion at all drive levels, and drivers never did anything nasty with increasing excursion, a 12" driver is going to need approx 1.8x the linear excursion capability to match the output of an 18" driver. This is not impossible, but it is not likely for two drivers based on the same underlying motor geometry. Depending on how much output you truly need, this may not be a big deal. --- A tapped horn style design has two excursion minima, and provides better excursion protection for the driver. Tapped horns are not immune from air nonlinearities as the output increases, but generally fair better in this regard than a vented box. Air velocities are globally lower than the typical ported box, and the maximum velocities are near the large mouth, and low near the small throat. [/QUOTE]
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