Systems List › DIY › Fi Car Audio SP4 18D1 Sealed
NotesThe measurements for this system start with the impedance curve. This is a dual 1ohm voice coil rated driver so with the coils in series the impedance minimum is a quite low 1.4 ohms. This rises to about 2ohms at 10Hz and peaks at 15 ohms at 32Hz. The minimum impedance above the system resonance occurs at 53Hz where it is about 2.6 ohms. The impedance then rapidly rises towards 200Hz where it reaches 8 ohms. The sharp rise in the impedance curve above the system resonance indicates that the driver will have a peaked response and a significant upper frequency roll off.
The system sensitivity measured at 1 meter using a 1.42 volt input, which equates to 1 watt nominal into a 2ohm load, indicates just as suspected that the system has a peaky response curve and a rolled off upper bass range. The 10 meter sensitivity measurement with a 14.2 volt input reinforces the result. The average sensitivity of the two measurement types is roughly 80dB at 20hz which is not bad for a sealed system of this size, rising to 90dB at 35Hz, peaking at almost 93dB at 45Hz and then falling about 8dB total to just 85dB at 100Hz. The overall response shape, the distortion data and the impedance measurements suggest that this driver would be best utilized low passed under 100 to 120Hz. The time domain data for this system does indicate that there is a hint of delayed energy near 45Hz but it is doubtful it would be enough to be audible in any case. The group delay for the system never reaches even 1 cycle.
The high power measurements start with a baseline sweep that causes the system to produce roughly 90dB at 50Hz at 2 meters from the M30 microphone. This required an input voltage of 2 volts for this system. The SP4 responds in a linear fashion to the first 20dB increase in output demand where the compression of the output is less than 1dB over the full measurement bandwidth. The next 5dB increase in demand from the driver corresponds to a voltage input that is now increased to about 36 volts. At this level the SP4 remains uncompressed and just reaches 1dB of compression over the 45-70Hz range. The next 5dB increase for the next sweep requires an input voltage of 63.3 volts and the SP4 is now exhibiting notable compression of the output of about 1dB in the deep bass and 2dB in the upper bass range. At this level the SP4 was not exhibiting any egregious bad noises so a further increase of 3dB was tried. This required an input voltage of just under 90 volts. At this level the SP4 driver finally made some distress and obvious heavily distorted noises. The output compression had now become significant as well reaching about 2dB over a large part of the deep bass range and up to 3dB near 40-60Hz. At this voltage drive level the SP4 produced just under 109dB at 20Hz and topped out near 120dB from 35-45Hz. The upper bass response during this sweep gradually reduced to about 115dB or less from 80-120Hz.
The distortion measurements are taken at the same drive levels as those used for the output compression results. The 20 volt measurement shows that at this level the driver is linear and relatively clean with less than 5% THD above 24Hz. The 35.6 volts measurement shows the THD increasing a bit but remaining below 8% above 24Hz and increasing to 15-25% below 20Hz. Increasing the drive signal to 63.3 volts causes the distortion to increase greatly as the driver is now being pushed to high excursion with a lot of current. The THD climbs and is now 11% or less above 26Hz but rises to the 40-70% range below 20Hz as the driver is clearly past its linear excursion envelope. The final 89.4 volts measurement pushes the SP$ driver very close to its excursion mechanical limits in the deep bass and this is clearly shown by the THD climbing to the 100% range below 16Hz. Above 30Hz the THD is still held down at 14% or less. The THD is primarily composed of the 3rd harmonic below 20Hz and above 150Hz. The 2nd harmonic is the primary source of THD over the majority of the 20-100Hz bandwidth.
CEA-2010 style burst testing revealed some interesting things. The SP4 would not produce a passing result at 10Hz. The 3rd harmonic always seemed to be high at this frequency. If distortion was ignored the system would eke out 97.3dB at 10Hz but with massive distortion. A passing result was possible at 12.5Hz where the SP4 was able to record just above 95dB. If distortion was ignored completely a result of over 101dB at 12.5Hz was possible but with 100% THD. The distortion dropped and the output increased dramatically moving up in frequency. At 20Hz the SP4 was nearly able to reach 110dB with passing distortion and 110.6dB driven flat out with distortion reaching over 40%. From 25-50Hz the SP4 system was limited by the driver excursion rather than distortion. There was some behavior during this testing that was a bit odd. While doing the burst tests and edging up to the limits of the SP4 driver the sound would abruptly change to an odd overtone and the K20 amp would suddenly protect and mute the outputs for about 2 seconds. This was with the amplifier signal lights indicating that the output was well below clipping the amplifier output stages. At first it was thought that this may simply be too much current demand from the amplifier since it is being bridged into a load with a minimum resistance of less than 2 ohms. However after observing the behavior this seems unlikely as the Powersoft K series amplifiers have never exhibited this behavior with burst testing or even when completely destroying a driver resulting in a shorted load during a measurement sweep. Nor has it been observed when bridged into other 2ohm nominal loads. If the signal level was backed off what would be 0.3dB or so, the issue would go away. This behavior was repeated at all of the lowest frequency bands from 10Hz up to and including 50Hz. Looking at the impedance curve of the SP4 in the sealed test box it can be noted that the impedance between 25-40Hz is actually about 6ohms or higher which should not pose a problem. At the 31.5Hz band the impedance is about 15 ohms before any heating of the system, which should pose no problem at all for the amplifier so this points to something occurring with the driver itself. Perhaps the coil was contacting something in the motor or a lead wire was shorting. Whatever the cause it is clearly the limit of the SP4 driver. After talking with the owner of the driver he commented that he has seen this issue himself with the other SP4 drivers he owns and with a different amplifier powering them, so it is a driver related issue it seems. Perhaps a short that develops at high excursion or some sort of motor design property. However this issue was only when driving the SP4 to extreme excursions that were close to some sort of mechanical limit for the driver anyway so it should not be an issue in any normal use. At 80Hz and above the amplifier ran out of juice before the driver ran out of excursion so the output gets up into the 125dB range with the amplifier clipping.