[BentZero]

After disconnecting the center speaker the JL 600/6 seems like the wrong direction to go when it's time for my Hi-Fi audio upgrade. I was looking at the JL 750/5 and it has the same 75w RMS at 4 ohms for the front and rear speakers. It also has a 300w at 2ohm sub output. The sub output would allow me to add a real sub at a later date, but I wonder if I'd be able to power the stock subs in the meantime.

[djarchow]

I have the HK system so take this with a grain of salt. I pulled my drivers and did Thiele Small measurements on them as well as frequency response and distortion measurements in a test enclosure. Based on what I saw, the HK woofers could handle probably at most 100 watts or less assuming you had a built in high pass filter limiting all the frequencies below 50-60 hz.

Running full range, distortion began to rise quite a bit above I think 11 volts which corresponded to about 20 watts. I then limited the sweep to above 50 hz and distortion was better until I hit about 16 volts/40 watts and my amp started distorting due to the 2 ohm impedance.

I would assume that the hifi woofers/subs are even less capable than the HK drivers but can't be sure. No matter what, you don't want to run these without a high pass filter to kill the low bass. The underseat drivers are woofers not subwoofers and don't have the excursion to handle the low stuff or a lot of power.

[BentZero]

Nice, thanks for the detailed response. i assumed that I'd have to turn the gain waaaaaaaaay down for the under seat "subs". Perhaps a more prudent path would be to power the under seat "subs" with the 5th channel and save the sub section for a dedicated trunk mounted sub.

[Viffermike]

Originally Posted by djarchow

I have the HK system so take this with a grain of salt. I pulled my drivers and did Thiele Small measurements on them as well as frequency response and distortion measurements in a test enclosure. Based on what I saw, the HK woofers could handle probably at most 100 watts or less assuming you had a built in high pass filter limiting all the frequencies below 50-60 hz.

Running full range, distortion began to rise quite a bit above I think 11 volts which corresponded to about 20 watts. I then limited the sweep to above 50 hz and distortion was better until I hit about 16 volts/40 watts and my amp started distorting due to the 2 ohm impedance.

I would assume that the hifi woofers/subs are even less capable than the HK drivers but can't be sure. No matter what, you don't want to run these without a high pass filter to kill the low bass. The underseat drivers are woofers not subwoofers and don't have the excursion to handle the low stuff or a lot of power.

BIG time kudos, djarchow . This is very valuable info, and verifies most of what I personally suspected (or already knew) about the OEM subs ...

... the biggest takeaway of which is they are not true subs. Not with that small of a power-handling capability. They are mid-bass drivers meant to fill in low end that the 4" OEM full-rangers (Hi-Fi) and mid drivers (HK) can't reproduce.

20 watts RMS power handling for any speaker meant to reproduce loudly and clearly is not that good. That rating for a bass driver is, well, downright sad. It's bad even at 40 watts RMS -- acceptable, but still bad.

BentZero : Those JL Audio amps have built-in high-pass filters on the sub channel. If you're set on the /5 model, I'd either run a single sub off of it or two 4-ohm subs run in series. The latter won't seem to be as loud, but it actually will be since you'd be moving twice the air with two vs. one.

[djarchow]

Originally Posted by Viffermike

20 watts RMS power handling for any speaker meant to reproduce loudly and clearly is not that good. That rating for a bass driver is, well, downright sad. It's bad even at 40 watts RMS -- acceptable, but still bad.
.

Viffermike,

I know you know most of this. I am going into more detail for others as there is a lot of mis-information out there. FYI I am planning to do a full write-up and measurement on the stock drivers and amp when I have time.

Actually I was fairly impressed with the underseat woofers for what they are; a woofer, not a subwoofer. Most people get all worked up about how much power a driver can handle. There are two things that determine that:

One is thermal; at what point are you simply putting too much power into the driver and run the risk of damaging the voice coil from thermal overload. This is the number you always see listed on drive spec sheets, though many car audio drivers are highly overrated in this respect.

Then there is excursion; how much the cone moves in and out while playing. Excursion limits are usially defined as how much the cone can move before distortion starts to set in (xmax in mm) or you damage the driver from the voice coil hitting the back plate or the suspension (surround and spider) simply won't let you move any further regardless of the amount of power you pump in (xmech in mm). Excursion, not power input is generally the thing that leads to most of the distortion we end up hearing in a woofer once we start to hit the limits of the driver.

Excursion tends to go up as frequency goes down. That is why a true subwoofer will have much higher xmax than a woofer and will have a surround and spider which allow more movement as well as a longer voice coil to match that excursion at lower frequencies. So depending on the range of frequencies you are playing, often a driver will hit excursion limits long before you hit thermal limits.

A good example is the Hertz 4" coaxial you have in your car (I have the non coax version of the woofer). It is rated at 40 watts continuous and has 1.5mm of xmax. In our doors with 40 watts, it will hit its excursion limits below 240 Hz. At 40 watts into 100 Hz it will be trying to do almost 6mm of excursion one way and at best, sound like crap, and at worst possibly damage the driver. To keep the driver below it's excursion limits at 100hz you could only feed the driver 3 watts. This isn't a limitation of this particular driver, or the stock BMW mids. It is a limitation of any 4" driver. Even high end 4" drivers like Morel, Focal, Audio Frog, etc. only have a few mm of excursion and that is what limits how loud they can play at lower frequencies.

The HK underseat woofers are no different. Use them in the frequency range they are designed for (> 60 Hz) and performance is actually pretty good. They are really efficient so will play loudly without a lot of input power and distortion is actually pretty low assuming you use a steep high pass filter like the stock amp does of about 60 hz and at least a 3rd order slope.

As for the 20 watts and 40 watt limits I mentioned above, the 20 watts was where I started to see excursion related distortion trying to produce a 20-30hz signal. Once I limited my sweep to over 60 hz, my measurement amp simply couldn't supply enough current to drive the 2 ohm driver above 40 watts, as it isn't rated at 2 ohms at all, and so amp was distorting, not necessarily the driver. If I pull the drivers out of the car again, I may hook them up to the Krell amp in my home theater and remeasure the woofer as that amp can easily drive over 500 watts into 2 ohm load. My guess is that the stock HK woofers could easily handle 70-80 watts at frequencies over 70 hz. depending on the enclosure they are in.

Unless you get a true subwoofer, like the SWS 8" (which has other issues) none of the 8" woofers that will fit under our seats (Jehnert, German Maestro, Focal, etc) will play subwoofer frequencies at any real volume level without distorting. Bottom line, if you want to play music with sub-bass frequencies at any reasonable level, you need a subwoofer.

Regards,

Dennis

[Viffermike]

Originally Posted by djarchow

Viffermike,

I know you know most of this. I am going into more detail for others as there is a lot of mis-information out there. FYI I am planning to do a full write-up and measurement on the stock drivers and amp when I have time.

Actually I was fairly impressed with the underseat woofers for what they are; a woofer, not a subwoofer. Most people get all worked up about how much power a driver can handle. There are two things that determine that:

One is thermal; at what point are you simply putting too much power into the driver and run the risk of damaging the voice coil from thermal overload. This is the number you always see listed on drive spec sheets, though many car audio drivers are highly overrated in this respect.

Then there is excursion; how much the cone moves in and out while playing. Excursion limits are usially defined as how much the cone can move before distortion starts to set in (xmax in mm) or you damage the driver from the voice coil hitting the back plate or the suspension (surround and spider) simply won't let you move any further regardless of the amount of power you pump in (xmech in mm). Excursion, not power input is generally the thing that leads to most of the distortion we end up hearing in a woofer once we start to hit the limits of the driver.

Excursion tends to go up as frequency goes down. That is why a true subwoofer will have much higher xmax than a woofer and will have a surround and spider which allow more movement as well as a longer voice coil to match that excursion at lower frequencies. So depending on the range of frequencies you are playing, often a driver will hit excursion limits long before you hit thermal limits.

A good example is the Hertz 4" coaxial you have in your car (I have the non coax version of the woofer). It is rated at 40 watts continuous and has 1.5mm of xmax. In our doors with 40 watts, it will hit its excursion limits below 240 Hz. At 40 watts into 100 Hz it will be trying to do almost 6mm of excursion one way and at best, sound like crap, and at worst possibly damage the driver. To keep the driver below it's excursion limits at 100hz you could only feed the driver 3 watts. This isn't a limitation of this particular driver, or the stock BMW mids. It is a limitation of any 4" driver. Even high end 4" drivers like Morel, Focal, Audio Frog, etc. only have a few mm of excursion and that is what limits how loud they can play at lower frequencies.

The HK underseat woofers are no different. Use them in the frequency range they are designed for (> 60 Hz) and performance is actually pretty good. They are really efficient so will play loudly without a lot of input power and distortion is actually pretty low assuming you use a steep high pass filter like the stock amp does of about 60 hz and at least a 3rd order slope.

As for the 20 watts and 40 watt limits I mentioned above, the 20 watts was where I started to see excursion related distortion trying to produce a 20-30hz signal. Once I limited my sweep to over 60 hz, my measurement amp simply couldn't supply enough current to drive the 2 ohm driver above 40 watts, as it isn't rated at 2 ohms at all, and so amp was distorting, not necessarily the driver. If I pull the drivers out of the car again, I may hook them up to the Krell amp in my home theater and remeasure the woofer as that amp can easily drive over 500 watts into 2 ohm load. My guess is that the stock HK woofers could easily handle 70-80 watts at frequencies over 70 hz. depending on the enclosure they are in.

Unless you get a true subwoofer, like the SWS 8" (which has other issues) none of the 8" woofers that will fit under our seats (Jehnert, German Maestro, Focal, etc) will play subwoofer frequencies at any real volume level without distorting. Bottom line, if you want to play music with sub-bass frequencies at any reasonable level, you need a subwoofer.

Regards,

Dennis

All great points, and as you stated, most is beyond what the casual music reproduction fan knows or cares about. That said, multiple drivers can offset xmax limitations to an extent -- which is a big reason why BMW likely uses two underseat units instead of just one.

Bringing that point back to the 4" drivers -- yep, yep, yep. All very true. Thing is, sensitivity makes a huge difference in how much power a speaker structure needs to reach a certain volume as well; that's when the thermal issue becomes, well, more of one. What I personally love about my Hertzs is that it strikes a balance between xmax capability (1.5mm is pretty good for that size of a mids driver), sensitivity, and power handling for our particular application -- and why the design reproduces bass down to nearly 100hz without significant rolloff at 1 watt. The sensitivity of the driver is such that the speaker actually will only see more than 3 watts or so very rarely, so distortion only happens when the speaker is overloaded by source material complexity -- or sound overloaded because of poor source reproduction or recording.

That's why I refuse to buy a speaker by a manufacturer that won't share complete specifications of its products. Makers that don't provide this info usually have at least one severe deficiency to hide compared with competitors.

[djarchow]

Originally Posted by Viffermike

What I personally love about my Hertzs is that it strikes a balance between xmax capability (1.5mm is pretty good for that size of a mids driver), sensitivity, and power handling for our particular application -- and why the design reproduces bass down to nearly 100hz without significant rolloff at 1 watt. The sensitivity of the driver is such that the speaker actually will only see more than 3 watts or so very rarely, so distortion only happens when the speaker is overloaded by source material complexity -- or sound overloaded because of poor source reproduction or recording.

That's why I refuse to buy a speaker by a manufacturer that won't share complete specifications of its products. Makers that don't provide this info usually have at least one severe deficiency to hide compared with competitors.

I agree about wanting to see actual specs. Home audio driver mfg are much better about publishing specs than car audio. Actually the spec Hertz lists is a bit misleading. The 91db they quote is not at 1/w, it is at 2.83 volts and that is is actually 86.7 db 1/w. I measured all four of my Hertz EMV 100.5 using the Dayton Audio Tester and they were all very close to each other with an average of 86.3 db at 1 watt at one meter. Still 86 db is pretty good and you are right, higher sensitivity means lower power requirements.

My BMW HK mids averaged 84db at 1 watt at one meter. The HK underseat subs were 88.3 db at 1 watt at one meter.

Regards,

Dennis

[Migz71]

Sorry to bump this thread but I am after some advice! Is there such things as capacitors or resistors for cutting high frequencies to the underseat subwoofers as my amp only has hpf for these channels for the underseat subwoofers! I would like to crossover at 250hz if possible? Anything out there that I can get other than buying a active crossover? Cheers