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Bob

You may be surprised to hear this, Bob, but if a 100 Hz crossover has been working well in your system with your old sub, it will likely provide a similar result with your new JBL sub, with the difference being deeper and more powerful bass at the low end. Assuming both subwoofers are capable of operating well into the low-bass range of your main speakers, you'll want to emulate the gap between them that currently exists and has been successful for you.

To help understand this, take a look at the frequency response graph above, generated from measurements recently taken for the Orb Audio People's Choice subwoofer/satellite speaker system we reviewed. This shows a graphic representation of the sound level (i.e., the volume output) across the full range of audio frequencies for all four speakers in the system. We're interested in the top two traces, the blue one for the subwoofer and the purple one for the main left/right bookshelf speakers. The ideal for any full-range speaker or speaker system is a more or less flat response curve that doesn't excessively accentuate or detenuate any particular band of frequencies, though the reality is that all speakers have some peaks and valleys in their output, and some speaker designers would argue that a perfectly flat response wouldn't sound pleasing or natural to many listeners.

In particular, take a look at the upper range of where the subwoofer starts to drop off, and the low end frequencies where the main speaker starts to generate flat output. You'll notice that there's a slight gap in between the curves where the sonic output of the system in total appears to be about 3 to 4 decibels down from the peak levels of the subwoofer and satellite on either side. If you draw a line down from that trough to the frequency scale on the bottom, it coincides with the 195 Hz we measured as the -3dB point for the main satellite. We provide this measurement because -3dB is generally accepted as the place beyond which a detenuation of output becomes audible. So you ideally never want to see a valley or trough in the response of a system that goes much beyond that, or it's liable to have noticeable consequences.

In this case, the tiny satellite speakers in the Orb system have barely enough low end output to mate with the subwoofer's high end output without a noticeable sonic gap, at least in the quasi-anechoic environment we measure in. That's a fancy way of saying we take the room out of the equation when we measure, along with any bass reinforcement or attenuation your room boundaries, floor coverings, furniture, etc. might provide at your prime listening position. Factor in those variables, and you might very well have more overlap in your room than you see here on the graph. In fact, some bass reinforcement is likely, though where in the frequency range it occurs in your room is anyone's guess.

So what does this have to do with setting your sub crossover? Ideally, what you're trying to achieve in making that adjustment is a nice even overlap between the sub and main speakers. In your case, if your main speakers really do go down to around 60 Hz before hitting their -3dB point, I would start by setting the crossover in my receiver to the THX recommended standard of 80 Hz, which should give you a nice overlap in the transition range. You can go up from there and try that against your tried and true 100 Hz. That said, those room characteristics mentioned above can dramatically affect subwoofer performance, along with the position of the sub in the room. Only by experimentation and a fair amount of tuning with different program material will you settle on the ideal crossover point and volume setting for any subwoofer, so be prepared to spend some time getting this right.

One last point: if you use your receiver or surround processor for the sub crossover duties, which we generally recommend, you'll want to either turn the crossover knob on your subwoofer fully clockwise to open it up and take its filter out of the signal path, or engage the crossover bypass. For this JBL sub, you would do that by choosing the LFE option on the LFE/Normal selector switch, which takes the internal crossover out of the loop.

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