I just got the NHT Sub 2 which I am really excited about. It is a powered sub with 2 10" drivers in a sealed enclosure. One thing that is really cool about it is that it comes with an outboard controller that has settings for the subwoofer level, fully variable phase, low pass crossover frequency, and hi-pass crossover frequency. The hi-pass crossover is used to eliminate low frequencies that would otherwise be sent to the main speakers, as they will be covered by the subwoofer instead. The low-pass crossover has a fixed slope of 18 db/octave and the hi-pass is 12 db/octave.
One's first thought might be to set the hi and low pass crossovers to the same frequency. However, at that frequency the combined output from the main speakers and the subwoofer would be + 3 db, correct? As an example, let's say that we want the subwoofer to handle frequencies below 60 hz and the main monitors to handle everything above that. My thought is that the crossovers should be set so that both the sub and the mains are each 3 db down at 60 hz. Taking the slopes into account, this would mean that low-pass should be set at about 51 hz and the hi-pass at about 68 hz.
Of course, I am making the assumption that both the subwoofer and main speakers have a perfectly flat reponse, which of course they do not. Also, I am not taking room acoustics into account. Any thoughts? Any suggestions on properly setting the phase? Is it as easy as adjusting it so that it sounds the loudest from the monitoring position and then turning the level down so that it smoothly blends in with the main monitors?
Subwoofer Crossover Settings
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TapeOpAndy
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Perhaps I am being a bit "overgeeky." That's my nature, but I don't really think it's for no reason. For a subwoofer to blend in smoothly, it would make sense to avoid having it overlap with the same frequencies that the main monitors are covering, as this can introduce werid phasing issues in addition to spikes in the response. By saying that a LPF is commonly 3 db down at the crossover frequency, are you implying that linear filtering starts taking place at a lower frequency? In this case, with a slope of 18 db/octave, at about 51 hz?
Last edited by btswire on Wed Mar 05, 2008 2:03 pm, edited 1 time in total.
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TapeOpAndy
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Sorry for not being clear!!! Let me clarify.
I agree that it's extremely important to be geeky about subwoofer configuration. In fact, I would argue that before you even decide what crossover frequency to utilize, you must spend some time measuring for room modes and then deciding not only crossover frequency, but strategic placement of the sub.
In fact, in order to set phase correctly, you will have to first choose a position for the sub that results in the flattest frequency-domain response (where it doesn't over-excite room modes and it doesn't sit in any low-order null points), and then you can adjust for phase (the group delay relative to the other speakers at mix position).
With that said...
1. When a filter is said to have a cutoff frequency of XX Hz, that means its response is 3 dB from nominal at XX Hz. In other words, if the filter was labeled correctly, a LPF or HPF at 60 Hz, no matter what its slope, will be 3 dB down at 60 Hz. (It's actually the square root of 1/2 the voltage of the passband, which is very close to 3 dB.)
2. If your room suffers from peaks and dips in the low end (which most rooms do), then the response of your speakers and sub will not be exactly 3 dB down at the filter cutoff frequencies.
Therefore, unless you have do a high-resolution analysis of your room response (both in frequency and in phase-domain), calculating what crossover frequencies to use based on the slope of ideal filters is a geeky but not very useful exercise.
I agree that it's extremely important to be geeky about subwoofer configuration. In fact, I would argue that before you even decide what crossover frequency to utilize, you must spend some time measuring for room modes and then deciding not only crossover frequency, but strategic placement of the sub.
In fact, in order to set phase correctly, you will have to first choose a position for the sub that results in the flattest frequency-domain response (where it doesn't over-excite room modes and it doesn't sit in any low-order null points), and then you can adjust for phase (the group delay relative to the other speakers at mix position).
With that said...
1. When a filter is said to have a cutoff frequency of XX Hz, that means its response is 3 dB from nominal at XX Hz. In other words, if the filter was labeled correctly, a LPF or HPF at 60 Hz, no matter what its slope, will be 3 dB down at 60 Hz. (It's actually the square root of 1/2 the voltage of the passband, which is very close to 3 dB.)
2. If your room suffers from peaks and dips in the low end (which most rooms do), then the response of your speakers and sub will not be exactly 3 dB down at the filter cutoff frequencies.
Therefore, unless you have do a high-resolution analysis of your room response (both in frequency and in phase-domain), calculating what crossover frequencies to use based on the slope of ideal filters is a geeky but not very useful exercise.
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TapeOpAndy
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Best way to set phase (once everything is positioned correctly) is to create an overlap between the subwoofer pass band and the main monitor passband. Then output a sine wave within the shared pass band (so that both the sub and the mains are outputting the sine wave). Vary the phase until you have maximum cancellation at mix position. Then flip the polarity of the sub (or change its phase by 180 deg, if there isn't a dedicated polarity switch).
I'm going to assume you don't have a graphic equalizer with a pink noise generator, spectrum analyzer and microphone. You can pick these up on ebay pretty cheap and they can be a big help getting you in the ballpark.
You pretty much need to forget all the geeky stuff. I am a geek's geek but I've set up subwoofers in several different systems and there's virtually no way to do it except for using your ears. It takes at least a couple hours to do this correctly and there's no shortcut that I know of.
The advise above is correct on setting phase. Do that first. Next, set the subwoofer to run full range. Move the sub to a few different locations in the room, play some bass heavy music, and note the difference in sound. Try a corner, a wall in a couple locations, and try moving the sub a foot or two out from the wall. When the sub is at each location walk around the room slowly and listen for peaks and valleys in bass response. Pay particular attention to your listening position and note whether it seems to have average bass or is in a peak or trough of bass response. You may end up moving your listening position and your main speakers as a result of all this screwing around.
You should begin to get a feel for where the sub has the smoothest response and how your listening position correlates with that response. When you've found a good position for the sub you will start working with the filters. Work with the low pass filter first. Find the spot where, when you turn the sub on and off, you get just the low bass without adding anything to the midbass from your monitors. Put on several CDs that have a range of bass sounds and experiment with the filter and the sub volume so that when you turn it on it is just adding the additional bass that you feel sounds right. Lastly, experiment with the high pass filter and adjust it so that you are not getting any rumbling effect from music with a lot of low bass. The bass should sound tight but feel like it's going all the way down as low as you can hear. If you've got your sub set up correctly you won't notice it as a separate speaker. It will subtly blend in with your monitors.
I know this is unscientific but you are going to have to suppress your inner geek and use your best listening skills to set your sub up right. Block out an entire afternoon. This takes a while.
You pretty much need to forget all the geeky stuff. I am a geek's geek but I've set up subwoofers in several different systems and there's virtually no way to do it except for using your ears. It takes at least a couple hours to do this correctly and there's no shortcut that I know of.
The advise above is correct on setting phase. Do that first. Next, set the subwoofer to run full range. Move the sub to a few different locations in the room, play some bass heavy music, and note the difference in sound. Try a corner, a wall in a couple locations, and try moving the sub a foot or two out from the wall. When the sub is at each location walk around the room slowly and listen for peaks and valleys in bass response. Pay particular attention to your listening position and note whether it seems to have average bass or is in a peak or trough of bass response. You may end up moving your listening position and your main speakers as a result of all this screwing around.
You should begin to get a feel for where the sub has the smoothest response and how your listening position correlates with that response. When you've found a good position for the sub you will start working with the filters. Work with the low pass filter first. Find the spot where, when you turn the sub on and off, you get just the low bass without adding anything to the midbass from your monitors. Put on several CDs that have a range of bass sounds and experiment with the filter and the sub volume so that when you turn it on it is just adding the additional bass that you feel sounds right. Lastly, experiment with the high pass filter and adjust it so that you are not getting any rumbling effect from music with a lot of low bass. The bass should sound tight but feel like it's going all the way down as low as you can hear. If you've got your sub set up correctly you won't notice it as a separate speaker. It will subtly blend in with your monitors.
I know this is unscientific but you are going to have to suppress your inner geek and use your best listening skills to set your sub up right. Block out an entire afternoon. This takes a while.
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