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Discussion Starter · #1 ·
Greetings.
I recently bought a Denon AVR-1610 receiver with Polk Audio PSW110 subwoofer. I am currently using these with an old set of CA Newton MC100 satellites. I'm trying to tune the crossover point and the sub's LPF so that I obtain a good SQ with this configuration. For this, I'm using REW Generator to reproduce lower frequencies (20-100 Hz).

However, I'm seeing that crossover is not functioning in a way I'd expect. With speakers set to 'Small', I expect the receiver to "cut-off" all frequencies lower than the set value and re-direct that to my subwoofer. This is not happening. With crossover set to 150 HZ, subwoofer switched off, generating log/meas sweep from 250 Hz to 30 Hz, I can hear my front satellites producing frequencies right down to around 85 Hz. What am I missing?

My setup:
- PC connected to Denon AVR-1610 via digital coaxial
- PSW110 connected via receiver pre-out
- All speakers set to 'Small'
- LFE+Main, LPF with varying settings tried: 80 Hz to 150 Hz
- Crossover tried with varying settings: 80 Hz to 150 Hz
- Tried with Audyssey MultiEQ on and off with similar results


Any inputs are much appreciated.


Thanks,
Naveen
 

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Welcome to the Forum, naveen!

However, I'm seeing that crossover is not functioning in a way I'd expect. With speakers set to 'Small', I expect the receiver to "cut-off" all frequencies lower than the set value and re-direct that to my subwoofer.
The crossover is not a “brick wall” that shunts all frequencies above or below the crossover frequency. Beyond the crossover point, response drops at a pre-determined rate per octave. If you see a crossover designated as “24 dB/octave,” that means that the signal is -24 dB an octave out from the crossover frequency, -48 dB two octaves out, etc. In your case, one octave below 150 Hz is 75 Hz, so at 75 Hz the signal should register ~24 dB lower that it would at 150 Hz. You should be able to measure that in your room from the speakers, with either frequency-specific sine wave tones at 150 and 75 Hz. Of course, any dips or peaks in response at the measured frequencies caused by the room will affect the reading, so you might not register exactly what the crossover’s rated slope.

Regards,
Wayne
 

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Discussion Starter · #3 ·
Welcome to the Forum, naveen!
Thanks!

The crossover is not a “brick wall” that shunts all frequencies above or below the crossover frequency. Beyond the crossover point, response drops at a pre-determined rate per octave.
That's very interesting, considering that none of the A/V receivers' manuals seem to bother mentioning this! I was able to verify this like you suggest - sine wave tones at 120 Hz with crossover set to 250 Hz and 120 Hz. I guess it does make sense if the subwoofer has to smoothly roll over to the mains, though I thought the overlap would likely be ~10 Hz or so. Thanks for the explanation.

One other thing, how does the subwoofer (receiver's sub-out, rather) behave w.r.t the crossover? Does it start attenuating *after* the crossover point extending to an octave or two more, or it attenuates leading up to the crossover point such that the crossover becomes a brick-wall?

And, how does the Low-Pass Filter (LPF) setting impact any of this (if at all)?


Thanks,
Naveen
 

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One other thing, how does the subwoofer (receiver's sub-out, rather) behave w.r.t the crossover?
It continually drops each octave further from the crossover frequency just like the mains do. If your crossover is set at 150 Hz, the sub’s signal will be ~24 dB lower one octave higher (300 Hz) and ~48 dB lower two octaves out (600 Hz) – assuming a crossover with a 24 dB/octave slope that is...

And, how does the Low-Pass Filter (LPF) setting impact any of this (if at all)?
Not sure I get the question? The receiver’s sub-out is a low pass filter. “Low pass” means it’s passing through only the low frequency signals.

Perhaps this picture of a low pass filter will give you a visual of what a filter is doing to the signal. The red line shows the roll-out of a first-order filter (-6 dB/octave). Response for second, third, fourth and fifth orders are shown as well (-12, -18, -24, -30 dB/octave).




Does it start attenuating *after* the crossover point extending to an octave or two more, or it attenuates leading up to the crossover point such that the crossover becomes a brick-wall?
The slope begins to drop a bit before the designated crossover frequency, how much so depending on the filter’s mathematical characteristics.

The picture below shows the electrical response of two types of filters, Linkwitz-Riley (red) and Butterworth (blue). The Butterworth high and low pass filters are -3 dB at the crossover frequency, while the L-R is -6 dB.

The crossover’s electrical response will affect the acoustical response of the speakers (i.e. measured in-room). Butterworth filters have a 3 dB boost around the crossover frequency, while L-R measure flat around the crossover frequency. As you can tell from this picture, as well as the one above, the filter’s response at no point ever becomes a “brick wall.”




Regards,
Wayne
 

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Discussion Starter · #5 ·
And I thought crossover was a simple concept all along...

I did some reading on this and I found the following very useful:
http://en.wikipedia.org/wiki/Audio_crossover
http://www.rane.com/note160.html

With respect to crossover between a subwoofer and the mains though, it seems like the primary aspect to consider is the immediate frequency band around the crossover point rather than octaves. That is, with a crossover at 150 Hz, say, 30/40/50/60/... Hz around that point is important (100/120 Hz to 200/180 Hz). More generically, the point at which the high and low pass filters *start* attenuating the signal is important. The reason I say this is because of the limited response of the mains around the crossover point.

My mains are rated down to 100 Hz while my sub is rated up to 250 Hz. If we consider 1 octave, I'd have to set my crossover at 200 Hz (1 octave out from my mains), but then my subwoofer will not be able to go 1 octave out to 400 Hz. Similarly with a crossover at 120 Hz. Considering overlap of a band of frequencies around the crossover would mean a crossover at 120, 150 or 200 Hz. At this point, the issue of directional sound crops up eliminating use of 200 Hz crossover (Most will probably argue against use of 150 Hz crossover too, but with my not-so-good mains, I'll consider it with a preference for 120 Hz crossover)

And, how does the Low-Pass Filter (LPF) setting impact any of this (if at all)?
Not sure I get the question?
I was referring to a setting on my receiver - looking at the manual again, it looks like it is applied only for the LFE signal (LPF for LFE). So, it shouldn't have any impact on the crossover characteristics.


Thanks very much,
Naveen
 

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As it happens, the general advice in the Audyssey-related threads to set the crossover at 80Hz worked well for me. It turns out that, where my front left and right are positioned, REW shows they have nulls around 65Hz and 70Hz, probably related to reflections. Similarly, the sub located closer to the wall shows a null around 90Hz. So with the crossover at 80Hz, the sub is doing most of the work below that point and covers the dips in the mains, and vice versa, above 90Hz, the mains are doing most of the work so the null in the sub response has less affect.

If you try looking at this by using REW for full range scans of the sub with the mains, don't be surprised to see the sub level appear 3dB elevated over the mains together. I was puzzled by this for quite a while, until I finally figured out that the mains together were losing 3dB from interference between the two of them, 3dB that is not lost when the two channels are added electrically in a single sub. If you drive just one channel at a time, the response curve should show relatively even peaks, and you can see the cumulative result of the crossover and make an informed choice among what's available.

Bill
 

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Discussion Starter · #7 ·
Very interesting, thanks for sharing. I am itching to lay my hands on a RS meter to get started with REW.

- Naveen
 
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