Subwofer's LPF frequency response

[alexk0il]

Got a new BK XXLS400 sub, still learning how to set it up in my room. Anyway, found a strange behavior of the frequency filter of the sub while setting the crossover with my speakers.


I am generating 200, 100 and 60Hz audio signals on my phone and stream them to my AMP. I measure then the sub output attenuation vs sub's LPF settings.
Here is my setup:
Android frequency generator app -> BT LDAC receiver -> DAC -> Analogue Amp.

Here are the results:

My concerns are as following:

• The total attenuation range is about 20dB for 200Hz, and only 11dB for 60Hz. I'd expect it to be in the range above 40-60dB at least
• The 60Hz signal is attenuated by -3db at ~110-120Hz. I'd expect to see the LPF to kick in around 60dB
• 200Hz signal is still leaking at 40Hz. In fact, there is no difference after the 80Hz filtering, the attenuation doesn't change in the 80-40Hz area

Does it look normal and I need to readjust my expectations, or do my concerns have some merit?

Thanks

 

[sigbergaudio]

I might be slow but I don't understand the graph. Is the X-axis the wrong way? Why is 180hz at 0dB attentuation?

 

[sigbergaudio]

A general comment (despite not understanding the graph), a typical LPF is either 12dB/octave or 24dB octave.

So if the crossover is at 60hz, you would expect 120hz to be either 12dB or 24dB down, and you'd expect 240hz to be either 24dB or 48dB down.

 

[alexk0il]

I might be slow but I don't understand the graph. Is the X-axis the wrong way? Why is 180hz at 0dB attentuation?

I measure how the signal is attenuated at dufferent LPF cutoff frequencies. 180Hz is the max freq I can set on my sub.

Whatever the signal at 180Hz id, I take it as a reference (0dB) and measure the attenuation while decreasing the cutoff freq all the way down to 40Hz (my sub's min setting).

Hope it makes sense.

 

[sigbergaudio]

I measure how the signal is attenuated at dufferent LPF cutoff frequencies. 180Hz is the max freq I can set on my sub.

Whatever the signal at 180Hz id, I take it as a reference (0dB) and measure the attenuation while decreasing the cutoff freq all the way down to 40Hz (my sub's min setting).

Hope it makes sense.

Maybe. Confusing to not have the frequency response as the graph. Would it be possible to map the actual frequency response as the graph instead, so my brain doesn't have to hurt? and then you can have one graph for 40, one for 80 and one for 120hz cutoff frequency?

 

[alexk0il]

A general comment (despite not understanding the graph), a typical LPF is either 12dB/octave or 24dB octave.

So if the crossover is at 60hz, you would expect 120hz to be either 12dB or 24dB down, and you'd expect 240hz to be either 24dB or 48dB down.

Have a look at 200Hz line. There should be a 12/24dB difference between 40Hz and 80Hz cut off filters. In reality the is no difference at all.

 

[alexk0il]

Maybe. Confusing to not have the frequency response as the graph. Would it be possible to map the actual frequency response as the graph instead, so my brain doesn't have to hurt? and then you can have one graph for 40, one for 80 and one for 120hz cutoff frequency?

I don't have a frequency scanner. But please see my other comment, it should still make sense.

 

[sigbergaudio]

I don't have a frequency scanner. But please see my other comment, it should still make sense.

Looks like you plotted this manually, so you could do that the other way around. But anyway, the sub probably have some kind of analog filter that stops filtering at around 20dB then or something.

 

[alexk0il]

Looks like you plotted this manually, so you could do that the other way around. But anyway, the sub probably have some kind of analog filter that stops filtering at around 20dB then or something.

Would I not be measuring the frequency response of my amp at the same time? Can't guarantee that my 45 year old Dynaharmony G-Class amp has a flat frequency response in that range.

I'm using the "high speaker inputs" to feed my sub so it is "colored" by the amp and it's built in equalizer. No way to bypass the preamp unfortunately. Likely to get +/-3dB inaccuracies at least.

It shouldn't matter with my measurements, I normalize the signal to 180Hz starting point at a given signal frequency.

 

[sigbergaudio]

I'm probably still slow but I have no idea what you did to plot that graph.

What I'm just saying is that it would be easier to understand if you used the X-axis to show which frequency response you are measuring, as opposed to which LPF you have set.

 

[sigbergaudio]

Either way I guess it is what it is. This is how your sub works.

 

[NTK]

May be I am missing interpreting them, I also couldn't make much sense out of your curves.

For exmaple, for the 100 Hz (red) curve, my understanding is that you played a constant 100 Hz tone, and only adjusted the LPF cutoff frequency from 180 Hz down to 40 Hz, and measured the SPL output with the various LPF cutoff settings.

What I found strange is that at LPF cutoff of 170 Hz, you measured no attenuation (0 dB). However, when the cutoff is 100 Hz, you had -12 dB attenuation at 100 Hz. That's too much attenuation when the signal frequency is the same as the cutoff frequency. For most filters, the attenuation at cutoff is 3 dB, and for Linkwitz-Riley it is 6 dB.

Ditto for 60 Hz (gold) curve, which also was 12 dB down at cutoff frequency, and was 3 dB down at 1 octave (120 Hz) above cutoff. That's far too much attenuation for those cutoff frequencies.

 

[Sancus]

I'll be honest I don't understand the needless self-infliction of pain with setups like these. Everybody with a sub should have a UMIK1 to measure with REW and a miniDSP(or similar DSP tool) to implement the crossover and EQ, IMO. It's worth the 300 bucks or so, trust me.

 

[alexk0il]

My understanding is that you played a constant 100 Hz tone, and only adjusted the LPF cutoff frequency from 180 Hz down to 40 Hz, and measured the SPL output with the various LPF cutoff settings.

Correct. Since my uncalibrated microphone, a vintage amp and room modes have a non-flat frequency responce I normalized the measurements to the SPL level at 180 Hz to get the attenuation relative to that level.

What I found strange is that at LPF cutoff of 170 Hz, you measured no attenuation (0 dB). However, when the cutoff is 100 Hz, you had -12 dB attenuation at 100 Hz. That's too much attenuation when the signal frequency is the same as the cutoff frequency. For most filters, the attenuation at cutoff is 3 dB, and for Linkwitz-Riley it is 6 dB.

Yes, the sub is advertised to have (quoting the product info) "Linkwitz Riley Correction which adjusts the system Q and lower frequency response so the subwoofer sounds like it is has larger cabinet".

One of my concerns is that I would expect to see a 6/12/24dB attenuation between 80Hz and 40Hz cutoff, yet the 200Hz signal stays unattenuated in that range.

Ditto for 60 Hz (gold) curve, which also was 12 dB down at cutoff frequency, and was 3 dB down at 1 octave (120 Hz) above cutoff. That's far too much attenuation for those cutoff frequencies.

I think you are saying that the measurements look a bit unusual.

 

[NTK]

These are my theoretically calculated curves with second order Linkwitz-Riley (LR2) and fourth order (LR4) using your format. You can see the 6 dB attenuation when the signal frequency matched the cutoff. Also, for the 200 Hz and 100 Hz curve, at 1 octave below cutoff (i.e. 100 Hz and 50 Hz, respectively), the attenuation is -14 dB for the LR2 and -25 dB for the LR4. (These numbers are calculated exactly using their transfer functions, and at 1 octave below cutoff are a little higher than the nominal approximations of -12 dB/octave and -24 dB/octave.)

These curves are very different from your measurements. When we comparing attenuations, room response shouldn't matter if the sub and the mic weren't moved.

 

[sam_adams]

What we expect to see:

12 dB/octave:

24 dB per octave:

For completeness:

12db per octave high pass:

24db per octave high pass:

 

[sigbergaudio]

Correct. Since my uncalibrated microphone, a vintage amp and room modes have a non-flat frequency responce I normalized the measurements to the SPL level at 180 Hz to get the attenuation relative to that level.


Yes, the sub is advertised to have (quoting the product info) "Linkwitz Riley Correction which adjusts the system Q and lower frequency response so the subwoofer sounds like it is has larger cabinet".

One of my concerns is that I would expect to see a 6/12/24dB attenuation between 80Hz and 40Hz cutoff, yet the 200Hz signal stays unattenuated in that range.




I think you are saying that the measurements look a bit unusual.

The quote from the product info doesn't really have to do with the LPF filters.

The weirdest thing with your measurements is as pointed out that at 100hz frequency you have lots of attentuation when the crossover is set to 100hz. We'd expect it to be -3dB or -6dB down at 100hz with 100hz crossover, but your graph is showing -12dB. It's tempting to assume something is wrong with your measurements (as opposed to the sub). And also the fact that it flattens out is somewhat strange, but perhaps the nature of the probably analog filter applied (I'm not an expert in analog crossovers).

Perhaps you should send this to the manufacturer for comment (possibly including link to this thread).

 

[wwenze]

Can u just use white noise please

 

[alexk0il]

Can u just use white noise please

I can try, any recommendations for a decent Android white noise generator? I can see a lot of apps to help with sleeping, not sure how white the noise they generate.

Also, my mic has is uncalibrated and very likely not to have a flat freq response.

 

[harkpabst]

Your entire setup used for measuring and for listening to music needs more detailed explanations. Same as in that other forum where people don't like to measure things as much as here (according to your perception).

Theory of low-pass (and high-pass) filters has been explained in some detail. What is still up to debate is how you did perform your measurements and why you even thinks there's something wrong with your BK electronics XXLS400.