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Calculating High Pass Filter & Input Power for Subwoofer Below Recommended HPF S
Ben Sparkes:
Chris, Art, thankyou both for your replies. Safe to say I have learnt far more on this forum just within 3 replies than I ever have browsing forums such as r/soundsystem!
I have done a little more experimentation in Hornresp which has raised 2 questions:
I have noticed that when comparing between a stack of 1 and 4 horns with identical input and crossover settings (HPF=32hz, 24db/oct L/R, 97.98vRMS), the peak excursion is slightly reduced in a stack of 4 compared to 1 (by just over 1mm). What is the scientific reason behind this change? I assume this is one of the benefits people are referring to when talking about "coupling horns"? Do the nearby drivers place resistive load on the physical movement of the cones due to the air pressure around the front of the stack? I also noticed this same effect when comparing between half space and eighth space.
The black line represents displacement in a stack of 4, and the grey line is in a stack of 1.
Hornresp only allows me to model between 1 and 9 parallel speakers, but I assume this effect would scale up with larger arrays (12, 16, etc). I can also see how this allows us to lower the HPF frequency while still staying within the Xmax limit of the driver. Are there any physical limits to this effect or does it continue to infinity? (Assuming some hypothetical array of 200 boxes, for example).
My 2nd question pertains more to real world applications of alternative processing. Under some circumstances (ie. small venues or outdoor spaces) it may be more beneficial to sacrafice some maximum SPL for better frequency response & low end response. In the following graph I have modelled the normal processing of this box (grey line, as mentioned above) against a different setting (black line, decreased power & lower HPF; 50v RMS input, 20hz HPF 24db/oct). With the lower input power the peak excursion is still within the 9.8mm spec of the original setting. While the graph is quite hard to read (my poor Photoshop skills showing here) this frequency response nets us a 1.5-2db loss at the tuning frequency (33-34hz), and reduces the response between 50 and 80hz by 4-6dB, while achieving roughly a 5.5dB boost at 20hz. My question here would be; would this ~5dB increase at the ultra-low end net any perceivable improvement in actual sound quality? Ie. is it worth the effort of implementing such a setting versus simply decreasing the volume to a more appropriate level?
My thought process here is for situations where we would not be running these cabinets at full tilt anyway (ie. smaller venues where 146dB [measured in eighth space] is simply painful) or outdoor settings where there is a concern for noise complaints (across flat ground such high SPL is audible for 10+mi, which we were unfortunately informed first hand by the local police :o). Situations where we have a poor quality/limiter power service may also apply here (as lower amplifier output is required).
Thanks again for your input!
Ivan Beaver:
--- Quote from: Ben Sparkes on January 06, 2022, 02:19:44 PM ---Chris, Art, thankyou both for your replies. Safe to say I have learnt far more on this forum just within 3 replies than I ever have browsing forums such as r/soundsystem!
I have done a little more experimentation in Hornresp which has raised 2 questions:
I have noticed that when comparing between a stack of 1 and 4 horns with identical input and crossover settings (HPF=32hz, 24db/oct L/R, 97.98vRMS), the peak excursion is slightly reduced in a stack of 4 compared to 1 (by just over 1mm). What is the scientific reason behind this change? I assume this is one of the benefits people are referring to when talking about "coupling horns"? Do the nearby drivers place resistive load on the physical movement of the cones due to the air pressure around the front of the stack? I also noticed this same effect when comparing between half space and eighth space.
The black line represents displacement in a stack of 4, and the grey line is in a stack of 1.
Hornresp only allows me to model between 1 and 9 parallel speakers, but I assume this effect would scale up with larger arrays (12, 16, etc). I can also see how this allows us to lower the HPF frequency while still staying within the Xmax limit of the driver. Are there any physical limits to this effect or does it continue to infinity? (Assuming some hypothetical array of 200 boxes, for example).
My 2nd question pertains more to real world applications of alternative processing. Under some circumstances (ie. small venues or outdoor spaces) it may be more beneficial to sacrafice some maximum SPL for better frequency response & low end response. In the following graph I have modelled the normal processing of this box (grey line, as mentioned above) against a different setting (black line, decreased power & lower HPF; 50v RMS input, 20hz HPF 24db/oct). With the lower input power the peak excursion is still within the 9.8mm spec of the original setting. While the graph is quite hard to read (my poor Photoshop skills showing here) this frequency response nets us a 1.5-2db loss at the tuning frequency (33-34hz), and reduces the response between 50 and 80hz by 4-6dB, while achieving roughly a 5.5dB boost at 20hz. My question here would be; would this ~5dB increase at the ultra-low end net any perceivable improvement in actual sound quality? Ie. is it worth the effort of implementing such a setting versus simply decreasing the volume to a more appropriate level?
My thought process here is for situations where we would not be running these cabinets at full tilt anyway (ie. smaller venues where 146dB [measured in eighth space] is simply painful) or outdoor settings where there is a concern for noise complaints (across flat ground such high SPL is audible for 10+mi, which we were unfortunately informed first hand by the local police :o). Situations where we have a poor quality/limiter power service may also apply here (as lower amplifier output is required).
Thanks again for your input!
--- End quote ---
Especially with horns, they "feel" the radiation resistance of each other when stacked together. This is because the horn mouths couple together to effectively form a larger horn mouth for each cabinet.
This also helps to smooth out the response, due to the use a horns that are actually to small for the freq they are reproducing. As is typical in touring type products.
As to your second question, yes you can run the cabinets lower, at a lower level and get a different "feel". The wider freq response (assuming there are instruments there to provide the content) can provide an added "weight" or fullness to the sound.
Just be sure to keep the drivers within their limits, before damage or excessive distortion occurs.
It is all about tradeoffs. In this case, you are trading maximum SPL for freq extension.
David Morison:
--- Quote from: Ben Sparkes on January 06, 2022, 02:19:44 PM ---Hornresp only allows me to model between 1 and 9 parallel speakers, but I assume this effect would scale up with larger arrays (12, 16, etc). I can also see how this allows us to lower the HPF frequency while still staying within the Xmax limit of the driver. Are there any physical limits to this effect or does it continue to infinity? (Assuming some hypothetical array of 200 boxes, for example).
--- End quote ---
It'll be a curve of diminishing returns, as the boxes will have to be within a reasonable fraction of a wavelength of each other in order to couple.
So, going from 100 to 200 boxes will make much less difference than going from say, 4 to 8.
Note you can do multiples of up to 81 elements in Hornresp as you can do combined series-parallel wiring ;)
--- Quote ---In the following graph I have modelled the normal processing of this box (grey line, as mentioned above) against a different setting (black line, decreased power & lower HPF; 50v RMS input, 20hz HPF 24db/oct). With the lower input power the peak excursion is still within the 9.8mm spec of the original setting. While the graph is quite hard to read (my poor Photoshop skills showing here) this frequency response nets us a 1.5-2db loss at the tuning frequency (33-34hz), and reduces the response between 50 and 80hz by 4-6dB, while achieving roughly a 5.5dB boost at 20hz. My question here would be; would this ~5dB increase at the ultra-low end net any perceivable improvement in actual sound quality?
--- End quote ---
I don't think so, but if you have enough time to test it for yourself, don't let me stop you.
My main thought is that while 20Hz does indeed get louder, it's still 20-odd dB down from the levels at 30+Hz, so will still be pretty hard to notice.
HTH,
David.
Chris Grimshaw:
--- Quote from: David Morison on January 07, 2022, 08:38:19 AM ---
My main thought is that while 20Hz does indeed get louder, it's still 20-odd dB down from the levels at 30+Hz, so will still be pretty hard to notice.
--- End quote ---
+1. The change in level at 20Hz is inconsequential.
If the -3dB point (and/or -10dB point) were being moved down by an octave or so, then I'd say this sort of processing is worthwhile. The difference in those frequency response curves, however, could be done with a ~5dB cut at about 80Hz.
Chris
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