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In Reply to: RE: perfect phase response with built-in 3-order hipass filter??? posted by mäç on August 16, 2009 at 18:14:55
it's physics. For those who believe time and phase-alignment is essential, it is accepted that time and phase-alignment is less relevant in the lower bass regions as presented by the loudspeaker itself.
Why? Well, first of all a single bass note wavelength will not fit into a room. Let's look a the wavelengths of various notes:
First Key of a piano - 27.5 Hz - 12.5 meters
Middle C - 261.63 Hz - 1.31 meters
Last key of a piano - 4186.01 Hz - 0.082 meters
You can see that wavelength really starts to increase in the mid-bass region of the audio spectrum (as frequency decreases). There's no way A(0) will fit within my room (so it wraps all around, and I hope that my room is not some nice, neat fraction of this wavelength that will create a node or a null in my listening position).
Also, since bass is less localizable (if that's a word), what you perceive as low-end is dominated by other factors (room loading, geometry and absorption, etc. This leads back to point #1.
Does this make sense?
I am a huge time and phase-alignment proponent, and I use my 2 powered subs both destructively and constructively to smooth out the sub-60 Hz response in my listening space.
Follow Ups:
Have you ever run a square wave through a 3rd order Butterworth filter tuned to 50Hz? Have you really seen no phase shift at higher frequencies, say in the 800-1KhZ range?
Furthermore, why do some loudspeaker designers (who know their stuff) fuss so much about sub woofer integration when the frequencies are generally below 100Hz? Bases on your response, it would seem to that it wouldn't matter much due to the size of the wavelength. Maybe you know something that they don'?
Richard Bass Nut's "near perfect" full range speaker has a built-in 2nd order high-pass filter also known as natural rolloff. The effects of the resulting phase distortion are seen well into the midband. A mangled square wave at 50hz admist rolloff would cause distortion in the midband at up to 500hz.
Most "1st order designers" do not mention their designs are only TRUE acoustic 1st order where the drivers are not rolling off. Once the drivers roll off, you get all kinds of mayhem going on. Running a mid or midbass "full range" with no low pass filter, for example, means the driver is allowed to pass it's natural rolloff which is 12db/octave - not the 6db/octave required.
What would be better, in my mind, is WILLFULLY coming up with a 4th order acoustic response and just removing the phase roll with forward-reverse digital signal processing. It does work. If you're listening to CD or PCM you've already got a filter in your DAC. What's a bit more DSP to achieve transient perfect reproduction? Might be a worthwhile trade if you ask me.
I tested the Thuneau Allocator and with phase correction enabled, the crossover (by itself) is truly transient perfect with the Arbitrator properly dialed in - aka reproduces square waves at ALL frequencies. The trick is to get the electrical transfer functions to combine with the drivers natural rolloffs / response to yield the 4th order acoustic transfer function.
1st order speakers and full range cone-type speakers are (IMHO) over-rated and are only truly transient perfect for portions of their bandwidth where drivers are in the linear (non-rolloff) region of their respective bandwidths. They are moreso "just another way to skin the cat" with their own set of limitations and exceptions - just like every other design out there.
Cheers,
Presto
...since I've been in a EE lab. Like the Indigo Girls once sang "Got my paper, and I was free."
As for your other question, I can't speak for "some" loudspeaker designers (actually I can't speak for any), but I think I addressed your question in my initial post - the majority of variables that significantly impact your sub-bass experience are dictated by elements outside of the speaker desginer's control. One bass response does not fit all, and is exactly why (IMHO) Richard Vandersteen integrates an 11-band EQ into the powered subs in the Quatro, 5A and 7 series speakers. Plus, there are a lot of crappy subs out there. Not sure that I can tell you much more that isn't obvious.
Sub integration ain't easy. No one said it was. If it's done right, it's spectacular. Maybe I'm an idiot and have no idea what I'm talking about, but my system sounds amazing to me and, quite frankly, that's really all that matters.
Also, it would be helpful if you register here at AA.
So, the topic's been pretty well covered. Couple points:
There IS phase shift with a first order crossover, just not nearly as much. A true first order alignment is clean in the time domain, but 'true' first order designs are extremely rare for a variety of reasons. Directivity changes make the slow rolloffs tough to achieve across a broader response window, and both drivers must be exceedingly well behaved. Do you know a lot of 12"s that are clean at 5kHz?
Bass alignment causes phase shift in all speakers. Infinite baffle designs are the best in this regard for audibility sake, generally, as there's no raising of the Fc and thus the phase shift takes place around and about Fs rather than a higher Fc. This applies to open baffle as well, though the summation of the front and rear waves makes matters somewhat more complex. In any case, it's low enough that it's less of an issue of concern than a midwoof/tweeter crossover, which is bouncing around in or on the high end of the vocal band.
Bass is supposed to sound big. 6.5" is not a woofer size.
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