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Trdat

Back to Transients. Understanding "Q" and myths of bass.

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I opted to put some of these questions in this forum as I read the article bass myths from the website. Although, I agree and understand all the concepts in the article many other concepts have floated around in other articles and forums. Either I have had a misunderstanding or there is two sides of the coin to the argument or the supposed argument isn't well defined. 

Now, there is no point in advertising what I know, lets just say I am not an amateur but in saying that obviously what i don't know is not tangible in terms of books or proper explanations in forums.

Lets get to the point, I have just discovered that according to "some"(by all means possible experts) to improve transients all your doing is really cutting off or reducing the low end bass wavelengths from the extension. Which eliminates those frequencies and in turn gives you less overhang or better transients. Now this correlates to the bass myths article where its mentioned that "The fastest sounding bass is when there is no bass."

Now, I know there is no such thing as fast bass but there definitely is overhang. I have improved my subs from .707 to a .58 QTC and you can feel significantly less overhang. 

Where I am going with this is lets put aside the transients for a bass cab or a subwoofer. If the above is true then by simply crossing over your PA cab with a sub woofer will improve transients to the PA cab right?. My question is, isn't designing the enclosure alignment important in knowing how the bass will react in terms of overhang? For instance for a mid cab playing from 500hz and above and we prefer better transients does the qtc play a role in the overhang? If the cab has nothing to do with long wavelengths  then I would suppose QTC has no role right?(according to the above theory)

   

 

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I think you may be referring to group delay.  See if the following article addresses some of your thoughts on the matter.

http://stereointegrity.com/wp-content/uploads/SIGroupDelay.pdf

A subwoofer is the entire system of driver, cabinet, amplifier, and signal (processing).  My 24” subs are flat to 7 Hz in most rooms and I’ve played them at shows up to 120Hz.  People were pretty incredulous that a 24” sub could play that cleanly and “tightly” up to 120Hz.  I haven’t really compared my subs to other subs in terms of group delay but it is one of the graphs that I look at when designing a subwoofer and setting the DSP for the driver to the enclosure.

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Awesome question!

Quote

"The fastest sounding bass is when there is no bass."

This is not correct.  The fastest sounding bass is the *widest bandwidth bass*.  Technically, bass gets faster with more extension in either direction.  The audible overhang is caused by ringing from one or more interacting resonances.  In an outdoor setting, the major resonance will be for the sub itself at the bottom of its bandwidth.  The sound and audibility of this resonance will depend on it's shape and bandwidth and how this resonance interacts with the content itself.  It's complicated, and I'm not confident that I can give a straight answer as to what bandwidth/Qtc is optimal or whatever.  It's something I plan to study in the future.  Throw the sub into a small room, and now you shaping effects from the room plus room resonances that may also interact with everything else.   So here things get even more complicated.  If you run a typical "Room EQ" systems to try to "fix" the room, you're adding even more variables, and not necessarily improving the characteristics that matter that most.

When I speak of resonance interaction, I'm talking about the consequences for perception, which can be very complicated and non-linear.  A common misconception is that narrow bandwidth (or high Q) resonances are more audible because they ring for longer.  It is true that high Q resonance ring longer; however, perception seems to be much more sensitive to medium-wide (or medium-low Q) resonances.  The lowish Q resonances can dramatically amplify or suppress the audibility of coincident high Q resonances as well, and are probably of primary importance to perceived transient response.  Sorry I can't say much more than this, despite having worked for years on developing my own methods for optimizing bass response.  I am very satisfied with the results I'm getting, but I'm a long way away from being able to state a set of "rules" for what matters to perception.

I am currently running a system with 5 Hz extension and a very generous "house curve".  As optimized, the room contributions are nearly inaudible, especially in the seating area.  It's as good or better than any outdoor system I've heard, and it's rather stunning that such is possible inside a small listening room.  Extra extension in the soundtrack makes it even better.  Most movie soundtracks are processed throughout with subharmonic synthesizers which can restore ULF bass content that had been stripped by filters earlier in the recording chain.  Sadly, most such movies get filtered again after the SH synth, but if the downstream filter is mild or can be reversed with BEQ, it makes a *big* difference to the bass for me.  Even if it's filtered at say 20 Hz instead of 40 Hz, a SH synth augmented kick drum just slams so much better.  I hear and feel the SH synth contribution literally everywhere, even in normal dialog (!), and it improves realism if it's not overdone.

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On 2/8/2020 at 5:04 AM, Trdat said:

Where I am going with this is lets put aside the transients for a bass cab or a subwoofer. If the above is true then by simply crossing over your PA cab with a sub woofer will improve transients to the PA cab right?

 

 

So you're saying that the transient intermodulation distortion can be mitigated when feeding a speaker a smaller bandwidth signal? This should be true, but I can't attest that this effect is audible or if it's even a problem at all, as long as the speaker is being driven within its linear operation range. I don't know if this effect is even relevant for loudspeakers and I haven't managed to find much about this on the internet. I've only come across TIM measurements of amps. And slew rate of amps is typically not affecting audio quality, unless you're planning to do something wierd.

I don't think it's possible to improve the other two aspects of the transient response (ringing and group delay) with a simple HPF.

 

@SME I'd agree that a resonance of lower Q is more audible than a resonance of higher Q with the same gain, but I think that the resonance of higher Q will be more annoying for the listener. When I'm working on a mix I almost never use narrow additive EQ points, because those tend to sound unnatural (unless you're going for a specific stylistic effect).

 

 

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4 hours ago, peniku8 said:

@SME I'd agree that a resonance of lower Q is more audible than a resonance of higher Q with the same gain, but I think that the resonance of higher Q will be more annoying for the listener. When I'm working on a mix I almost never use narrow additive EQ points, because those tend to sound unnatural (unless you're going for a specific stylistic effect)

I would say that higher Q resonances tend to be more *obvious* even though the lower Q resonances may be more *audible*.  The subtle distinction here is important.  It is also my experience that both low Q and high Q flaws can cause annoyance.  The high Q flaws are much easier to identify though.

I agree that there's almost never a good reason to use a narrow-band boost when producing a soundtrack.  However, any high-pass or shelf filter which abruptly cuts the bass below some frequency tends to have a similar effect.  And these are actually quite common with real-life content.  Most music has significant filtering at 30 or 40 Hz, and tracks with hotter bass often ring a fair bit down there.  Most of the time though, it sounds OK for the actual track, like it was done that way on purpose.  That is, the overhang often compliments the rhythm.  OTOH there are movies that really ramp up the bass and then cut off steeply, which causes an annoying and repetitive one-note rumble to follow every sound.  These are unfortunately quite common.

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16 hours ago, SME said:

Awesome question!

This is not correct.  The fastest sounding bass is the *widest bandwidth bass*.  Technically, bass gets faster with more extension in either direction. 

You are taking the quote out of context a bit. I think we are talking about 2 different things. Objective signal reproduction capability of the speaker vs subjective perception of the reproduced spectral content. 

Comparing the technical reproduction of a system that is flat from 0.1-100kHz to one that is sort of flat from 40-15kHz with some resonance issues is a slightly different comparison. Yes technically the wide bandwidth system can better reproduce the input signal (assuming it also has equivalent headroom, dispersion, distortion, yadda yadda)  Objectively that is correct however I'd bet that depending on content a large percentage may still pick the more "flawed" less capable system as having tighter or faster bass in a blind comparison.

 Why is there a common perception that a smaller sealed sub will be tighter or more articulate than a much larger vented sub? This didn't just appear out of nowhere. I don't believe it has to do with ringing of the port either. It has to do with much greater low frequency energy, headroom and extension IMO. Technically and objectively the large ported sub probably has a great advantage at reproducing the signal. My comment is meant to illustrate people's subjective preferences and the fallacies of looking at things like the shortness of an impulse to gauge bass tightness. Bass frequencies are not fast. Low frequency wavelengths are inherently slow by definition. The initial positive pressure and the stop of the signal can be abrupt and fast which is related to ringing, resonance and overall bandwidth and FR but a certain amount of time is required for a frequency to develop and this takes longer as frequency decreases. As deeper frequencies are included in the signal the signal takes longer to complete. How can adding more low frequency extension and energy make the length of the signal faster? Alternatively many people will pick bass response that is boosted some in the 60-150Hz range as tighter. The result is the same. 

The way something sounds to someone is subjective to them. If you blind test 30 people and start with a system that has extension to 5Hz and content that is full bandwidth and do nothing more than gently roll off the the lowest frequencies ever higher for each test most of the people will not pick the most full bandwidth one as the fastest or tightest sounding. Because it isn't. It contains more of the low frequency energy. It IS slower and thicker sounding. The energy has changed to include more LF's and deeper extension. I've done this test with friends before.

 

 

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4 hours ago, Ricci said:

You are taking the quote out of context a bit. I think we are talking about 2 different things. Objective signal reproduction capability of the speaker vs subjective perception of the reproduced spectral content.

I'm trying to address both the physical and subjective aspects, but perhaps my posts have not been clear enough.

4 hours ago, Ricci said:

Comparing the technical reproduction of a system that is flat from 0.1-100kHz to one that is sort of flat from 40-15kHz with some resonance issues is a slightly different comparison. Yes technically the wide bandwidth system can better reproduce the input signal (assuming it also has equivalent headroom, dispersion, distortion, yadda yadda)  Objectively that is correct however I'd bet that depending on content a large percentage may still pick the more "flawed" less capable system as having tighter or faster bass in a blind comparison.

In theory, the wider bandwidth system will be faster.  In practice, the results depend a lot on details like what the speaker+ sub system is doing, what the room is doing, and how perception resolves all of that.  Without being more specific, I can only conclude that this blind comparison could go either way.

4 hours ago, Ricci said:

Why is there a common perception that a smaller sealed sub will be tighter or more articulate than a much larger vented sub? This didn't just appear out of nowhere. I don't believe it has to do with ringing of the port either. It has to do with much greater low frequency energy, headroom and extension IMO. Technically and objectively the large ported sub probably has a great advantage at reproducing the signal. My comment is meant to illustrate people's subjective preferences and the fallacies of looking at things like the shortness of an impulse to gauge bass tightness. Bass frequencies are not fast. Low frequency wavelengths are inherently slow by definition. The initial positive pressure and the stop of the signal can be abrupt and fast which is related to ringing, resonance and overall bandwidth and FR but a certain amount of time is required for a frequency to develop and this takes longer as frequency decreases. As deeper frequencies are included in the signal the signal takes longer to complete. How can adding more low frequency extension and energy make the length of the signal faster? Alternatively many people will pick bass response that is boosted some in the 60-150Hz range as tighter. The result is the same.

I'm assuming that headroom within the nominal passband is unlimited in order to simplify the discussion.  Obviously *in practice* a vented sub will probably reproduce the signal better than a sealed sub if things are being pushed.  That's a whole other discussion and an important one, but I don't think it the one the OP is focused on.  It's not what I focus on, given my 4x21" system which is operating well below its limits 99.99% of the time.

So sticking to just the linear / low-level operation stuff, it's not necessarily the physical ringing per se that's making the vented sub sound distinct from the sealed sub.  The physical ringing is (assuming small physical distance between driver and vent) one and the same with the physical impulse/frequency response of the sub.  Assuming no major playback chain EQ, that response interacts with the room to yield a composite response, and that composite response is perceived somewhat consistently throughout the room, despite dramatic variation in literal measured FR/IR.  There are *a lot* of details here that affect what is ultimately perceived.  Though the differences in response shape between vented and sealed are likely to still play a big part in this, just as the spectral balance of a soundtrack has a big affect on how it sounds when played back.

4 hours ago, Ricci said:

The way something sounds to someone is subjective to them. If you blind test 30 people and start with a system that has extension to 5Hz and content that is full bandwidth and do nothing more than gently roll off the the lowest frequencies ever higher for each test most of the people will not pick the most full bandwidth one as the fastest or tightest sounding. Because it isn't. It contains more of the low frequency energy. It IS slower and thicker sounding. The energy has changed to include more LF's and deeper extension. I've done this test with friends before

I'm not sure what you mean by gently, but I'll assume you mean using a filter that has minimal perceptual consequence around the transition area.  That being the case and on an ideal system, I expect the main thing that would change is apparent *weight*, which is actually not related to apparent speed.  However when you take into account speaker/sub resonances (including at the bandwidth limit of the combination) and room-induced resonances, then a broadly increased emphasis of LFs is likely to amplify the audibility of those problems, which can definitely increase apparent thickness, overhang, and apparent slowness of the perceived sound.

Try not to get confused by the fact that lower frequencies oscillate more slowly.  The "speed" is not determined by how fast the lowest frequencies oscillate.  The "speed" is determined by the composite sound.  If you start with a unit impulse, you have a sound with equal balance across the spectrum.  If you apply a high pass filter in order to reduce the low-frequency part of the spectrum, then you actually elongate the impulse substantially, and this manifests as ringing around the cut frequency.  The lower you cut, the less the amplitude of the ringing is relative to the initial impulse.

To pick a real life example.  If you hear a large helicopter overhead with blades that have a strong 8 Hz fundamental, is the bass you are hearing and feeling "slow"?  Not really, unless it's really far away so that all you can hear is the rumble.  In reality the sound is more of a chopping sound, rich in higher harmonics that make it quite fast even though it repeats at 8 Hz.

I can play e.g. "Ready: Player One" with and without Bass EQ.  The movie is steeply filtered at 20 Hz, and without Bass EQ all the sounds have a notable overhang, especially the big hits.  If I add Bass EQ, the overhang is gone and the sound becomes more realistic.  The air has a kind of palatability, as though you're really present.  It's apparent throughout the track because the movie uses SH synth very well.  The big hits don't ring, they just pummel you.  If you've heard a real-life car crash, you know what I mean.  Likewise with big fireworks.  There's an *oomph* that the sound has, in addition to the *slam* factor.  It still happens *fast*, but has a ton of weight too.

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SME

I think we are on the same page. I was just clarifying what was meant by my comments in the bass myths section. They are primarily addressed at the subjective terms that "audiophiles" & average people throw around about bass and the causes of those terms. Also the trend of visually comparing subwoofer impulse responses. 

Linking the subjective to the objective is where the fun really is. This is a DEEP subject well beyond the scope of 2 paragraphs in the bass myths article. 

For example wide range dynamic capability is often overlooked entirely when discussions of transient response, or accuracy arise but it is absolutely critical to good sound. 

I agree with you. I run 8 sealed RF 19's which loaf along at most any level I care to use in my home. Extension is relatively flat down to the 6Hz range. Electronics roll off starts to stack up below that point. Anyway... I'm familiar with the type of bass reproduction you are describing. Technically and subjectively I agree. 

With that said the bass systems that we have are the 0.1%. Most people have never heard this type of system. If they have they may not have been able to spend real time to get used to it. 

When we go back and consider the types of mass produced systems most people have, the rooms, FR's and listeners who use terms like speed, fast, weight, boomy etc in reviews and descriptions and then begin to consider WHY the small sealed subwoofer, or open baffle is often described as "tight" or "fast" while the much bigger, deep tuned vented sub is considered to be more booming and "loose". Objectively the measurements will show that the vented sub would show the smaller sealed sub or OB to be at a large disadvantage in: Distortion, dynamic range capability and deep bass SPL capability, etc...The only clear objective wins for the small sealed sub would typically be well below the vent tuning of the vented sub, lack of vent noises and group delay near the tuning of the vented sub. Some of these may even be questionable advantages depending on the DSP applied to the sealed sub and the particular vented sub in question.

The question isn't whether these subjective terms are right or wrong but why / when they are used. My opinion is that it is not directly related to ringing of the vent on the ported sub, or sub or room resonances, etc, in most cases. IMO it primarily has to do with overall bass level, extension and FR shape. It is also my opinion that visually comparing subwoofer impulse responses as a measure of "speed" or "tightness" is worthless unless the impulses are measured with both subs having matching FR and output level. The sub with greater extension and low frequency energy (Better at being a sub!) will show a longer slower impulse measurement. 

 

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15 hours ago, Ricci said:

I think we are on the same page. I was just clarifying what was meant by my comments in the bass myths section. They are primarily addressed at the subjective terms that "audiophiles" & average people throw around about bass and the causes of those terms. Also the trend of visually comparing subwoofer impulse responses. 

Linking the subjective to the objective is where the fun really is. This is a DEEP subject well beyond the scope of 2 paragraphs in the bass myths article. 

For example wide range dynamic capability is often overlooked entirely when discussions of transient response, or accuracy arise but it is absolutely critical to good sound.

[...]

Well, I think most of those subjective terms actually depend strongly on *linear* aspects of response, which is to say those that are relevant when the sub is playing well below its max output.  This implies that impulse response *is* very important.  However, the problem with *visually* comparing impulse responses is that the differences that are perceptually most important not readily visible in IR plots.

15 hours ago, Ricci said:

[...]

The question isn't whether these subjective terms are right or wrong but why / when they are used. My opinion is that it is not directly related to ringing of the vent on the ported sub, or sub or room resonances, etc, in most cases. IMO it primarily has to do with overall bass level, extension and FR shape. It is also my opinion that visually comparing subwoofer impulse responses as a measure of "speed" or "tightness" is worthless unless the impulses are measured with both subs having matching FR and output level. The sub with greater extension and low frequency energy (Better at being a sub!) will show a longer slower impulse measurement.

IR "speed"/ringing and FR extension/shape are essentially two different ways of analyzing the same characteristics of a *linear* system.  This is precisely true for minimum phase (MP) systems, and most subs (measured anechoically) and other physical producers of sound have MP behavior.  Audio systems with multiple crossovers are not necessarily MP, but they may be close enough for all intents and purposes.  I'm also not aware of evidence that slight excess group delay is especially audible, as long as there is no substantial pre-ringing.

So when talking about MP or near MP response, both IR and FR magnitude are just different ways of analyzing at the same fundamental characteristics.  A flat FR implies a "perfect" IR, and vice versa.  A sealed sub with a 2nd order roll-off in the FR will exhibit an elongation of the IR with possible ringing around the transition frequency (depending on how high the Q is).  A vented sub with a 4th order roll-off (plus whatever electronic HPF is applied) will typically exhibit more elongation and/or ringing in the IR than a sealed sub with 2nd order roll-off.

Now one can ask, is it the "frequency response shape" or the "time domain ringing" that's audible?  The answer is that they are different realizations of the same phenomenon, and perceptually speaking this phenomenon can cause both emphasis of continuous notes around the cut-off frequency and ringing and/or overhang in that region on transients.  And again, things get a lot more interesting and potentially hard to predict once you throw in room resonances and whatever spectral shaping was applied to the content.

What's clear to me is that certain roll-off shapes have a lot more or less audible impact than others, and I haven't really worked out the details.  Any overhang or ringing I hear in music is rarely offensive to me.  Movies are another story, and I think the problem there is that soundtracks tend to have an extreme ramp in balance towards the bottom which dramatically amplifies the resonance at the cut-off frequency of the filter they use.  This happens in part because cinemas and dub-stages get calibrated with pitiful bass levels.  Of course if you use a sub that rolls off at or above the cut-off frequency in the track, you'll probably get even more exaggeration of the sub's roll-off characteristic.

Part of the trick with music may be that impulse sounds in real life aren't really pure.  Real life kick drums have many natural resonances, and so the emphasis, overhang, or ringing from an HPF on, e.g. a kick drum in a track is likely to just sound like it's a natural part of the kick drum sound.  Often this is heavily stylized, and so becomes an essential part of the sound rather than a "flaw".  Note also that almost all digital content is distorted on purpose to add higher harmonics / "spectral replicas" of the original lower frequency information.  This tends to "tighten" and "brighten" the sound overall, increasing loudness and improving perceived quality.  Such processing is likely to be downstream of any HPF, so the HPF resonance on the kick drum also gets "replicated" at higher harmonic frequencies, which makes it sound more like it's part of the sound of the drum when it's played back.

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