Help comparing subs and understanding graphs

[Brewjitsu]

So THD isn't something that I need to be concerned about if HT is my interest? Only music?

[shadyJ]

THD is something you should be concerned about if you care about accuracy, regardless of type of content. 

[Bossobass Dave]

Axiom's tests of a wide range of male and female listeners of various ages with normal hearing showed that low-frequency distortion from a subwoofer or wide-range speaker with music signals is undetectable until it reaches gross levels approaching or exceeding the music playback levels. Only in the midrange does our hearing threshold for distortion detection become more acute. For detecting distortion at levels of less than 10%, the test frequencies had to be greater than 500 Hz. At 40 Hz, listeners accepted 100% distortion before they complained. The noise test tones had to reach 8,000 Hz and above before 1% distortion became audible, such is the masking effect of music. Anecdotal reports of listeners' ability to hear low frequency distortion with music programming are unsupported by the Axiom tests, at least until the distortion meets or exceeds the actual music playback level.

 

 

Here's a link to a study conducted by Axiom focused on harmonic distortion with music as the source (the above quote is taken from the conclusion of the listening test):

 

http://www.axiomaudio.com/distortion

 

The battle will rage for eternity in most likelihood. My comment was referring only to the subwoofer bandwidth using full bandwidth LFE from blockbuster movie soundtracks as the source. THD <100% is not detectable by anyone without a reference to immediately compare it to. Of that fact, I am certain and will gladly bet the farm if there be any takers.

 

It has always been my opinion, and I've posted it a go-zillion times, that lopping off a few octaves of the source by using a resonant alignment sub and/or having a hugely distorted response (+/- 10dB or a 15dB house curve) at the seats far less desirable (and far more distortion) than having full bandwidth capability with some harmonic distortion <30 Hz.

 

Those who disagree can chime in with their reasoning and data.

 

I don't recommend commercial subwoofers. I only commented on the 2 choices offered for comparison. I have, of course, a limited edition of the Raptor and have included a single module version of that offer. The amplifier is outboard and is available to operate with any mains in whatever country. It's footprint is smaller than the subs you mention in this thread and it's performance is better in every category.

 

That opinion, of course, is purely unmitigated bias based in fact.

 

[shadyJ]

The Axiom test in that link was not even testing distortion, their methodology was all wrong. It doesn't prove anything with respect to distortion that I can see, other than bass frequencies can mask a whole lot. And of course Axiom would say distortion doesn't matter; the one occasion their sub was tested for distortion, it yielded very poor measurements! While human hearing is relatively tolerant to distortion in bass frequencies (thanks to masking), not to mention most of what is heard in deep frequencies is effects noises where who would know if it is distortion or not, there have been one study that showed surprising sensitivity to distortion in bass frequencies, at least to pure tones. The only serious study of perception of distortion in low frequencies is Louis Fielder and Eric Benjamin's 1988 study. I don't consider anywhere near a complete study of the subject, but it is the only testing that exists in this area, surprisingly. You can see the results of their experiments on this page, if you examine Table 2. You can see the upward spread of masking and also the effects of the equal loudness curves, both in deep frequencies. Note how much distortion is masked by lower harmonics versus higher harmonics. Interesting stuff!

[SME]

I agree that the Axiom testing is worthless, and I believe distortion does matter.  However, it is a very complicated subject, and it's not really possible to say things like "THD at 1% if OK but THD at 5% sounds bad".  In fact, harmonic distortion is probably a lot less audible than other forms of non-linear distortion like intermodular distortion.  However, it's the underlying non-linearity that really matters because it contributes to both harmonic and intermodular distortion, depending on the content being played.  So THD measurements are still useful as an indication of how much underlying non-linearity is present that will likely contribute to other, more objectionable, forms of distortion.

 

But don't forget about linear distortion, which is present equally at all playback levels (unlike non-linear distortion which typically rises with playback level) and is typically more severe (in terms of %) than non-linear distortion in typical playback scenarios.  Linear distortion can also amplify non-linear distortion considerably.  Linear distortion is caused by uneven response in the speakers, speaker/room interaction, and the effects of room acoustics in the listening area.  Real life rooms often show bass peaks upwards of 20 dB or higher than the rest of the response.  This is basically 1000% distortion.  Suppose you had a 20 dB bass peak at 60 Hz relative to 20 Hz and you played a 20 Hz tone that would have produced 10% 3rd harmonic (i.e., at 60 Hz) in outdoor measurements.  However, in the room, that 60 Hz harmonics gets amplified by 10 times, even if you were to use EQ to reduce the level of 60 Hz when played from the sub intentionally.  So THD that was 10% outdoors becomes 100% indoors.  So what do you do?  You can try to reduce the distortion in the sub, but even if you knocked it down to only 1% (good luck doing that at 20 Hz), your in-room THD will still be 10%.  So you can see that it actually makes a lot of sense to try to solve the linear distortion problem using bass traps or other room treatments to knock down that 60 Hz peak.

 

So in summary, while I believe non-linear distortion matters, in most cases in which the equipment isn't being pushed to its limits, it probably matters a lot less than the linear distortion induced by the room acoustics.

[Bossobass Dave]

The Axiom test in that link was not even testing distortion, their methodology was all wrong. It doesn't prove anything with respect to distortion that I can see, other than bass frequencies can mask a whole lot. And of course Axiom would say distortion doesn't matter; the one occasion their sub was tested for distortion, it yielded very poor measurements! While human hearing is relatively tolerant to distortion in bass frequencies (thanks to masking), not to mention most of what is heard in deep frequencies is effects noises where who would know if it is distortion or not, there have been one study that showed surprising sensitivity to distortion in bass frequencies, at least to pure tones. The only serious study of perception of distortion in low frequencies is Louis Fielder and Eric Benjamin's 1988 study. I don't consider anywhere near a complete study of the subject, but it is the only testing that exists in this area, surprisingly. You can see the results of their experiments on this page, if you examine Table 2. You can see the upward spread of masking and also the effects of the equal loudness curves, both in deep frequencies. Note how much distortion is masked by lower harmonics versus higher harmonics. Interesting stuff!

 

And, the cited test results don't consider room gain, which reduces HD as a % quite a bit.

 

This reduction by room gain effect shows up in all of my tests done in-room using BHD Irene scene as source. Josh tested the FI SSD 18 which shows gobs of HD <30 Hz, where room gain begins. It couldn't even post a passing number at 10 Hz! My first Blackbird subs were loaded with the 15" version of that driver, but look at the HD recorded at the seats while playing the scene at reference level. The 6.28 Hz fundamental is almost a full octave below the 10 Hz GP CEA burst and shows nearly zero 2HD of that fundamental:

 

 

By contrast, here's another well known member's rendition of the scene with multiple 18" high end drivers in a sealed alignment, but... he's running the subs at +15dB hot and has the first fundamental filtered out by design (for SPL headroom). I didn't calc the HD numbers for this one, but the bigger Q is: which presentation is more distorted and is the HD in the blackbird test using a known relatively very high HD tested driver at reference level in-room audible?:

 

 

Even if room gain didn't attenuate the HD as a % by boosting the fundamental by +20dB or so (typical), 2HD is 13 Hz and 3HD is 18.6 Hz, both infrasonic frequencies.

 

[shadyJ]

I take your point that harmonic distortion isn't something to be terribly concerned about - but only at this level of performance. Multiple heavy-duty drivers driven by a high-performance amp is going to do pretty good with respect to distortion, especially with room gain. Probably most of us in this forum have very good performance with respect to distortion, or at least THD. 

[Ricci]

But don't forget about linear distortion, which is present equally at all playback levels (unlike non-linear distortion which typically rises with playback level) and is typically more severe (in terms of %) than non-linear distortion in typical playback scenarios.  Linear distortion can also amplify non-linear distortion considerably.  Linear distortion is caused by uneven response in the speakers, speaker/room interaction, and the effects of room acoustics in the listening area.  Real life rooms often show bass peaks upwards of 20 dB or higher than the rest of the response.  This is basically 1000% distortion.  Suppose you had a 20 dB bass peak at 60 Hz relative to 20 Hz and you played a 20 Hz tone that would have produced 10% 3rd harmonic (i.e., at 60 Hz) in outdoor measurements.  However, in the room, that 60 Hz harmonics gets amplified by 10 times, even if you were to use EQ to reduce the level of 60 Hz when played from the sub intentionally.  So THD that was 10% outdoors becomes 100% indoors.  So what do you do?  You can try to reduce the distortion in the sub, but even if you knocked it down to only 1% (good luck doing that at 20 Hz), your in-room THD will still be 10%.  So you can see that it actually makes a lot of sense to try to solve the linear distortion problem using bass traps or other room treatments to knock down that 60 Hz peak.

 

So in summary, while I believe non-linear distortion matters, in most cases in which the equipment isn't being pushed to its limits, it probably matters a lot less than the linear distortion induced by the room acoustics.

 

 

This is a very good point. I find that outdoors and in bigger spaces with less low bass lift THD matters much more and is more audible. In a vehicle or smaller room it is much less important to me especially in the deep bass. Deep bass distortion is typically very high but it gets reduced dramatically in smaller acoustic spaces. This is part of the reason that I like sealed or IB systems for smaller spaces like cars and HT rooms but I prefer horns and vented cabs for bigger more open spaces like venues and outdoors where you don't get the same amount of boost to the low end and reduction in distortion. I actually tried a high power sealed system for a while in a big open space in comparison to a horn bass system. I can't think of many other people who have. I also find that mechanical and other non harmonic distortions are more offensive than HD. This type of noise is also easier to hear outdoors or in a much bigger environment partly because the system has to be pushed much harder for the same output level.

 

As you say in a small room like most HT's are, it isn't the deep bass THD that is a problem but the peaks in the response at the LP boosting the distortion harmonics. Typically there will be one to 3 of these in the bass/ mid-bass  range of most domestic listening rooms. These can greatly boost any harmonic that falls into that range and EQ does nothing against it. My situation has one right around 45Hz. It will boost the 9th harmonic of 5Hz, the 5th harmonic of 9Hz, 3rd HD of 15Hz 2nd of 22.5Hz etc. It clearly shows up in my distortion measurements. For bass the 3rd and a bit of the 2nd are mostly the issue unless you are really driving the system a bit too far or you have a particularly nasty peak boosting the 4th or 5th HD.

[Bossobass Dave]

Years ago, I conducted an experiment with harmonic distortion. I asked a group of young musicians (meaning they were more accustomed to pitch and had good hearing) what they thought about different presentations of a clip of music that was only drum kit and bass guitar. I did the tracks myself so there would be no bias as to what the clip "should" sound like. I used one sub that had very high HD and another sub that presented virtually zero HD. I asked the group to comment on which they preferred and why.

 

The drummer in the group actually clearly preferred the high HD presentation. His comments were along the lines of "tighter", "quicker", "more attack". The rest of the group was very mixed and showed no clear preference. But, everyone in the group clearly heard a difference between the 2 presentations.

 

After that, I used the scene from Daredevil wherein the kid wakes up blind in the hospital with a series of LFE transients that got louder and louder. This test was in 2 parts. First, I used signal shaping to equalize the no HD, full bandwidth sub to the bandwidth limited high HD sub. The result was that there was no one in the group who had a clear preference and most said the effect was the same from both subs.

 

The 2nd part was between the low HD sub run full bandwidth vs the rolled off high HD sub. The result was unanimous and the subjective descriptives were numerous, from "weight" to "awesome" and everything in between. This was the first in a series of many sessions that asked people to comment on any perceived difference between a full BW presentation and a rolled off presentation by inserting a HPF at various frequencies during playback of a ULF soundtrack clip. Every test, regardless of participant variation, has always been unanimous that yes, they perceive a difference and yes, it is significant, subjective descriptions aside.

 

In another exercise, I mic'd the pluck of the low E string of a bass guitar with no electronic processing. I was surprised to see that harmonics of the fundamental appeared as loud or louder than the fundamental! When adding processing, harmonic distortion is a standard operating procedure and the resulting harmonic composition is what makes one bass guitar sound different than the next bass guitar. Nevertheless, once the final processing is done and the recording is mastered, if the system used to reproduce the recording adds significant harmonics, the difference is readily audible. At what % depends on the listener. Most pundits have settled on 10% THD in the subwoofer band, YMMV.

 

But, when it comes to a full subwoofer bandwidth LFE from a soundtrack, there is no frame of reference and the full spectrum non-spacing of the frequencies in the effects make it virtually impossible to hear harmonics that don't belong.

 

All of the evidence I've gathered supports the idea that in-room frequency response linearity and bandwidth are far more important than harmonic distortion measured outdoors at GP. That's why I prefer a few dB less output with full bandwidth over a few dB more output with a truncated bandwidth and multiples to tame wildly distorted in-room FR and to arrive at the desired playback levels. That's the essence of the ported v sealed story.

[Kvalsvoll]

I tend to agree with bosso here that harmonic distortion is not very important with real content.

 

But the problem SME and Ricci shows with the distortion increasing due resonances also has another issue - resonances last longer in time, and will dominate what is heard, and even if those resonances seems to be reduced or removed after eq, they still exist and will ring for any signal created after the eq.

[maxmercy]

I think both HD, IMD and resonance control are important, and very few room layouts/system designs can minimize all 3.  Even though HD is best heard with pure tones, I think can pick out a system starting to strain vs loafing at a particular SPL, but I have never set up a proper A/B comparo (add one other thing to the to-do list).  But I have also never quantified the HD or resonance behavior of a system starting to strain.  Are we listening to increased resonance ring, increased cone breakup, HD, or increasing compression or a combo of all 4 or more that tells us "that speaker is getting driven with too much power".  Compression brought on by beginning to reach the excursion limits of a driver will in a way emulate tube overdrive/distortion.  While some speakers with inherent HD products (Carmody's Classix series) can sound better than a more accurate system with bad recordings, it's with very good recordings that the differences may more easily be heard, IF the HD present is the only thing changed.

 

Too many variables at play in the system to know for sure.  But I do know that IME going from an 87dB sensitive conventional system to a 93dB sensitive horn and array system, playing back known content at the same SPL was a night and day difference.  The room resonances didn't change, as I used the same layout.

 

JSS

[SME]

The problem here is that you can't really characterize distortion on real-world content using THD measurements because THD measurements *require* pure sine waves.  What THD measurements give you is insight into the underlying non-linearity of the system that is responsible for contributing distortion, harmonic or otherwise, to content that places similar demands on the equipment.

 

That actually makes things very tricky.  For example, a pure sine wave at 10 Hz that moves a woofer beyond Xmax will produce a lot of HD.  Real life content with 10 Hz content at the same level but other higher frequency content present at the same time may not push the woofer beyond Xmax at all.  It depends on the relative levels.  Paradoxically, I can imagine situations in which the higher frequencies are present at a lower level, allowing the woofer to achieve *more* excursion and therefore causing those higher frequencies to be more distorted than if they were played at a higher level.  This assumes that the non-linearity is primarily due to Kms or BL non-linearity, but related to inductance or current or thermal considerations?  Then the pure sine waves, which actually help to ventilate the voice coil, might be a lot cleaner than a complex signal like pink noise, which may not move the woofer and ventilate the voice coil much at all.

[Ricci]

Harmonic distortion is only one tiny piece of the puzzle. There are so many other kinds of distortion. Non harmonic noises and mechanical sounds or resonances are the easiest to hear for me and the most distracting. IMD distortion, distortion in the time domain (The acoustics of the space are HUGE here typically causing smearing and spreading of the signal over certain frequency ranges.) Compression of the signal peaks and the inability to reproduce the full output requirement is a distortion and one of the worst in my opinion. Technically even EQ'ing the level of the mid, bass or treble to taste is a form of signal distortion. From a technical standpoint low distortion is the ability to reproduce the electrical signal waveform exactly at your position.

[Bossobass Dave]

The problem here is that you can't really characterize distortion on real-world content using THD measurements because THD measurements *require* pure sine waves.  What THD measurements give you is insight into the underlying non-linearity of the system that is responsible for contributing distortion, harmonic or otherwise, to content that places similar demands on the equipment.

 

That actually makes things very tricky.  For example, a pure sine wave at 10 Hz that moves a woofer beyond Xmax will produce a lot of HD.  Real life content with 10 Hz content at the same level but other higher frequency content present at the same time may not push the woofer beyond Xmax at all.  It depends on the relative levels.  Paradoxically, I can imagine situations in which the higher frequencies are present at a lower level, allowing the woofer to achieve *more* excursion and therefore causing those higher frequencies to be more distorted than if they were played at a higher level.  This assumes that the non-linearity is primarily due to Kms or BL non-linearity, but related to inductance or current or thermal considerations?  Then the pure sine waves, which actually help to ventilate the voice coil, might be a lot cleaner than a complex signal like pink noise, which may not move the woofer and ventilate the voice coil much at all.

 

HD measurements do not require pure sine wave signal input. The term simply changes from "Total Harmonic Distortion" or "THD+N" to "Inter Modulation Distortion".

 

The 3 are directly related.

 

For example, a pure sine wave at 10 Hz that moves a woofer beyond Xmax will produce a lot of HD.  Real life content with 10 Hz content at the same level but other higher frequency content present at the same time may not push the woofer beyond Xmax at all.

 

 

Real life content input at the same frequency and level will affect equal throw vs sine wave input. Simultaneous higher frequencies will not affect throw either way.

 

As I've shown and Josh mentioned, in-room response is the major determinant of HD (THD, THD+N and IMD) as a %.

 

Here's one proof from an in-room measurement. Slartibartfast, from the old AVTalk/Ilkka measurements days, measured his Velo outdoors/GP and then brought the sub inside to measure the same 2 metrics of focus, FR at the mic and subsequent HD. Notice how the high HD at the low end disappears as a % with room gain. Also notice the same effect on narrower bands of response; when there is a dip, HD increases as a % and where there is a peak, HD decreases as a %:

 

[Ricci]

I've got a few sets of data like that from my rooms. Yep...I'll post some of it if I get a chance. Shows much the same thing in a nutshell.

[Brewjitsu]

Ok question for you knowledgeable folk.

 

Everyone well almost everyone keeps saying for a large area such as mine I need a ported sub.

If you have two subs to compare and one is ported and one is sealed but they both produce the same frequency response, cea measurements is the ported still a better option? Why?

[dgage]

"Everyone" is saying ported since you have a large room and ported GENERALLY has more output compared to an equivalent sealed, at least above port tune.  They are two different designs though with pros and cons to each.  The following are generalities and in my mind I'm comparing a ported 18" sub to a sealed 18" sub.  By the way, when pushed, the two sub types will NOT have equivalent frequency response.  And if they do have similar frequency response then you're not pushing them hard, which is actually a good thing.  But this means, especially for sealed, that you have multiple subs or don't belong on this board because you aren't listening to the bass loud enough.

 

Ported Pros:

• Less power required to hit a certain output level
• Uses the port to assist in providing output, which is why you will get more output down to the port tuning frequency

Ported Cons:

• Requires a larger enclosure than equivalent sealed
• More complex design and build
• Need to make sure your port is tuned correctly and of sufficient size to ensure you don't get port noise (chuffing)
• Need to put a high pass filter below port tune to protect the woofer.

Sealed Pros:

• Simpler and more forgiving design and build
• Extends deeper (below 20Hz)
• May sound better when pushed hard (no port noises, only have to worry about keeping within excursion limits)

Sealed Cons:

• Requires more power to deliver the deeper output
• Will require more subwoofers to equal the output of a ported

 

I'm sure there are more for each but I've got to go eat pizza.   This should get you started.

[Brewjitsu]

I know we shouldn't compare cea measurements form companies with the measurements we get here because there is different people, equipment and environmental conditions contributing to those results. But what would be the worse case scenario? If we are comparing more reputable companies like psa, reaction, Rythmik etc with results from here would worse case scenarios be 1 db difference 2 db difference or more?