Vented vs TH Sound Quality
Many of you have listened to both vented boxes and tapped horns. Would you care to list the reasons you like the sound of one versus the other? And would you speculate on why this is?
Here's the reason I bring this up:
I've built subs for about 20 years. Over that span, the one sub that I'm really happy with is my TH-Mini 'clone.' I've built a LOT of bandpass boxes, but I've never managed to build one that didn't sound sluggish, at least to a degree. Probably the most successful bandpass box I've built was one that was part of a multi-sub Geddes type setup, where the BP boxes deep extension complemented the Mini's lack thereof.
But by itself, I've never built a bandpass that was satisfying.
I've built a handful of vented subs. Not entirely sure why I've shied away from them, perhaps they're too simple? Can't say I was completely unhappy with them, but horn subs seem more 'fun' for DIY.
I've built a few sealed subs, but shied away from them because the Mini's output seems 'effortless' compared to the sealed subs. Perhaps lower distortion?
Now the obvious solution would be to stick with tapped horns. But my main gripe with those is SIZE. The Geddes multi-sub approach really sounds great, but I can't afford the space for three huge tapped horns. So I'm considering a vented box instead.
I've studied all the literature exhaustively, and I'm really struggling to understand why the tapped horns sound good. Here's a list of ideas I've had; would be curious if some of you who've built both types would chime in.
1) Personally, I prefer the sound of my TH-Mini to the sound of *all* the bandpass boxes I've built. Every time I've tried to integrate a bandpass box with one of my systems, it's felt like the bass is 'out of sync' with everything else.
Theoretically, a horn should have superior phase response to a vent, because the output of the vent is 180 degrees out of phase, while the output of a horn is just 90.
Unfortunately, I don't this this tells the whole story. It's not as simple as "horns are superior to ports."
For instance, in the measurements of the Rhythmik vented box and the Danley DTS-10 tapped horn, we see that the tapped horn has huge spikes in the group delay curve at 55hz and 102hz.
Here are some ideas I'm kicking around, on why tapped horns sound so good. These are only hypotheses, and I'd love to get some opinions on what you think of YOUR tapped horns.
Idea 1 : Tapped Horns sound good because the phase distortions are harmonic.
If you would, take a moment and think about a drum beat. The beat will have a fundamental at a specific frequency, along with harmonics. For instance, a 40hz drum beat will have harmonics at 80hz, 120hz, 160hz, etc.
My 'hunch' is that tapped horns may sound good because their problems are harmonic. For instance, the Danley DTS-10 has a big spike in the group delay curve at 55hz, but it's mirrored by a spike about one octave higher. So if there's a drum 'whack' at 55hz, it's harmonics will *also* be delayed.
This is different than the vented or bandpass boxes, where the phase shift is centered on one frequency. For instance, in a bandpass box tuned to 40hz the group delay at 40hz will be *significantly* higher than the group delay at 80hz. Perhaps this explains why bp boxes sound 'slow' to me?
Also, note that the amount of harmonic correlation between the peaks is related to the asymmetry of the woofers distance from the throat and the mouth. In other words, if the woofer in your tapped horn is very far from the mouth, but very close to the throat, the peaks will be *less* harmonic. And if you're going to have these distortion, it's probably better that they're harmonic rather than NOT harmonic. (IE, it's better to have a peak in the group delay at 40 and 80hz than at 33hz and 80hz, because music itself is harmonic. Unless you don't listen to music on your subs :) )
Idea 2 : Tapped Horns sound good because they have low harmonic distortion.
I'd love it if it were this simple. That tapped horns sound good because they have low distortion. I'm not 100% convinced that this is why they sound good however.
For instance, I once built a tapped horn with multiple woofers, then converted the same sub to push-pull. The push pull sub sounded cleaner. But the problems that I hear with bandpass subs don't have to do with them sounding grungey; it has to do with them sounding sluggish.
(Note that the distortion that I reference in idea 1 is distortion in the group delay curve; the distortion that I reference in idea 2 is the type of distortion we're most accustomed to discussing, which is distortion in the frequency response curve of the sub.
Idea 3 - I've just been using crummy ports
This is an idea I'm really excited about. The idea that my bandpass boxes sounded bad not because of a fundamental problem with the box type, but due to the use of crummy ports.
There's a JBL paper which gets into the details on this. It's a very good read. We find that:
1) As you increase the power on a ported speaker the port frequency changes. Which means that the frequency response of ported speaker changes with volume. And this affects *all* speakers with ports, including the bandpass types.
2) I was surprised to see that ports generate harmonic distortion. I didn't know this, in my mind I imagined that the port output was basically the frequency it's tuned to, and that's it.
3) Possibly the most frustrating aspect of the paper is that there isn't one 'perfect' port design; you really have to optimize the port for the application. (IE, a high power port behaves differently than a low-distortion port.)
4) I was really surprised to see how little power it took to make the port start misbehaving. Particularly since I routinely used 500-1000 watts in my car subs. If I'm reading the literature right, a ported box with a port that's too small basically works WORSE than a sealed box at high power.
5) The bigger the port, the better. And flares generally help.
6) One of the odder findings was that you could reduce distortion by baffling the port *inside* the box. I've never seen this done. It might explain why some prefer the sound of slot ported subs.
Long story short - perhaps tapped horns sound good because they're a lot like a vented box where the port is so big, the woofer is literally sitting in the port.
Here's some info from Geddes on bandpass subs:
Bandpass subwoofers and Geddes' design
"The response given above was basically correct, a bandpass does put a resonance chamber on the front of a closed box loudspeaker and this does increase the efficiency. It is a fairly narrow band design, which is fine for a sub, but I don't give any importance to group delay at LFs in small rooms.
They are hard to get working correctly, I will agree with that, but I have had a lot of experience with them so I can seem to make them work.
They are "passive" in that the amps are external. I have not found plate amps to be reliable and they are too limited in the controls. I recommend a DCX2496 and some rack amps for the LF subs.
I also don't worry too much about "frequency response" of a sub because the room changes it completely anyways. How the subs work in the room at the choosen location is what matters and this is far more determined by the room and placement than by any free field measurement of the subs. In the end the sub gets EQ'd all over the place anyways. What IS required is a great deal of linear output capability and bandpass does this when a robust driver is used. I measure this as a near field measurement of the output, but not anything free field.
I used multiple ports because there was no place to put one. Large areas require long lengths, but a small area has too much turbulence, noise and distortion. So I needed to devide the poart into smaller pieces that would fit.
The sub is about 18 x 16 x 14 inches and weighs about 45 lbs.
Phase, Time and Distortion in Loudspeakers
Some good info on group delay from a great site.
"Group delay refers to the delay experienced by one group of frequencies with respect to another. All filters, including loudspeaker enclosures, introduce group delay in the audio signal. To gain a basic understanding, imagine a system where the treble is delayed by (say) 30 seconds after the midrange. That this would be very audible and highly disconcerting is obvious. That is the essence of group delay, and fortunately no audio product will be as bad as the example.
It would be very nice to know the threshold of audibility of group delay with respect to frequency, but this remains an area where not a great deal seems to have been done. No extensive data is available and so far, the best table is from Blauert and Laws ...
500Hz 3.2 ms
1kHz 2 ms
2kHz 1 ms
4kHz 1.5 ms
8kHz 2 ms
Table 5 - Group Delay Audibility Thresholds
Given that the minimum audible group delay is claimed to be 1ms at 2kHz, that amounts to a physical driver displacement of 345mm - assuming the velocity of sound to be 345m/s (22°C at sea level). No (sensible) speaker system will ever have that much delay, so for the most part group delay should not cause any audible problems.
One area that is of some concern is bass. The table doesn't show anything below 500Hz, but comments about "slow bass" can be found all over the Net and in magazines etc. It seems probable that some bass alignments do indeed exceed the threshold of audibility, and this would account for the comments. Bandpass enclosures in particular seem to suffer from the slow bass syndrome, with people commenting that the bass is "a day late" . I think we can safely assume that this is a slight exaggeration, but these enclosures do seem to exhibit characteristics that would explain the idea of slow bass. Since bass in isolation cannot be fast, the only answer is that it is delayed compared to the rest of the system.
It is not unusual for even a vented box to have a group delay of perhaps 20-30ms, and while a tad shy of a day, it's still quite a long time in audio reproduction. By comparison, a 24dB/octave Linkwitz Riley crossover network has a group delay of 480us (see table).
I wanted to throw in a piece of information that I haven't seen discussed in the debate about the audibility of group delay.
What if we discussed the delay in terms of music?
Here's what I mean by this. Let's say you are listening to a band that's playing a drum beat at a rate of 120 beats per minute. I would argue that nearly anyone could perceive if the beat varied by ten percent. IE, if the beat went from 120 beats per minute to 132 beats per minute, I'll bet you could perceive it. In fact, I wouldn't be surprised if you could perceive a difference of five beats per minute.
So how does that work out in milliseconds?
At 120 beats per minute, that is one beat every 8.3333 milliseconds. If you're listening to that band and you can perceive them shift from 120 to 132 beats per minute, than you're perceiving a change in beats from one beat every 8.3333 milliseconds to one beat every 7.5758 milliseconds.
That works out to a difference of just 0.758 milliseconds.(!!!)
That's quite short.
Can we perceive a difference of 0.758 milliseconds? I'd argue that if you can hear a band shift from 120 to 132 beats per minute, then the answer is YES
Of course, the big "X Factor" here is harmonics. For instance, I've noticed that the harmonics of bass notes are far more apparent over headphones, where we're removing a huge number of variables out of this equation. (IE headphones eliminate the room, eliminate a crossover point, eliminate phase shift at xover, eliminate reflections, eliminates diffraction.)
An adequate sized port will not change Fb enough to notice over the linear range of a speaker.
Tapped horns can have more output for a given excursion than BR, but also have more distortion for a given excursion.
Distortion sounds louder, and louder LF is becoming more and more usual, a 10-20 dB LF "haystack" below 100 Hz is not unusual any more.
You have seen baffling the port inside the box before in post #7 of this thread, and drew it yourself in #9:
Looking at the phase response of the Keystone TH using a BC18SW115-4 and a BR cabinet using the same speaker tuned to the same FB, we see they are 30 degrees apart at 125 Hz, but at 40 Hz, the BR is 180 degrees "behind" (210-30=180).
Assuming both are time aligned at a crossover point of 125 Hz, that lag of 1000 ms /40Hz =25 ms per cycle, one half cycle (180 degrees) is 12.5 ms, one beat at 80 B.P.M.
So the 40 Hz BR LF response is more than a beat slow compared to the TH LF at 120 BPM (8.33ms).
Since bandpass cabinets typically have around 180 degree phase shift near the upper bandpass, usually very close to the intended crossover frequency, the lag is even worse, and alignment is more of a compromise.
They "seem to work", but don't sound coherent.
All of what you describe is why I stick with low Q sealed. Effortless, clean, small, no port problems. It is the quality of the driver I find most important.
You've pretty much summed it up right here.
Also, non-horn designs don't require electronic delay to time align with your main speakers.
Perhaps you have never heard a good and properly sized bass reflex sub? They are lots of bad ones around....
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