15Apr99
Chuck wrote:
>Hi folks: I built an 8 cubic foot sub around ACIs SV-12 driver.
Recently
>I encountered a loud popping noise. I asked ACI what they thought (they
confirmed my suspicions
>that I was boosting it too much at 16 Hz, I was hoping it was something else).
If you have any
>other opinions/ideas feel free. I'm more than willing to troubleshoot.
Chuck has his SV-12 in a very large closed box with Q(tc) = .57 This closed
box is a good candidate to be converted to a vented box, especially since he
is having an excursion problem.
Using my WinSpeakerz demo version for the SV-12 we can look at some vented
box conversions. (You can download this SV-12 demo at either the True Audio
web site or the ACI web site)
First, let's note that the 8 cu ft closed box results in a -3 dB corner at
about 32 Hz. (for a half space load)
Try venting the box with F(B) = 14 Hz. The response is now extended to 22
Hz and the excursion is reduced in the range from 12 Hz to about 30 Hz. In
particular the excursion in the 16 Hz range is almost cut in half.
Meanwhile with this reduced excursion the system is producing about 5 dB
more output at 16 Hz. Specifically, the response at 16 Hz is up from -10 dB
to -5 dB. I suspect this will cure Chuck's excursion problem nicely while
giving him further bass extension as a bonus.
How much does it cost. :-(
Not much at all! The physical conversion consists of adding two 3" diameter
(i.d.) tubes each 21.3 inches long. This will give him 14.14 sq inches of
vent area which should be adequate for this system.
In terms of the system response the price Chuck pays is am increase in group
delay below about 30 Hz. At 20 Hz the group delay increases from about 8.5
milliseconds for the closed box to about 17 ms for the vented box. This
does not seem too big a penalty for the increased output capability and
extended bass.
Chuck, If you try this vented box conversion please let us know what
happens. You might also look at tunings as high as 18 Hz for different
design tradeoffs.
Regards,
John
16Apr99
In response to the above Brian wrote:
>Good idea. The only problem I see with this is that those excursion
>problems may be caused by signals ABOVE the tuning frequency of 16 Hz. That
>Chuck is boosting the response at 16 Hz doesn't necessarily mean that the
>box is seeing a 16 Hz signal.
>
>At what point does the power handling response for the sealed and vented box
>variants appreciably diverge (i.e. by 3dB) above the suggested Fb?
Brian has a good point. If Chuck is using an "octave" equalizer then he is
boosting a pretty wide band with that 16 Hz slider. Even so, the vented box
conversion I proposed (F(B) = 14 Hz) will only improve the situation for all
signals above 12 Hz. Simulations show that only below 12 Hz does the
excursion increase over that for the closed box. Fortunately, as Brian
hints, there is little signal content in this low range.
Lest anyone think this case proves the argument "vented is better than
closed" let me remind us of the group delay penalty for the vented box. The
vented box has progressively more group delay or "time smear" below 50Hz
than the closed box. Do I dare to say the vented box bass is just a tad
'slower'! At 20 Hz the vented box has 17 ms delay vs 8.5 ms for the closed
box. But this extra delay would not keep me from making the conversion!
You can always close the vent to "undo" the conversion. Something I
compulsively do with all my vented boxes in the course of initial checkout.
In order to answer Brian's last question about at what frequency the vented
box achieves a 3 dB advantage I increased the input power to the vented box
(using the SV-12 WinSpeakerz demo) from 100 W to 200 W and compared this
vented system to the closed box at 100W input. The excursion curves crossed
at 12.5 Hz and 18 Hz. This means that between 12.5 and 18 Hz the vented
system can handle twice the input power of the closed box. (actually more
at the middle freqs in this range) But we have more happening here than
just the excursion difference. We also have the output difference to
consider. Not only does the vented box handle 3 dB more input power
(mechanical limit only) at 18 Hz but the vented system also has 4 dB more
output for a total difference of 7 dB. Let's ask the question 'over what
range does the vented box have at least a 3 dB output advantage over the
closed box'. The answer to this, considering both increased output and
reduced excursion, is approximately 12 to 35 Hz.
It looks like in THIS PARTICULAR CASE the vented box has an overall 3 dB
advantage below 35 Hz and down to about 12 Hz. Excursion is reduced
significantly starting at 100 Hz. What makes this conversion to a vented
system so compelling is the fact that THIS vented box has an excursion
advantage to such a low frequency (12 Hz). Often vented boxes get into
trouble when the tuning frequency [F(B)] is sufficiently high that the
system will see significant program material below the F(B). This 8 ft
SV-12 subwoofer should work quite nicely as a vented box. Don't just take my
word for it. Anyone who wishes can perform this comparison for themselves
by downloading the SV-12 demo version of WinSpeakerz at the True Audio site.
So Chuck . . . any results yet?? Brian, do you see any negatives to the
vented system that I've missed?
Regards,
John
16Apr99
In response to the above Clark asked:
>Would someone please explain how a 10 msec difference in group delay at 20
>Hz is audible in a room where reverberation time is in the 1000 msec range?
>
>Is there any difference in group delay above 50 Hz?
I didn't and wouldn't claim that the group delay differences between the
closed and vented boxes under discussion are audible. Rather, I suspect
that IF they are audible they lie very close to the threshold of audibility
at most. In fact, the group delay difference, by itself, might well be
inaudible. But there can be little doubt that the frequency response
difference will be audible with program material that includes content down
to 20 Hz.
The difference in group delay between THESE closed and vented systems at 50
Hz is about 0.3 ms (2.6 Vs 2.9 ms) and decreases rapidly at higher
frequencies. The little bit of data available on group delay audibility
thresholds suggests a threshold of audibility of 3.2 ms at 500 Hz and
increasing about half again for every octave of falling frequency.
Likewise, the threshold drops at higher frequencies. A quick finger count
extrapolation gives: about 6.4 ms at 200 Hz, about 12.8ms at 100 Hz, about
25ms at 50 Hz and probably something like 50 ms at 20 Hz. Now this is
extrapolating quite a bit from the published data (Blauert & Laws, JASA, May
1978) but it should not be in gross error. This would suggest that both the
vented (17 ms) and closed (8.5 ms) system group delays at 20 Hz are well
below my 'guestimated' audibility threshold of 50 ms.
Now 1000 ms (1 second) of broadband delay would be clearly audible as a
distinct echo. How that much delay in just the bass range would probably be
somewhat less obvious but almost certainly still plainly audible. As a
reference we might note that 30 ms or so is enough delay to begin affecting
musicians performance. I kind of use 30 ms as a rule-of-thumb for the
threshold of delay audibility. Certainly I would not want any more delay
than that from my loudspeakers. But is 17 ms at 20 Hz audible? I don't
know. When Chuck does the "undo" test by plugging the vent he will hear
the
delay change from 8.5 ms to 17 ms (at 20 Hz) but he will also hear the
frequency response change. I'd say the freq response shift will be the more
audible effect by a considerable margin. But the transient response will
definitely be slightly affected by the delay.
Chuck . . . anything to report? no pressure. . . no pressure :-)
Regards,
John
20Apr99
In response to the above Greg wrote:
>In a previous thread on 'fast bass' the consensus was that group delay was
>one of the culprits of why a vented design tended to be inferior sonically
>to a sealed design. If that's true, then some portion of whatever the time
>frame is for LF group delay of a vented design is audible down to the limits
>of our hearing.
>
>Some old data I've seen for time aligning horns shows audible delay of 1ms
>at 2khz, dropping to 4ms at 250hz, or 1ms / octave. This falls in line with
>your ~3ms at 500hz. So we get 8ms at 15.6hz, assuming our ears don't exhibit
>a much steeper rolloff in the lowest frequencies as you suggest. If it does,
>then I don't see how group delay can be the major contributor to the reason
>why we prefer sealed to vented, or why a vented design sounds more like a
>sealed when tuned to below our audible threshold.
I definitely agree that group delay is probably a factor in audible
differences between closed and vented boxes but I can't agree with the
suggestion that 'vented systems sound inferior to closed systems'. It
really just depends on the SPECIFIC systems under comparison. Given he
freedom to compare any two systems it would be easy to give examples of
excellent vented boxes that are superior to other poorly designed closed
boxes but it wouldn't prove that vented is superior to closed in general. It
would be just a specific example of one system vs. another.
Most often the debates of closed vs. vented are based on the same driver (as
in Chuck's 8 cu ft system). Even here I am reluctant to say closed is always
better. As we've seen in the earlier discussion Chuck has had an audible
excursion problem under 'large signal' conditions. The conversion of his
closed box system to a 4th order vented system appears to offer both
increased SPL and reduced excursion. This could well make the difference in
whether Chuck experiences bottoming when he repeats his 'large signal' test.
If so, this comparison boils down to the vented system being able to play
louder and with more extended bass than the closed box system. The "price"
is the degraded group delay response for the vented system compared to the
closed box. The essential question remains: is this small group delay
difference audible? For me, the honest answer is "I don't know".
I've done
many listening comparisons of a given enclosure switched between closed and
vented modes. I always hear a significant difference but I realize that
there are two components of that difference: frequency response and delay
response. I'm of the opinion that the frequency response difference is the
primary audible difference for these comparisons. I have also done many
listening test where the only change was the introduction of an all-pass
filter. In other words, where the only difference was the introduction of a
specific amount of group delay without and frequency response difference. I
have never heard a difference in this test using full range source material
played over loudspeakers. I don't suggest this is proof of anything, but it
gives me the basis for the opinion that small amounts of group delay are
inaudible.
We need a better yardstick by which to measure delay effects in a more
consistent fashion. Over the years the audio community has attempted to
interpret time delay effects in terms of:
(1) phase response
This never seems to lead to useful interpretations of the data except for
the case where signals are summed. I have heard many professional audio
engineers debate the significance of phase response to no conclusion.
Manufacturers only seem to use phase response data to create a smoke storm
of disinformation in the marketplace.
(2) group delay response
More recently the audio community has moved the discussion of delay effects
from the domain of phase response to that of group delay response,
especially as it concerns speakers. I think this is a good thing and that
it takes us closer to an understanding of delay effects. Still though, we
can't look at a speaker's group delay response and see where we have an
audible effect as we can easily do with a frequency response. But it is
much more meaningful than looking at a phase response plot.
After reviewing the "Blauert and Laws" data again I decided to look at the
data in terms of the period, T, of the test frequency waveform. Here is
what I saw:
Frequency (Hz) Threshold (ms)
Threshold in T (periods of the
test frequency)
8k Hz
2 ms
16 T
4k Hz
1.5 ms
6 T
2k Hz
1 ms
2 T
1k Hz
2 ms
2 T
500 Hz
3.2 ms
1.6 T
(I hope my text chart comes through OK)
Each threshold was converted to a "normalized" threshold by dividing the
threshold by the period of the test signal. For example, at 1k Hz the
period of the test signal is 1ms (1/1000). The B&L threshold of 2 ms
divided by 1 ms equals 2 periods or 2 cycles.
It is interesting to note the fairly consistent thresholds in terms of T at
the three lower data points. The Blauert and Laws data seem to suggest a
somewhat consistent threshold for delay audibility of about 2 cycles or less
at any frequency between about 500 and 2k Hz.
I think that normalizing the delay data in terms of the waveform period
(i.e. cycles) may be a better way to view delay effects that makes
audibility more apparent than either phase response or group delay response.
I think that if we were to compare this "Period Normalized Group Delay" for
the closed and vented speakers under discussion we would be better able to
"see" the delay difference between these systems in a more meaningful way.
Here are some significant points for these two systems:
Frequency Closed Box
Delay (T) Vented Box Delay (T)
100
.070 cycles
.074 cycles
50
.135 cycles
.149 cycles
20
.176 cycles
.338 cycles
If, in fact the audible threshold is something like 2 cycles as the Blauert
and Laws data suggest then the delay of both these systems would seem to be
below audibility.
But what if our thresholds are less than 2 cycles and more like 1 cycle or
1/2 cycle. Then the vented system nears the threshold of audibility . . .
but only at 20 Hz or below. At 50 or 100 Hz both systems would appear to be
safely 'out of the woods' so to speak.
It would be interesting to design an all-pass filter that reproduces the
group delay of each of these systems and then perform listening tests with
the most difficult program material (clicks and pops) to see if just the
group delay were audible.
Well, let's bring this little rambling discussion to a close. What have I
proven? Nothing. But I hope I've given us some more fodder to chew in this
closed vs. vented discussion. I do think that "cycles of delay" is
probably
a better view of delay effects than either phase or group delay. Once we
have some confidence in where these delay thresholds are located we can
design our speakers with ever more confidence in their sonic transparency.
Regards,
John
/////////////////////////////////////
John L. Murphy
Physicist/Audio Engineer
True Audio
https://www.trueaudio.com
Check out my new book "Introduction to Loudspeaker Design" at Amazon.com
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