'Fast' bass drivers and how to find them - diyAudio
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Old 28th June 2003, 02:48 PM   #1
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Default 'Fast' bass drivers and how to find them

We hear a lot about 'fast bass' and 'floppy bass'. It sounds odd to me that sometimes the 'floppy bass' comment is about speakers with good specs. So can we not see the specs and determine if the bass will be 'fast ' or not?
Reflex enclosures (ducted) may be a bit more difficult to analyse with respect to 'fast' bass.
Sealed enclosures should be easier to check out.So a driver with good mechanical and electrical damping should be able to produce 'fast bass' in a suitable enclosure (say Qtc = 0.5 to 0.6 ).
The Q after all is a measure of the transient response of the driver which in turn tells us how the bass will be.
So can we say that a driver with say :
Fs = 28Hz
Qms=1.69
Qes=0.41
(Qts=0.33)
In a suitable box we could get a Qtc of 0.6 and so should have 'fast bass'.
So I would conclude that this driver must be improperly used if it produces 'floppy bass' in a sealed enclosure.Reflex enclosures are much harder to work with.

The other comment that I would like to make is that it would be good to get a bass driver with low Qms because that is the inherent mechanical damping of the driver itself . This means that this driver ( also with an overall low Qts!)compared to a driver with say a Qms of 3.6 would sound 'faster' even if the external connecting wire and output impedance of the amp were high at say 1 ohm ! The reason being that it (Qms) contributes more to the overall Qts/Qtc and so is not affected as much by the external contributors to the overall Q of the system. The external factors generally increasing Q and making it more 'floppy'.

I base this view on the very tight bass that I have heard from subs with very low Qms ( in a suitable sealed box of course). The electronics was solid state and quite dated.
Any comments?
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Old 28th June 2003, 05:14 PM   #2
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I think it has a lot to do with enclosures... Passive radiator enclosures sound really smeared to me. Right now I'm listening to a pair of ported studio monitors that respond down to 45hz (not terribly low, but with room gain, not bad) and when I want faster or tighter bass, I just plug the ports with socks It really works. It also cuts off the LF response so if I want to use a subwoofer and make my total SPL go up, I can.

I've heard really good things about dipole subs, and I'm about to build one (today or tomorrow, I think) so I'll let you know what results I get.
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Old 28th June 2003, 05:18 PM   #3
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I too agree that it has to do mostly with the enclosure.

I was just about to say that dipoles seem to have very "fast" bass, some may think this because they roll off at quite a high frequency. But if you use equilization to keep them flat 'til the really low frequencies, you'll see just how "fast" and "tight" dipole bass can be.
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Old 28th June 2003, 06:43 PM   #4
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OK guys. Some terminology. Fast is not an appropriate term for trying to describe what you are trying to describe (use that term over on the basslist and expect to be buried under a firestorm of responses). Fast is really a frequency thing. HF can be fast, low frequencies not. For a bass driver to be "fast" it has to be very extended range, and then it still is not the bass that is fast.

What you are trying to describe is an ability to accelerate (ie control), and a lack of group delay.

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Old 28th June 2003, 10:29 PM   #5
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Yes acceleration. Time domain. That's it. A fast woofer is a tweeter

But how to determine this? The usual meassurements do not reveal acceleration behavior. Would be a reversed waterfall show what we want to know? Are low Rms or high qms an indicator for it (a "fast" woofer) ?
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Old 29th June 2003, 08:58 AM   #6
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I think fast bass from a subwoofer depends entirely on how carefully you match the midbass. Ever heard a system with "fast" bass? I'll bet if you ask the owner to turn off everything but the sub, it won't sound fast anymore.
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Old 29th June 2003, 09:33 AM   #7
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Default Transient attack

I think we all agree that it is partly the HF element of the bass note that contributes to the bass sounding 'tight' or 'fast'.
So the part of the system that reproduces the HF part of the bass note must do it accurately and aslo be accurate in 'time'.
That will be done by the rest of the system - mids and tweeter.

Even if that is OK the bass driver will still have to be very good at 'starting' and stopping'. That I guess will be the mechanical equivalent of 'slew rate'. This factor I have never seen measured (as "mm/second"). The bass driver will have to slew only as fast as the fundamental note in the bass ( say 40Hz) . But the bass note ( of say the drum ) has a very sharp rise time and is faster than the slew rate for the 40Hz signal. That will be the HF part of the bass note and it will be filtered out ( in the crossover) and fed to the rest of the system. This must add with the fundamental note eventually without any time delay. Only then will the bass note have the attack of the original signal.

So 'tight' bass needs a very competently designed bass AND HF section with great attention to time delays in each section. This is also why simple crossovers (6db/octave) used with good drivers have very good transient attack in the bass.

So after getting good drivers it will be quite difficult to make a 'good' crossover to ensure that you do not loose out on the capabilities of the speaker - due to time smear . You could get a flat frequency response over the band but will it stand up to a coherent transient signal ? That will determine the 'tight' bass and other finer details of the image.
Suddenly the crossover looks far more difficult that it appeared to be ?
Cheers.
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Old 29th June 2003, 10:08 AM   #8
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Want some "fast" bass - lol

make a line source of smaller drivers... obviously must be capable of some low end extension.
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Old 30th June 2003, 04:29 AM   #9
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I don't know the reason for this, but I am experiencing very clean "fast" bass with a very old Electro-Voice 12" inch fullrange driver (LS-12) in an open baffle. When played without the subs, it is "faster" sounding then any box speaker or multi-speaker I've ever owned above 60 Hz (and I've owned a bunch). I have four bass drivers on each baffle surrounding the EV's (two 12" and two 10") that, when turned on, extend the bass below about 80 Hz without sounding a bit slower. The EV's specs aren't that impressive, and the bass drivers are just old mis-matched units I had laying around. The bass drivers are boosted quite a bit. The bass drivers that I have used in regular boxes didn't sound this "fast". When put in the OB and coupled with the EV fullranger, they become different speakers altogether. Part of it may be the EV with no crossover, but I suspect that much of it is due to the open baffle. It's an example of the sum being greater then the parts.
I'd suggest everyone should experiment with open baffle bass sometime (not to mention the great mids that OB has).
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Old 30th June 2003, 09:13 AM   #10
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Default Re: Transient attack

Quote:
Originally posted by ashok
I think we all agree that it is partly the HF element of the bass note that contributes to the bass sounding 'tight' or 'fast'.
So the part of the system that reproduces the HF part of the bass note must do it accurately and aslo be accurate in 'time'.
That will be done by the rest of the system - mids and tweeter.

Even if that is OK the bass driver will still have to be very good at 'starting' and stopping'. That I guess will be the mechanical equivalent of 'slew rate'. This factor I have never seen measured (as "mm/second&quot. The bass driver will have to slew only as fast as the fundamental note in the bass ( say 40Hz) . But the bass note ( of say the drum ) has a very sharp rise time and is faster than the slew rate for the 40Hz signal. That will be the HF part of the bass note and it will be filtered out ( in the crossover) and fed to the rest of the system. This must add with the fundamental note eventually without any time delay. Only then will the bass note have the attack of the original signal.

So 'tight' bass needs a very competently designed bass AND HF section with great attention to time delays in each section. This is also why simple crossovers (6db/octave) used with good drivers have very good transient attack in the bass.

So after getting good drivers it will be quite difficult to make a 'good' crossover to ensure that you do not loose out on the capabilities of the speaker - due to time smear . You could get a flat frequency response over the band but will it stand up to a coherent transient signal ? That will determine the 'tight' bass and other finer details of the image.
Suddenly the crossover looks far more difficult that it appeared to be ?
Cheers.
We're still not there yet. A fast risetime 40 Hz note will be divided by the xover network: the fundamental (plus maybe 2nd, part of 3rd harmonic) will be routed to the bass driver, the attack/release edge will be routed to the mid/tweeter.

If you simulate a 40Hz burst through a say 1kHz lowpass, you will see that the sharp edges at the start and stop points are rounded: there is no need for the bass driver to start/stop quickly, because that part of the signal is not getting to it. That is what the xover is for.

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