How can we make transient response, low end excursion utilization, negative port effects, and in room FR better? Since they are all significantly affected by the tuning frequency of the enclosure, the answer is to lower the tune beyond what is typically used. Using a very low tune keeps the responsibility of output away from the port over a larger range of bass frequencies, allowing port output to come into play only when the driver starts showing signs of becoming stressed. A logical goal is to make the driver solely responsible for output down to at least ~30hz, as the frequencies above that are where the bulk of musical bass lies and where accuracy is most critical. Using the one octave estimation, this would ideally put the desired tune at no higher than 15hz. The lower tune means that the 4th order roll off (which occurs below tuning) occurs at a much lower frequency as well, and a high pass filter and its accompanying negative side effects can be avoided.
With very low tuning, the negative effects associated with port use don't get a chance to play a role, and the subwoofer and bass quality in a proper design using a well built enclosure are a direct representation of of what the chosen driver is capable of. The more capable the driver, the better the accuracy will be. The ill effects of the port are pushed down into frequencies so low that they basically become non issues, as those frequencies are mostly tactile as opposed to audible - our sensitivity to those frequencies is extremely low
As you can see, sensitivity decreases exponentially in subwoofer frequencies. If distortion is kept in check, the main issue that will affect frequencies up to an octave higher or more is the transient response. This can be attacked two fold. the lower tune pushes the degradation of transient response down into lower frequencies, where sensitivity is decreased. This can be observed in measurements of a subwoofer using multiple tuning options.
There are obstacles that accompany using a really low tune - nothing comes for free. The port issues can potentially become amplified, as we're now looking at pushing much more air through the same port area - compression and chuffing can become major problems - and since the port actually has to be longer to create that lower tune, the first port resonance can be low enough as to be audible based on the subwoofer's operating range. In addition, sensitivity in the musical range can take a hit, and the driver output doesn't match up well with the port output - this means the FR is no longer flat. How can this be alleviated? Use a larger enclosure.
The larger enclosure will increase the low end sensitivity, allowing you to keep a flatter response into lower frequencies. By fine adjusting the enclosure volume and tune, you can mold an anechoic FR that is flat throughout the bulk of the bass range with a shallow roll off starting at ~25-30hz, extending down to the tuning point, with the 4th order roll off occurring somewhere below that.
The larger enclosure also enables you to use a much larger diameter port - this reduces audible chuffing and output compression limits and essentially eliminates them as a concern altogether, as the limits will often be well below what the larger port allows. Add a rounder to the outer layer of MDF to act as a DIY semi flare, and the audible chuffing limit is a thing of the past. Some have even built their own MDF flares to use on the internal port opening as well.
The increased low end sensitivity (less enclosure spring force) of a larger enclosure means less power is needed to achieve the same output levels at low frequencies than in a smaller enclosure. So this time, we are trading some higher frequency headroom for low frequency extension like before, but we're not getting the disproportionally high rise in THD like a traditional sealed sub due to the port output playing a role. We are also getting much deeper, stronger bass. Some have inferred that a really low tune causes higher THD levels above tuning at equal output levels because more excursion is needed to reach those same levels. This is not an accurate assessment. In the higher frequencies, when port output isn’t coming into play, output is solely a result of air displacement from the driver, just like having the enclosure sealed. Because of this, the same amount of excursion will always result in the same amount of output – all that is changed with larger enclosures and lower tunes is the sensitivity, or how much power is needed to achieve the needed excursion. In addition, since all of the negative effects of traditional ported design have been pushed deep down into lower frequencies, and less power is needed, you gain large advantages in reduced distortion.
To visualize this concept at its most extreme, model two enclosures of the same size using the same driver, one ported and one sealed. Feed them both with the same amount of power and tune the ported sub to ~2hz. From about 7hz on up, they behave identically in just about every performance parameter – FR, group delay, output, and excursion, and the port velocity from the ported design won’t yet have come in to play. What does this mean? This reinforces that when port output is not in play, a ported and sealed sub behave virtually identically. The key is to utilize that port output while minimizing the noise distortions and transient response degradation as much as possible so that they are inaudible,
reaping all of the benefits without suffering the negative side effects - having your cake and eating it too. The LLT design allows you to do just this. The resulting natural FR works better in room than any other alignment in terms of achieving a flat FR naturally, and for the usable extension you are getting, the THD stays lower than sealed at any given output level, as the sealed sub isn’t getting any relief. Unless one is using enough drivers to get them flat with low distortion to 1hz at their desired playback levels while sealed (very inefficient usage), there is always room for improvement with a port.
There has to be a drawback in getting all of these benefits though, and there is, though for some it's not really a drawback. The size of the LLT has to be large, sometimes very large. ~300 effective liters for most 15" drivers is common, and ~650 effective liters for 18" drivers. Not everyone is willing to live with this.
I've been asked what separates a LLT from a standard extended bass shelf design explained
here. A LLT matches that description and also meets the following minimum requirements.
- Tuning <=15hz
- Cylindrical port diameter >= 6” for 15" drivers and >=8" for 18" drivers
- Port opening clearance equal to at least the port diameter in all directions
- First port resonance >=190hz
- No discrete high pass filter
- FR with a ~4db/octave rolloff between the tuning frequency and the lowest room node
- Minimal resonance enclosure (use of cylindrical concrete forms made of paper – generically referred to
as Sonotube - for the enclosure makes this easy)
When finalizing your design, you may often find that adding an additional X liters lets you extend 1hz deeper or gives you 1db more headroom - that may be the case, but keep overall performance balance in mind as well. What does that extra volume do in relation to excursion demands and max headroom from the amp you have chosen? How about port velocity? I recommend going big, but you'll eventually reach a point of diminishing returns, and past that, performance actually takes a hit - keep that in mind.
In terms of power, recent measurements performed by Ilkka and the realization that real world playback does not consist of continuous tones have caused me to reconsider my previous amp limiting guidelines. I used to think that one should aim for an amount of power that straddles the rated xmax of a driver in the given enclosure as seen in simulations without exceeding it. This guideline was faulty for three reasons. 1) The xmech, or mechanical limit of a driver's suspension, is much higher than the xmax, usually 1.5x or more. This means there is freedom for more excursion at the cost of nonlinearities, but that extra cushion of excursion is better put to use than having to clip your amp, as the resulting signal sent to the voice coil can be damaging. 2) Simulations show excursion used with a given amount of power based on a continuous playback of each frequency, like a sine wave. This is not an accurate representation of how the driver will behave with real music or movies, as few if any have extended continous tones. Excursion use will be less than the simulations predict with real material at a given power level. 3) A driver's suspension, even if very loose, will tend to resist movement more as it approaches its limits. This acts as another layer of protection.
Because of this, I now recommend using as much power as is economically feasible, with the only caution being fear of frying your voice coil. If you are constantly pushing your driver to extreme playback levels, you will need to be weary of this happening. It's better to put your money towards a second sub than it is to put it towards an immensely powerful amplifier, as the better amp will only get you so far without risk of damage.
Now go build your LLT
12-30-08, 11:01 PM
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