Charlie,
From Data Bass "notes":
"The long term output compression sweeps start at 2.31 volts input measured at a 2m distance. This system showed excellent linearity and freedom from compression through the next five 5dB increases in the signal..... At this point the system is producing near 110dB at 20Hz, just under 120dB at 40Hz and hitting 130dB towards the end of the measurement."
One can add 6 dB to the two meter figures for an equivalent one meter SPL level.
Art
Thanks for pointing that out, Art. Absolutely correct.
Still, what a waste. The driver (one of them) has a 99dB@1m,1W sensitivity, but in this cabinet that drops to about 20dB, or to 79dB/W! This is why I find most of the B&C drivers useless as subwoofers. Qts is often 0.2 or so, which is way too low and produces the drooping frequency response that makes them very inefficient transducers at very low frequencies. This driver (the 21iPAL) were really designed for high SPL above 40Hz, not for true subwoofer use down to 20Hz.
Also, practically speaking, the low impedance is not helpful. Many amps (apart from car audio) are struggling to output full power at 20Hz into 4 Ohms, never mind 2 Ohms.
A couple of these:
Faital Pro 18XL1800 18" High Power Subwoofer Speakers. Faital Pro 18XL1800 Waterproof Speakers. Faital Pro 18XL1800 18" for ported bass reflex subwoofer speaker cabinets. Lightweight neodymium magnet - Faital Pro 18XL1800 3600 watt high power woofer
might be a better option for low bass, down to the low 20s, in a large sealed or vented box.
Well, time will tell..
Sorry- I wasn't talking about the bass, I was making a joke that was a BIT of a stretch- you're in Hungary, where housing is relatively affordable relative to many other places, so you can afford a nice large basement, but then.... you'll be... (in) Hungary..... hence unsatiated.
Pretty bad stuff, but you miss 100% of the shots you don't take
Martin have a box with -3dB = 30Hz using the 21" IPAL but its some kind of weird horn bass reflex hybrid. The specs are not complete enough to see what kind of LF output its capable of:
http://www.martin-audio-japan.com/userguides/GUIDES/MANUALS/ASXugPre.pdf
far too expensive for most though.
Keep in mind that the cab housing both 21's was a 24" cube (61cm). Each driver was seeing about 80L or less. This is a miniscule space for a 21" driver. The output headroom available from this 24" cube at any bass frequency is tremendous as is the power handling and distortion performance. It's not a waste if you need maximum performance from minimum space.
No driver or combination of drivers are going to have a significantly higher voltage sensitivity at 20Hz in a sealed cab of the same air volume. You will never get even 90dB@20Hz (1W equivalent) voltage sensitivity out of a 160L sealed cab. This is HIL in effect.
Sensitivity is not efficiency. Raw response shape is not efficiency either. Back in the day signal processing involved analog circuits and was much more costly and difficult to implement. A flat response shape was needed above all else right out of the box. Things have changed greatly especially in the bass range since most subs end up being active / processed systems. I don't worry so much about raw voltage response, especially with sealed subs. F3 is literally one of the least relevant data points for me when designing sealed subs.
It is incorrect to say that low Qts drivers are low efficiency at low frequencies. A lot of people make this mistake. It goes back to the old school of thought that response shape and sensitivity are indicative of efficiency or performance at low frequencies. They are not. Companies like B&C, Powersoft, Faital, Beyma, 18Sound, Lavoce, etc...Have huge R&D budgets, brilliant engineers and state of the art development and testing systems. There are good reasons that many modern professional bass drivers are trending towards very high efficiency / low Qts.
You are right to say that was a very small cab for those drivers, or any subwoofer that is supposed to have high output at low frequency. HIL is exactly it. That's why I suggested a large cabinet with two 18s.
I disagree with you on all of this part of your post, above. Whether you are looking at sensitivity or efficiency you absolutely cannot just look at the passband value. If your goal is a high output true subwoofer, then output at 20Hz is more important than passband efficiency. You cannot just DSP your way out of it and throw insane power at the driver and expect great things to happen. I think that is somewhat naive. And this is coming from a proponent of DSP in loudspeakers.
The iPAL driver is an excellent example of what I am saying above - the 20Hz value is 20dB less than the passband value. The driver is not intended to be a deep sealed sub, rather it's for bass down to 40Hz in a vented box. I suggest that it is better to find a more suitable driver having a lower passband efficiency but with a lower Fs and higher Qts and implement it such that the 20Hz SPL with the same input power was greater. Maximum SPL is one thing, and the iPAL has a lot of displacement capability, but who needs over 135dB at 100Hz??? You need SPL at 20Hz if you are really making an over the top sub.
This driver is inefficient at 20Hz, period. You can help that a bit by using a CB with a larger volume per driver, but the response will still droop badly. With lots of power and DSP correction you can flatten out the response curve but you are not making the driver any more efficient at 20Hz by doing that.
Well, I discovered the same thing using software simulation, you can get more output in a smaller package with high efficiency, low qts drivers. Some people are really thinking old school here, with DSP we don't need flat frequency response, just low distortion, high output capability and displacement. LaVoce and B&C 18 inch ferrite drivers can give me much more of that than something like Scanspeak and Peerless 12 inch drivers, while costing the same, 250€. I can have much greater displacement and much lower distortion with 21 inch B&C DS drivers for the same price of a SB Acoustics 15 inch driver, all for 400€. Opening my budget even more. for 600€ I can get that 19mm mechanical excursion 18 inch BMS driver which is much better than the 13 inch Scanspeak driver.
The high BL woofer always wins in output for per actual watts dissipated for a given displacement and box volume.
AES E-Library >> Direct-Radiator Loudspeaker Systems with High Bl
The papers go into great detail about the advantages.
That kind of argument does not have a good smell.
Manufacturers have their own imperatives that may not correspond to what you want. Aren't those low Qs coming about by having massive VCs to handle massive signals and consequently cone assemblies that weigh more than half a pound?
I've posted the Lotus versus Corvette analogy too many times to repeat here in detail. The Lotus designers use a "virtuous circle" of reducing vehicle weight iteratively to achieve a car with great handling.
I think there is a false equivalence of (1) small sealed boxes with giant EQ and (2) big boxes with small EQ. While you can coerce a small box speaker to make sine-like motions at 20 Hz and have a swell FR using lots of EQ, are there penalties in distortion, transient behaviour, linearity of degenerative feedback from enclosed air, etc?
B.
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The answer to your final question is a 'yes' for the distortion penalty part.
The distortion that is created when a driver is used in a "too small" box is due to the different pressures on either side of the cone during excursion and can be calculated. Use the "closed-box1.xls" spreadsheet on this page:
SPLmax
Replace the bold entries under "driver data" and "box data" only, the rest of the cells will re-calculate accordingly.
For distortion look at the cells beneath cell U14, below the heading "Box air spring distortion at Xmax". This is ONLY the contribution of air non-linearity to distortion and assumes an otherwise distortionless driver. Real world distortion will be higher.
The spreadsheet also lets you calculate SPL vs frequency given a particular amplifier's max voltage and current capability. Results are very informative, one way or the other. It's a great tool for the DIYer.
http://www.extra.research.philips.com/hera/people/aarts/RMA_papers/aar03c4.pdf
Direct link to the paper.
For those complaining about equalisation you can also just add mass to the cone as is shown at the end of the paper, at the expense of higher frequency efficiency. The paper also shows this type of high BL driver is a poor choice for vented systems. I can also state that from playing around in simulations this type of driver performs poorly in 4th order band pass systems. If you look on data-bass you will see that this type of driver does perform well in tapped horns if more efficient but larger speakers are desirable.
Direct link to the paper.
For those complaining about equalisation you can also just add mass to the cone as is shown at the end of the paper, at the expense of higher frequency efficiency. The paper also shows this type of high BL driver is a poor choice for vented systems. I can also state that from playing around in simulations this type of driver performs poorly in 4th order band pass systems. If you look on data-bass you will see that this type of driver does perform well in tapped horns if more efficient but larger speakers are desirable.
You see, the bass or subbass does not need a closed box, since such speakers give out a deep, drawn-out bass and not a panch midbass of a closed box. In addition, the coil for the speakers is very hot at work, so it can fail when the box is closed, so they are almost all used as a ported box for air circulation and in a closed box there is no air circulation but only compressing and unclenching it as if we were squeezing a closed water bottle with the hand and when the lid is open the air can pass back and forth cooling the coil so that not from a good life pro speakers are made like ported.
A good home speaker for a closed box:
Aurum Cantus AC300/75C2C 12" Woofer
and ready acoustics from the manufacturer with the same 12" although there is also ported:
Aurum cantus
someone also wanted to make acoustics with this speaker in a closed box:
New to DIY, 1st Project & Need help with Speaker Selection
I will use an example from that paper to show the point I am trying to make about low-Q (high BL) drivers, and how they are inefficient in the bass region. You can read the paper for the full details. I have attached Figure 2. What I write below is only for that case that such a high-BL, low Qts driver is used in a closed box as a subwoofer (that's the subject of this thread).
Figure 2 shows the response from two drivers, both in a 25 liter box: one has a "normal" 8 N/m BL value (thick line) and the other has a very (in my opinion) high BL value of 40 N/m (thin line). The high BL driver has a stronger motor, and this causes the Qts value to be much lower. This causes a "premature" rolloff at 6dB per octave at the low end of the passband, and this finally becomes the ultimate 12dB/octave rolloff at much lower frequencies.
To me Figure 2 shows what I usually find with low Qts drivers: their efficiency is LESS than a higher Qts driver in the bass range in a sealed box. You can see this in the range of about 10Hz to 400Hz in the example, where the thin line is lower than the thick one. This is exactly the range (10-100Hz at least) for low bass. I really do not see why this is "better"!
You will need MORE POWER for the same SPL and you will have to apply equalization. Why not use a more appropriate driver in the first place?
The next argument seems to be that "you can put the driver in a much smaller box" and then EQ it to be the same as the "normal BL" driver. They show an example where the box size was reduced from 25 liters to ONE LITER! Sure you can, but as I pointed out in my recent post, when you do that you INCREASE DISTORTION and this is especially true for a subwoofer where the cone is making large excursions and the box is "small" compared to the total swept volume (Sd*Xmax). If your goal is to move the most hair by playing your subwoofer through an open car window, then OK this might be what you are looking for. If you are not after that crappy "blatt-blatt" hyper-distorted car-audio bass sound, then you might want to consider distortion in your approach. These authors did not seem to take note of it, or any other repercussion of putting a driver in a very small box. Perhaps sound quality was not of concern to them?
Now let's think about where these high-BL drivers might be useful - it will be at higher frequencies where you can indeed put the driver in a small box and make use of the high BL and high passband efficiency. At 200-300Hz and above, these drivers, in a small(er) box, can really be super efficient! But not when they are deployed as subwoofer, in the lowest frequencies. This application is really best when used for midbass and higher frequencies, or when the driver is large perhaps above 100Hz.
Finally, why the authors bothered to include section 7 showing that very low-Qts drivers are unsuitable in a vented box is beyond me. That is common knowledge. Such a driver will always result in a peaking response around the box tuning, and that will ring badly. That has been known since Thiele's work in the 1950s.
Attachments
I think we need some new terminology to unwrap "sensitivity", "efficiency", "loudness" and so on*. CharlieLaub (who has posted some very worthwhile contributions here) shows that sometimes you can get more loudness with a weaker motor. Something doesn't sound right there.
Maybe I'm spending too much time dreaming about motorcycles this time of year, but really really odd to me that (speaker) drivers aren't also rated by "horsepower per cone weight" or some direct index of oomph. Why doesn't that make sense?
If more BL doesn't make a speaker better then something is very wrong with our design logic when a smaller BL sims-out to a better FR.
B.
*yes, I know there are reliable definitions of these terms but they still may not be saying what we want to communicate
The problem is that the nomenclature is too simplistic. What exactly IS the senstivity when the frequency response varies all of the place? Usually (historically) the voltage sensitivity was taken over some mid-passband average, or at 1kHz.
What we are talking about in this application (a subwoofer) is happening only near the resonance region. The closest ROT would be Hoffman's Iron Law, but even that falls down because what is F3 in this case? HIL essentially assumes a "maximally flat" response, not a super droopy one. The lone remaining metric IS the frequency response with a known power input (e.g. at 1W or 2.83V). Clearly the low Q driver is not at all efficient, not anything close to its passband efficiency, at low frequencies. That is precisely where we will be operating it in this application. You don't get to make use of the high passband efficiency, because you are not operating the driver in its passband.
Maybe an analogy would be something like "you can increase horsepower, but at the expense of torque". Then I ask you: which motor will get the bike from a standstill to 100kph faster?
I'm not convinced. With everything else held constant, when you increase motor strength the passband efficiency goes up but Qts goes down and the low end response droops. To a degree this is OK - a mild DSP correction is fine. But when the Qts drops to 0.2 then things have gone too far.
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