Thermal compression in domestic conditions is almost a myth and even if it ever happens, it should be inaudible due the time constants involved.
The dB loss you quote needs to supply the driver with a continuous signal for a long time.
The graph neither shows FR nor magnitude response. It represents maximum short term burst using shaped sine bursts of 6.5 cycles each at 1/3 octave centers per CEA2010.
Ignore away, it appears to suit your posting style. 😉
There is no "of course" in your assumption. It's displacement that relates to output, not cone area. The OP mentions 8" vs a single 15". 4-8s are required to equal a single 15 in Sd and that "of course" assumes equal Xmax.
The entire point of this discussion is to answer the OP's Q; whether or not a multiple smaller driver system can sound like a single larger driver system. Maybe I missed the part about cone area. 🙄
You're assuming a perfectly flat FR and I assume you mean harmonic distortion.
Note, for example, the rise in THD at 93 Hz from 7% outdoors with a flat FR to over 45% in-room because of the dip at that frequency in the measured FR at the LP. The harmonics at that frequency would go from being insignificant to extremely audible.
Conversely, if there is a peak in FR, the harmonics as a % are drastically reduced, which is the same effect with room gain (cabin gain in a vehicle) where the fundamental is boosted when the harmonics are not, drastically reducing THD as a %, which can also clearly be seen in the graphs.
Harmonic distortion is not a serious concern with modern subwoofer drivers. If it is, you'd need to rethink your design. OTOH, a +6dB peak in FR in the subwoofer bandwidth most certainly is audible.
A lot of published research shows the dramatic benefits. I did a bit of testing in the world's largest anechoic chamber (Bell Labs, Murray Hill) long, long ago. But easy to hear the difference on a short test pulse between "clooook" and "click." Very hard to make it work on a commercial basis and folks can destroy a fine woofer or two in their experiments.
Ben
Uuuhhh, what? 😛
Hey Lem, put a -3dB high shelf filter at 3k on your whole system and tell me you can't hear frequency response.
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Back to the OP question . . .
What was it again? 😉
Assuming that the 8" will output enough SPL at the lowest frequency you need, then it will be sufficient. I suspect that one 8" driver may not be enough, based on my personal experience with subs over the years.
First sub: from the old 1970s Linkwitz article, containing 2 KEF B139 drivers, eq down to about 30Hz. Sounded good, but when we moved to a larger house, not enough output.
Second sub: Four 12" in one box, one to each side of the box. Eq to about 30Hz. Plenty of headroom, excellent sound. Happy with it. Moved again.
Third subs: Four 10", one in each decorative column running in stereo. Eq to about 30Hz. Ample headroom, haven't run out of XMax yet. Totally happy with sound. Room is quite large.
What was it again? 😉
Assuming that the 8" will output enough SPL at the lowest frequency you need, then it will be sufficient. I suspect that one 8" driver may not be enough, based on my personal experience with subs over the years.
First sub: from the old 1970s Linkwitz article, containing 2 KEF B139 drivers, eq down to about 30Hz. Sounded good, but when we moved to a larger house, not enough output.
Second sub: Four 12" in one box, one to each side of the box. Eq to about 30Hz. Plenty of headroom, excellent sound. Happy with it. Moved again.
Third subs: Four 10", one in each decorative column running in stereo. Eq to about 30Hz. Ample headroom, haven't run out of XMax yet. Totally happy with sound. Room is quite large.
Beat the dead horse people we all know inside that this is the truth
IF you think you're hearing something different, and they "measure the same" you're either placebo effecting yourself or you're not measuring the right thing. Different designs have different delay, driver waterfall decay, distortion et. al. More smaller woofers is always going to have the potential to produce a flatter response in room due to having additional placement options and is typically easier to move said multiples of subs.
Assumptions have to be made for this discussion to focus on the OP's Q.
In the immortal words of the late Tom Nousaine: "Bass is Bass."
Data usually trumps opinions, so having a quick jaunt over to Data Bass and nicking a bit of Josh Ricci's excellent body of work, I took the CEA2010 Max Burst data for a 12 " driver and an 18" driver, each in a 1X sealed alignment and both driven by an amplifier that had lots of headroom.
Since what we hear is frequency response and in the cited case comparison both subs have virtually identical native FR and non-linearities are below the industry standards maximums for both subs, using 4 of the 12" drivers and level matching the 2 subs (a 1X18 vs a 4x12), there will be no discernible difference in playback of recorded program.
The 4 x 12" system is capable of higher output in dBSPL with equal measured non-linearity.
The 4x12" system can be configured in a dual-dual-opposed-driver system whereas the 1X18 cannot.
The 4x12 system can be separately built and placed to most likely result in a more accurate FR at the listening position (according to Toole, Geddes, et al) and the 1x18 cannot.
The 4 VCs in the 12s will heat up less than the single VC in the 18 = less power compression for equal output.
The 4x12 will require less excursion than the 18 for equal output = lower THD and affords one the option to further reduce THD by use of push-pull.
The facts are what they are. The majority of people draw their own conclusions based on what they think they are hearing. Actual data will always trump those unqualified opinions.
In the end, "Bass is Bass". If there is an audible difference, there is a measurable difference. When using world class drivers (an assumption that must be made or the discussion goes off into a good driver vs multiple bad driver discussion) in an actual room (or vehicle cabin) with boundaries, it is always gross differences in FR because one (also) assumes that the listener is not pushing either system beyond its designed capability in the comparison.
IF you think you're hearing something different, and they "measure the same" you're either placebo effecting yourself or you're not measuring the right thing. Different designs have different delay, driver waterfall decay, distortion et. al. More smaller woofers is always going to have the potential to produce a flatter response in room due to having additional placement options and is typically easier to move said multiples of subs.
This is basic psychoacoustics, not a silly argument. I won't even fight it because you're missing the point. Instead, I'll just link you:
Equal-loudness contour - Wikipedia, the free encyclopedia
Loudness - Wikipedia, the free encyclopedia
SPL's perceptual analog is loudness. You should not be new to this fact.
Likewise, frequency's analog is pitch. It's the brain's ability to order sounds on a scale from low to high. Again, this is how two speakers can have the same frequency response, but if one produces more distortion, you can tell that one as sounding differently. There is more high frequency content which your brain atrributes to having a higher frequency than the input signal. So no, you are not hearing the loudspeaker's frequency response otherwise they would sound the same. It is only pitch perception which allows you to determine this difference. Understanding how the mind processes sound makes for a better loudspeaker designer. 🙂
I too feel hot VCs (in normal home use) are mythical or at least not important. But do you have a reference-link relating to realistic domestic-level tests? If heating mattered, which I doubt, multiple small drivers would be better. (Some folks also argue that VC heating, if it mattered much, would disrupt VC-based motional feedback.)
For those whose minds like to think statistically, multiple small subs help average out all kinds of anomalies ranging from manufacturing of drivers to room location to cone break-up, etc.... as well as mounting half facing each direction.
Speaking of myths or things that are not important, somebody also mentioned "room gain" another enduring myth (except for hermetically sealed small cars or people who live in shipping containers).
Ben
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Just to point this out, room gain is a real thing, backed by physics and measured by thousands. It's why my T60 measures flat to 24 Hz instead of rolling off by the ground plane measurement of ~5 dB down from 50 Hz. Also, Power compression is also easily measured by running sine wave sweeps at various power levels, the difference between + 3 dB and twice as much power (up to the thermal and mechanical limit of the driver) and what it actually achieves -- is power compression. Most high end manufacturers post this information in their driver specs, meaning they will say power compression at 1/8th power = .5 dB 1/4 power = 2 dB full power =3.4 dB. You can't ignore science and you're a fool to call them myths.
There are many causes for hearing bass enlargement in a room (as compared to mounting your woofer in your neighbourhood park). The term "room gain" refers only to the effect of playing bass in a SEALED volume, like inside a small, tightly sealed car.
Yes, the physics is real, but the physics will tell you that the effect of "room gain" in your living room is inconsequentlial (unless you live in a tightly sealed shipping container).
Yes, the physics of heat on copper wire resistance is real but small. But I doubt there is any detectable effect at domestic playing levels and maybe barely none at all unless you have one of those long-ago old-fashioned low-level crossovers.
Has there ever been a grown-ups thread at this forum discussing what matters in priority? Can't count how many pictures I've seen here of rooms that look like Frankenstein acoustics. True, we each have our little favourite experiments (I'm putting super absorbents behind my ESL tweets in a few minutes) but also we need to keep the priorities in mind.
Ben
A few more details about my question.
First, I am asking this question here because I have not found convincing evidence in the car audio forums. So a am trying to drag a larger net to figure out why this may be the case. One of the major issues with mobile audio is space, more so than in almost any other environment. As a result, many people have tried different things to get the most use out of the limited space. One way to do this is to multiples of smaller drivers (usually direct radiators), that are easier to package, and often require less space, instead of a single larger driver.
With that said, many people who have years of experience designing, building, and competing in sq competitions have ditched the multiple driver approach. The statement that is often made is that multiple drivers, sound like multiple drivers and not a larger one. This isn't about spl, it is a question of sound quality. What they claim is missing is "weight", "grip" and visceral impact in the subbass, bass, and midbass range when using multiples over larger cones. These are subjective terms, I am trying to see if there is any science behind it.
In the mobile environment, using multiple subs in different locations does not make much difference, the wavelength are simply too long. So, it's not am issue of room modes <80hz. Spreading midbass placement may show some benefit however, for the same reason multiple subs even out room modes in larger spaces.
In my case, I am using two PPSL manifolds with 4 8" subs each. The manifolds allow me to pack the most Sd in the smallest package, with the benefit of force cancellation and distortion reduction. The subs I am using are Dayton Audio DCS205-4 8" Classic Subwoofer 4 Ohm. 8" subs are good for comparison because Sd of 4 x 8" subs is equivalent to Sd of most 15" subs. For any reasonable listening level, xmax is not a limiting factor for the manifolds. One manifold should net 106db at 30hz, before adding in the 15-20db (121-126 db total @ 30hz) of cabin gain we get down there, while remaining within xmax.
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Just to put this to rest. Subwoofers are non-linear devices, at a certain point at certain frequencies it will no longer make more sound but will convert that energy into heat, or some fraction thereof. You won't be able to hear this difference because the point where compression is >2-3 dB is typically near max power which unless you run pro audio you probably never touch. Inside it is typically unnecessary to do so unless you have a large room and too few cabinets.
In your hypothetical scenario of two drivers with the same frequency response, I agree that you wouldn't hear a difference in frequency response. But if you're trying to convince me that hearing THD is easier to point out by ear than hearing the frequency response of two different drivers, I strongly disagree.
I think Bosso's post is exactly right on point with the question at hand. I don't think that you fully understand what he was showing us.
Thermal compression is acknowledged by 18sound and Selenium (JBL) in some of their PDFs , call it marketing or not .... Some of these extremely well designed drivers are suffering 3db or more loss at full power, and considering that this came from the manufacturer it was probably under ideal conditions that favor the driver.... 3db is the difference between 500 watts and 1000 watts ... You can decide on whether or not you think that is significant....Its debatable..... However, 6db power compression is possible with some lesser drivers, and thats the difference between 250 watts and 1000 watts and that is CERTAINLY significant ...
1) There is no "science" to subjective terms, but "weight", "grip" and "visceral impact" in general describe "more low frequency SPL" to most sub fans. More clean LF SPL probably gets votes, though more even order distortion sounds louder, and is preferred by some).
Who judges SQ, and what are the judging parameters?
2) I recently built some PPSL cabinets using Dayton drivers, the measured distortion was higher than non-PP designs I had built using Eminence and B&C drivers. Oh well.
3) The 8" Daytons you use have 8.8 mm Xmax, fairly much for an 8", but very last century in terms of 15" displacement, some of which are running in the 30+mm range.
Doubling Xmax results in 6 dB more output level, so your 4x8" can easily be outrun by a single 15", and down at 20 Hz, it only takes 5 dB increase in level to sound twice as loud. Some drivers may have enough Xmax to sound almost four times as loud, hard to win against that, depending on the answer to my question above, and the definition of "reasonable listening level", which is a totally subjective term.
121 dBSPL at 30 Hz sounds about as loud as 90 dBSPL at 4000Hz.
If the judges have noise induced hearing loss, their hearing could be -40 or worse at 4kHz...
Art
"Lemans23" made a point earlier about how small drivers need extremely loose suspensions in order to achieve low FS figures, which is true, and a super loose suspension is less-than-desirable, i agree, and i would like to add that manufacturers can also achieve a lower FS by making the cone/moving mass heavier, which of course is terrible because it generally lowers efficiency ....
BUT
There are ways around these challenges.
If a designer is willing to try a type of cabinet alignment that is quarter wave loaded like a TL (qwp) or even better yet a Tapped-Pipe (Transflex/Air Coupler) it can really make using multiple smaller drivers in a bass cabinet much more worth building because not only is there gain (a bonus) but the drivers can have an FS much higher than the FB (box fundamental) .... So lets say for example we wanted to build a subwoofer that plays flat-ish down to 40hz then we could use drivers that have a free-air resonance of up to 65hz which means that suspensions can still be kept tight and moving mass can also be kept fairly light... Efficiency can be high .... Combined motor strength can really add up too ..
BUT
There are ways around these challenges.
If a designer is willing to try a type of cabinet alignment that is quarter wave loaded like a TL (qwp) or even better yet a Tapped-Pipe (Transflex/Air Coupler) it can really make using multiple smaller drivers in a bass cabinet much more worth building because not only is there gain (a bonus) but the drivers can have an FS much higher than the FB (box fundamental) .... So lets say for example we wanted to build a subwoofer that plays flat-ish down to 40hz then we could use drivers that have a free-air resonance of up to 65hz which means that suspensions can still be kept tight and moving mass can also be kept fairly light... Efficiency can be high .... Combined motor strength can really add up too ..
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Of course, (in regards to the above) good luck on finding a driver that possesses both an FS of 65hz along with a high xmax figure ..... The affordable options become very scarce ..
Never said subjective terms are science, but that does not mean there isn't science behind the use of the subjective terms. Also, these terms are applied to midbass frequencies too, not just LF, as I stated. In fact, many of the attempts with multiple drivers were with midbass mating to compression drivers and horns.
I don't know about last century, but I doubt those subs are last century. Maybe last decade. I am not sure what the point is of that comment. I know they don't have 30mm of xmax, but hey don't cost $700 either. I wouldn't have the space for something like that anyway, with 8" magnet depth alone. Still not sure what is your point. Of course Vd trumps, that's not the argument. Pretty sure I made that clear, and it is odd that was missed considering your focus on semantics.
Just for reference sake, most people running dual 15" IB (my manifolds are also IB) report that only a few mm of cone movement is present at listening levels. I would count "reasonable listening levels" as anything below the threshold of risk of permanent hearing damage at 1 hour. Are we done with the semantic gymnastics? I take it you meant well in case
Again, this does not appear to be SPL related, as I stated.