frequency vs size question

[Professor smith]

hi,

Most of the comercial hifi subs use single 12 or 15 inch drive units some use double. And they all claim to go very low in the region of 15hz. What size drive unit and enclosure do we really need to get a flat response to say 17hz?

How accurate are the manufacturers claims?

How does the room size affect the response?

[picowallspeaker]

How does the room size affect the response?

Low frequency waves are very long compared to high frequencies ,and to complete a full cycle it takes time ,and space .As a 20 Hz tone needs about
10 meters to complete 1 cycle ,you'll understand why room affects the reproduction.

How accurate are the manufacturers claims

Depends . Large scale production makes sure that a project has to pass many stages before exiting the factory. Advertising is another thing.

Most of the comercial hifi subs use single 12 or 15 inch drive units some use double. And they all claim to go very low in the region of 15hz. What size drive unit and enclosure do we really need to get a flat response to say 17hz?

The larger the driver ,the easier it reproduces bass freq .That is related to wave length . But there are other issues , like cone excursion and power compression ,and heat . Once you solve these ,the box and the amps are bonuses !

[chris661]

With subwoofers like that, they'll never go very loud and that low (displacement limited), even with the 1kW of amplifier power.
The power is needed to overcome the deficiencies of trying to run a sealed box as a subwoofer. They're not very efficient, but at least they go low. It's basically Hoffman's Iron Law in effect - small size, efficiency, low frequency extension. Pick two. In this case, they've left out efficiency, which is understandable as amplifier power is cheap nowadays.

I know B&W have switches to pick between maximum LF extension and maximum SPL. And the claims are usually either in-room, or the -10dB point. In the case of the latter, my sub will go to 21Hz, using a pair of 8" drivers and 100w.

[ccdoggy]

By default i never trust manufacturer's claims. Those measurements could have been made in a smallish room with optimal placement of the sub and the mic. They are usually pretty good at giving you an estimate but i never ever trust them. There are just too many ways to play with the numbers and room to get what marketing wants.

I am pretty pessimistic arnt i...

[chris661]

Probably better to be a bit pessimistic and not waste money than to be over-optimistic and have no money and a lot of "potentially useful" gear.

[Brett]

Originally Posted by Professor smith

hi,

Most of the comercial hifi subs use single 12 or 15 inch drive units some use double. And they all claim to go very low in the region of 15hz. What size drive unit and enclosure do we really need to get a flat response to say 17hz?

How accurate are the manufacturers claims?

How does the room size affect the response?

The trouble with most FR specs is they are made at low power. You could probably design a 5" driver to do 15Hz, but as SPL capability at a given frequency is determined by driver volume displacement (Sd x Xmax) then the theoretical 5" driver will be limited in output. There are quite a few modern sub drivers with good motors and large diaphragms and Xmax capability.

What's well worth reading are Illka's sub tests, to see what some commercial and DIY systems are actually capable of, then figure out what you want/need and start modelling. His tests are a couple of years old now so don't include some drivers like the Maelstroms.

[Legis]

Originally Posted by Brett

His tests are a couple of years old now so don't include some drivers like the Maelstroms.

These test would be the most interesting though, has anybody else tested them? I would like to know how much of the excursion can be used with <10% THD. One good rival would be BMS 18N860. BMS's drivers have very linear motor, which I know from experience

[Brett]

Originally Posted by Legis

These test would be the most interesting though, has anybody else tested them? I would like to know how much of the excursion can be used with <10% THD. One good rival would be BMS 18N860. BMS's drivers have very linear motor, which I know from experience

Kevin has posted Xmax is specified at 70% Bl and the curves of Bl/X have been very good.

The BMS looks good but they are designed for 2 different markets and it has 19mm vs 33mm Xmax so which would be better will be determined by application.

[Legis]

Originally Posted by Brett

Kevin has posted Xmax is specified at 70% Bl and the curves of Bl/X have been very good.

The BMS looks good but they are designed for 2 different markets and it has 19mm vs 33mm Xmax so which would be better will be determined by application.

Yep, the conservatively calculated (lengh of the VC - magnetic gap height) xmax of 18N860 is +/- 19mm. In maelmstoms the xmax is not stated the same way.

The cone of 18N860 can be moved approx +/- 30mm before the suspension starts to limit the excursion: YouTube - BMS 18N860 20Hz sine tone#3

In maelmstoms the biggest possible (not actual) source of distortion could be, I think, that the medium-BL motor has to move/drive very heavy cone against the air of a closed box. In this vid 18 inch Mael makes strange noise at quite moderate excursion (approx half of the specified +/- 32mm): YouTube - maelstrom 18 gen II 20 hz testtone bass test.MP4

According to Hifimaailma's (Finnish hifi magazine) test, SDX15 which also has XBL^2-motor, can move approx +/- 8mm in 150l@20hz bass reflex and +/- 7mm in 100l closed box before it hits 10% distortion limit. In BR the woofer generated 110dB pressure at 40Hz in half space and in closed box 106dB (limited to 10% distortion). Maximum pressures according to WinISD would have been 118dB and 115,5dB accordingly.

One can look the distortion performance of 18N860 at 1000 watts from the manufacturer's spec sheet. In the distortion graph the THD% is approx -20dB/10% at the whole spectrum, while the SPL at 40Hz is approx 122dB (the 170l@32Hz box is far from being optimal since the response starts to drop so early).

It's true that XBL^2 motors have very long excursion indeed. I'm however only interested of the linear excursion (THD% <10%), which can be, and usually is, quite a different story than calculated or claimed xmax (I'm not saying that this is the case with Maels).

From these points of view I would be very interested to see actual measurements of Maelmstroms. Measuring is fairly easy, one can do it at back yard with laptop, measurement mic, mic preamp and ARTA/Holm Impulse.

[Brett]

Originally Posted by Legis

From these points of view I would be very interested to see actual measurements of Maelmstroms. Measuring is fairly easy, one can do it at back yard with laptop, measurement mic, mic preamp and ARTA/Holm Impulse.

I shall await your tests then. The M18 is available from the UK and the BMS is distributed through the EU. I cannot get the 18N860 as the US distributor does not return calls and the EU distributors are expensive both in price and shipping (yes I understand to consider VAT). When I got my 4590 directly from BMS I paid more in fees/charges at this end than I did for the drivers. The 860 does not fit into any of the construction plans I have either.