Driver choice for a shallow subwoofer

[tktran303]

https://audioxpress.com/article/test-bench-scan-speak-32w-4878t00-subwoofer
http://website.audiocomponents.nl/download/scan-speak/32W-4878T00 - Klang Ton 2013-2.pdf
http://website.audiocomponents.nl/download/scan-speak/32W-4878T00 - Hobby HiFi 2013-5.pdf

The missing data point is the SB SB34SWPL…

 

[ai1]

... Put another way, our ability to hear the test frequency noise tones at frequencies of 40 Hz and below is extremely crude. Indeed, the results show we are virtually deaf to these distortions at those frequencies. Even in the mid-bass at 280 Hz and lower, the noise can be around -14 dB (20% distortion), about half as loud as the music itself, before we hear it.
https://www.axiomaudio.com/blog/distortion

I can only agree with that---I made a very average score at klippel distortion test (that for music, but pretty good in two-/multitone test). However, if neither low distortion nor high SPL are not objectives anymore, I wonder what they might be.

BTW, below are speakerboxlite.com simulation results for SB34SWPL (red curves), UM10-22 (green), L26ROY (blue) and RSS315HO (orange) in a 28L int vol sealed box. And they are almost the same in 40L or 50L box (well, except that in larger volumes cone displacement of L26ROY and RSS315HO exceeds Xmax, while SB34 and UM10 still have a 2-3mm margin).

And thank you for the simulation link, presenting several simulations on one plot is a nice feature. And I will probably go ahead with a smaller box--as all the wood is sent overseas, plywood prices are skyrocketing here;-)

 

[Chrise36]

I can only agree with that---I made a very average score at klippel distortion test (that for music, but pretty good in two-/multitone test). However, if neither low distortion nor high SPL are not objectives anymore, I wonder what they might be.

BTW, below are speakerboxlite.com simulation results for SB34SWPL (red curves), UM10-22 (green), L26ROY (blue) and RSS315HO (orange) in a 28L int vol sealed box. And they are almost the same in 40L or 50L box (well, except that in larger volumes cone displacement of L26ROY and RSS315HO exceeds Xmax, while SB34 and UM10 still have a 2-3mm margin).

And thank you for the simulation link, presenting several simulations on one plot is a nice feature. And I will probably go ahead with a smaller box--as all the wood is sent overseas, plywood prices are skyrocketing here;-)

View attachment 128819

If you build one the frequency response will be all over the place and if you test this with sweeps it is clearly audible if you have nulls in your listening position in my case the frequencies around 40 hz are almost not existant.You need more than one subwoofer to even this out and dsp is not going to help because cancellation will be there despite the power you give. Also every subwoofer gives you another 3 to 6 db so you gain spl with the same thd.Also from the thiele small factors the SEAS does better in a vented box.For closed you need a driver with Qts higher than 0.45.In my opinion try with two Pioneer and if you dont like them sell them they cost 100 euro together.If you want tight bass you can make a bit smaller boxes and this will help with power capability.The um10 is probably more capable of punishment in high spl.If you can measure your room and find the nulls and run the sweep say from 20hz to 250hz you can here the problems clearly.

 

[jhaider]

The price of Scan-Speak revelators being way above my constraint, right now, the contenders are (in alphabetic order)

• SB Acoustics SB34SWPL76-4
• Scan-Speak 32W/4878T00
• SEAS XM001-04 L26ROY

Is the ScanSpeak Discovery evolution of Peerless XXLS (name escapes me at the moment) or Tymphany/Peerless's replacement (SDF or something like that - it has a larger diameter voicecoil and new basket, but only available in 10") available to you?

Both of these should be much cheaper than the Revelator, but are still very good drive units. For a shallow subwoofer they have a huge design benefit - no polepiece vent. The pole is vented through holes in the cone under the dustcap.

 

[Chrise36]

Have a look at this:

 

[ai1]

If you build one the frequency response will be all over the place and if you test this with sweeps it is clearly audible if you have nulls in your listening position in my case the frequencies around 40 hz are almost not existant.You need more than one subwoofer to even this out and dsp is not going to help because cancellation will be there despite the power you give. Also every subwoofer gives you another 3 to 6 db so you gain spl with the same thd.

Sure, that is exactly the idea---to build 2 subwoofers to complement the main speakers which already have "some bass'' (2x10" XXLS 835016 each). Just as an indication, here are current REW measurements at the listening position (1/12 octave smoothing): without EQ (green line) and with some minimal EQ (red one).

For a moment, I have not thought much about bass management implementation.

 

[ai1]

Is the ScanSpeak Discovery evolution of Peerless XXLS (name escapes me at the moment) or Tymphany/Peerless's replacement (SDF or something like that - it has a larger diameter voicecoil and new basket, but only available in 10") available to you?

You are right, and I see now that there was a copy/paste error in my initial message---the scan-speak driver I consider is Discovery 30W/4558t00, not 4878 which is Revelator line. In fact, I have 2x10" XXLS 835016 in each Nao Note, and scan-speak's indeed look "familiar."
I have seen that mention of good ventilation of Scan-speak drivers. I am a bit confused with the derivative Peerless->Scan-speak->SB Acoustics, I assumed that SB34 inherited that feature, but according to the image below it has a pole vent.


I wonder how much space should be left for the pole vent? Finally, this may become a critical parameter.

 

[Chrise36]

Sure, that is exactly the idea---to build 2 subwoofers to complement the main speakers which already have "some bass'' (2x10" XXLS 835016 each). Just as an indication, here are current REW measurements at the listening position (1/12 octave smoothing): without EQ (green line) and with some minimal EQ (red one).
View attachment 128851
For a moment, I have not thought much about bass management implementation.

The subs must be placed in different spots than the speakers that cancellation is not going to dissapear. I feel you could benefit from a bandpass sub to fill that.

 

[HoweSound]

The Dayton RSS2665HF, might be a less expensive alternative to the SEAS L26ROY:

Dayton Audio RSS265HF-4 10" Reference HF Subwoofer 4 Ohm (parts-express.com)

 

[Rick Sykora]

You are right, and I see now that there was a copy/paste error in my initial message---the scan-speak driver I consider is Discovery 30W/4558t00, not 4878 which is Revelator line. In fact, I have 2x10" XXLS 835016 in each Nao Note, and scan-speak's indeed look "familiar."
I have seen that mention of good ventilation of Scan-speak drivers. I am a bit confused with the derivative Peerless->Scan-speak->SB Acoustics, I assumed that SB34 inherited that feature, but according to the image below it has a pole vent.

View attachment 128860
I wonder how much space should be left for the pole vent? Finally, this may become a critical parameter.

Maybe even more importantly, did you check that these drivers can be downward-firing?

 

[ai1]

Maybe even more importantly, did you check that these drivers can be downward-firing?

Oh, hope this is not a joke! Where this information may be found?

 

[egellings]

I don't know just what changes in suspension components would be needed to make a woofer capable of either down firing operation or facing the listener-firing operation.

 

[Wolf]

This mainly has to do with high-Mms woofers, or even larger drivers. The mass of the cone and suspension is heavy enough to actually cause the suspension to sag and move the at-rest position of the drivers closer to the floor. This means the available Xmax will be reduced in the forward direction, thereby making the driver much less efficient at doing its job.

This is not a myth or joke.

Pasted from AVS forum:
You can calculate the sag of a driver from:


Percentage of Sag = 24,849 / ( Xmax * Fs²)


where


- 24,849 is a constant value based on the relationship of acceleration due to gravity and Pi.

- Xmax is the maximum linear excursion of a loudspeaker voice coil while remaining within the magnetic flux field (mm).

- Fs is the free-air resonant frequency of the woofer (Hz).


The following is the same formula, including the relationship of acceleration and Pi in this case:


Percentage of Sag = 981,000 / (Xmax * (2 * Pi * Fs)²)


where


- 981,000 is acceleration due to gravity (mm/S²) * 100 (for the percentage).

- Xmax is the maximum linear excursion of a loudspeaker voice coil while remaining within the magnetic flux field (mm). - Fs is the free-air resonant frequency of the woofer (Hz)

- Pi is the ratio of a circle's circumference and diameter, usually rounded to 3.14.


As a general rule of thumb, any time the sag exceeds 5% of the driver's Xmax, it's not recommended for a down-firing subwoofer application."

 

[egellings]

So floor or upward facing might cause symmetrical suspension sag. Voice coil stays coaxial to the gap components. Facing the listener position might produce enough non axial sag to cause the voice coil to rub in the gap. Don't know which I'd rather get stuck with.

 

[Wolf]

Sag with a driver firing fore or aft usually is a non-issue. With the higher end subs using dual spiders and input terminals and tinsels balanced to either side of the cone, this really becomes no issue down the road. The spider, even when singular, is usually stiff enough to not sag of shift in one direction.

I know some of the drivers on the market need rotated every few years to keep them from rubbing, but I find their construction is sub-par for longevity compared to the units we're discussing here.

 

[egellings]

I have a subwoofer in my setup, and as an annual thing, I remove the driver and put it back in upside down. Then the house gets a thorough vacuuming. I can't ever recall a speaker actually having had that problem where rubbing actually happened, though. It feels better to flip the woofer, even if it's not really helping in any way. Kinda like the dog that circles 3 times before lying down, maybe.

 

[xarkkon]

am thinking of building a shallow down firing subwoofer too! how shallow are you thinking of?

from my very basic research around, I will likely settle on an Ultimax 10" sealed (thanks to advice from Wolf!), but will need to take time out to figure out the cabinet design.

 

[ai1]

For all considered/suggested drivers except for 30w4558 the sag is less than 5% (<2% for UM10 and 4,5% for SB34). In fact, Xmax=12,5 mm is the lower limit for Fs=20Hz to get 5% sag. Scan-speak 30w4558 has Xmax=12,5mm with fs=17Hz resulting in 6,9% sag. Does this mean it is eliminated?

I had in mind boxes which are 18cm deep. If I use UM10-22 this means 1,5cm space behind the pole vent. Will it be enough for proper ventilation keeping in mind 450W max power by minidsp-pwr? I assume that one should also leave enough space around the driver, no felt behind the motor, etc.

 

[ai1]

There is much discussion about the minimal space behind the driver magnet and the rear wall of the box. Opinions vary from 1,25-1,5cm to twice the diameter of the vent. From images of UM10-22 I have found, the latter would imply 6-7cm behind the magnet, with the box at least 22-23cm deep. This is clearly well beyond my geometry constraint. Life is not easy...

That being said, I wonder if such ventilation is needed for the driver which will hardly ever receive more than 10W of RMS...

 

[kipman725]

you can put it close to the back wall, probably within a vent diameter. When I have run drivers in free air a pair of paper back books under the magent has all that is required to raise the driver off the ground far enough that its behaviour does not deviate significantly from holding the driver horizontally.