20 Hz HT bass horn

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
I have plans to move into a new place that will allow me to (finally) build a bass horn :D

The room: 3.9 x 4.2m with ~2.7m ceiling

Drivers: AE Speakers AV12 x 2
Hornresp parameters for both drivers:
SD 996
BL 15
CMS 1.25E-04
RMS 13.70
MMD 456
LE 1.48
RE 1.49

I've modelled quite a few different drivers, and these drivers not only model similar to the Lab driver in the Lab horn, but for this application I get better results than I can model for any driver.

My best results are with these numbers:
compression ratio of 2:1 hence throat of 500 cm2 and a mouth of 12000 cm2 and an axial length of 6m, a rear sealed chamber of 80L (40L per driver)

Shown in the image is the design against the wall:

An externally hosted image should be here but it was not working when we last tested it.


Lab horn shown for comparison. On the right a side elevation is shown. A fixed screen will be included as well as a concealed AV rack. Distance from mouth to side wall is ~840mm.

Hornresp model:

An externally hosted image should be here but it was not working when we last tested it.


Excursion is shown lighter. IIRC that is 100w input power.

Hornresp input parameters:

An externally hosted image should be here but it was not working when we last tested it.


I've modelled it as a single horn, but it will be built as two horns stacked, each with 6m length but half mouth size.

Now for some questions ...

How do I work out how close I need to be to the corner for it to be considered corner loaded?

I have tried to keep some space on the sides as I want some clearance around the mains.

Can anyone else get a better result with some changes? This is the best result I've got. I'll consider making it bigger if I get significant improvement, but so far I've found it has to be a LOT bigger to be significantly better.

Can anyone point out any weaknesses in the design?


Is it worth push pull mounting? If I do this I will have to have a throat chamber due to the magnet - does this have any acoustic penalty?

Any suggestions on how I might better utilise the corner loading situation?

Is 18mm MDF enough? Yes I know ply would be better, but this is huge and ply is going to be expensive. Hence I've opted to brace extensively, use curves where possible and make the parts connected closely to the driver ULTRA stiff (not shown in the schematic yet).

Please note: schematic drawing only - I will simplify it for construction, although I intend to keep the gentle curves for rigidity.
 
My amp is overkill for this horn - Europower EP2500 which supplies 650w to each driver. With that kind of power, my sims show 135db within xmax limits, but it only really needs 100w to reach 120db at 20 Hz.

For a future upgrade I have considered later adding some other small sealed subs in locations to smooth out room response, and could use one channel of that amp. They would need to be very good to match the low distortion of the bass horn. This is the approach that Earl Geddes of Gedlee uses and recommends.
 
Is it worth push pull mounting?

I think this depends on the design. When you calculate Vb so that it'll provide the same pressure to the diaphragm as the horn (so equal pressure on both sides of the driver), then you could probably use isobaric configuration. But when you try to get Vb as small as possible to reduce excursion, then the drivers will see different (unequal) pressure, so that the isobaric configuration won't work as intended.

Is 18mm MDF enough?

When comparing 18mm birch plywood with 18mm MDF, then ply will provide more damping around the 30-90 Hz region. Anyway, 18 mm MDF is enough in my opinion (also, the horn will be very heavy when using thicker material).
 
paulspencer said:
My amp is overkill for this horn - Europower EP2500 which supplies 650w to each driver. With that kind of power, my sims show 135db within xmax limits, but it only really needs 100w to reach 120db at 20 Hz.

Holy smokes! How much extension do you have at 135dB? Still down to 20hz? And is that accounting for room gain?

That thing could really make you say "Oops, I crapped my pants!"

P.
 
G'day Paul

This is a VERY interesting project. It will likely be your last sub, if you get it right, so it's worth doing properly.

A few small changes I'd suggest, firstly, include the front chamber volume in your model. There is at least the cone volume that should be taken into account.

Secondly, the spacing to the right wall should allow the expansion to continue as smoothly as possible into the room. It appears your sub is a little far from the right wall to accomplish this. Comparing my models with real horns in real rooms suggests this is a valid approach and should also be included in your hornresponse model. Have a look at the 18" basshorn on my webpage at http://diy.cowanaudio.com I have not yet built this horn, but many others have and their measurements show it behaves at least as well as the model. I have built the 40Hz basshorn on my webpage, and in a real room it works better than modelled, despite having included the last conical section (which is not in the box) and is modelled for 1/8th space. You have modelled your horn as radiating into an eighth space environment, when it is actually more constrained than that. My guess is that you will have an f3 in the mid/high teens.

Don't forget to delay your mains by your horns path length for proper integration. Also you might want to start planning your new main speakers now :D

Cheers

William Cowan
 
May I ask anyone interested to jump in here with their take on the pros and cons of horn loaded subwoofers? I've been dying to ask this and waiting for an appropriate thread.

There is a mind-numbing amount of subjective nonsense on the web devoted to (relatively) different approaches for home subwoofer designs. Any real measurable facts that can be brought to light here? I'd love to make one of these my next project.

...or is this a hijacking that should be taken elsewhere?
(dammit - now where's my sock-puppet?)
 
cowanaudio said:
the spacing to the right wall should allow the expansion to continue as smoothly as possible into the room. It appears your sub is a little far from the right wall to accomplish this.

I agree. e.g. A tractrix expansion turns over around itself, but an exponential continues outward/requires continuity.

bluebeard said:
May I ask anyone interested to jump in here with their take on the pros and cons of horn loaded subwoofers?

Not meaning to hijack, but quickness/effortlessness, and if done a certain way you can beat the room by joining with it (if you take the meaning).
 
G'day Bluebeard

Here's my take


Pros:

Low distortion (Harmonic, IMD, compression)
Low group delay
High output
High efficiency
Small size for given output

Cons:

Large size for LF cutoff
Complex to build
Harder to design than SB/VB
Fixed delay that must be accounted for
Hard to go back to non horn bass :D

Cheers

William Cowan
 
Some imperial units...

room size (http://www.digitaldutch.com/unitconverter/)

12.8' deep (front to back wall)
13.8' wide (across the wall on which the sub will be placed with screen)
~9' ceiling

each bass horn is
1.6' deep (projects into room from the front wall) / 20"
8.5' wide / 102"
4.3' high / 51"

throat 8 sq ft for each box of 16 total
mouth 393 sq ft total
axial length 236" or 20'

for anything else:
http://www.digitaldutch.com/unitconverter/

I think this depends on the design. When you calculate Vb so that it'll provide the same pressure to the diaphragm as the horn (so equal pressure on both sides of the driver), then you could probably use isobaric configuration. But when you try to get Vb as small as possible to reduce excursion, then the drivers will see different (unequal) pressure, so that the isobaric configuration won't work as intended.

How do you calculate the VB which will give equal pressure? Isn't this going to vary with frequency? I have chosen the volume to get it reasonably flat. I know Tom Danley prefers to keep it small as he believes this gives lower distortion as the air mass is more linear then the driver suspension. I'm aiming for a balance here - a larger volume yields less of a peak in response so it is easier to eq flat. Excursion is not a problem since xmax is not reached until output exceeds 130 db without any room gain assistantce.

There will only be one driver per box, hence I won't be using isobarik configuration - there is no need. By push pull, I mean one driver will be mounted so that the output from the rear of the driver will be loaded into the horn, and the polarity reversed. This would involve a throat chamber.

Holy smokes! How much extension do you have at 135dB? Still down to 20hz? And is that accounting for room gain?

135 db in the midbass, slightly less at 20 Hz - probably about 130 db but that is with high power and there would be power compression. No room gain is allowed for here, but the room is lightly constructed, probably with 19mm pine strip flooring under the carpet, 10mm plasterboard (drywall), framed construction and very light cladding on exterior walls. My current room has similar construction and size and there is actually NO ROOMGAIN.

This is based on the settings in Ultracurve to eq it flat. Around 30-40 Hz there is eq cut to remove a 35 Hz room mode and below that the response drops off rapidly. In a room with the kind of room gain most would predict, my sealed subs (F3 ~ 40 Hz) would need little eq to get down to 20 Hz, but in fact more than that max available is required (>15db).

William,
Thanks for the suggestion. I'll adjust my model and experiment with a front chamber.

Would it make sense to add a triangle shaped box to the corner as (from plan view) to integrate the corner?

I've adoped Tom Danley's suggestion on corner loading with a fillet in the corner, and re-modelled in hornresp and the increased horn mouth area and length gives a smoother response - now +/-1db from 20 - 80 Hz without allowing for the throat chamber.

TD on corner loading:
http://web1.prosoundweb.com/community/read.php?board=3&id=143250

I modelled with a throat chamber of 7.5L and it had no significant impact on the results. This was assumed to be the volume on air within the cone (1L) and the volume of air from the middle of the cone to the closed end (6.5L).

Another question:
Is the aspect ratio going to be a problem?
At the start of the horn path, it is ~ 55 x 464mm and this changes to 1264 x 464mm. I have read that parallel sides introduces resonances as well, which make a horn less smooth than modelled. Thoughts anyone?

Is there a bettter way to design the throad and the way the driver loads into it?
 
G'day again Paul

Answering a few of your questions:

The parallel walls have no effect at the frequencies of interest. (<80Hz)

The triangle in the corner will have little to no effect based on my experience with my 40Hz horn.

Also I see the real horn path being ~1M-2M longer than your corner loading picture shows. I'd be having a close look at the expansion all the way out to the other side of the box. The mouth grows significantly in that 1-2 meters. You might see that the mouth is then almost the diagonal in the room, but this is my point exactly! The room plays a VERY big part in the performance of a bass horn. If you can adjust the contour of the horn to make the transition into the room as smooth as possible, you could probably do a 15Hz flair freq in the same size box you plan on building. When I was playing with modelling and building horns, I thought this effect was too good to be true, but it is the only way I have been able to explain the performance my tiny horn achieved indoors. (Outside , on it's own it's worse than useless) I have stiff walls near the mouth of the horn, you might need to do the same. The smooth transition of the flair into the room is worth getting right. You really will be sitting in the mouth of the horn.

Cheers

William Cowan
 
Paul,

Your rendering is awesome. Looking forward to seeing your finished product.

http://www.digitaldutch.com/unitconverter/

I just wanted to share google's unit conversion function for those who might not know about it. I find it extremely handy and use google as my primary calculator most of the time now.

You can use google to do many common conversions.
Syntax examples:
1 foot to meters
47 liters to cubic feet
14 lbs to stones
32 degrees in radians

Google also recognizes constants like pi and e.
It also recognizes ^ and **[/] as the power function and sqrt() as the square root function.

You all probably have the idea by now.

Cheers, Roger
 
William,

I'd be happy if it works as you say, it would be a bonus. The room in question has typical timber framed construction with 10mm plasterboard walls/ceiling and timber flooring under the carpet. Exterior walls have a very light cladding system. As a result I expect the room will be essentially like a large bass trap - absorbing a fair amount of bass, and letting a lot of it go through the walls. I don't believe I'll have any real room gain, based on my experience with my current rooom with similar construction if not a little more solid. This makes it hard to predict how much the walls will work as an extension of the horn.

Perhaps I could extend it further by beefing up the walls possibly.

If you can adjust the contour of the horn to make the transition into the room as smooth as possible, you could probably do a 15Hz flair freq in the same size box you plan on building.

Sounds good to me, I'd REALLY like it to get down to 15 Hz! I've heard a system that got down below 20 Hz and it did seem to have just a little bit extra for some LFE.

How do you suggest I make the transition into the room smooth? The horn mouth will be very close to floor to ceiling with just a small allowance for the fact that I have to be able to jack up the second box - 200 kg or more of box up high in the air - quite a feat to install in a room! I think the ceiling is about 2.7m or more and the mouth height will be about 2.6m.

I really like the look of that curve.

What curve is that?

Your rendering is awesome. Looking forward to seeing your finished product.

Thanks. I look forward to hearing it! At the moment I can only imagine what it might sound like.

Tehrasha,
Thanks for the link. Interesting thread. Looks like we have the same idea from what you wrote (big bass horn behind screen with 2 x 12" drivers). To keep this thread on track, I've commented in your thread.
 
cowanaudio said:

Also I see the real horn path being ~1M-2M longer than your corner loading picture shows. I'd be having a close look at the expansion all the way out to the other side of the box. The mouth grows significantly in that 1-2 meters. You might see that the mouth is then almost the diagonal in the room, but this is my point exactly! The room plays a VERY big part in the performance of a bass horn.


William,

I think this was the subject of a thread a long time ago - the 1/8 space loading already takes the extra expansion into account. You can't take the corner loading AND room expansion together, and the original simulation was correct. Sure, you will get "cabin gain" depending on room dimensions that will increase efficiency down low, but it is NOT simulated the way you are doing it.

This has also been brought up by others on other forums who have seen your site.

Cheers
Graeme
 
G'day Graeme

I know this has been the centre of much debate, but modelling for 1/8th space goes nowhere close to showing how the horn will behave in a domestic environment. An 1/8th space model does not take into account the ceiling. In Paul’s case, the room only expands in two directions from the horn, it does not expand vertically. An 1/8th space environment expands in three directions from the mouth of the horn. Perhaps another approach would be to include the whole room in the horn model, calling the room diagonal the mouth in Paul’s case, and then model for half space.

A more accurate representation of an 1/8th space environment might be a large hall, with a very high roof, where the subwoofer is sitting in the corner.

You have a few horn subs in your collection, how about you drag in the horn with the smallest mouth and measure it in the corner of a small (20-30m^2) room. You'll be very surprised how different it performs compared to out doors or in a hall. The differences are far more than can be accounted for by room gain. The ripple caused by the small mouth also disappears. Room gain alone would not cause this to happen.

I do respect your knowledge when it comes to horn loaded loudspeakers in a professional environment. You were building properly designed horn loaded bins before they became popular, but I'm not sure what your experience is in the domestic environment. When it comes to bass in a small room, you really can get a free lunch if you balance the trade offs.

Cheers

William Cowan
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.