Horn Sub-bass

[AndrewT]

Some horn sub-bass project design thoughts;
Flare constant to suit 15Hz (exponential)
Mouth area to suit 20Hz into corner (pi/2 solid radians)
Throat about cone area/2
Qts less than 0.3
BL greater than 25
Frequency range about 3 octaves (20Hz to 160Hz)
Mouth area and flare constant chosen to give flat response to 20 Hz and total horn length = 6m to 7m, mouth=3.6sqm throat = 0.04sqm (15inch driver)
Q1 What effect does Fs have on lo end response?
Q2 Some horn web sites quote Fh=2*Fs/Qts and Fl=Fs*Qts/2. Are these true?
Q3 how flat is the frequency response between Fl and Fh?
Q4 No box loading on rear of cone- just venting into the loft space?
thanks in anticipation
Andrew T.

[slowmotion]

Hi AndrewT, all

Nice little horn you've got there, Andrew,
you live in one of them scottish castles?

Sounds like a great project, if you've got the room for it.
Just make the horn big enough. If you start to compromise on size
you'll never know where you end up.
Seems like you're on the right track, tho, so here goes:

Q1: You can safely put Fl below Fs, other things matter more, IMHO.
Q2: Mostly no, there's much more to it than that.
Q3: Depends on the horn, the driver and the room
Q4: I would absolutely load the rear of the cone.

There are others on this forum who know more about horns than I ,
so there may be other opinions, too

I you haven't allready , get the Hornresponse program.
Use it.

cheers

PS : May I ask what the rest of the system is?

[Tim Moorman]

Andrew,

Quite the project - big enough to walk into!
I'll add a bit to slomo's comments, based on limited experience.

Horn Respose can take you a long way with a minimum of calculation.

The throat area, flare rate, and mouth are all inter-related to create a resonant system. Varying one will have an effect on the others. The throat size will govern compression, and the load on the driver to some extent. Your ratio of cone to throat looks about right. If you model some differing ratios in horn response, you will see the overall effect is balancing efficiency with compression. My advice is use no more compression than necessary to get the job done. Drivers of poor quality may not have strength to sustain compression loading for any length of time under severe conditions (driven hard).

Q1 - It's part of the overall picture. That is, you are really creating a resonant system that will effectively lower Fs of the driver. You do not want the driver Fs below Fc of the horn, but low enough to be useful.

Q2 - Somewhat true, although I've seen Qes substituted for Qts by people like Bruce Edgar. Pre-dates horn response by a large margin, and primarily a guideline. Check Marshall Leach's work for the extreme math solutions.

Q3 - If you keep the pathway length and mouth near ideal, you can get very flat response. But with size compromises, you will see roll-off starting perhaps 5-10 Hz above Fc.

Q4 - The rear chamber is used to basically extend response lower by creating a damping condition that counteracts the reactance of the horn as it starts to unload near Fc. Big impedance swings are evident nearing cut-off. Helpful for better low end response. Also, once unloaded(below Fc), a horn rolls off at 12 dB, or slower than the 24 dB of a vented system.

The odd thing is, I know of people who have built horn subs with a 15" vintage driver in mind, and as part of the modeling program, then switched to another 15" driver entirely (same general characteristics), and it worked equally well.

Tim

[mike.e]

Note that the ideal driver may NOT be a high BL low Qts highly damped one. ML util will reveal the mathematically correct driver to use (labhorn)

Q4 No box loading on rear of cone- just venting into the loft space?

Not ideal.
1-youd have to use atleast 2nd order filtering to avoid destroying the driver below Fc

2-Reactance annulling is impossible

3-The driver,being hornloaded on one side,will give rise to audible distortion on the rarefaction wave. Some people enjoy this sound - scoop owners. Personally I aim for the most high fidelity sound - not the 'happy' sound

A large rear chamber of even 200litres is in order

If you require hornresp modeling,or cad help,Im happy to lend a hand.

Regards
Mike.e

labhorn link

[AndrewT]

Hi all, usefull replies so far. Thanks
Existing equipment/room
Sources
Sugden CD, Gyro LP, Quad FM.
Amplifiers
Croft valve pre or Conrad Johnson PF1
Sub-bass power Crimson 1704 modded to 4003
Bass power Crimson 1704 modded to 2004
Mid power Michell Alecto mk1
Treble power WAD kel80 (valve classAB) or Sugden Class A
Speakers
WAD kls3gold passive xover just now, eventually active.
Room 7.6m * 4.6m * 3.5m high
loft space above adjacent bedroom reserved for this horn project.
Fc & Fs; if I choose a driver with Fs=40Hz, Horn Fc=15Hz then will the horn work a full octave below this i.e. down to 20Hz?
As the horn unloads below 20Hz and the speaker loses the resistive reactance of the horn, at what rate would you recommend rolling off the bass response? 1pole, 2 pole or 4 pole? and starting at 15Hz or 10Hz or what? or just let the infinite baffle box do it at 2pole?
Where can I download hornresponse and MLutil?
regards Andrew T.

[Tim Moorman]

Andrew,

Use a search engine...

Horn Response

http://www.users.bigpond.com/dmcbean/

[slowmotion]

Hi Andrew,

Hows it going with the planning?


I have a small question, If you don't mind:
You listed your speakers,
but I am sort of ignorant about speaker brands,
so I'm a bit uncertain
if they are horns or direct radiators.
Sometimes it is difficult to get a good result
if one mix the two different speaker types.

cheers

[AndrewT]

Hi,
all my existing speakers are direct radiators.
This project will be my first dip into horn technology.
I hope the integration of horn to direct works out. In my favour may be that horn below 120 to 150Hz only will be easier on the ear???
I do not want to ditch all my speakers at present but after I've heard this I might want to convert to horn everything!
regards Andrew T.

[Brett]

Originally posted by AndrewT
Hi,
all my existing speakers are direct radiators.

You have my condolences.

This project will be my first dip into horn technology.
I hope the integration of horn to direct works out. In my favour may be that horn below 120 to 150Hz only will be easier on the ear???

Under 60-80Hz will be easier.

I do not want to ditch all my speakers at present but after I've heard this I might want to convert to horn everything!

I did, and couldn't foresee going back to DR's: long term, that is. I'm suffering thru some now because I'm away from home for a period and my horn rig takes up more space than my car, let alone fit into it.

[slowmotion]

Yes, but that is also dependent on the walls and windows
of the listening room.
With brick or cement walls there's no problem,
you get support from the walls.

On the other hand if you have a room like mine ,
where the walls are made of wood,
most of the low bass goes right through the walls.

That may be a problem or a blessing,
depends on which way you look at it

cheers