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Old 15th August 2005, 01:43 AM   #21
GM is offline GM  United States
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Default Re: adapt a horn to the environment...

Greets!

Once the filter chamber is deleted, it's no longer a typical HI-FI BLH, but a zero compression horn (expanding TL, aka Voigt pipe) that can be either considered a dipole front or back loaded depending on its configuration.

If each gainstage was smaller than the previous one then it would act as a very resonant stepped reverse tapered TL, applying additional pressure on the back of the driver. If the last gainstage were attached to a horn lens throat......... So why not just shape the horn flare to do this? Look at the RCA's catenoid flare: http://home.att.net/~lkalin/fabrication.html

Yes, in a typical HI-FI room the acoustics are indeed quite different than in a very large one or especially outdoors. Down low the room dominates, so for best performance the room's gain curve should be measured and the horn's response designed to have ~the inverse of it. IOW, where the room has a peak, the horn should have a null and vice versa down to the point where the room's gain begins climbing, so yes, the mouth would need to be somewhat smaller than optimum to get this 'ripple'. At this point you want the horn to be rolling off at about the same rate it's rising.

This all sounds good in theory, but tough to achieve in practice, so most folks just make the horn as long/big as can be tolerated and the response is what it is, with some using digital EQ to smooth it out.

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Old 15th August 2005, 10:48 AM   #22
Cortez is offline Cortez  Hungary
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Default Re: Re: adapt a horn to the environment...

Quote:
Once the filter chamber is deleted, it's no longer a typical HI-FI BLH, but a zero compression horn
And what is your view about a design like this ?

Quote:
This all sounds good in theory, but tough to achieve in practice, so most folks just make the horn as long/big as can be tolerated and the response is what it is, with some using digital EQ to smooth it out.
No, not the ripple was the point, but the inertia of a closed airspace.
A horn, designed to outdoor use (huge sizes with ideal flare, etc...)
cant work properly in a small room. It isnt enough strong to compress the air.
But if the horn (maybe the throat and the mouth too) is smaller, it would have more power against the walls.
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Old 15th August 2005, 12:49 PM   #23
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Decreasing the mouth size in this manner is defeating the object, you're going to end up with a transmission line.
When you say 'inertia of airspace', are you referring to the acoustic impedance of the room as seen by the speaker and enclosure?
The whole point of a back loaded horn design is to create a great deal of gain in the bass region for drivers that are highly damped, by coupling a small moving cone to a large volume of air. This is the requirement even if your room is only small, it's much bigger than the cone displacement volume). Look at the response curves of the fostex driver range, note their Q values and then see what fostex recommend as enclosures. The lower the Q value (damping factor), the more gain the cabinet will have. If the Q of the driver is higher than about 0.3, then a horn isn't really required for hi fi use, unless it's part of a multi way system, as the bass output from the horn will be too high in comparison to the directly radiated middle and high frequencies (unless these too are more efficiently coupled to the listening area).

Outdoor, stadium, Nightclub, theatre and concert P.A. type horn speakers are large, don't usually go very low, and have very high sensitivity over a small bandwidth compared to hi fi horns, and are used in arrays with lots of differing driver/enclosure combinations to cover various frequencies, with active filters and multiple dedicated amplifiers to suite their application.

Most single driver hi fi back loaded horns will perform over a greater bandwidth, with a compomise in efficiency, but this isn't a problem, as the bass horn output only needs to match the sensitivity of the driver in the unassisted mid and treble regions for a balanced overall respose. As a back horn design is a compomise between bandwidth and efficiency, and the efficiency doesn't need to be great, the bandwidth is quite broad and usually performs well into the higher frequencies that are not required from it, where the drivers sensitivity has increased, hence the compression chamber.
Take away that cusion of air, and you'll and up with a big hump in the mid bass and beyond, possibly emphasised by the room modes of your listening environment.
A back loaded horn needs to balance:-
Efficiency (Sd to throat ratio and flare) to match the mid and high frequencies,
Bass extension (length and flare),
Low pass filter (compression chamber) to remove excessive mid bass etc. and of course domestic acceptability.
Typical successful designs balance these variations in a manner that is generally approved by many people, and versatile enough to be located in many sized rooms.
If you're building a bass horn speaker for a smaller room, make either a small one like the cheap trick or fostex recommended 103 or 108, of a larger one with a little less efficiency, to tame overall gain in the confined room volume.

By the way, a PA bass horn will perform exactly the same indoors (near field response), but the enclosed airspace, reflected sound waves and room modes would create a dangerous place to be without ear defenders if fully driven!






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Old 15th August 2005, 05:01 PM   #24
Cortez is offline Cortez  Hungary
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Quote:
Decreasing the mouth size in this manner is defeating the object, you're going to end up with a transmission line.
I didnt say to decrease it to a straight line.
Rather throat: Sd/3, mouth: Sd*8

Quote:
When you say 'inertia of airspace', are you referring to the acoustic impedance of the room as seen by the speaker and enclosure?
Yes.

Quote:
The whole point of a back loaded horn design is to create a great deal of gain in the bass region for drivers that are highly damped, by coupling a small moving cone to a large volume of air.
I know, but note, that already a BR with a little size port can help a lot in bass.
Relate to this, 8*Sd is enough large, isnt ?
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Old 16th August 2005, 07:31 AM   #25
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Rather throat: Sd/3, mouth: Sd*8

With most full range drivers, a throat this small and a mouth this large would be very efficient (depending on flare type and length), increasing the risk of over powering bass in relative to the upper frequencies.
I once made this mistake with a 12cm driver enclosure- throat being Sd/2 and mouth at a massive 25*Sd with a hybrid flare and around 1/4 Fo in length. The efficiency was too high for the room I had intended to use it (due to the Sd/throat ratio not mouth size), the resulting bass output was overpowering the other frequencies greatly.
It sounded far better in a much more damped room with lots of soft furnishings and a deep pile carpet, but still bass heavy.
The problem was that the Q value of the driver was higher than stated in the documentation provided, so the bass gain of the horn was not required.

You mention that a reflex enclosure greatly enhances bass output, but the way in which the port resonates is unlike a horn system.
The port/enclosure volume is tuned to resonate at a specific frequncy and contributes to the overall output of the system over a very limited frequency range, and the maximum linear output from the port is directly related to its size, unlike a horn.

P.A. bass systems often have very large ports or multiple ports so that they remain linear even at high S.P.L.'s.

Shove a sock in the port of most home reflex speakers and you conceive a drop in bass output at normal listening levels. However, lower bass frequencies will usually raise a little, due to blocking up the short circuit, and making the speaker aa acoustic suspension type.

At high levels with a small port, you probably wont notice much of a difference when the sock goes in, due to the port output being comparatively low in relation to the cone output.

I'm still rather partial to acoustic suspension systems for this reason, as well as the superior transient response.








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Old 16th August 2005, 10:21 AM   #26
Cortez is offline Cortez  Hungary
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> With most full range drivers, a throat this small and a mouth
> this large would be very efficient (depending on flare type and
> length), increasing the risk of over powering bass in relative to
> the upper frequencies.
Well, i'am not afraid of this. The bass isnt enough for me now.
My horn is ca. 2m long. What length do you advise ?
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Old 17th August 2005, 03:33 AM   #27
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Default Re: Re: Re: adapt a horn to the environment...

Quote:
Originally posted by Cortez

And what is your view about a design like this ?


No, not the ripple was the point, but the inertia of a closed airspace.
A horn, designed to outdoor use (huge sizes with ideal flare, etc...)
cant work properly in a small room. It isnt enough strong to compress the air.
But if the horn (maybe the throat and the mouth too) is smaller, it would have more power against the walls.
Greets!

It has its uses when done properly, but it's not going to compete with either type of BLH unless you accept its highly resonant response.

Like I said, you want the horn to be rolling off with increasing room gain so it doesn't increase room 'compression'.

GM
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Old 17th August 2005, 04:35 AM   #28
Cortez is offline Cortez  Hungary
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I concentrate in general to the low end, aside from the other resonants above.
My main goal is to build an enclosure wich works effectively in a 30-100Hz region
as wide and as smooth as possible. That's why i'am interested in Karlson designs too,
because there the widerange operation can be achieved. And against the wavy FR
is will i plan to use acoustical filters like a muffler, that low-pass up to 100-200Hz.

With smaller horn sizes adapt to the room size, i meaned for: lets take a big horn,
with relative big rear chamber, and big throat and mouth area. Now lets keep the
length and the flare rate from this horn design, but lets decrease it (the rear chamber too)
proportionately. Wath would happen, what would be different now ?
(Because you sais all the time, that the rolloff depends on the mouth size)
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Old 17th August 2005, 06:01 AM   #29
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Ok, so for example (2m length) you decrease the throat area. This will increase the efficiency of the horn, and decrease it's active bandwidth.
Now you have a mid bass hump.
You also reduce the mouth area. This shifts the lower cutoff frequency and you loose lower bass tones. It also increases the 'comb effect' as the flare rate is reduced causing a more uneven response through the active range.
Now you decrease the compression chamber. This will filter out less of the upper frequencies from being loaded by the horn. Now the mid bass hump is bigger and wider.
If by chance, one of the reinforced harmonics in the mid bass output corresponds to a room mode in your smaller room, you will conceive a loud single note bassline, with lots of missing bass notes.
If you could expect reasonable performance by doing this, I should think you might see more enclosures built like this.
If you want to experiment with the effects of playing with the horn geometry, try the applet at www.ishtek.com/software_ts.html
I've built a pair of horns based on the results from this applet, and the program seems quite accurate.



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Old 17th August 2005, 06:02 PM   #30
Cortez is offline Cortez  Hungary
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Ok, thx, i will try this program!
Otherwise can you tell me, how the horn's long defines the operation ?
Lets fix the rear chamber's volume, the throat area and the mouth area too.
Now just our horn's long is variable. I'am curious specially for the operation, not just the results.

Isnt there a site somewhere on the net about horns with FEA ? (Final Element Method)
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