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[Marcel Groen]

Hi snowflake,

I also don`t know how to get that extra loading under the passband off the horn. I you look to the impedance chart of the TH115 i think that the first two resonantpeaks from the right are those from the real horn. The one on the left must have a relationship with the pressure that comes from the front of the speaker. The pathlengt of the horn must be calculated in otherwise you could not get the resonantpeak at the point you wanted to be if you model this type of loudspeaker.
But if you calculated it with only the speed of sound, like you mentioned, you could not see a relationship between the pressure on the back of the driver and the pressures that comes from the front. Now i am not to familiar with phase, but it could be that some delay is accounted in that is caused by phaseshifts. I don`t know what the phase will do under the passband of the horn. And can real acoustic phase be modeled with a equivalent circuit(Leach).
At the time the speaker is moving backwards there must be pressure on the back of the diagram, at least i think so. When the full pressure(from the front) must come on the back of the driver i don`t, maybe at the time when it`s going back or maybe at his neutral position.
This pressure could be calculated like a sort of back emf in the acoustic circuit, just like the acoustic/mechanic circuit introduce the back emf to electrical circuit.
Maybe i am totally on the wrong road, i am just trying to understand it with logical explanations.

Marcel    

Ps. my englisch is not very good.  

[Michael_Elliston¶]

snowflake wrote on Tue, 20 September 2005 08:03

I have been trying to figure out what is happening inside the tapped horn designs for a while.  On a few of the pictures you can see either where one side of the driver is mounted near the mouth or where there is an access panel which presumably gives access to one or other side of the driver.  I can't see any relation between the distance of the back of the driver from the mouth and the frequency response or impedance chart. In some of the designs the back side of the driver is so near the mouth it can't be horn loaded and its output must be insignificant compared to what is coming down the horn. I have seen a design where the back chamber has a port that enters the horn further back than the driver http://www.audioasylum.com/forums/hug/messages/87962.html but Tom Danley replied to that thread suggesting that it is difficult to load a vent with a horn because of the frequency dependent loading of a horn.  Perhaps you could use several ports all loading over a very small frequency range. Perhaps the tapped horn uses several small apertures to form the horn throat.  I'm not sure if it is viable but could the size of the apertures be used to offset the low impedance presented by the horn at certain frequencies? Phil

Hi Phil,Well I  dont know if it applies here,but when a reflex system has several different ports,they sum to one 'inbetween port'

Im really interested in this stuff too. I expect some FEA would help.

Mike.e

[Tom Danley]

Hi guys

A bunch of people asked questions, I will try to answer.

I don’t think it would be fun or fair to just explain how to build a Tapped horn although Walt seems very close.
Rather, what I can do is to mention a few “clues” or things, which lead to the approach.

When working with the Unity and later Synergy horns, it was clear that the acoustic passageway called a horn could be driven from a location other than at the “small end” with only a few differences in operation, all of which can be modeled.

The bass reflex approach provides a phase inversion (at resonance) which allows the rear radiation to be additive with the front.
This approach depends heavily on a high Qb in order to have a high Q in operation, horn loading a port produces a significant acoustic load which greatly alters the Qb.
Bottom line is the port stops working properly if significantly loaded by the horn.

The real problem with a horn that is too small is that the radiation load presented to the driver changes too much vs frequency, in a perfect horn (the kind assumed in horn theory) the radiation resistance is constant.
Peaks in the output are normally associated with dips in the impedance which causes “more” power to be delivered, producing the peaks.

The Tapped horn approach can be smaller because at the low cutoff, the drivers appears to have a smaller radiating area (but the same motor), which raises the electrical impedance, lowering the peak.
Above the low cutoff peak, the driver can appear to have as much as twice its original radiator area (still on the same motor) which raises the dip by having a greater area / load.

The idea in the patent application is that the drivers acoustic source impedance tracks the horns acoustic load impedance (when everything is right) which results in a MUCH flatter response and greater efficiency than a conventionally driven horn the same size could produce.


One cool benefit of the approach is seen when one compares the group delay of a Vented box and Tapped horn with an identical low cutoff.
The PB-12 tower for example has a measured group delay which hovers around “zero” down to about 30 Hz.  
A vented box with the same low cutoff would already have about 10 –12 ms GD at 30 Hz.     One can picture the effect of GD more clearly when you recall that this is like moving your speaker to the rear in time at a rate of 13.5 inches per millisecond of delay.

The other effect of the Tapped horn can be realized when you consider that alone, the driver in the Tower has a sensitivity of about 88 dB 1 W while it is a minimum of 95 dB 1W 1M when in the Tapped horn.

If you guys are inclined to and are near Atlanta, call the shop and go get a demo and see what you think.
Best,

Tom Danley

Danley Sound Labs

Stephan,  So far as why is the Sky Blue?, I would guess it has something to do with the fact that liquid oxygen is also sky blue.
Liquid oxygen has gotten a bad rap too, look what you can do with it.
Hungry? Fire up the bbq. Yumm.

http://www.doeblitz.net/ghg/

[Phil Pope]

I'm still struggling on the tapped horn but I do know why the sky is blue.  Dust in the atmoshphere scatters the shorter blue wavelengths in sunlight to a greater degree than the longer red wavelengths.

Phil

[Tom Danley]

Hi
True, but the link is more fun than talking about Rayleigh scattering.
Cheers,

Tom

[Timber_MG]

Is it usefull to look at only the m/nodal behaviour of a pipe fired into one end with a rear chamber "tapping" into this pipe (tapered or not) at a position/positions conducive to your requirements? The "tap" if caught at a pressure maximum would act like a flute and dampen an undesired mode and likewise for the driver's relative poition to the closed end of this pipe and one has to go and position the driver and "tap"in a geometrically fortunate position?

The braodband dampening would be similar to an open tranmission line due to the signifficant differene in arrival time of the rear-wave?

[Walt de Jong]

Hello,

I think it is quite useful to use the (one dimensional) wave equation to understand the principle behind tapped horn:

dp^2/dt^2=c^2 x dp^2/dx^4

With c=speed of sound (this wave equation can be found by solving the mass, impulse and energy balance equations for a small control volume of air in the horn)

The most easy way to solve this by assuming a solution in the form:

P(x,t)=X(x) x T(t)

By using the correct boundary values you will find that X(x) describes the modeshapes in the horn. T(t) gives the timeresponse. For a non-constant cross-sectional area solving this partial differential equation is not easy but it can be done. In fact the principle of 'nodes' in a transmissionline is a result from solving the wave equation. I think most people don't realise this. If you know whats behind the obvious (logical) theory of nodes it much more easy to study new principles such as the tapped horn.

Best regards,

Walt

[drewgandy]

Tom Danley wrote on Wed, 21 September 2005 12:16

Hi guys The PB-12 tower for example has a measured group delay which hovers around “zero” down to about 30 Hz.   A vented box with the same low cutoff would already have about 10 –12 ms GD at 30 Hz.     One can picture the effect of GD more clearly when you recall that this is like moving your speaker to the rear in time at a rate of 13.5 inches per millisecond of delay.

Perhap I'm not fluent enough in group delay but I don't understand how it can "hover around zero" when measured outside the enclosure.  Isn't there some delay at least based on the distance of the driver from the mouth?  

[Mark Seaton]

chmed wrote on Tue, 27 September 2005 22:49

Perhap I'm not fluent enough in group delay but I don't understand how it can "hover around zero" when measured outside the enclosure.  Isn't there some delay at least based on the distance of the driver from the mouth?

Might it make more sense if it was stated as "was pretty much constant" through the stated range?  

Most measurement systems allow you to define some t0 to get a more useful view of the data, and in some cases, to make the measurement possible at all.  Effectvely you are just removing the time of flight, or fixed delays in the system.  In most LF systems that time of flight is greater than the physical distance from speaker to mic, and even more so in most bass horns.

[Tom Danley]

Hi guys

Mark answered the question but I can give a bit more explanation.
Group delay began as a descriptor of the difference between the delay at one frequency compared to another.  
The fixed delay component,  related to time of flight and driver internal delay is not part of Group Delay.
As a result, a drivers GD is unchanged by changing the mic to speaker distance.
While a horn can have a larger fixed delay (due to the additional path length), they can also have less group delay than a direct radiator / vented system, as in the PB-12 Tapped horn.

The part about GD which may not be obvious is that all filters have group delay too, not just a speaker. In fact, any normal filter than changes a signal’s amplitude, also produces GD.

Cheers,

Tom Danley