A quiz for you folks

[454Casull]

1) Is it possible to select a conventional electrodynamic driver and operate it, in a music-reproduction application, in such a way that it is able to maintain true omni behaviour (no reflectors) to a higher frequency than what the frame size, and thus baffle size, would dictate?

2) A small driver is placed into the end of a tightly-fitting pipe so as to minimize its apparent baffle size, as the Pluto (and others) have. The driver suffers significantly higher distortion when ventilation around the frame is poor, as is normally the case with small drivers. Does the addition of felt or similar absorptive material as a lining inside the pipe near the driver help, or hurt the driver?

FYI - I do not know the answer to either. My first thought for 1) was to fire the driver into a pipe narrower than the diaphragm so that the sound wave at the end sees a much smaller "baffle", allowing for a higher baffle step. Doing this, though, gets you into pipe resonance, even if you severely limit the passband.

[AllenB]

I assume that when you say omni with a cone driver, you are looking at the pluto. I don't see why you cannot create a small baffle opening like you suggest. Think of it as a horn throat, pointing upwards. Your driver operating as a compression driver. You could create a 360 degree horn by laying a disc above it.

[whgeiger]

Originally Posted by 454Casull

1) Is it possible to select a conventional electrodynamic driver and operate it, in a music-reproduction application, in such a way that it is able to maintain true omni behaviour (no reflectors) to a higher frequency than what the frame size, and thus baffle size, would dictate?


2) A small driver is placed into the end of a tightly-fitting pipe so as to minimize its apparent baffle size, as the Pluto (and others) have. The driver suffers significantly higher distortion when ventilation around the frame is poor, as is normally the case with small drivers. Does the addition of felt or similar absorptive material as a lining inside the pipe near the driver help, or hurt the driver?


FYI - I do not know the answer to either. My first thought for 1) was to fire the driver into a pipe narrower than the diaphragm so that the sound wave at the end sees a much smaller "baffle", allowing for a higher baffle step. Doing this, though, gets you into pipe resonance, even if you severely limit the passband.

... to both questions. But that answer is of nominal utility, so:

1. Only point sources are omni-directional (radiate complete spherical wave fronts). When signal wave length becomes smaller than driver diaphragm dimensions, the driver will no longer exhibit point source behavior. At the same time this is precisely where the onset enclosure and diaphragm resonances become a concern as well. You can enclose the driver any way you want, but that will not materially change the physics of the matter.

2. To dissipate heat away from the a driver motor, use a metal enclosure, heat conducting gaskets and heat sink the voice coil. Use a BR design to get air flow inside the enclosure. Line/fill the enclosure as needed for acoustic reasons as the heat problem has already been addressed. n.b., One of the reasons for voice coil overheating is that the driver is not responding to the signal sent to it. So condition the signal and send the filtered out-of-band components to driver(s) designed to handle them.

Regards,

WHG

[454Casull]

Originally Posted by AllenB

I assume that when you say omni with a cone driver, you are looking at the pluto. I don't see why you cannot create a small baffle opening like you suggest. Think of it as a horn throat, pointing upwards. Your driver operating as a compression driver. You could create a 360 degree horn by laying a disc above it.

It is not the size of the opening but the size of the apparent baffle that determines the baffle step transition. Won't matter what size hole you use, if your "baffle" is still the same diameter as your driver.

By putting in the pipe, the driver is far away enough that the sound will see, practically speaking, only the end of the pipe.

[454Casull]

Originally Posted by whgeiger

... to both questions. But that answer is of nominal utility, so:

1. Only point sources are omni-directional (radiate complete spherical wave fronts). When signal wave length becomes smaller than driver diaphragm dimensions, the driver will no longer exhibit point source behavior. At the same time this is precisely where the onset enclosure and diaphragm resonances become a concern as well. You can enclose the driver any way you want, but that will not materially change the physics of the matter.

Yes, were I to use a bare driver I would LP before it became directional... but this usually happens at a frequency higher than that for baffle step. So anyway, how would you accomplish this? I am quite interested and may well use your technique in my next system.

2. To dissipate heat away from the a driver motor, use a metal enclosure, heat conducting gaskets and heat sink the voice coil. Use a BR design to get air flow inside the enclosure. Line/fill the enclosure as needed for acoustic reasons as the heat problem has already been addressed. n.b., One of the reasons for voice coil overheating is that the driver is not responding to the signal sent to it. So condition the signal and send the filtered out-of-band components to driver(s) designed to handle them.

Regards,

WHG

Certainly one can allow the motor structure to release heat more readily, which is good for long-term power handling, but is peak power handling not determined mostly by the voice coil? All the resistive heating that happens in an instant is generated throughout only the copper.

[AllenB]

Originally Posted by 454Casull

It is not the size of the opening but the size of the apparent baffle that determines the baffle step transition. Won't matter what size hole you use, if your "baffle" is still the same diameter as your driver.

By putting in the pipe, the driver is far away enough that the sound will see, practically speaking, only the end of the pipe.

A baffle step is a change in directivity. If we can ensure a fixed omnidirectivity, that implies a freedom of baffle step issues, does it not?

[crazyhub]

Originally Posted by 454Casull

2) A small driver is placed into the end of a tightly-fitting pipe so as to minimize its apparent baffle size, as the Pluto (and others) have. The driver suffers significantly higher distortion when ventilation around the frame is poor, as is normally the case with small drivers.

Hi,

Higher distorsions aren't obligatory due to VC heat in such tightly-fitting enclosures and may be linear ones:
chamfering driver holes
And not to forget the study of midrange enclosures by Jim Moriyasu.

[454Casull]

Originally Posted by AllenB

A baffle step is a change in directivity. If we can ensure a fixed omnidirectivity, that implies a freedom of baffle step issues, does it not?

Omni from a regular electrodynamic driver comes from two things:

1) Assuming pistonic behaviour (which we want), the driver must be playing low enough that the reproduced wavelengths are significantly smaller than the size of the diaphragm. The rule of thumb that I can recall is the diaphragm should be larger than 1.0x (?) highest wavelength, or directivity narrows.

2) The midpoint of baffle step happens at approximately 115 (m/Hz) / baffle width; the passband should terminate at or before this point.

Both, not either, are required for omni.

[AllenB]

If you fire the driver into a narrow tube, you could circumvent the drivers directivity by using a phase plug (as used in compression drivers).

Then you are left with a tube and I would think a 360 degree horn would terminate this so it may sound good.

Of course cone breakup will be close behind.

[454Casull]

The tube will have resonances if it is to be otherwise useful:

Acoustic resonance - Wikipedia, the free encyclopedia

This is what I alluded to in my first post. Ordinarily the driver is low-passed before resonances are excited but it is not possible in my application. Perhaps some form of TQWT would help but I do not know how to use those.

Oh, and no phase plug necessary as the wavelengths are long enough that the diaphragm's shape doesn't matter.