Driver to vent spacing

[danielm]

With the driver and vent on the front baffle is there an optimum spacing between the two?

It seems to me there would be in phase and out of phase conditions radiated from the vent within the passband.

Or, does that fact that all the sound emits from one source, the cone, prevent this?

[jimbo1968]

Same question from me, but with the details that my vents are a ring of eg 15mm holes around a CHR-70.

[albin]

I once dripped water into different shaped bowls.Then you could think about that trick of putting a speaker at your listening position and your head in the room where it sounds best.My thought was to put the opening of a vent(or line) kind of mutually opposite to the driver in the box.But I'm no 'spert or anything.
regards Albin

[Scottmoose]

Originally Posted by jimbo1968

Same question from me, but with the details that my vents are a ring of eg 15mm holes around a CHR-70.

Oooh. A Thuras style box. Nice touch.

Basically, it depends on the type of enclosure, the dimensions of said cabinet & various other factors.

[event horizon]

I don't think that there is any optimal spacing when considering frequencies where the driver is effectively unload by the enclosure & port. Below Fb it won't make any real difference as the driver will still be unloaded.

However, it will make a difference if the driver is expected to do some upper bass & midrange duty to. If the port is close to the bass driver there is the distinct possiblity of the rear radiation from the driver reflecting off of the back of the enclosure & straight out of the port

We all know that reflex enclosures tend to have minimal damping material in the centre of the enclosure (or they wouldn't work too well) & it tends to be concentrated on the enclosure walls. This isn't ideal as it tends to be quite thin so there is the possibility of this happening.

So it's best to keep the port a good distance from the driver if it'll be doing midrange duties to.

IMO

[danielm]

Thanks.

The freq range of 200-600hz coming from the port is my concern. There could be distructive summing.

Maybe if the port area is large enough it could be made long with a few bends to act as a bp filter.

Maybe I'll just make up a test box with a moveable port and find out.

[danielm]

I guess a passive radiator would solve this, huh.

[GM]

Originally Posted by danielm

With the driver and vent on the front baffle is there an optimum spacing between the two?

It seems to me there would be in phase and out of phase conditions radiated from the vent within the passband.

Or, does that fact that all the sound emits from one source, the cone, prevent this?

Depends on the cab's size, shape as Scott noted.

There normally is, but if properly designed/implemented only its fundamental will be audible and the phase rotation will be below most folk's threshold.

The vent is a half WL resonator, so has a separate air mass plug with a fundamental and both even and odd harmonic structure, so is a musical instrument in its own right.

GM

[GM]

Originally Posted by event horizon

However, it will make a difference if the driver is expected to do some upper bass & midrange duty to. If the port is close to the bass driver there is the distinct possiblity of the rear radiation from the driver reflecting off of the back of the enclosure & straight out of the port

So it's best to keep the port a good distance from the driver if it'll be doing midrange duties to.

Hmm, as presented, this is impossible since the vent's air mass plug will stop it. What it can do though is excite it if the reflection wave is low enough in frequency, but this normally can only occur where the vent is directly behind, and far enough away from, the driver, such as in some tube sub designs.

Actually, for a true reflex it's best to have it as close as practical to both maximize vent efficiency and driver damping, though for sub duty the WLs are so long that vent location isn't an issue as long as the cab doesn't have a high enough aspect ratio to generate 1/4 WL action in the vent's pass-band. If the vent is large and/or long though, its pipe harmonics can audibly comb filter with the driver's output.

The Onken is a good example. Indeed, many folks like it for this very reason as it adds a bit of 'richness'/'fullness' to the lower mids while a well damped reflex, MLTL, etc. will be more accurate yet can sound too 'dry'/'analytical' for some.

GM

[GM]

Originally Posted by danielm

I guess a passive radiator would solve this, huh.

True, but not worth the expense unless a really large, long vent is required and even then, making a TQWT may be more economical overall if keeping the cab small isn't a priority.

GM