Distance for the JBL 2405 to change phase?

[ngccglp]

Hi,

May I know what is the distance I need to move the JBL 2405 for the phase to shift from 0 degrees to 180 degrees? I noticed a significant change in sound by just moving my 2405 about 1 inch forward.

Thanks.

[Jan Daugaard]

Hi Ngccglp,

at 7 KHz -- the lowest recommendable cross-over frequency for a 2405H --, the wavelength is 344 m/s / 7000 Hz = 5 cm, so a 180° phase change is achieved by moving the 2405H 2.5 cm (1 inch) forwards or backwards.

[frank23]

phase, wavelenght and the distance you set them back are related

there is no "one distance" for 180degr phase shift, because it is different for every frequency [one of 7k was calculated above]

what you can achieve by moving it, is acoustical alignment with other drivers in your speaker

maybe the change you heard was because the acoustic alignment was wrong in the first place

however, this is a very complex subject because designers calculate for phase shift due to offset, phase inversion for +/- invert connection and for phase shift caused by crossovers in the crossover range to reach a certain phase characteristic

so a non-acoustical-aligned driver can still have zero phase shift at the crossover frequency, although it might look completely wrong

when working with simple crossovers [6dB for instance, like I use them], acoustical alignment is very nice to have

impulse behaviour makes it even more complex, there you can understand that it is very desireable to have the drivers aligned acoustically as the distance the impulse has to travel from acoustical center to your ears is equal for all drivers and so the impuls at the speaker arrives as an impuls at your ears

frank

[ngccglp]

Thanks guys,

So its correct that the phase shift can take place within a few centimeters. I tried wiring the tweeter out of phase and the sound appears to be more coherent now.

[jbl2441project]

I have four pcs. of the JBL 2405s and i was thinking of using them together with my 2441 drivers crossing over at 8khz , i was wondering if changing its polarity would make it work mounted on the front of the baffle, or should i put in the same plane of the voice coil of the 2441? in phase? is one enough together with one 2441 or can two work as long as they are angled so they dont phase out?

[Allanvh5150]

And to make the problem more difficult to figure out, depending on the crossover that you are using you may get 90, 180 or some other value before you even connect the driver. The allignment of the drivers is more to get the sound from each driver getting to the listening position at the same time, the term is "time allignment". As someone already metioned the phase angle is is stated at the crossover point at one given frequency. If the drivers are time alligned they will all be in phase but after only one wavelength they are all out of phase again. Make sense?:)