Adding Time Delay to a Crossover

[69stingray]

Is there any information available on how to add a time delay to a crossover? For example, if one was designing a bipole speaker and wanted to delay the forward firing speakers by a couple milliseconds.

Is this even possible without and active crossover?

[richie00boy]

It's not worth trying passive. You can physically step (or angle) the baffles, other than that go active. Active is far better anyway

[phase_accurate]

speakers by a couple milliseconds

Do you really need milliseconds ? That is quite a lot. 1 ms is the same as a displacement of 30 cm approx.

Regards

Charles

[69stingray]

Do you really need milliseconds ? That is quite a lot. 1 ms is the same as a displacement of 30 cm approx.

I was thinking about how one would go about designing a bipole speaker with rear firing drivers. The sound from the rear firing driver would have to travel to the wall (approx. 1 ft lets say) and then back to the front of the speaker (approx. another 1 ft less say).

Is there a way to add a delay to the front firing speakers of approx. 0.001765 sec with a passive crossover?

This is more of an educational post for me. I assume there is a way to do it. Just have not read or seen anything on it.

[phase_accurate]

Is there a way to add a delay to the front firing speakers of approx. 0.001765 sec with a passive crossover?

1.) No this is definitely beyond the capabilities of passive crossovers.

2.) Why do you want to do this at all ? I assume it is the woofer that you want to build a bipole, isn't it ?

Regards

Charles

[AndrewT]

Hi,
the rear firing woofer is mostly omnidirectional.
The soundwave does not need to bounce off the adjacent wall to return to the front.
The sound wave simply travels around the speaker box and the shortest path length from front to back is the delay involved.

The delay you need will be quite a bit less than you thought necessary.

[kevinkr]

I think the bipole speaker design 69stringray is asking about might actually be intended to operate full range in which case a delay between the front and rear drivers might be required.

IMO I don't think passive XO all pass filters are practical and may not even be possible. Active versions which are possible would be a huge pain to design, with lots of stages to get a consistent delay across the band of interest and not very tunable either. Seems like this would be something best done with DSP.

[myhrrhleine]

What you'll need to do is design an 'all-pass' phase shifting network.
It'll give you the delay you seek.

It's usually considered best if you use a digital delay line or 'bucket-brigade' line, but those are low voltage/current solutions.

[Tenson]

Indeed, I asked a similar question before. All-pass is the passive answer.

However, the result will not be what you imagine. By delaying one part of a bi-pole design you will move the null that usually occurs at the sides. By changing the delay you will move the null towards either front or back of the cabinet. This is how some PA speakers get cardioid bass response with delayed rear woofers.

http://www.meyersound.com/mseries/m3d-sub/

Basically put, you don’t need to bother. The slightly delayed rear reflections is what gives bi-poles there great soundstaging and imaging. It gives your mind spatial location information.

[Zaph]

Here's what a passive ladder delay network looks like, at least as it's laid out in SoundEasy. Values L6, C8, C9 and L10 are the circuit. (everything else in this network is response shaping and impedance control)

I occasionally do these on LR2 designs with large woofers mounted on flat (non-tilted) baffles. It's a way to make up for the difference in acoustic centers. I've never published a design like that because of it's complexity and the tendency of people to look at the crossover and say "WTF is that?"

If you use Soundeasy, you can easilly model bipoles using the virtual front panel and system frequency domain. There's a couple ways to do it, but the way I've done it is to measure a front woofer from a fixed distance (say 1m or a 1/2 m) then measure the rear woofer, but from the front at the same distance from that woofer. On the front panel, move the back woofer Z location to where it actually is. Assuming all the phase information is accurate, what you see when you look at the response combined... is the reason I don't recommend bipoles. Big nulls in the response that are not only measureable, but easily audible. But, using the system frequency domain, from there you can model in the ladder delay network to minimize the nulls if you wanted. The values will be huge and the result will still be far from perfect. Plus when you put the speaker in the room, the tonal balance usually needs a lot of work. Let's just put it this way: mounting a woofer on the back covering the same range as a woofer on the front is *NOT* the best way to resolve baffle step compensation.

All that said, I think rear firing speakers have value, but they are best used above the baffle step frequency. Then you will get the in-room soundfield that type of design can offer. It's not entirely accurate reproduction, but it can be enjoyable anyway.