Are you experiencing latent low-end? Out-of-phase phatness? Fear not! On this episode of Ask The Expert, Otto talks you through some very simple tweaks and rules of thumb that ensure your subwoofer will play in phase and in time. Every time.
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What is the total latency of Sub 6?
First of all, it makes sense to point out that Sub 6 is a DSP-based subwoofer, which carries certain benefits, for example the fact that you can tailor and optimise the output for the driver inside that cabinet. One of the drawbacks of using a DSP (Digital signal processing) is that it can cause what we call latency.
Latency refers to the total delay of the signal from when it enters the subwoofer until it comes out of the speaker driver. The longer the latency time is, the harder it is to align the subwoofer with the loudspeakers – due to the timing of the sound coming out of the subwoofer being misaligned with the sound coming out of the loudspeaker.
The closer the speaker and the subwoofer are in time, the easier it is to correlate the sound between them, and the better the overall presentation is, as a result. It goes without saying then, that we want to achieve as low latency as possible in our subwoofer. How can we do this?
In a surround sound system, you will often use an AV receiver, and in these devices you can often enter the distance between each speaker and the listening position, so that the AV receiver compensates for that difference in distance.
This becomes slightly more complicated when the subwoofer employs DSP – because the DSP increases latency – and so you need to take this into account when entering the distance in your AV receiver.
In the case of Sub 6, we are fortunate to have an extremely fast DSP, meaning that the total latency of the Sub 6 is only 2.5 milliseconds – this is extremely low for a DSP-based subwoofer, and makes it a lot easier to integrate it both in stereo systems, but also in surround sound systems.
This 2.5 millisecond latency amounts to the same as a distance of 85 centimeters, so – when entering the distance figures into your AV receiver – you will need to add an additional 85 centimeters, which will compensate for this 2.5 milliseconds of latency caused by the DSP, and allow the subwoofer to play in time with the other speakers.
Phase?
The next thing we want is for the subwoofer to properly correlate with the speakers, and for this we need the speakers and the subwoofer to be in phase (see fig1.).
In phase means that the sound from both all sources is arriving to your ears at the same time or – more accurately – that the sound waves are moving forwards and backwards together in sync.
If the subwoofer and the speaker are within about 60 degrees of each other in terms of phase, they will correlate within about 1 dB. 60 degrees of phase amounts to a certain time difference. If we take 40 Hz as an example, 60 degrees of phase is actually around 4 milliseconds. This means that when the subwoofer only has 2.5 milliseconds by itself, we still have 1.5 milliseconds to play with. In conclusion, as long as the subwoofer is within about half a metre of the speaker, they will essentially be in phase.
fig 1.
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