You've heard this: A ported box will never be as tight as a sealed box. The fact that the group delay is so much larger tells you everything you need to know! With a sealed enclosure you get closer to zero group delay!
So while it's true that a ported box will have more group delay than a sealed box, you can design a ported box so the group delay is imperceptible. In other words, since you don't sense it, the ported box will sound just as tight as a sealed box.
In this blog post, we'll talk about what group delay is, why it happens, and why group delay is larger in a ported system. We’ll also discuss some findings from “back in the day” where engineers smarter than me unlocked acceptable group delay numbers!
Let's jump right in:
Wikipedia has this to say about Group Delay:
Group delay describes the time shift of the envelope of a wave packet, a "pack" or "group" of oscillations that travel together, centered around one frequency.
These delays are usually frequency-dependent, which means that different frequency components experience different delays. As a result, the signal's waveform experiences distortion as it passes through the system. This distortion can cause problems such as poor fidelity analog audio…
In other words, group delay is when a signal is distorted through a time shift. This time shift in a subwoofer system happens because air compresses and rarefies. This compression and rarefication happens in a sealed box, too. It’s just that adding a port adds another mass of air to compress and rarefy, which adds complexity.
How does this manifest as distortion? Think about the natural frequency and harmonics generated by plucking a bass guitar. Some of the sounds are going to arrive in your ears later than others. In the case of a sealed enclosure, the lowest frequency arrives last. In the case of a ported enclosure, The frequency centered around port tuning arrives at your ears last.
So how do we control group delay in a ported enclosure? We make sure port tuning and enclosure volume are a good fit for the driver.
But let’s understand something: just because the group delay in a ported enclosure is higher than the group delay in a sealed enclosure, that does not mean your ears can discern it. There is a threshold of audible group delay, and this threshold increases as frequency drops. At 60hz, the threshold of audible group delay is eleven and a half milliseconds. At 50hz, the threshold of audible group delay is thirteen milliseconds, at 40hz, the threshold is 15ms, at 30hz the threshold is 18.5ms, and at 20hz are 25ms.
Controlling Group Delay
The tuning frequency will affect group delay. If we maintain enclosure volume and increase tuning frequency, we will increase group delay.
Increasing enclosure volume will also increase group delay. The opposite is also true. Decreasing ported enclosure volume will also decrease group delay.
Common Mistakes and Solutions
The biggest mistake you can make when you design your subwoofer box is tuning for big SPL centered around one frequency. These will have a distinct ring and a lot of perceptible group delay. Okay, this is only a mistake if you’re building a box for music. If you’re building a one-note wonder for SPL trophies, group delay isn’t an issue. We don’t care about group delay if you’re burping for trophies with limited power.
Another mistake is using a prefabricated ported box whose specs are unknown. Some of these have a relatively high tuning frequency to maximize output. The high frequency will blow past the audible threshold at the high frequency and produce bad sound. This is likely the source of the myth that ported enclosures sound bad.
The easiest way to minimize audible group delay is to use an enclosure volume and tuning recommended by the driver manufacturer. They know the product better than we do, that’s for certain. They also don’t want their audience to say “Yeah, I used their enclosure specs and it sounded awful.” because the next sentence would be something like “Go with another subwoofer driver”
While I want everyone to experiment on their car audio journeys, I do want to point out that people smarter than you and I have provided some guidelines for easy success. In the case of a tight-sounding ported enclosure that romps while sounding as tight as a live acoustic performance, we can have our cake and eat it, too. Just keep the group delay within acceptable guidelines. Remember that acceptable group delay thresholds, measured in milliseconds, will increase as frequency drops and that increasing enclosure size or raising tuning frequency (or both) in our ported enclosures will increase group delay. Conversely, reducing enclosure volume or lowering tuning frequency will reduce group delay. And we can model group delay within WinISD.
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