How accurate does bass reflex tuning need to be? - diyAudio
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Old 24th May 2012, 12:34 PM   #1
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Default How accurate does bass reflex tuning need to be?

How accurately do I need to build the volume of my bass cabinet?

If I am aiming for 29 litres, say, what is the level of give and take that is allowable?

I know that I need to deduct the volume of the bass driver, and the reflex port.

This brings me to my next question: How do I measure the right length of reflex tube? If they were just tubes, this would be simple, but most have flared ends. Do I ignore the flares and just measure the parallel sided dimension? Again, what accuracy is needed?

If I am 10% out in cab volume or tube length for example, what would be the affect to the tuning?

I will be using a ScanSpeak 22W8851 driver. Strassacker recommends a 29 litre cabinet with a HP70 reflex tube 28cm long. This would achieve 40/30Hz (-3dB/-8dB).
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Old 24th May 2012, 12:45 PM   #2
Boscoe is offline Boscoe  United Kingdom
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Try it? Have you got WinISD? Try the different volumes and lengths and see how they affect performance. I would also say that the room conditions will change the response more than the odd centimeter on the tube length.
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Old 24th May 2012, 12:54 PM   #3
AndrewT is offline AndrewT  Scotland
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My first reflex (ported box) did not perform all that well. I built it exactly to the design manual that came with the driver.

Since then I have always "tuned" the reflex using adjustable length ports.

I have come to believe that the "ideal" response that is predicted by the graphs (whether by software or by manual calculation) is not exactly achievable with a preset fixed length port.
In addition the amplifier and the cables and the room interact with the response and this requires further "tuning" to get a sensible response that gives a realistic output arriving at your ears.
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Old 24th May 2012, 01:02 PM   #4
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10% is fine , no audible differences , but measurable for sure . room temperature , barometric pressure and humidity can change your driver's T/S parameters up and down for at least the same percentage .

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Rens
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Old 24th May 2012, 04:07 PM   #5
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To be accurate, you should measure the impedance and see and listing !
The tuning depends on what is in the box ...
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Old 24th May 2012, 04:12 PM   #6
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Quote:
Originally Posted by jerome69 View Post
To be accurate, you should measure the impedance and see and listing !
The tuning depends on what is in the box ...
This is what I did.

I used an AC ammeter (Fluke multimeter), and used a frequency generator to find where current was maximum. At that point, impedance is minimum, which is indicative of port tuning.
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Old 24th May 2012, 04:22 PM   #7
AndrewT is offline AndrewT  Scotland
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But that tells you or the room nothing about what Qbox is. Is Qbox ~0.6, or 0.7, or 0.8, or 0.9, or 1.0, or good God, I hope you don't aim for Qbox>1.0

I hate high Q boxes. They never sound "right" on a wide range of music and voice.
I prefer lower Q and fairly recently discovered that low Q boxes can also sound quite accurate.
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Old 24th May 2012, 04:40 PM   #8
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Quote:
Originally Posted by AndrewT View Post
But that tells you or the room nothing about what Qbox is. Is Qbox ~0.6, or 0.7, or 0.8, or 0.9, or 1.0, or good God, I hope you don't aim for Qbox>1.0

I hate high Q boxes. They never sound "right" on a wide range of music and voice.
I prefer lower Q and fairly recently discovered that low Q boxes can also sound quite accurate.
So how do you find the 'Q' of a box?
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Old 24th May 2012, 04:53 PM   #9
AndrewT is offline AndrewT  Scotland
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A bit like testing for Q when doing the T/S parameter measurement.

The spread of the voltage around the peak gives the height to width ratio of the peak. That is roughly the Q of the port output. It's similar for a sealed box. No idea for a TH.
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Old 24th May 2012, 08:21 PM   #10
tvrgeek is offline tvrgeek  United States
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Better than 20% you are kidding to yourself. Voice coil temp, surround variation, humidity, BP and so on. Of course, we try for better. We try for 5%. BTW, I too like low Q. .6 seems about right. WT-2 is an easy measurement tool.
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