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Balcro

"Port chuffing" - when & where does it matter?

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When modelling speaker drive units in winISD I understand that to avoid Port Chuffing you should keep Air Velocity below approx 18 M/s.

Mostly, higher Air Velocities occur down at 22 - 35Hz or so, while the level @ 100Hz usually stays at about 3 - 5 M/s

When modelling, I would set the input power level to either full rated power throughout the response range (for those units that can achieve it) or at the
bottom of the traditional power-handling "60Hz dip" so that "xmax" is not exceeded.  

What would be good / best  practice? Any comments, technical or otherwise.

 

Balcro.

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In theory you want no more than 20 M/s within the speaker pass band at full excursion, but temper that with knowing most of the power from the electric bass is an octave up from the fundamental frequency of the note you're playing.

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Posted (edited)

I'd completely agree with Bill on this, which perhaps doesn't move things on but Stevie and I disagree for example, he's much more worried about chuffing than I am. He's demonstrated to me that one of my cabs chuffed (is that a word) at only 10W, however I've gigged that cab for years without ever noticing any unusual sounds. I'd have argued that at those frequencies the output could easily be 18dB down so that's equivalent to 640W of broadband power but I was startled to se that so little power was needed at low frequencies. If you used a conventional bass control however 12dB of boost which would be greatest at the extreme frequencies would make it interesting. One thing we did notice however is that port area used in winISD for calculation of port velocity isn't the only important factor, we found four small diameter ports chuffed much earlier than a single larger diameter port of slightly smaller cross section.

In reality it's a compromise you kind of work out yourself depending upon the design goals you set out. I'd be much less cavalier with a hi fi cab than a bass cab. Often the constraint is the size of the port and the size of the cab. With a small cab a port which kept within your 18M/s could be almost as big as the cab at 40Hz, where would you put it? Another reason for an HPF.

Edited by Phil Starr

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16 hours ago, Bill Fitzmaurice said:

In theory you want no more than 20 M/s within the speaker pass band at full excursion, but temper that with knowing most of the power from the electric bass is an octave up from the fundamental frequency of the note you're playing.

Thanks Bill,

It looks as if I am in the right area with 18 M/S in the pass band.

Balcro

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Posted (edited)
16 hours ago, Phil Starr said:

I'd completely agree with Bill on this, which perhaps doesn't move things on but Stevie and I disagree for example, he's much more worried about chuffing than I am. He's demonstrated to me that one of my cabs chuffed (is that a word) at only 10W, however I've gigged that cab for years without ever noticing any unusual sounds. I'd have argued that at those frequencies the output could easily be 18dB down so that's equivalent to 640W of broadband power but I was startled to se that so little power was needed at low frequencies. If you used a conventional bass control however 12dB of boost which would be greatest at the extreme frequencies would make it interesting. One thing we did notice however is that port area used in winISD for calculation of port velocity isn't the only important factor, we found four small diameter ports chuffed much earlier than a single larger diameter port of slightly smaller cross section.

In reality it's a compromise you kind of work out yourself depending upon the design goals you set out. I'd be much less cavalier with a hi fi cab than a bass cab. Often the constraint is the size of the port and the size of the cab. With a small cab a port which kept within your 18M/s could be almost as big as the cab at 40Hz, where would you put it? Another reason for an HPF.

THanks Phil,

I take your point about the problem with multiple ports. There's probably some deep turbulence / friction physics going on there, because the addition of more ports seems to be increasingly non-linear where resultant port length is concerned.

I've just tried adding some filters to a specimen frequency response and the difference can be dramatic. Air Velocity was reduced from 20 down to 13.2 @ 40 Hz and the reduced Cone Excursion below 35Hz was even better. A definite alternative solution for chuffing, particularly in small boxes. Two bonuses for the price of one.

Balcro.

Edited by Balcro
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17 hours ago, Phil Starr said:

He's demonstrated to me that one of my cabs chuffed (is that a word) at only 10W, however I've gigged that cab for years without ever noticing any unusual sounds.

 

If it's not at a level that's noticeable in use, is there really a problem there?  I get that from an engineering POV it's desirable to minimise it, but if you're juggling port velocity against the size of ports you can fit, surely some turbulence below the point it where causes audible artifacts could be an acceptable compromise. 

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I'm really surprised HPF's aren't more widespread or cheaper as an fx pedal. It's a lot simpler to implement than even a simple tone control. Behringer do a graphic for £18 for example and I don't think the BD121 is much more. There seems to be a real blind spot about it.

 

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Another question for Bill.

Can you expand upon your meaning of passband. I've looked up the meaning of "passband" but it can be a very wide or very narrow range.

In respect of a single bass driver, when you said "within the passband at full excursion" do you mean from the low point where xmax is reached up to the top of the range or is there some other range of frequency response. Thanks.

Balcro.

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Pass band is where the speaker primarily operates. For instance, with a four string you wouldn't care about port velocity below 40Hz, as there's nothing there at sufficient levels to excite it. Full excursion is at the voltage required to reach xmax at the peak above Fb. Not at Fb of course, as that's where excursion is at a minimum. What you don't care about is velocity at maximum Pe voltage, unless you have one of those rare drivers that doesn't reach xmax well below maximum Pe.

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5 hours ago, Bill Fitzmaurice said:

Pass band is where the speaker primarily operates. For instance, with a four string you wouldn't care about port velocity below 40Hz, as there's nothing there at sufficient levels to excite it. Full excursion is at the voltage required to reach xmax at the peak above Fb. Not at Fb of course, as that's where excursion is at a minimum. What you don't care about is velocity at maximum Pe voltage, unless you have one of those rare drivers that doesn't reach xmax well below maximum Pe.

THanks for that.

Balcro

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Bill is  correct that there is little energy bill 40 Hz but with most cabinets which are tuned to give high-efficiency in the passband,  the efficiency below 40Hz is high and gets higher as frequency decreases in a bass reflex cabinet. That is why Phil Starr suggests a High Pass Filter in the signal chain.

Chuffing may cause some compression and purists would be alarmed at that. My cab has the single large port that Stevie specified but I played through Phil’s rig at a gig on Friday and I heard nothing untoward.

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