Are there any rules of thumb for designing a subwoofer port when it comes to port air velocity? I use Bassbox Pro 6. It's not a superfancy program but it does show me air velocity giving the box I design, driver, and power. I notice is changes with port area which seems obvious enough. How do I know how low is low enough that I wont be listening to that "chuffing" sound instead of my music?
between .15 and .2
This IS dependent on power input as well - so whats true for 1 watt will not be for 64 watts.
BTW, most people will never supply more than 64-128 watts to a subwoofer. So a look at 128 watts should be more than sufficient.
(..note that this doesn't mean that you shouldn't have more power for your amplifier. The reason for more power in a sub amp has nothing to do with the wattage, but rather it suggests a higher current power supply which should be more capable of dealing with driver created back emf.)
This IS dependent on power input as well - so whats true for 1 watt will not be for 64 watts.
BTW, most people will never supply more than 64-128 watts to a subwoofer. So a look at 128 watts should be more than sufficient.
(..note that this doesn't mean that you shouldn't have more power for your amplifier. The reason for more power in a sub amp has nothing to do with the wattage, but rather it suggests a higher current power supply which should be more capable of dealing with driver created back emf.)
The generally accepted rule of thumb for preventing turbulence and port compression is to design for a max velocity of 5% of the speed of sound, or about 17 meters per second (~55 feet/sec). This may be increased (some suggest doubled) if a flare of a large enough radius is added to both entrance and exit.
If you are not afraid of a (very) little math, do a search on the site for a formula I posted that can calculate port diameter based on a specific SPL goal and max desired air velocity.
BTW, ignore the back emf nonsense in the previous post.
If you are not afraid of a (very) little math, do a search on the site for a formula I posted that can calculate port diameter based on a specific SPL goal and max desired air velocity.
BTW, ignore the back emf nonsense in the previous post.
Hmm, no offence but if you were able to say with authority that back EMF is/isn't a problem then you wouldn't be asking about vent velocities.
The issue with back EMF is a kind of red herring. The actual issue is that phase shift due to voicecoil inductance around the tuning frequency can give misleading measurements if you simply take the point of minimum impedance (as measured on a simple meter, not a complex impedance meter) to be the tuning frequency.
The issue with back EMF is a kind of red herring. The actual issue is that phase shift due to voicecoil inductance around the tuning frequency can give misleading measurements if you simply take the point of minimum impedance (as measured on a simple meter, not a complex impedance meter) to be the tuning frequency.
ScottG said:
Hmm. 0.15 to 0.2 - wouldn't it be helpful to quote units?
ScottG said:
So what you are saying is that if I put more power into my speakers, the port velocity will go up? Glad you cleared that up for me.
ScottG said:
There's a lot of assumptions in there. Hard to say if that is good advice without knowing the application.
ScottG said:
Aside from the complex power (capacitive/inductive load) issues, there isn't that much going on here. Good amplifiers are designed to run into loudspeaker loads.
It is nonetheless good advice to have some headroom.
"The reason for more power in a sub amp has nothing to do with the wattage, but rather it suggests a higher current power supply which should be more capable of dealing with driver created back emf."
High current is good, but not for the stated reason. Back EMF is maximum at the driver/box resonance, which would be called Fc in a sealed system (but which is still there in vented), giving an impedance peak and a *minimum* in current draw.
And driver velocity and back-EMF is at a minimum at Fb.
"Back EMF is very important and can change the tuning drastically."
How so?
In any case, the whole issue of measurement anomalies can be sidestepped by putting your finger on the cone and feeling for minimum displacement, which tells you the true freq (OK, you have to measure that with a meter) of Fb.
High current is good, but not for the stated reason. Back EMF is maximum at the driver/box resonance, which would be called Fc in a sealed system (but which is still there in vented), giving an impedance peak and a *minimum* in current draw.
And driver velocity and back-EMF is at a minimum at Fb.
"Back EMF is very important and can change the tuning drastically."
How so?
In any case, the whole issue of measurement anomalies can be sidestepped by putting your finger on the cone and feeling for minimum displacement, which tells you the true freq (OK, you have to measure that with a meter) of Fb.
Seriously, I dont think you guys should be bashing Scott. He's making very good points and you're just picking out details that anyone could just assume we knew.... I didn't understand what back EMF was... thought it had something to do with putting the port too close to a wall or something (i.e. back pressure.)
Someone kill this thread before someone gets hurt.
Someone kill this thread before someone gets hurt.
Ron E said:
Hmm. 0.15 to 0.2 - wouldn't it be helpful to quote units?
So what you are saying is that if I put more power into my speakers, the port velocity will go up? Glad you cleared that up for me.
There's a lot of assumptions in there. Hard to say if that is good advice without knowing the application.
Aside from the complex power (capacitive/inductive load) issues, there isn't that much going on here. Good amplifiers are designed to run into loudspeaker loads.
It is nonetheless good advice to have some headroom.
Did alexcd not state that he is using a software program that displayed port velocity? Might a user of such a program have differing results based on power input - and not no why? Is either of my suggestions in *ANY* way "off" from what a novice user, (at least with respect to port velocity), might experience?
In otherwords I gave him information that would be most pertinant to his situation. (..or at least I hoped that I did.)
Now considering room gain, port "gain", average driver efficiency, average tuning freq., etc. - please tell me how the 64-128 watts is "..a lot of assumption.." Remember now, we are specifically referencing a bass reflex system. More over, we are talking about a port tuning that will almost certainly be tuned at or below 20 Hz. This creates a "ceiling", with respect to enclosure size, for most designs strictly because of the port length an diameter. Now I'm not saying that assumption becomes a "rule", but I still believe that the assumption is valid for most bass reflex sub designs. Again, it was a generality - and I think its fairly obvious from my wording that it was intended to be a generality (..so it should not mislead any reader into thinking otherwise).
..and finally,
There is in fact quite a lot going on with driver induced back emf and its interaction with an amplifier. Even "good" amplifier manufacturers do little more than shoot for low output impeadance to give a voltage character to the output, and use an LCR (perhaps even several for varying impeadances) to simulate a real load to then derive distortion measurements. In fact many/most manufactures (even good ones) don't supply enough *continuous* current in the power supply for anywhere near the power levels rated as maximum for the amplifer. So while it is true that a good amplifier is "designed to run into loudspeaker loads", this does not mean that they are designed *well* to run into loudspeaker loads (..particularly where those loads become the most problematic - like where the driver's compliance and mass counter each other and generate back emf).
...................................
Oh, and noah - yes, I'm fully aware that current *draw* is at a minimum near peak impeadance.. You are not looking "deeply" enough into the problem (..i.e. you are thinking from a "foward" perspective, try looking at it from a "rearward" perspective).
alexcd said:
I have to disagree about him making "good points". Nobody is bashing anyone. Questioning, perhaps.....
ScottG said:
Sure alexcd has software, but 0.15 to 0.2 is not a good answer in any context without specifying units. Presuming you mean 0.15 to 0.2 "Mach", as provided by the ubiquitous, free WinISD, the answer is not even a good one.
ScottG said:
My comment was directed to the fact that that is about as obivous as anything could be. Before I ever had software or an engineering education it was quite obvious that port velocity increases with power input.
ScottG said:
ROTFL, no assumptions there at all. First you assumed average driver, average tuning, average efficiency. All I know is that the original poster asked about port velocity and you are then telling them what power level to design their port for - without any foreknowledge of the driver, application, etc. What if it is a 25 watt driver, or a 2000W driver in a car?
ScottG said:
Hence the question about the port velocity
ScottG said:
"You are not looking "deeply" enough into the problem (..i.e. you are thinking from a "foward" perspective, try looking at it from a "rearward" perspective)."
"I'll let Noah speak to that..."
I might if I knew what it meant. I get confused when more than two words per sentence are in quotes.
"I'll let Noah speak to that..."
I might if I knew what it meant. I get confused when more than two words per sentence are in quotes.
alexcd said:
Kids these days......
Ever read Harrison Bergeron?
http://en.wikipedia.org/wiki/Harrison_Bergeron
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