Using grains of rice for port tuning???

[gtirard]

I'm completely ignorant about this topic, I thought port tuning was something super scientific.

What do you think about this technique?

 

[LetItGrowTone]

It is better done outdoors, or in an anechoic chamber if you have one of those. EDIT: credit agedhorse for the advice, of course
Last time I did it indoors and missed by 3Hz (comparing the number I got with the number that the cab designer said it should be). EDIT: I meant that indoors was good enough for me that time, but it's possible for the error to be bigger if a reflection lines up just right.
I used this online tone generator.

If you do this, you could take the opportunity to note that the cab sounds worse below its tuning frequency, and come back and tell people that.

 

[Roxbororob]

Yes It works pretty well. The idea is to do a tone sweep to find the spot where the rice dances less. That gives you resonant frequency of the box. Then you can decide if that's optimal (or what you want) and adjust accordingly. Testing outdoors as @LetItGrowTone says is most accurate, but keeping in mind we gig indoors.

 

[Dino Boreanaz]

I agree with all of the above and will also note that I've successfully used salt for this in the past. I just posted this in another thread describing my experiences with this test.

 

[Tim Skaggs]

Works well. I’ve done it many times. The key is to continue turning down the tone source volume, and to keep finding the point where the cone moves the least. Adjust freq, turn down volume slightly, adjust freq, repeat.

Another thing I’ve used is very small pieces of rubber.

When you get to the freq which makes the substance (rice, etc) on the speaker cone as still as possible, that freq is the resonant freq of the box as it is when tested. You have to adjust port length to get the box tuned to the freq you want it to be for the speaker being used.

I like to use one large (adequate to avoid chuffing) port if the cabinet has the room available. I have set the box to the freq for the speaker being used, & then try slightly longer or shorter port tubes and play through the cab to see if I like something slightly different. I usually go with the freq (as close as I can get it) in the TS parameters for the speaker.

By the way, this IS super scientific, and a lot more accurate than calculating the port size / length from box dimensions & speaker displacement.

 

[agedhorse]

Yes It works pretty well. The idea is to do a tone sweep to find the spot where the rice dances less. That gives you resonant frequency of the box. Then you can decide if that's optimal (or what you want) and adjust accordingly. Testing outdoors as @LetItGrowTone says is most accurate, but keeping in mind we gig indoors.

Testing outdoors has nothing v to do with how we play the cabinet. It’s merely to extract the resonant frequency of the system and it’s done outdoors (or anechoic) in order to prevent reflected energy from skewing the results.

Works well. I’ve done it many times. The key is to continue turning down the tone source volume, and to keep finding the point where the cone moves the least. Adjust freq, turn down volume slightly, adjust freq, repeat.

Another thing I’ve used is very small pieces of rubber.

When you get to the freq which makes the substance (rice, etc) on the speaker cone as still as possible, that freq is the resonant freq of the box as it is when tested. You have to adjust port length to get the box tuned to the freq you want it to be for the speaker being used.

I like to use one large (adequate to avoid chuffing) port if the cabinet has the room available. I have set the box to the freq for the speaker being used, & then try slightly longer or shorter port tubes and play through the cab to see if I like something slightly different. I usually go with the freq (as close as I can get it) in the TS parameters for the speaker.

By the way, this IS super scientific, and a lot more accurate than calculating the port size / length from box dimensions & speaker displacement.

Calculating is just as scientific, one is a prediction the other is a measurement of the result.

 

[micguy]

This method (I have a degree in Acoustical Physics, and worked my entire career designing audio gear, so I've done stuff like this) works just fine. If you have good instrumentation, there are other ways to do this, but that doesn't mean this method isn't a good one.

That said, if you think about it for a bit, this method also explains why miking a cab gives you less than desirable response at low frequencies - at resonance, the woofer is not moving much, and thus is putting out very little acoustically - the port is doing the heavy lifting in terms of where the sound is coming out. So, if you put a mic in front of the woofer cone (where you get decent mids and a bit of high frequencies), you miss out on the low bass freq's. Why does the sound person want to put a DI on your bass? Because a DI will have good bass content in the signal - the mic, not so much.

 

[fdeck]

just don't use cooked rice.

 

[micguy]

Testing outdoors has nothing v to do with how we play the cabinet. It’s merely to extract the resonant frequency of the system and it’s done outdoors (or anechoic) in order to prevent reflected energy from skewing the results.

A good anechoic chamber avoids the need to do it outdoors - having worked at a company where we had a bunch of those, they are valuable, especially in a climate where it's not nice during big parts of the year. Of course, a good anechoic chamber isn't cheap or small - it won't fit in your garage.

 

[HolmeBass]

If the port needs to be bigger, rather than smaller, what happens then? Come to think of it, with the rice test, how do you know which direction the port size needs to go?

 

[fdeck]

If the port needs to be bigger, rather than smaller, what happens then? Come to think of it, with the rice test, how do you know which direction the port size needs to go?

Too much bigger, and you're SOL, though the interaction with the back wall can add a bit of effective length to a port.

However, the conventional wisdom is that you should design your port to have some room for adjustment after measurement. You can figure out the direction from the same modeling software that you used for designing the speaker. And I recommend trying more than one of the free programs and see if they agree, before building anything.

In addition, for us DIY'ers, I always recommend what I call "poor man's tolerance analysis," which is to vary each parameter a bit in the modeling program and see how much it actually changes the performance of the speaker. You can enter the measured tuning frequency into the design program and decide if it's actually worth correcting.

We have no choice but to use the tools that are available and practical. I do all of my testing indoors. I haven't blogged about it yet, but I think you can predict the overall response of a ported speaker in the bass region from a measured curve taken as close as possible to the cone. This is because the cone and port response are both complicated functions of all of the design parameters, but the relationship between the cone and port curves is a much simpler function of just one parameter -- the port tuning frequency.

 

[micguy]

I haven't blogged about it yet, but I think you can predict the overall response of a ported speaker in the bass region from a measured curve taken as close as possible to the cone. This is because the cone and port response are both complicated functions of all of the design parameters, but the relationship between the cone and port curves is a much simpler function of just one parameter -- the port tuning frequency.

It's more complicated that that - there are a handful of parameters that interact, including the Q of the driver, which can affect things quite a bit - a low Q driver will move less at the port resonance, but give more port output - the relationship between woofer output and port output that you speak of (that you are hoping is fixed) changes with that parameter. You might believe you can predict the output of a ported system from a nearfield measurement, but if you do enough experimentation, I'm pretty sure you'll back off of that assertion at some point.

 

[Element Zero]

Add my wife’s broccoli beef and we all eatin’ good.

 

[fdeck]

It's more complicated that that - there are a handful of parameters that interact, including the Q of the driver, which can affect things quite a bit - a low Q driver will move less at the port resonance, but give more port output - the relationship between woofer output and port output that you speak of (that you are hoping is fixed) changes with that parameter. You might believe you can predict the output of a ported system from a nearfield measurement, but if you do enough experimentation, I'm pretty sure you'll back off of that assertion at some point.

Indeed, it's only based on the equations at this point. I've done some measurements, but I face the same dilemma as all DIY'ers, which is that the basic electromechanical theory -- if the parameters are known -- is probably more accurate than our response measurements, in the bass region.

I owe everybody a blog article. It'll make sense when you see it.

 

[agedhorse]

This method (I have a degree in Acoustical Physics, and worked my entire career designing audio gear, so I've done stuff like this) works just fine. If you have good instrumentation, there are other ways to do this, but that doesn't mean this method isn't a good one.

That said, if you think about it for a bit, this method also explains why miking a cab gives you less than desirable response at low frequencies - at resonance, the woofer is not moving much, and thus is putting out very little acoustically - the port is doing the heavy lifting in terms of where the sound is coming out. So, if you put a mic in front of the woofer cone (where you get decent mids and a bit of high frequencies), you miss out on the low bass freq's. Why does the sound person want to put a DI on your bass? Because a DI will have good bass content in the signal - the mic, not so much.

Yup, DI low frequency extension is more constant than mixing, but the tradeoff is a more neutral mid tone (which may also be an advantage to some).

 

[beans-on-toast]

The right type of dry sand works well, as opposed to rice, when the speaker is test driven with a solid state amp.

 

[vvvmmm]

This method (I have a degree in Acoustical Physics, and worked my entire career designing audio gear, so I've done stuff like this) works just fine. If you have good instrumentation, there are other ways to do this, but that doesn't mean this method isn't a good one.

That said, if you think about it for a bit, this method also explains why miking a cab gives you less than desirable response at low frequencies - at resonance, the woofer is not moving much, and thus is putting out very little acoustically - the port is doing the heavy lifting in terms of where the sound is coming out. So, if you put a mic in front of the woofer cone (where you get decent mids and a bit of high frequencies), you miss out on the low bass freq's. Why does the sound person want to put a DI on your bass? Because a DI will have good bass content in the signal - the mic, not so much.

Seems that might be an argument for room-mic'ing (a good room). I gotta wonder - and I've never heard of this - is mic'ing the port of benefit? Of course, it's why I take a DI in parallel to whatever mic (I tend to use MD421 II, RE 320 or SM7b) into whatever pre (I like The Brick and Meeks, have used and liked 1176 straight in). And fwiw, lately I often don't use the DI at all by paralleling two different pres off of the mic - seems to sit pretty good with a bit of HPF, sometimes a lot.

 

[5StringBlues]

Use brown rice to help you dial in a more organic tone

 

[Clark W]

Use brown rice to help you dial in a more organic tone

What happens if you use wild rice?

 

[basscooker]

You should check out the videos of the ones where they put sand or rice on a flat plate and do frequency sweeps. Really cool.