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Helmholtz Resonator

 

Silver Member
Username: Alonzoub

Post Number: 114
Registered: Apr-10
mmmkay, this might be a slightly strange question but... Is it legit to use the equation for a Helmholtz resonator to calculate the tuning frequency of a ported enclosure?

I ask this because ive been using a little program I wrote to calculate box tuning for like a year now, but I misplaced the source code and don't remember where I got it. I am pretty sure it is derived from Helmholtz resonance though, I just want to make sure. I'm using PTC Mathcad for school a lot now and I want to re-write all of my box design programs in Mathcad so I can make graphs and what not.

This is the Helmholtz Resonance formula:
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F = Resonant Frequency
v = speed of sound
A = throat area
V = cavity volume
L = throat length

Thanks in advance for any help.
 

Gold Member
Username: The_image_dynamic

San Diego, California

Post Number: 5952
Registered: Dec-06
I don't have time to actually give this much thought atm, but a ported sub box basically IS a Helmholtz Resonator, so it makes sense.
 

Gold Member
Username: Southernrebel

Monroe, Louisiana Team Audible...

Post Number: 8495
Registered: Mar-04
http://www.diysubwoofers.org/misc/portcal.htm

That should help.
 

Silver Member
Username: Alonzoub

Post Number: 115
Registered: Apr-10
alright cool, ill make sure to make sure to check that diysubs formula against helmoltz later, and thanks brad thats what I thought.
 

Silver Member
Username: Alonzoub

Post Number: 116
Registered: Apr-10
Just want to follow up here in case someone in the future happens to find this thread through google or the search function.

The formula that Canaan linked checks out to be the exact same formula as Helmholtz Resonance except with end-correction added in.

The one strange thing though, they assume the speed of sound to be 343.93 m/s. I find this strange because the speed of sound at sea level is 340.29 m/s, and the speed of sound at the average altitude of the US is somewhere around 337 m/s... so by that logic, you would have to be somewhere on dry land that is over 1000 feet below sea level to get the speed of sound to be 343.93 m/s. It doesn't affect the formula too much though (less than 1% error) just found that interesting...

Thanks again for the link.
 

Gold Member
Username: Southernrebel

Monroe, Louisiana Team Audible...

Post Number: 8496
Registered: Mar-04
Elevation, temperature and humidity also play a part in tuning/speed of sound.

The speed of sound 'assumed' in the equations from the link I posted are based on ~70deg F 'at sea level'.

If they were going by 'standard laboratory conditions"
AKA, 50% relative humidity, 20*C, 100kPa. The speed of sound is 343m/s
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