volume velocity

Thread view:

Enable EMail Alerts

Start New Thread

Thread rating:

richard pickworth - 22 Jul 2005 16:52 GMT

In a duct with a side - branch, would the amount of sound in the branch
depend on constant sum of volume velocity in the duct system?
yours
Richard

Reply to this Message

Angelo Campanella - 22 Jul 2005 19:24 GMT

> In a duct with a side - branch, would the amount of sound in the branch
> depend on constant sum of volume velocity in the duct system?

    In HVAC technology, the sound power (essentially sound pressure times
volume velocity) is the variable that is treated, with areas being the
controlling factor. Right, wrong, or only approximate, this method
declares that the sound power divides as the areas.

    Thus, if a 1 ft^2 area source duct is tapped by a o.5 ft^2 duct, 2/3rds
of the sound power continues to proceed down the continuing source duct,
while 1/3 of the source sound power proceeds down the duct that was
joined to the source duct.

    A more detailed analysis of the acoustic impedance of the joint and
subsequent duct can produce a different division, likely frequency
dependent, but that is not widely known or used as far as I know.

        Angelo Campanella

Reply to this Message

richard pickworth - 08 Aug 2005 18:03 GMT

I wonder if you could stick a section of duct with a fan in the middle in an
anechoic chamber, then stick a microphone in at one end and measure the
sound pressure (in octaves), then assuming a plane wave in the duct (which
seems resonable), calculate the sound power using characteristic impeadance.
Assuming you don't destroy the anechoic chamber in the process (oops!).
yours
Richard

>> In a duct with a side - branch, would the amount of sound in the branch
>> depend on constant sum of volume velocity in the duct system?
[quoted text clipped - 14 lines]
>
> Angelo Campanella

Reply to this Message

ari - 09 Aug 2005 08:21 GMT

> I wonder if you could stick a section of duct with a fan in the middle in an
> anechoic chamber, then stick a microphone in at one end and measure the
> sound pressure (in octaves), then assuming a plane wave in the duct (which
> seems resonable), calculate the sound power using characteristic impeadance.
> Assuming you don't destroy the anechoic chamber in the process (oops!).

The impedance method has been applied to fans. KTH in Stockholm has been
doing a lot with the approach during last decades the in MWL. One of
latest was EU project NABUCCO. My understanding is that it is more
suitable to centrifugal than axial fans. Actually i would be very
careful with the axial fan applications.

When it comes to upstream and down stream differences Neise wrote some
20...25 years ago that to get the total fan sound power level, 2.4
dB(A?) should be added to upstream standardized measurements to get the
total level. Flow velocities don't usually exceed 30 m/s and generally
below 20 m/s so doppler effects are usually not considered.

The plane wave assumption obviously depends on the duct diameter and the
accuracy aimed at. The Swedes have dealt with higher modes basically in
the same manner as plane wave (meaning that they need require a mic per
mode, if I remember correctly. You end up needing app. 15 mics on a
frequency double the plane wave "cut off". Again if i remember
correctly.)

> yours
> Richard
[quoted text clipped - 6 lines]
>>controlling factor. Right, wrong, or only approximate, this method
>>declares that the sound power divides as the areas.

Even more simple way is that sound power is logaritmic sum of sound
pressure level and cross section area, which leads to same assumption of
the sound power dividing according to branch area proportion(s).

>>Thus, if a 1 ft^2 area source duct is tapped by a o.5 ft^2 duct, 2/3rds of
>>the sound power continues to proceed down the continuing source duct,
[quoted text clipped - 4 lines]
>>subsequent duct can produce a different division, likely frequency
>>dependent, but that is not widely known or used as far as I know.

If you wanna go to impedance approach you should know the rest of the
system (and fan) as well, since there are "local" and "system" effects
which are "seen" by the approach.

But obviously everything comes down to how detailed knowledge you are
after...

>>Angelo Campanella

BR,

ari

Reply to this Message

richard pickworth - 08 Aug 2005 21:41 GMT

I wonder if there would be a difference in the measurement up stream or down
stream of the fan, due to convection (dopler shift?).
Also I wonder if the tonal content at the blade passage frequency would
affect the measurement?
yours
Richard Pickworth

>> In a duct with a side - branch, would the amount of sound in the branch
>> depend on constant sum of volume velocity in the duct system?
[quoted text clipped - 14 lines]
>
> Angelo Campanella

Reply to this Message

Natural Science Forum / Physics / Acoustics / August 2005

volume velocity