If I put a Helmholtz resonator in a sound field, how do I calculate
the amplitude of the air column in the neck of the resonator, given
that I know the sound pressure without the resonator in place?

The sound pressure divided with the acoustic impedance of the
resonator, would give me the flow, but then I would have to know the
sound pressure with the resonator in place, so this is not what I am
looking for.

Thx

"Svante" wrote in message
om...
If I put a Helmholtz resonator in a sound field, how do I calculate
the amplitude of the air column in the neck of the resonator, given
that I know the sound pressure without the resonator in place?

The sound pressure divided with the acoustic impedance of the
resonator, would give me the flow, but then I would have to know the
sound pressure with the resonator in place, so this is not what I am
looking for.

Thx

the colour of the Cheese !!!!........

"Svante" wrote in message
om...
If I put a Helmholtz resonator in a sound field, how do I calculate
the amplitude of the air column in the neck of the resonator, given
that I know the sound pressure without the resonator in place?

The sound pressure divided with the acoustic impedance of the
resonator, would give me the flow, but then I would have to know the
sound pressure with the resonator in place, so this is not what I am
looking for.

Thx

the colour of the Cheese !!!!........

"Svante" wrote in message
om...
If I put a Helmholtz resonator in a sound field, how do I calculate
the amplitude of the air column in the neck of the resonator, given
that I know the sound pressure without the resonator in place?

The sound pressure divided with the acoustic impedance of the
resonator, would give me the flow, but then I would have to know the
sound pressure with the resonator in place, so this is not what I am
looking for.

Thx

alternativaly try this

http://www.phys.unsw.edu.au/~jw/Helmholtz.html

"Svante" wrote in message
om...
If I put a Helmholtz resonator in a sound field, how do I calculate
the amplitude of the air column in the neck of the resonator, given
that I know the sound pressure without the resonator in place?

The sound pressure divided with the acoustic impedance of the
resonator, would give me the flow, but then I would have to know the
sound pressure with the resonator in place, so this is not what I am
looking for.

Thx

alternativaly try this

http://www.phys.unsw.edu.au/~jw/Helmholtz.html

"malcolm" wrote in message news:MQSyb.389687$Fm2.401038@attbi_s04...
"Svante" wrote in message
om...
If I put a Helmholtz resonator in a sound field, how do I calculate
the amplitude of the air column in the neck of the resonator, given
that I know the sound pressure without the resonator in place?

The sound pressure divided with the acoustic impedance of the
resonator, would give me the flow, but then I would have to know the
sound pressure with the resonator in place, so this is not what I am
looking for.

Thx

alternativaly try this

http://www.phys.unsw.edu.au/~jw/Helmholtz.html

This was a nice page about the Helmholtz resonator, but it did not
answer the question above.

1. I know the sound pressure at some position.
2. If I put a helmholtz resonator there, what flow can I expect
through the neck of the resonator.

"malcolm" wrote in message news:MQSyb.389687$Fm2.401038@attbi_s04...
"Svante" wrote in message
om...
If I put a Helmholtz resonator in a sound field, how do I calculate
the amplitude of the air column in the neck of the resonator, given
that I know the sound pressure without the resonator in place?

The sound pressure divided with the acoustic impedance of the
resonator, would give me the flow, but then I would have to know the
sound pressure with the resonator in place, so this is not what I am
looking for.

Thx

alternativaly try this

http://www.phys.unsw.edu.au/~jw/Helmholtz.html

This was a nice page about the Helmholtz resonator, but it did not
answer the question above.

1. I know the sound pressure at some position.
2. If I put a helmholtz resonator there, what flow can I expect
through the neck of the resonator.

On 2 Dec 2003 11:30:47 -0800, (Svante)
wrote:

"malcolm" wrote in message news:MQSyb.389687$Fm2.401038@attbi_s04...
"Svante" wrote in message
om...
If I put a Helmholtz resonator in a sound field, how do I calculate
the amplitude of the air column in the neck of the resonator, given
that I know the sound pressure without the resonator in place?

The sound pressure divided with the acoustic impedance of the
resonator, would give me the flow, but then I would have to know the
sound pressure with the resonator in place, so this is not what I am
looking for.

Thx

alternativaly try this

http://www.phys.unsw.edu.au/~jw/Helmholtz.html

This was a nice page about the Helmholtz resonator, but it did not
answer the question above.

1. I know the sound pressure at some position.
2. If I put a helmholtz resonator there, what flow can I expect
through the neck of the resonator.

I give up!
John,s dad say: "Man that multiply by Q get more than he started with,
but same dimensions"

On 2 Dec 2003 11:30:47 -0800, (Svante)
wrote:

"malcolm" wrote in message news:MQSyb.389687$Fm2.401038@attbi_s04...
"Svante" wrote in message
om...
If I put a Helmholtz resonator in a sound field, how do I calculate
the amplitude of the air column in the neck of the resonator, given
that I know the sound pressure without the resonator in place?

The sound pressure divided with the acoustic impedance of the
resonator, would give me the flow, but then I would have to know the
sound pressure with the resonator in place, so this is not what I am
looking for.

Thx

alternativaly try this

http://www.phys.unsw.edu.au/~jw/Helmholtz.html

This was a nice page about the Helmholtz resonator, but it did not
answer the question above.

1. I know the sound pressure at some position.
2. If I put a helmholtz resonator there, what flow can I expect
through the neck of the resonator.

I give up!
John,s dad say: "Man that multiply by Q get more than he started with,
but same dimensions"

ow (Goofball_star_dot_etal) wrote in message ...
On 2 Dec 2003 11:30:47 -0800, (Svante)
wrote:

"malcolm" wrote in message news:MQSyb.389687$Fm2.401038@attbi_s04...
"Svante" wrote in message
om...
If I put a Helmholtz resonator in a sound field, how do I calculate
the amplitude of the air column in the neck of the resonator, given
that I know the sound pressure without the resonator in place?

The sound pressure divided with the acoustic impedance of the
resonator, would give me the flow, but then I would have to know the
sound pressure with the resonator in place, so this is not what I am
looking for.

Thx

alternativaly try this

http://www.phys.unsw.edu.au/~jw/Helmholtz.html

This was a nice page about the Helmholtz resonator, but it did not
answer the question above.

1. I know the sound pressure at some position.
2. If I put a helmholtz resonator there, what flow can I expect
through the neck of the resonator.

I give up!

Would this indicate that you have given it a try, but not reached any
conclusion? If so would you share your thoughts?

John,s dad say: "Man that multiply by Q get more than he started with,
but same dimensions"

Yeah, but in this case I think the sound level DROPS with increased Q.
Helmholtz resonators are used in this way for ABSORBATION and I would
like to understand this better (ie quantitatively)

I was thinking in these lines:
For some reason I assumed a point source with volume flow Q1 at
distance r1 to the resonator
At the resonator the sound pressure would be p1=Q1/r1*rho0*jw/(4*pi)
If the resonator air plug moves it would also produce a pressure
p2=Q2*Zap, where Q2 is the flow through the port and Zap is the
radiation impedance.

The actual pressure outside the resonator would be the sum of these;
p1+p2, which is also Q2*Zah where Zah is the acoustic impedance of the
resonator as seen from outside the resonator.

From these equations Q2 can be derived as

Q2=-Q1*(rho0*jw)/(4*pi*r1*(Zah+Zap))

Is this right? I feel somewhat uncomfortable with that Q2 would go
towards infinity when r1-0, but maybe that is because of the
abstraction of the point source.

ow (Goofball_star_dot_etal) wrote in message ...
On 2 Dec 2003 11:30:47 -0800, (Svante)
wrote:

"malcolm" wrote in message news:MQSyb.389687$Fm2.401038@attbi_s04...
"Svante" wrote in message
om...
If I put a Helmholtz resonator in a sound field, how do I calculate
the amplitude of the air column in the neck of the resonator, given
that I know the sound pressure without the resonator in place?

The sound pressure divided with the acoustic impedance of the
resonator, would give me the flow, but then I would have to know the
sound pressure with the resonator in place, so this is not what I am
looking for.

Thx

alternativaly try this

http://www.phys.unsw.edu.au/~jw/Helmholtz.html

This was a nice page about the Helmholtz resonator, but it did not
answer the question above.

1. I know the sound pressure at some position.
2. If I put a helmholtz resonator there, what flow can I expect
through the neck of the resonator.

I give up!

Would this indicate that you have given it a try, but not reached any
conclusion? If so would you share your thoughts?

John,s dad say: "Man that multiply by Q get more than he started with,
but same dimensions"

Yeah, but in this case I think the sound level DROPS with increased Q.
Helmholtz resonators are used in this way for ABSORBATION and I would
like to understand this better (ie quantitatively)

I was thinking in these lines:
For some reason I assumed a point source with volume flow Q1 at
distance r1 to the resonator
At the resonator the sound pressure would be p1=Q1/r1*rho0*jw/(4*pi)
If the resonator air plug moves it would also produce a pressure
p2=Q2*Zap, where Q2 is the flow through the port and Zap is the
radiation impedance.

The actual pressure outside the resonator would be the sum of these;
p1+p2, which is also Q2*Zah where Zah is the acoustic impedance of the
resonator as seen from outside the resonator.

From these equations Q2 can be derived as

Q2=-Q1*(rho0*jw)/(4*pi*r1*(Zah+Zap))

Is this right? I feel somewhat uncomfortable with that Q2 would go
towards infinity when r1-0, but maybe that is because of the
abstraction of the point source.

ow (Goofball_star_dot_etal) wrote in message ...
On 2 Dec 2003 11:30:47 -0800, (Svante)
wrote:

"malcolm" wrote in message news:MQSyb.389687$Fm2.401038@attbi_s04...
"Svante" wrote in message
om...
If I put a Helmholtz resonator in a sound field, how do I calculate
the amplitude of the air column in the neck of the resonator, given
that I know the sound pressure without the resonator in place?

The sound pressure divided with the acoustic impedance of the
resonator, would give me the flow, but then I would have to know the
sound pressure with the resonator in place, so this is not what I am
looking for.

Thx

alternativaly try this

http://www.phys.unsw.edu.au/~jw/Helmholtz.html

This was a nice page about the Helmholtz resonator, but it did not
answer the question above.

1. I know the sound pressure at some position.
2. If I put a helmholtz resonator there, what flow can I expect
through the neck of the resonator.

I give up!

Would this indicate that you have given it a try, but not reached any
conclusion? If so would you share your thoughts?

John,s dad say: "Man that multiply by Q get more than he started with,
but same dimensions"

Yeah, but in this case I think the sound level DROPS with increased Q.
Helmholtz resonators are used in this way for ABSORBATION and I would
like to understand this better (ie quantitatively)

I was thinking in these lines:
For some reason I assumed a point source with volume flow Q1 at
distance r1 to the resonator
At the resonator the sound pressure would be p1=Q1/r1*rho0*jw/(4*pi)
If the resonator air plug moves it would also produce a pressure
p2=Q2*Zap, where Q2 is the flow through the port and Zap is the
radiation impedance.

The actual pressure outside the resonator would be the sum of these;
p1+p2, which is also Q2*Zah where Zah is the acoustic impedance of the
resonator as seen from outside the resonator.

From these equations Q2 can be derived as

Q2=-Q1*(rho0*jw)/(4*pi*r1*(Zah+Zap))

Is this right? I feel somewhat uncomfortable with that Q2 would go
towards infinity when r1-0, but maybe that is because of the
abstraction of the point source.

On 4 Dec 2003 05:22:45 -0800, (Svante)
wrote:

(Goofball_star_dot_etal) wrote in message ...
On 2 Dec 2003 11:30:47 -0800, (Svante)
wrote:

"malcolm" wrote in message news:MQSyb.389687$Fm2.401038@attbi_s04...
"Svante" wrote in message
om...
If I put a Helmholtz resonator in a sound field, how do I calculate
the amplitude of the air column in the neck of the resonator, given
that I know the sound pressure without the resonator in place?

The sound pressure divided with the acoustic impedance of the
resonator, would give me the flow, but then I would have to know the
sound pressure with the resonator in place, so this is not what I am
looking for.

Thx

alternativaly try this

http://www.phys.unsw.edu.au/~jw/Helmholtz.html

This was a nice page about the Helmholtz resonator, but it did not
answer the question above.

1. I know the sound pressure at some position.
2. If I put a helmholtz resonator there, what flow can I expect
through the neck of the resonator.

I give up!

Would this indicate that you have given it a try, but not reached any
conclusion? If so would you share your thoughts?


I know nothing about acoustics. I just guessed that the pressure
inside would be mulitplied by Q, the flow in the resonator air plug
multiplied by Q and the pressure outside divided by Q, all compared to
the values without the resonator present. (Where Q is the 'Q' of a
tuned circuit not flow)

John,s dad say: "Man that multiply by Q get more than he started with,
but same dimensions"

Yeah, but in this case I think the sound level DROPS with increased Q.
Helmholtz resonators are used in this way for ABSORBATION and I would
like to understand this better (ie quantitatively)

It is not a good time for me to guess in public, I think. I would be
tempted to measure Q and the sound pressure and inside the resonator.


I was thinking in these lines:
For some reason I assumed a point source with volume flow Q1 at
distance r1 to the resonator
At the resonator the sound pressure would be p1=Q1/r1*rho0*jw/(4*pi)
If the resonator air plug moves it would also produce a pressure
p2=Q2*Zap, where Q2 is the flow through the port and Zap is the
radiation impedance.

The actual pressure outside the resonator would be the sum of these;
p1+p2, which is also Q2*Zah where Zah is the acoustic impedance of the
resonator as seen from outside the resonator.

From these equations Q2 can be derived as

Q2=-Q1*(rho0*jw)/(4*pi*r1*(Zah+Zap))


I'll try. ..


p1=Q1/r1*rho0*jw/(4*pi)
p2=Q2*Zap

Let A=rho0*jw/(4*pi*r1)

p1=Q1*A
Q2*Zah=p1+p2
Q2*Zah=Q1*A+Q2*Zap
Q2=Q1*A/Zah + Q2*Zap/Zah
Q2(1- Zap /Zah) = Q1*A/Zah
Q2(Zah-Zap)/Zah = Q1*A/Zah
Q2 = Q1*A/(Zah-Zap)

Hmm, I probably should have changed signs somewhere.


Is this right? I feel somewhat uncomfortable with that Q2 would go
towards infinity when r1-0, but maybe that is because of the
abstraction of the point source.

I think so.

On 4 Dec 2003 05:22:45 -0800, (Svante)
wrote:

(Goofball_star_dot_etal) wrote in message ...
On 2 Dec 2003 11:30:47 -0800, (Svante)
wrote:

"malcolm" wrote in message news:MQSyb.389687$Fm2.401038@attbi_s04...
"Svante" wrote in message
om...
If I put a Helmholtz resonator in a sound field, how do I calculate
the amplitude of the air column in the neck of the resonator, given
that I know the sound pressure without the resonator in place?

The sound pressure divided with the acoustic impedance of the
resonator, would give me the flow, but then I would have to know the
sound pressure with the resonator in place, so this is not what I am
looking for.

Thx

alternativaly try this

http://www.phys.unsw.edu.au/~jw/Helmholtz.html

This was a nice page about the Helmholtz resonator, but it did not
answer the question above.

1. I know the sound pressure at some position.
2. If I put a helmholtz resonator there, what flow can I expect
through the neck of the resonator.

I give up!

Would this indicate that you have given it a try, but not reached any
conclusion? If so would you share your thoughts?


I know nothing about acoustics. I just guessed that the pressure
inside would be mulitplied by Q, the flow in the resonator air plug
multiplied by Q and the pressure outside divided by Q, all compared to
the values without the resonator present. (Where Q is the 'Q' of a
tuned circuit not flow)

John,s dad say: "Man that multiply by Q get more than he started with,
but same dimensions"

Yeah, but in this case I think the sound level DROPS with increased Q.
Helmholtz resonators are used in this way for ABSORBATION and I would
like to understand this better (ie quantitatively)

It is not a good time for me to guess in public, I think. I would be
tempted to measure Q and the sound pressure and inside the resonator.


I was thinking in these lines:
For some reason I assumed a point source with volume flow Q1 at
distance r1 to the resonator
At the resonator the sound pressure would be p1=Q1/r1*rho0*jw/(4*pi)
If the resonator air plug moves it would also produce a pressure
p2=Q2*Zap, where Q2 is the flow through the port and Zap is the
radiation impedance.

The actual pressure outside the resonator would be the sum of these;
p1+p2, which is also Q2*Zah where Zah is the acoustic impedance of the
resonator as seen from outside the resonator.

From these equations Q2 can be derived as

Q2=-Q1*(rho0*jw)/(4*pi*r1*(Zah+Zap))


I'll try. ..


p1=Q1/r1*rho0*jw/(4*pi)
p2=Q2*Zap

Let A=rho0*jw/(4*pi*r1)

p1=Q1*A
Q2*Zah=p1+p2
Q2*Zah=Q1*A+Q2*Zap
Q2=Q1*A/Zah + Q2*Zap/Zah
Q2(1- Zap /Zah) = Q1*A/Zah
Q2(Zah-Zap)/Zah = Q1*A/Zah
Q2 = Q1*A/(Zah-Zap)

Hmm, I probably should have changed signs somewhere.


Is this right? I feel somewhat uncomfortable with that Q2 would go
towards infinity when r1-0, but maybe that is because of the
abstraction of the point source.

I think so.

On 4 Dec 2003 05:22:45 -0800, (Svante)
wrote:

(Goofball_star_dot_etal) wrote in message ...
On 2 Dec 2003 11:30:47 -0800, (Svante)
wrote:

"malcolm" wrote in message news:MQSyb.389687$Fm2.401038@attbi_s04...
"Svante" wrote in message
om...
If I put a Helmholtz resonator in a sound field, how do I calculate
the amplitude of the air column in the neck of the resonator, given
that I know the sound pressure without the resonator in place?

The sound pressure divided with the acoustic impedance of the
resonator, would give me the flow, but then I would have to know the
sound pressure with the resonator in place, so this is not what I am
looking for.

Thx

alternativaly try this

http://www.phys.unsw.edu.au/~jw/Helmholtz.html

This was a nice page about the Helmholtz resonator, but it did not
answer the question above.

1. I know the sound pressure at some position.
2. If I put a helmholtz resonator there, what flow can I expect
through the neck of the resonator.

I give up!

Would this indicate that you have given it a try, but not reached any
conclusion? If so would you share your thoughts?


I know nothing about acoustics. I just guessed that the pressure
inside would be mulitplied by Q, the flow in the resonator air plug
multiplied by Q and the pressure outside divided by Q, all compared to
the values without the resonator present. (Where Q is the 'Q' of a
tuned circuit not flow)

John,s dad say: "Man that multiply by Q get more than he started with,
but same dimensions"

Yeah, but in this case I think the sound level DROPS with increased Q.
Helmholtz resonators are used in this way for ABSORBATION and I would
like to understand this better (ie quantitatively)

It is not a good time for me to guess in public, I think. I would be
tempted to measure Q and the sound pressure and inside the resonator.


I was thinking in these lines:
For some reason I assumed a point source with volume flow Q1 at
distance r1 to the resonator
At the resonator the sound pressure would be p1=Q1/r1*rho0*jw/(4*pi)
If the resonator air plug moves it would also produce a pressure
p2=Q2*Zap, where Q2 is the flow through the port and Zap is the
radiation impedance.

The actual pressure outside the resonator would be the sum of these;
p1+p2, which is also Q2*Zah where Zah is the acoustic impedance of the
resonator as seen from outside the resonator.

From these equations Q2 can be derived as

Q2=-Q1*(rho0*jw)/(4*pi*r1*(Zah+Zap))


I'll try. ..


p1=Q1/r1*rho0*jw/(4*pi)
p2=Q2*Zap

Let A=rho0*jw/(4*pi*r1)

p1=Q1*A
Q2*Zah=p1+p2
Q2*Zah=Q1*A+Q2*Zap
Q2=Q1*A/Zah + Q2*Zap/Zah
Q2(1- Zap /Zah) = Q1*A/Zah
Q2(Zah-Zap)/Zah = Q1*A/Zah
Q2 = Q1*A/(Zah-Zap)

Hmm, I probably should have changed signs somewhere.


Is this right? I feel somewhat uncomfortable with that Q2 would go
towards infinity when r1-0, but maybe that is because of the
abstraction of the point source.

I think so.

ow (Goofball_star_dot_etal) wrote in message ...
On 4 Dec 2003 05:22:45 -0800, (Svante)
wrote:

(Goofball_star_dot_etal) wrote in message ...
On 2 Dec 2003 11:30:47 -0800, (Svante)
wrote:

"malcolm" wrote in message news:MQSyb.389687$Fm2.401038@attbi_s04...
"Svante" wrote in message
om...
If I put a Helmholtz resonator in a sound field, how do I calculate
the amplitude of the air column in the neck of the resonator, given
that I know the sound pressure without the resonator in place?

The sound pressure divided with the acoustic impedance of the
resonator, would give me the flow, but then I would have to know the
sound pressure with the resonator in place, so this is not what I am
looking for.

Thx

alternativaly try this

http://www.phys.unsw.edu.au/~jw/Helmholtz.html

This was a nice page about the Helmholtz resonator, but it did not
answer the question above.

1. I know the sound pressure at some position.
2. If I put a helmholtz resonator there, what flow can I expect
through the neck of the resonator.

I give up!

Would this indicate that you have given it a try, but not reached any
conclusion? If so would you share your thoughts?


I know nothing about acoustics.

Yeah, right... :-)

I just guessed that the pressure
inside would be mulitplied by Q, the flow in the resonator air plug
multiplied by Q and the pressure outside divided by Q, all compared to
the values without the resonator present. (Where Q is the 'Q' of a
tuned circuit not flow)

OK! That seems reasonable, I didn't think of using the INSIDE pressure.


John,s dad say: "Man that multiply by Q get more than he started with,
but same dimensions"

Yeah, but in this case I think the sound level DROPS with increased Q.
Helmholtz resonators are used in this way for ABSORBATION and I would
like to understand this better (ie quantitatively)

It is not a good time for me to guess in public, I think. I would be
tempted to measure Q and the sound pressure and inside the resonator.


I was thinking in these lines:
For some reason I assumed a point source with volume flow Q1 at
distance r1 to the resonator
At the resonator the sound pressure would be p1=Q1/r1*rho0*jw/(4*pi)
If the resonator air plug moves it would also produce a pressure
p2=Q2*Zap, where Q2 is the flow through the port and Zap is the
radiation impedance.

The actual pressure outside the resonator would be the sum of these;
p1+p2, which is also Q2*Zah where Zah is the acoustic impedance of the
resonator as seen from outside the resonator.

From these equations Q2 can be derived as

Q2=-Q1*(rho0*jw)/(4*pi*r1*(Zah+Zap))


I'll try. ..


p1=Q1/r1*rho0*jw/(4*pi)
p2=Q2*Zap

Let A=rho0*jw/(4*pi*r1)

p1=Q1*A
Q2*Zah=p1+p2
Q2*Zah=Q1*A+Q2*Zap
Q2=Q1*A/Zah + Q2*Zap/Zah
Q2(1- Zap /Zah) = Q1*A/Zah
Q2(Zah-Zap)/Zah = Q1*A/Zah
Q2 = Q1*A/(Zah-Zap)

Hmm, I probably should have changed signs somewhere.


If both Q's are defined outwards, then -Q2*Zah=p1+p2, so there is the sign.


Is this right? I feel somewhat uncomfortable with that Q2 would go
towards infinity when r1-0, but maybe that is because of the
abstraction of the point source.

I think so.

Thanks for the input!

ow (Goofball_star_dot_etal) wrote in message ...
On 4 Dec 2003 05:22:45 -0800, (Svante)
wrote:

(Goofball_star_dot_etal) wrote in message ...
On 2 Dec 2003 11:30:47 -0800, (Svante)
wrote:

"malcolm" wrote in message news:MQSyb.389687$Fm2.401038@attbi_s04...
"Svante" wrote in message
om...
If I put a Helmholtz resonator in a sound field, how do I calculate
the amplitude of the air column in the neck of the resonator, given
that I know the sound pressure without the resonator in place?

The sound pressure divided with the acoustic impedance of the
resonator, would give me the flow, but then I would have to know the
sound pressure with the resonator in place, so this is not what I am
looking for.

Thx

alternativaly try this

http://www.phys.unsw.edu.au/~jw/Helmholtz.html

This was a nice page about the Helmholtz resonator, but it did not
answer the question above.

1. I know the sound pressure at some position.
2. If I put a helmholtz resonator there, what flow can I expect
through the neck of the resonator.

I give up!

Would this indicate that you have given it a try, but not reached any
conclusion? If so would you share your thoughts?


I know nothing about acoustics.

Yeah, right... :-)

I just guessed that the pressure
inside would be mulitplied by Q, the flow in the resonator air plug
multiplied by Q and the pressure outside divided by Q, all compared to
the values without the resonator present. (Where Q is the 'Q' of a
tuned circuit not flow)

OK! That seems reasonable, I didn't think of using the INSIDE pressure.


John,s dad say: "Man that multiply by Q get more than he started with,
but same dimensions"

Yeah, but in this case I think the sound level DROPS with increased Q.
Helmholtz resonators are used in this way for ABSORBATION and I would
like to understand this better (ie quantitatively)

It is not a good time for me to guess in public, I think. I would be
tempted to measure Q and the sound pressure and inside the resonator.


I was thinking in these lines:
For some reason I assumed a point source with volume flow Q1 at
distance r1 to the resonator
At the resonator the sound pressure would be p1=Q1/r1*rho0*jw/(4*pi)
If the resonator air plug moves it would also produce a pressure
p2=Q2*Zap, where Q2 is the flow through the port and Zap is the
radiation impedance.

The actual pressure outside the resonator would be the sum of these;
p1+p2, which is also Q2*Zah where Zah is the acoustic impedance of the
resonator as seen from outside the resonator.

From these equations Q2 can be derived as

Q2=-Q1*(rho0*jw)/(4*pi*r1*(Zah+Zap))


I'll try. ..


p1=Q1/r1*rho0*jw/(4*pi)
p2=Q2*Zap

Let A=rho0*jw/(4*pi*r1)

p1=Q1*A
Q2*Zah=p1+p2
Q2*Zah=Q1*A+Q2*Zap
Q2=Q1*A/Zah + Q2*Zap/Zah
Q2(1- Zap /Zah) = Q1*A/Zah
Q2(Zah-Zap)/Zah = Q1*A/Zah
Q2 = Q1*A/(Zah-Zap)

Hmm, I probably should have changed signs somewhere.


If both Q's are defined outwards, then -Q2*Zah=p1+p2, so there is the sign.


Is this right? I feel somewhat uncomfortable with that Q2 would go
towards infinity when r1-0, but maybe that is because of the
abstraction of the point source.

I think so.

Thanks for the input!

ow (Goofball_star_dot_etal) wrote in message ...
On 4 Dec 2003 05:22:45 -0800, (Svante)
wrote:

(Goofball_star_dot_etal) wrote in message ...
On 2 Dec 2003 11:30:47 -0800, (Svante)
wrote:

"malcolm" wrote in message news:MQSyb.389687$Fm2.401038@attbi_s04...
"Svante" wrote in message
om...
If I put a Helmholtz resonator in a sound field, how do I calculate
the amplitude of the air column in the neck of the resonator, given
that I know the sound pressure without the resonator in place?

The sound pressure divided with the acoustic impedance of the
resonator, would give me the flow, but then I would have to know the
sound pressure with the resonator in place, so this is not what I am
looking for.

Thx

alternativaly try this

http://www.phys.unsw.edu.au/~jw/Helmholtz.html

This was a nice page about the Helmholtz resonator, but it did not
answer the question above.

1. I know the sound pressure at some position.
2. If I put a helmholtz resonator there, what flow can I expect
through the neck of the resonator.

I give up!

Would this indicate that you have given it a try, but not reached any
conclusion? If so would you share your thoughts?


I know nothing about acoustics.

Yeah, right... :-)

I just guessed that the pressure
inside would be mulitplied by Q, the flow in the resonator air plug
multiplied by Q and the pressure outside divided by Q, all compared to
the values without the resonator present. (Where Q is the 'Q' of a
tuned circuit not flow)

OK! That seems reasonable, I didn't think of using the INSIDE pressure.


John,s dad say: "Man that multiply by Q get more than he started with,
but same dimensions"

Yeah, but in this case I think the sound level DROPS with increased Q.
Helmholtz resonators are used in this way for ABSORBATION and I would
like to understand this better (ie quantitatively)

It is not a good time for me to guess in public, I think. I would be
tempted to measure Q and the sound pressure and inside the resonator.


I was thinking in these lines:
For some reason I assumed a point source with volume flow Q1 at
distance r1 to the resonator
At the resonator the sound pressure would be p1=Q1/r1*rho0*jw/(4*pi)
If the resonator air plug moves it would also produce a pressure
p2=Q2*Zap, where Q2 is the flow through the port and Zap is the
radiation impedance.

The actual pressure outside the resonator would be the sum of these;
p1+p2, which is also Q2*Zah where Zah is the acoustic impedance of the
resonator as seen from outside the resonator.

From these equations Q2 can be derived as

Q2=-Q1*(rho0*jw)/(4*pi*r1*(Zah+Zap))


I'll try. ..


p1=Q1/r1*rho0*jw/(4*pi)
p2=Q2*Zap

Let A=rho0*jw/(4*pi*r1)

p1=Q1*A
Q2*Zah=p1+p2
Q2*Zah=Q1*A+Q2*Zap
Q2=Q1*A/Zah + Q2*Zap/Zah
Q2(1- Zap /Zah) = Q1*A/Zah
Q2(Zah-Zap)/Zah = Q1*A/Zah
Q2 = Q1*A/(Zah-Zap)

Hmm, I probably should have changed signs somewhere.


If both Q's are defined outwards, then -Q2*Zah=p1+p2, so there is the sign.


Is this right? I feel somewhat uncomfortable with that Q2 would go
towards infinity when r1-0, but maybe that is because of the
abstraction of the point source.

I think so.

Thanks for the input!