Sound absorbing materials?

[Bob Cain]

I am looking for a sound absorbing material or layers of
materials which I could place at one end of a 6' long 12"
diameter tube that would absorb as much of the sound field
coming down from the other end as possible, i.e. terminate
it. Leaving the terminating end open is an acceptable
option so that on the return path all that would need to be
attenuated would be the reflection from the impedence
mismatch at the terminating end. Any suggestions?


Thanks,

Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein

[Arny Krueger]

"Bob Cain" > wrote in message

> I am looking for a sound absorbing material or layers of
> materials which I could place at one end of a 6' long 12"
> diameter tube that would absorb as much of the sound field
> coming down from the other end as possible, i.e. terminate
> it. Leaving the terminating end open is an acceptable
> option so that on the return path all that would need to be
> attenuated would be the reflection from the impedance
> mismatch at the terminating end. Any suggestions?

I once attended a presentation by a person who described his attempts to
terminate a loudspeaker transmission line in such a way as to minimize the
creation of reflections. He used polyester fiber, combed out and wispy at
the entry to the absorptive area, and with increasing density down the
line.

[Bob Cain]

Arny Krueger wrote:

> "Bob Cain" > wrote in message
>
>
>>I am looking for a sound absorbing material or layers of
>>materials which I could place at one end of a 6' long 12"
>>diameter tube that would absorb as much of the sound field
>>coming down from the other end as possible, i.e. terminate
>>it. Leaving the terminating end open is an acceptable
>>option so that on the return path all that would need to be
>>attenuated would be the reflection from the impedance
>>mismatch at the terminating end. Any suggestions?
>
>
> I once attended a presentation by a person who described his attempts to
> terminate a loudspeaker transmission line in such a way as to minimize the
> creation of reflections. He used polyester fiber, combed out and wispy at
> the entry to the absorptive area, and with increasing density down the
> line.
>

Sort of a gradient index approach. That's kind of what I
figured would be best to minimize reflections from the
material interface itself. Do you have any idea as to where
to obtain fiber suitable to the task?


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein

[Tim Padrick]

Regular: http://www.poly-fil.com/consumer/CProducts.asp?itemid=fiberfill

Special: http://www.poly-fil.com/industrial/IProducts.asp?itemid=speakerman



"Bob Cain" > wrote in message
...
> Arny Krueger wrote:
>
> > "Bob Cain" > wrote in message
> >
> >
> >>I am looking for a sound absorbing material or layers of
> >>materials which I could place at one end of a 6' long 12"
> >>diameter tube that would absorb as much of the sound field
> >>coming down from the other end as possible, i.e. terminate
> >>it. Leaving the terminating end open is an acceptable
> >>option so that on the return path all that would need to be
> >>attenuated would be the reflection from the impedance
> >>mismatch at the terminating end. Any suggestions?
> >
> >
> > I once attended a presentation by a person who described his attempts
to
> > terminate a loudspeaker transmission line in such a way as to minimize
the
> > creation of reflections. He used polyester fiber, combed out and wispy
at
> > the entry to the absorptive area, and with increasing density down the
> > line.
> >
>
> Sort of a gradient index approach. That's kind of what I
> figured would be best to minimize reflections from the
> material interface itself. Do you have any idea as to where
> to obtain fiber suitable to the task?
>
>
> Bob
> --
>
> "Things should be described as simply as possible, but no
> simpler."
>
> A. Einstein

[Arny Krueger]

"Bob Cain" > wrote in message

> Arny Krueger wrote:
>
>> "Bob Cain" > wrote in message
>>
>>
>>> I am looking for a sound absorbing material or layers of
>>> materials which I could place at one end of a 6' long 12"
>>> diameter tube that would absorb as much of the sound field
>>> coming down from the other end as possible, i.e. terminate
>>> it. Leaving the terminating end open is an acceptable
>>> option so that on the return path all that would need to be
>>> attenuated would be the reflection from the impedance
>>> mismatch at the terminating end. Any suggestions?

>> I once attended a presentation by a person who described his
>> attempts to terminate a loudspeaker transmission line in such a way
>> as to minimize the creation of reflections. He used polyester fiber,
>> combed out and wispy at the entry to the absorptive area, and with
>> increasing density down the line.

> Sort of a gradient index approach. That's kind of what I
> figured would be best to minimize reflections from the
> material interface itself. Do you have any idea as to where
> to obtain fiber suitable to the task?

The guy who made this presentation was doing well at the time with trips to
his local fabric store. Come to think of it, he also was using wedges of
polyurethane foam from the same place.

[Bob Cain]

Tim Padrick wrote:

> Regular: http://www.poly-fil.com/consumer/CProducts.asp?itemid=fiberfill

Fantastic, thanks. They even offer R&D assistance.


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein

[Scott Dorsey]

Bob Cain > wrote:
>
>I am looking for a sound absorbing material or layers of
>materials which I could place at one end of a 6' long 12"
>diameter tube that would absorb as much of the sound field
>coming down from the other end as possible, i.e. terminate
>it. Leaving the terminating end open is an acceptable
>option so that on the return path all that would need to be
>attenuated would be the reflection from the impedence
>mismatch at the terminating end. Any suggestions?

What sound is coming down it, at what levels? If it's narrowband,
a tuned cavity might be a solution. If not, there are gadgets that
have reflective panels that reflect the sound back on on itself where
they cancel out, which are okay for repetitive noises. They are used
for marine diesel mufflers.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Bob Cain]

Scott Dorsey wrote:

> What sound is coming down it, at what levels? If it's narrowband,
> a tuned cavity might be a solution. If not, there are gadgets that
> have reflective panels that reflect the sound back on on itself where
> they cancel out, which are okay for repetitive noises. They are used
> for marine diesel mufflers.
> --scott

It's broadband, audio band that is. The goal is a small
"anechoic chamber" for plane wave microphone measurements.
I won't be doing linearity measurement so levels can be
fairly low.


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein

[Scott Dorsey]

Bob Cain > wrote:
>Scott Dorsey wrote:
>
>> What sound is coming down it, at what levels? If it's narrowband,
>> a tuned cavity might be a solution. If not, there are gadgets that
>> have reflective panels that reflect the sound back on on itself where
>> they cancel out, which are okay for repetitive noises. They are used
>> for marine diesel mufflers.
>
>It's broadband, audio band that is. The goal is a small
>"anechoic chamber" for plane wave microphone measurements.
>I won't be doing linearity measurement so levels can be
>fairly low.

What if you just poke the end of the tube out the window? In a quiet
rural area, that will work nicely and it'll go down to very low
frequencies. You may need some sort of flare at the end to make it match
properly, though.

You can think of the tube as being a transmission line, and what you want to
do is terminate the line so that there is no reflection back to the source.
This is easy to do at high frequencies, but difficult to do at low frequencies.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Bob Cain]

Scott Dorsey wrote:

> Bob Cain > wrote:
>

>
> You can think of the tube as being a transmission line, and what you want to
> do is terminate the line so that there is no reflection back to the source.
> This is easy to do at high frequencies, but difficult to do at low frequencies.

Exactly. There is a thread going now in the Aurelex subform
at http://www.recording.org/cgi-local/ubb/ultimatebb.cgi
between Eric Desart and I which is trying to pin this down
to an initial experimental setup.


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein