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Author Topic: Frequency depending resistor  (Read 1736 times)

Marcel de Graaf

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Frequency depending resistor
« on: August 23, 2018, 01:19:32 pm »

Hi All,

Maybe a somewhat off-topic question on this board, but in the end it has a link with acoustics.

Is there a way to make a frequency depending resistor (pure resistive)?

gr. Marcel
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Andrew Broughton

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Re: Frequency depending resistor
« Reply #1 on: August 23, 2018, 05:37:49 pm »

Frequency-dependant? You mean resistance varies by frequency? Like a capacitor or choke but without any inductance?
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Marcel de Graaf

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Re: Frequency depending resistor
« Reply #2 on: August 24, 2018, 01:44:22 pm »

Hi,

Indeed a frequency dependent restive load.

I have tought about a potmeter that will change its position with frequency, but can`t find a lot on the net about some sort of setup. I would like to build a little circuit which behave with characteristics of radiation impedance to do some test with different analyzers.

PS: maybe its more a question on a forum about electronics.
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John Roberts {JR}

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Re: Frequency depending resistor
« Reply #3 on: August 24, 2018, 02:29:13 pm »

Hi,

Indeed a frequency dependent restive load.
A resistive load has impedance that is unchanging with frequency, reactive components (like inductors or capacitors) change impedance with frequency.
Quote

I have tought about a potmeter that will change its position with frequency, but can`t find a lot on the net about some sort of setup.
potentiometer? You could servo a frequency sensor to motor controlled potentiometer but that seems like an expensive and difficult way to accomplish something.
Quote
I would like to build a little circuit which behave with characteristics of radiation impedance to do some test with different analyzers.
What kind of radiation? What kind of analyzers?
Quote
PS: maybe its more a question on a forum about electronics.
Maybe its a bot?

JR
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Marcel de Graaf

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Re: Frequency depending resistor
« Reply #4 on: August 24, 2018, 03:14:41 pm »

John,

Resistors does present a load that are unchanging with frequency. The analog model of the radiation impedance of a circular piston has a part which behave as a decreasing resistor with decreasing frequency.

http://www.fonema.se/mouthcorr/mouthcorr.htm (the plot from Beranek).

Its just this thing i would like to build with electronics parts.

I want to make some measurements with a changing resistive part in the circuit to see if acoustic analyzers like holmimpulse, smaart, tef etc....will measure the same. I want to do this in a electrical way so its easier to compare and under the same conditions.


marcel
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Langston Holland

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Re: Frequency depending resistor
« Reply #5 on: August 24, 2018, 09:51:26 pm »

Hi Marcel:

You will find more success in your quest if you clearly specify your purpose. :)

Because it's Friday night and I want to do something fun, and I'm a geek, see if the following is entertaining. Or not.

Is your goal to learn about radiation resistance with a given application in mind, or are you trying to qualify various measurement systems for that specific application, or something else?

If it's the former, you may be looking at the problem backwards. As you implied, opposition to the movement a given sized radiator changes as frequency changes. This opposition is the result of air pressure (SPL) divided by air flow (particle velocity). Electrically this is the same as voltage divided by current. In other words, impedance. The changes with frequency behave like a reactance even though the word resistance is used. That's why your linked article shows reactive components in the equivalent circuit diagram.

Since the radiator experiences a rapid loss in acoustic impedance as the wavelength of sound approaches and then exceeds its diameter, this is electrically equivalent to inductance. Again, your linked article shows inductance as the primary component of the equivalent circuit. This is your "frequency dependent resistor".

Now, if you're trying to compare and contrast acoustic measurement systems, measuring inductance isn't going to help. I'd suggest you use a test stimulus that more closely simulates acoustic behavior, such as a programmable reverb and echo effects unit. You will find all measurement systems will closely agree with fairly steady state signals, but begin to show differences as you change the stimulus over the time of acquisition. In other words, changes like you see when measuring loudspeakers and rooms. These differences you will notice are primarily due to (3) things:

1. How the measurement system selects (or allows the operator to select) the raw acoustic data (acquisition length and time domain windowing technique).

2. Whether the measurement system uses FFT or sinusoidal tracking methods (or both).

3. How the measurement system selects (or allows the operator to select) the transformed data for the purpose of displaying the impulse response or complex transfer function. This again involves data length and windowing techniques.

Almost ALL measurement systems do this stuff differently for different reasons. Thus you can get somewhat different plots on your screen with different systems using the same measurement event and each can be, and probably will be, correct! Smaart is a heck of a lot better than a scope for characterizing a sound system's correlation to human perception, but the scope is a heck of a lot better for studying one-shot events in a circuit.

It depends on your goal.

PS: If you want to get a better feel for Beranek's work on acoustic impedance as excerpted in your link, read Tom Danley and Doug Jones' chapter on Loudspeakers in the current (5th Ed) of "Handbook for Sound Engineers". Their explanation is way better. :)
« Last Edit: August 25, 2018, 04:23:19 pm by Langston Holland »
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Frank Koenig

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Re: Frequency depending resistor
« Reply #6 on: August 24, 2018, 10:47:21 pm »

Hi All,

Maybe a somewhat off-topic question on this board, but in the end it has a link with acoustics.

Is there a way to make a frequency depending resistor (pure resistive)?

gr. Marcel

Ferrite beads can be reasonably accurately modeled as frequency dependent resistors. That's what makes them useful as EMI suppressors. As frequency increases their resistance goes up without much inductance that could cause a resonance at a particular frequency. Why do you ask? -F
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Lyle Williams

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Re: Frequency depending resistor
« Reply #7 on: August 25, 2018, 05:39:41 am »

Frequency dependent resistors are called capacitors and inductors.  They have capacitance and inductance, not just pure resistance.

Components can be combined to create a purely resistive load, but only at a given frequency.

A wideband frequency-dependent resistor is generally implemented as a purely resistive input to an amplifier, which hides the characteristics of the later circuit components from the source.

Eg, a graphic EQ is a user-variable frequency dependent resistor.

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Marcel de Graaf

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Re: Frequency depending resistor
« Reply #8 on: August 25, 2018, 06:09:58 am »

Langston, Frank thnx for the input.

What made me triggered is on old post of TD:
http://mailman.soundlist.org/pipermail/sound/2007-January/027323.html

I find it interesting that a omni point source with flat magnitude, measures a (fixed) time difference of almost -90 degrees between a in- and output signal. His explanation why this is, made me visualize this in my head.

What i want to do is to build a circuit (as simple as possible) with some resistor, cap and inductors, that measures the magnitude into that load flat, but with a (fixed) phase delay between input and output. As i see it you have to use some kind of frequency depended resistor to achieve this.

The circuit is than a good reference for me to use different analyzers, to see how they measure and see the plot, from a known load. I know a can made a compare with an acoustic measurement, but i want to do the electrically, so i can really exclude any uncertainty.

gr. marcel
« Last Edit: August 25, 2018, 06:45:51 am by Marcel de Graaf »
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Frank Koenig

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Re: Frequency depending resistor
« Reply #9 on: August 25, 2018, 11:55:30 am »

What i want to do is to build a circuit (as simple as possible) with some resistor, cap and inductors, that measures the magnitude into that load flat, but with a (fixed) phase delay between input and output. As i see it you have to use some kind of frequency depended resistor to achieve this.

It sounds like what you're trying to do is to build an analog Hilbert transformer. If you hit the literature you'll likely find ways to approximate one over a limited range of frequencies. A straight integrator or differentiator will give you a 90 deg phase shift but, of course, the magnitude will vary as 6 dB / octave.

You're in the world of analog computing. Most folks doing this kind of work these days would model all this in software where everything becomes "a simple matter of programming" :) That's my first choice. -F
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