Electrical vs Acoustic damping

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Hello, all

Many woofers having a low Qts, their frequency range may be extended by means of the ~2 Ohm output impedance of a tube amplifier or an identical resistor added in series to the output of a transistor amplifier, at the expense of a larger enclosure size.

Consider, for example, a driver with a resonance frequency of 30 Hz, a Qts of 0.3 and a Vas of 50 litres. With a zero output impedance, a box of 11 litres is required to bring the total Q up to 0.707, in which case f-3 will be 75 Hz. But after adding a certain series resistance, the driver's Qtc may be raised to, say, 0.5 and a volume of 49 litres will be required for a flat frequency response with an f-3 of 45 Hz.

My question is: will the 11-litre and the 49-litre boxes be equally fast, i.e. have equally good impulse responses? Or in other words, are electrical and acoustic dampings thus interchangeable as long as the total system Q is the same? And if so, does it apply to vented enclosures as well?
 
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Thank you, Ron. Let me refine my question. Suppose I have a driver with a Qms of 4.0, a Qes of 0.5, an fs of 28 Hz, and a Vas of 20 liters. It will have a Butterworth frequency response with an f-3 of 46.5 Hz in a box of 13 liters.

Now suppose I have another driver, with the same parameters as above save it has a Qes of 0.25. By inserting a certain series resistance, or by using an amplifier with the corresponding output impedance, one can double this Qes and make it 0.5—equal to that of the first driver. With this setup, and in the same 13-liter box, the second driver will have the same Butterworth FR with an f-3 of 46.5 Hz.

Will these systems have identical impulse responses? Is it acceptable practice thus to extend the LF performance of loudspeakers by designing them to be used with a specific positive output impedance, regardless of the mid- and high-frequency effects due to the increasing Z, that is?

EDIT: There are many woofers with terribly low Qts values, around 0.21-0.3. Are not they begging for an increased output impedance?
 
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Same frequency response = same impulse response.
There is no gotcha from increased output impedance, other than reduced sensitivity.
Most speakers are designed for low output impedance because that is what they are driven with. If you design your own you can do what you want, this also becomes an option if your system is powered; although if powered, you can do more interesting things with filters, etc...
Many low Q woofers are designed either to use really small boxes, give a higher sensitivity spec, or to compensate somewhat for the series resistance of inductor coils, etc...
 
Hi Ant222,

Agree with Ron E:

b:)
 

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Thank you, Ron and bjorno, for your answers. If the impulse and frequency responses mutually determine each other, what about the transient response?

Is the sensitivity loss caused by the higher compliance of the acoustic suspension, and if so, is this loss uniform across the whole frequency range or only in the region of piston-like operation, i.e. below the break-up frequency?

What is a powered system? No, I do not intend to use filters beyond a first-order crossover.
 
Define "transient response". In the sense of how it is affected by T/S parameters, the only thing that matters is the area below ~200Hz.

Sensitivity loss is created by the loss of signal in series resistor

A powered system is one where you include a built-in amplifier.
 
Define "transient response". In the sense of how it is affected by T/S parameters, the only thing that matters is the area below ~200Hz.

Yeah. A rather vague term to characterize how closely the sound pressure produced by the loudspeaker tracks the incoming electrical signal.

Sensitivity loss is created by the loss of signal in series resistor

It is not important with a transistor amplifier because it is powerful enough, whereas a tube one will already have a positive output impedance.

A powered system is one where you include a built-in amplifier.

We call them active systems. It's not what I intend to do.
 
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