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Author Topic: Why can't I block out low frequency sounds as easily as high frequencies?  (Read 28120 times)

Offline Geezer

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elfabyanos: I think we should rule out the case of valve amps sounding better than solid-state amps when there's a significant degree of distortion - I entirely agree with what you say about our perception of odd and even ordered harmonic distortion - but ideally we want no distortion, at least in the reproduction of sound (as opposed to the creation of sound e.g. the 2nd order harmonic distortion in an electric/electronic instrument amplifier).

Other than that, the only things I'd like to add are that better quality components will have tighter tolerances and will be more linear, and that the power supplies will be likely to incorporate larger capacitors, capable of storing more energy and creating a greater reserve for when those large transients do occur.

Yes, the design of the amplifiers is very important, although the speakers and the acoustic characteristics of the room in which they are placed will probably have a far greater impact on the perceived sound quality than anything else. But what does this have to do with the original question?
 

Offline LeeE

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Geezer: yes, the impedance of the driver will vary with frequency, but not so far as to take into completely different realms of voltage and current delivery.

Lee - Take a butcher's at this. It may change your opinion. http://users.ece.gatech.edu/mleach/papers/vcinduc.pdf

Geez: I'm simply not prepared to wade through eight pages of an academical paper in search of what, exactly?

C'mon - don't expect me to put all the effort into proving myself wrong - that's your job.  And I'm not saying that the paper you referred to is not relevant, but just pick out and quote the bit of it that is relevant to this thread, and then give the link to the entire paper so I can verify it.
 

Offline Geezer

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Geezer: yes, the impedance of the driver will vary with frequency, but not so far as to take into completely different realms of voltage and current delivery.

Lee - Take a butcher's at this. It may change your opinion. http://users.ece.gatech.edu/mleach/papers/vcinduc.pdf

Geez: I'm simply not prepared to wade through eight pages of an academical paper in search of what, exactly?

C'mon - don't expect me to put all the effort into proving myself wrong - that's your job.  And I'm not saying that the paper you referred to is not relevant, but just pick out and quote the bit of it that is relevant to this thread, and then give the link to the entire paper so I can verify it.

I found the paper quite interesting. I thought you would too. However, if it's bamboozling you, just look at figs 9 and 11 and you can see how much the impedance increases with frequency. No big surprise really. A speaker has a lot of inductance and mechanical resonance, both of which have a significant impact on the impedance at various frequencies.

The bottom line is that you can infer almost nothing about the acoustic energy produced by a speaker based only on the amplitude of the signal across its terminals. Not only is it a very inefficient transducer, but it has a complex frequency response.

Slightly off topic of course, but that's one of the dirty little secrets of the audio equipment industry. You can produce an amplifier that does what an amplifier should do, i.e., faithfully reproduce the waveform that's stuck in the input with a much greater amplitude and capable of driving a low impedance load without distorting it. Unfortunately, the sound "quality" is heavily influenced by the speaker, but the amp has not the faintest clue about what's actually coming out of the speaker.

Turns out, if you introduce various types of distortion into the signal (valve characteristics etc etc etc) you can apparently "improve" the "quality". But the "quality" is largely subjective, and this is a really great thing for the audio industry because it allows them to eternally make "improvements" that obsolete previous generations of equipment just by making a few more spurious claims that no one will ever be able to really quantify - but it makes for good conversation down the pub.

Sorry, but I've been watching this circus for fifty years, and it just keeps going round and round and round.....

BTW - I have no particular interest in proving either of us right or wrong as you seem to suggest. I've already pointed out that I made some bad assumptions in this thread.
 

Offline elfabyanos

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Lee - Take a butcher's at this. It may change your opinion. newbielink:http://users.ece.gatech.edu/mleach/papers/vcinduc.pdf [nonactive]

Ignore this, there is nothing you can take away from it that will apply to a listening envirnment. A driver's free air resonance and impedence characteristics are of interest only to the loudspeaker's designers.

What your amplifier sees is the impedence curve for your loudspeaker system. The very same 8ohm driver in a closed cabinet might create an impedence of 40ohms at 50Hz, but in a bass-reflex the impendence at 50Hz might be 2.5ohms. Closed box cabinets create a high impendence at system resonance (Fs) off set by the resonance so in a well tuned system there is no significant increase or drop in perceieved output. Indeed there isn't in terms of energy transferred to the air as sound.

Bass reflex loudspeakers have an incredibly low impedence at the port tuning frequency, often accompanied by two peaks either side (in a well tuned system, in a bad one the peaks will be lop sided one way or another), which is typically also near fs. What happens is that the driver vibrates much less than if it was in a closed cabinet, but the air in the port resonates instead and takes over the audio output at those low frequencies. This is also the cause of the drop in impedence.
« Last Edit: 21/10/2010 11:17:24 by elfabyanos »
 

Offline Geezer

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Lee - Take a butcher's at this. It may change your opinion. http://users.ece.gatech.edu/mleach/papers/vcinduc.pdf

Ignore this, there is nothing you can take away from it that will apply to a listening envirnment. A driver's free air resonance and impedence characteristics are of interest only to the loudspeaker's designers.

What your amplifier sees is the impedence curve for your loudspeaker system. The very same 8ohm driver in a closed cabinet might create an impedence of 40ohms at 50Hz, but in a bass-reflex the impendence at 50Hz might be 2.5ohms. Closed box cabinets create a high impendence at system resonance (Fs) off set by the resonance so in a well tuned system there is no significant increase or drop in perceieved output. Indeed there isn't in terms of energy transferred to the air as sound.

Bass reflex loudspeakers have an incredibly low impedence at the port tuning frequency, often accompanied by two peaks either side (in a well tuned system, in a bad one the peaks will be lop sided one way or another), which is typically also near fs. What happens is that the driver vibrates much less than if it was in a closed cabinet, but the air in the port resonates instead and takes over the audio output at those low frequencies. This is also the cause of the drop in impedence.

Pardon me Elfabyanos,

But this thread is nothing to do with listening environment. The only reason this even came into the thread was to determine if there is a reasonable correlation between the power delivered to a speaker and the acoustic output. That does not seem to be the case.

I'd suggest you start another thread on the subject of audio quality.
 

Offline Geezer

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Anyhoo, despite your loud assertion that we should "ignore this", if you would care to take a minute to review the thread and understand the context in which the reference was provided, you might appreciate that it was to confirm how much the impedance of a speaker can vary with frequency, and how little that has to do with the so called "impedance" value stamped on the speaker.

What you are pointing out is that the enclosure design also has a considerable effect on the impedance presented to the amplifier, and that an enclosure might make it even more variable with frequency than the reference suggests.

Therefore, I suggest, we are actually in violent agreement.

 

Offline elfabyanos

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Yeah I think so.  Anyway, I didn't intend to take the thread OT, so to bring it back back I've dug out my college literature.

The explanation as to why low frequencies are hard to block - all sound that is transmitted through a wall is through re-radiation, requiring the wall to vibrate. The question is therefore why does the wall vibrate at lower frequencies as opposed to higher ones. All object tend to vibrate better at lower frequencies than higher ones, and this comes down to the physical properties of the medium. A solid object like a wall does not transmit high frequency sounds well for the same reason that it is a wall - i.e. it is solid and not compressible. If one imagines a wavelength that is twice the thickness of the wall, then to transmit such a sound the molecules on one surface of the wall would have to be traveling in precisely the opposite direction to the molecules on the surface on the other side.

The ability for a brick to have parts of it moving in opposite directions is low, indeed I would think that increase the level of such a sound wave high enough and the brick will just shatter. As the brick resists such movement the energy is instead reflected by the brick into the same room it came from.

Low frequency sounds have wavelengths much longer than the thickness of a wall, therefore the difference in velocities of the molecules in various parts of the brick is much smaller, the brick is therefore not resisting the movement as much, and less energy is reflected. Then the entire wall will accelerate as per mechanics as the force from the wave acts on the mass of the wall. The entire wall must have some give in it to allow it to vibrate, so that the entire wall bends in and out, and as such the middle of the wall which moves further will re-radiate more energy than at the corners where it is anchored to the other walls and rest of the building (in practice this would differ case by case, as buildings are not all as rigid as each other).

There is also a significant element somewhere in here about the velocity of the particles, as for a given audio energy the velocity increases with frequency. The higher speed has implications in terms of mechanics to explain why any medium transmits lower frequencies further than higher ones, as in losses through friction, but I don't know the maths well enough to back it up. Over the short distance of a wall thickness I think this effect would be negligible, but thats a hunch.
 

Offline Geezer

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Thanks Elfabyanos! I think that helps a lot.

Would it be correct to say that a wall can act as a sort of diaphragm at low frequencies, but it acts more like a reflector or energy absorber at high frequencies?
 

Offline maffsolo

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Thanks Elfabyanos! I think that helps a lot.

Would it be correct to say that a wall can act as a sort of diaphragm at low frequencies, but it acts more like a reflector or energy absorber at high frequencies?

Something like a passive flat panel woofer?
 

Offline elfabyanos

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Geezer - yes I would say thats a fair description.

Maffsolo - something like that, but passive radiators are more complicated as they affect the main driver directly as well, unlike the wall which is remote. Passive radiators in speakers are acoustically coupled to the main driver by the air in the speaker cabinet. The passive radiator and the main driver then become one system and the combined impedences and resonances become very different, often in non-intuitive directions.
 

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