[Geezer (OP)]

There may be another factor.

A higher frequency sound wave will interact with the wall in such a way that the maxima and minima of the wave encounter the wall at significantly different times - if you see what I mean.

So, the wall will receive a rather mixed signal as maxima and minima will be distributed across the surface of the wall. These will tend to interfere with, and cancel each other.

The lower the frequency, the greater the spatial separation between maxima and minima, so there will be less interference and canceling.

[tommya300]

http://en.wikipedia.org/wiki/Anechoic_chamber

Ah yes! We built an RF chamber for RF susceptibility and interference work, but it also worked quite well as an acoustic chamber.

I experienced a small full Anechoic/acoustic chamber. I felt a little bit dizzy at the first time entering and having the door shut. A bit claustrophobic?

[tommya300]

There may be another factor.

A higher frequency sound wave will interact with the wall in such a way that the maxima and minima of the wave encounter the wall at significantly different times - if you see what I mean.

So, the wall will receive a rather mixed signal as maxima and minima will be distributed across the surface of the wall. These will tend to interfere with, and cancel each other.

The lower the frequency, the greater the spatial separation between maxima and minima, so there will be less interference and canceling.

YES you hit it on the head. I seen this idea in my minds eye but could not put it to coherent words.

I believe the pyramid shape blocks of the acoustic chamber design enhances deflection not reflection  reducing the energy of the low freq longwaves

[tommya300]

Sure would be something to use a targeting optics to be able to view the vibration at a long distance, convert and invert the audio signal duplicate through DSP and send it back at the same magnitude to its audiable source. Somewhere along the path of travel the should have some canceling affect.
I don't think it would be a cost effective idea

[Geezer (OP)]

Sure would be something to use a targeting optics to be able to view the vibration at a long distance, convert and invert the audio signal duplicate through DSP and send it back at the same magnitude to its audiable source. Somewhere along the path of travel the should have some canceling affect.
I don't think it would be a cost effective idea

I think you can do something like that to eavsedrop on conversations by pointing a laser "microphone" at a window. Not that I actually have one or anything!

[RD]

... eavsedrop on conversations by pointing a laser "microphone" at a window.

Allegedly laser microphones can be jammed by taping a vibrating "marital aid" to the window  [:I]

[yor_on]

Use rubber, then let it bounce, ah, the sound?

Wasn't that what they used amongst other, trying to soundproof a room from those laser microphones? Stripes of rubber, hanging like a curtain?

[Geezer (OP)]

Another allusion to marital aids?

[yor_on]

No, I actually have some remembrance of them using this technique Geezer.
All Embasssy's have a room like that.

Don't know what they call it though?
the R*'er room

*Hurt*

[LeeE]

There is something a bit different from RF/light going on too, I think. Higher frequency light has more energy than lower frequency light. Sound sort of goes the other way. There is a lot of energy in a LF sound wave. That's why LF sounds can travel for much greater distances - I think!

Ah, this is where the amplitude comes in.  For a given amplitude HF sound does have more energy than LF sound but LF sounds are typically produced at much greater amplitudes than HF sounds.

If you watch an audiophile loudspeaker with the covers removed, while playing music with a high bass content, you'll be able to see the LF/bass driver move quite clearly but you'll not see any visible movement of the mid or HF units.  I've seen bass/LF cones travel over 1cm (each way) but the extremely light weight metal dome HF drivers commonly used in audiophile loudspeakers (which are typically around one inch/25mm in diameter) would collapse if they tried to match the travel of the bass/LF driver cones (even though HF domes have a much smaller area than LF cones they are trying to move the air much more quickly - in fact up to a thousand times more quickly - and they can only move that quickly be having very little inertia, which equals very little mass, which in turn means very little material).

[LeeE]

... eavsedrop on conversations by pointing a laser "microphone" at a window.

Allegedly laser microphones can be jammed by taping a vibrating "marital aid" to the window  [:I]

That shouldn't work, because the 'marital aid' will be operating at a much lower frequency than speech (Iirc, one of the limits of our sense of touch is that pressure pulses above ~20-30Hz in frequency are felt as a continuous pressure, which would make a HF 'marital aid' ineffective - speech is typically several hundred Hz and upwards).

In any case, laser microphones work by using the vibrating surface to modulate the stable laser frequency, so it would be trivial to filter out the stable LF frequency of the 'martial aid' as well.

[Geezer (OP)]

There is something a bit different from RF/light going on too, I think. Higher frequency light has more energy than lower frequency light. Sound sort of goes the other way. There is a lot of energy in a LF sound wave. That's why LF sounds can travel for much greater distances - I think!

Ah, this is where the amplitude comes in.  For a given amplitude HF sound does have more energy than LF sound but LF sounds are typically produced at much greater amplitudes than HF sounds.

If you watch an audiophile loudspeaker with the covers removed, while playing music with a high bass content, you'll be able to see the LF/bass driver move quite clearly but you'll not see any visible movement of the mid or HF units.  I've seen bass/LF cones travel over 1cm (each way) but the extremely light weight metal dome HF drivers commonly used in audiophile loudspeakers (which are typically around one inch/25mm in diameter) would collapse if they tried to match the travel of the bass/LF driver cones (even though HF domes have a much smaller area than LF cones they are trying to move the air much more quickly - in fact up to a thousand times more quickly - and they can only move that quickly be having very little inertia, which equals very little mass, which in turn means very little material).

Ah! Thanks Lee.

I do have a qestion though. I'm pretty sure most of the power from the amplifier is going to the LF speakers, whereas the HF speakers need hardly any power at all.

So, would that not mean there is a lot more energy produced by the LF speakers? Or are the HF speakers just much more efficient?

[LeeE]

I'm not sure that most of the energy from the power amplifier is going to the LF unit; in high-end audiophile systems using active crossovers you'll usually find that the same power amplifiers i.e. the same model, with the same power rating, are used for each frequency band and driver; the HF, Mid and LF drivers will have the same power available to them.

In an active crossover loudspeaker system the crossover sits between the preamplifier and the power amplifiers (note the plural), whereas a passive crossover sits between the power amplifier and the loudspeaker drivers.  A passive crossover then, must not only split the signal into separate frequency bands, before it is fed to the drivers, but it must do so upon a potentially very high power signal - up to several hundred Watts (mostly due to the currents being used, not the voltage) - without sapping an excessive amount of that signal energy.

An active crossover though, as it doesn't need to cope with such high currents, can be made with more finely produced and higher tolerance components, but it does then mean using a separate power amplifier for each of the active crossover frequency band outputs.  Active crossover audiophile stereo systems using six identical power amplifiers are not unknown - three per side, one each for LF, Mid & HF.

Active crossovers are also normally used in high-power P.A. and sound reinforcement systems too, where powers in the kilowatt range may be found - trying to pump a kW through some of the components in a passive crossover would make a good impression of a bar heater.

(sorry about having to digress into active/passive crossovers but the statement about using the multiple identical power amplifiers for each frequency band and driver might not have made much sense without understanding the system)

[Geezer (OP)]

I'm not sure that most of the energy from the power amplifier is going to the LF unit; in high-end audiophile systems using active crossovers you'll usually find that the same power amplifiers i.e. the same model, with the same power rating, are used for each frequency band and driver; the HF, Mid and LF drivers will have the same power available to them.

In an active crossover loudspeaker system the crossover sits between the preamplifier and the power amplifiers (note the plural), whereas a passive crossover sits between the power amplifier and the loudspeaker drivers.  A passive crossover then, must not only split the signal into separate frequency bands, before it is fed to the drivers, but it must do so upon a potentially very high power signal - up to several hundred Watts (mostly due to the currents being used, not the voltage) - without sapping an excessive amount of that signal energy.

An active crossover though, as it doesn't need to cope with such high currents, can be made with more finely produced and higher tolerance components, but it does then mean using a separate power amplifier for each of the active crossover frequency band outputs.  Active crossover audiophile stereo systems using six identical power amplifiers are not unknown - three per side, one each for LF, Mid & HF.

Active crossovers are also normally used in high-power P.A. and sound reinforcement systems too, where powers in the kilowatt range may be found - trying to pump a kW through some of the components in a passive crossover would make a good impression of a bar heater.

(sorry about having to digress into active/passive crossovers but the statement about using the multiple identical power amplifiers for each frequency band and driver might not have made much sense without understanding the system)

Thanks Lee!

I'm pretty sure the power input capacity of HF speakers is a lot less than the power input capacity of LF speakers (for a particular matched set of course), and I do know that heat dissipation can be a real issue with LF speakers, but I've never heard of it being an issue with HF speakers. I'm fairly confident the amp is dumping a lot more power into the low end.

Perhaps the coupling efficiency between the speaker and air is much greater at higher frequencies?

Slightly later:
Come to think of it, I seem to remember that there is an inverse relationship between the efficiency of LF speakers and the amount of distortion they produce. Unfortunately, I can't remember why! Possibly something to do with the stiffness of the cone?

[SeanB]

A simple reason for using identical amplifiers with a system using an active crossover is that by using identical amplifiers the distortion produced by the amplifier is identical for each channel, and is not so objectionable as a single amplifier that is sonically different. As well remember the midrange and tweeters will use power as well, although they generally dissipate more as sound than heat, whilst the big low end dissipates more power as heat than as sound.

As to blocking out low frequency, it is easier to block out high frequencies as you are effectively absorbing more energy per wave per unit thickness the higher in frequency you go, thus a rubber block that absorbs well at 10kHz would have to be 10 times thicker to offer the same absorbtion at 1kHz, and 100 times thicker at 100Hz.

[LeeE]

That may be a good point about the sound vs. heat produced, although iirc, ferrofluid was first used in tweeters to help dissipate heat (by thermally coupling the voice coil to the magnet), implying that heat dissipation was less of a problem in woofers, possibly because a woofer/LF driver won't need to move as quickly as an HF unit and could afford to use a heavier voice coil than a tweeter.  However, that does imply that similar levels of energy are being used in each driver, for if bi/tri amping works with identical amps then the individual drivers will have similar impedances and draw similar currents.

Also...

As to blocking out low frequency, it is easier to block out high frequencies as you are effectively absorbing more energy per wave per unit thickness the higher in frequency you go, thus a rubber block that absorbs well at 10kHz would have to be 10 times thicker to offer the same absorbtion at 1kHz, and 100 times thicker at 100Hz.

dunno if the relationship is so linear but looking at it from the energy angle is a very good point too.

[SeanB]

LeeE, ferrofluid is used in tweeters as they have a combination of a very small and low mass coil, but with a very high power load applied, as well as a very short travel. The fluid thus is able to stay in the gap, and is not going to suffer enough movement to create shear. In a woofer the coil travels a long way, but is a lot bigger, and has a much larger surface to radiate heat away to the pole pieces and the magnet basket. This means it can stay cool by radiating away heat from a large area, helped by the airflow from the cone pumping the air past the coil breaking any boundary layers there.

[Geezer (OP)]

I fear we may be on a bit of a snipe hunt here. The efficiency of loudspeakers (or drivers as they seem to call them these days) is so poor that it's probably useless to try to equate the power input to the acoustic wave pressure in a meaningful way.

It's coming back to me a bit now. Acoustic efficiency usually suffers at the expense of fidelity, which tends to explain why the most high fidelity systems need fairly gigantic power outputs, even though they are only being used in relatively small spaces. If you were ever able to convert, say 100 Watts of electrical energy into acoustic energy in your living room, you'd probably blow out your windows and seriously damage your hearing.

"Most loudspeakers are actually very inefficient transducers; only about 1% of the electrical energy sent by an amplifier to a typical home loudspeaker is converted to acoustic energy."

from - http://en.wikipedia.org/wiki/Loudspeaker#Efficiency_vs._sensitivity

[LeeE]

It's not just because of the relatively low acoustic efficiency of the loudspeakers that audiophile amplifiers tend to be relatively high powered, and in some cases they're not.

The real key is how much current a power amplifier can supply.  Generating the voltages, which dictate the amplitude of the cone/dome displacement and hence the audible volume, is relatively easy but the fidelity with which the profile of the cone/dome displacement follows that of the signal comes down to the magnitude of the force driving the cone/dome, which depends upon the current: a greater current flowing through the voice coil will have more authority and accelerate and decelerate the mass of the cone/dome against the air more quickly than a smaller current.

There are quite a few relatively low power (down to 10~15W) audiophile amplifiers, at least in terms of their continuous rating, but they are usually capable of delivering transient current levels well beyond those of a comparably powered non-audiophile amplifier.

[Geezer (OP)]

which depends upon the current: a greater current flowing through the voice coil will have more authority and accelerate and decelerate the mass of the cone/dome against the air more quickly than a smaller current.

Sort of. It's a function of the ampere-turns in the voice coil. You can achieve the same force with a lot less current if you have a lot more turns. You could build a speaker driver that used a relatively a small current, but it would have to operate at a much higher voltage, so it really is power that's applied to the speaker to do work on the air.

Of course, the load that the driver presents to the amp is rather complex. The coil produces a back EMF, and the impedance is anything but resistive, so the whole business tends to turn into a bit of a "black art", and quality becomes a bit subjective.

However, as I pointed out, I don't think we can infer too much about the actual acoustic energy based on the power that's being applied to the speakers drivers.