Questions I should know the answer to...but don't.

JJV

I thought this might be a useful thread to ask those difficult questions you would like answers to. To start the ball rolling.

How can electricity travel down a mic lead as 48 volt DC phantom power & travel back up in the opposite direction in the form of an analogous electrical signal?

If sound is caused by an object vibrating in air, what is vibrating when you blow across the top of a milk bottle or a flute?

Any (correct) answers appreciated.

eire1

JJV wrote: »

I thought this might be a useful thread to ask those difficult questions you would like answers to. To start the ball rolling.

How can electricity travel down a mic lead as 48 volt DC phantom power & travel back up in the opposite direction in the form of an analogous electrical signal?

Condenser mics have a transducer which converts sound into an electrical signal.

i57dwun4yb1pt8

1/ the dc is modulated by the mic capsule signal thats ac ,
think of the audio wave sitting on an x axis at + 48 instead of at 0

later the dc is removed by a capacitor and the signal is now an ac audio wave on the x = 0 axis



2/ the bottle top edge is vibrating , as well as the bottle ,
and being amplified by the body of the bottle resonating
if you hold the bottle hard - you wont get anything sound wise .

TroutMask

+ 48 volts is applied to pins 2 & 3 of the XLR via a pair of 6800 ohm resistors (these are simply put there for current limiting in the event that a bad cable is presented to the XLR). Since both the audio hot (pin 2) and cold (pin 3) receive the *same* voltage the audio signal is unaffected. There is, however, a DC block in the form of a pair of capacitors: one for the hot and one for the cold . DC current will not flow through the capacitors so the sensitive Pre is protected from any DC bias. The alternating audio signal flows through the DC blocking caps unhindered. At the mic, or active DI - blocking caps are again employed, and it is a simple matter for the engineer to design circuitry that can avail of this source of filtered DC power. Think of it as two circuits sharing the same cables - one which uses the DC blocking characeristics of the capacitor to pull off this efficient stunt.

flyswatter

What are programmable hardware synths without a keyboard called?

You know the ones that are just square shaped in size and you just tweak them. I presume midi keyboards are attached?

JJV

Synth modules I believe.

jiltloop

flyswatter wrote: »

What are programmable hardware synths without a keyboard called?

You know the ones that are just square shaped in size and you just tweak them. I presume midi keyboards are attached?

They can be called synth modules as stated above or more usually they would be called either a desktop synth or a rackmount synth, desktop just sits on your desk obviously and a rackmount has rack ears to allow it to be rackmounted (Ronseal or what?).

studiorat

JJV wrote: »

If sound is caused by an object vibrating in air, what is vibrating when you blow across the top of a milk bottle or a flute?

The air is vibrating. The way in which it vibrates in a bottle and a flute is slightly different since one is open at one end. But the principal the same.

If you have something like an organ it has two types of pipes, flue and reed. The flue is like your milk bottle where the air vibrates. The reed pipe has a reed in it which vibrates like a sax etc.

i57dwun4yb1pt8

^^ explain why there is little sound if you hold the the whole bottle with both hands ?

* answer the bottle is vibrating as well

JJV

Here's another one I struggle with. How does one work out the relevant watts & resistances when pairing amplifiers & speakers. For instance if a speaker has a resistance of 8 ohms how will this differ from a speaker with a rating of 4 ohms in terms of the amp's power? How does connecting 2 speakers in series affect the power of the amps etc. If anyone could point me in the direction of a good article on the web I would be most grateful.

studiorat

There's nothing to explain, it's not the case.

When you blow across the bottle the air begins by going into the bottle. Since there is nowhere for the air to escape pressure builds up. It continues to build until there is so much pressure that a jet of air is pushed out of the bottle. The air is pushed out until the pressure is low enough to suck air back in. This cycle is the vibration which causes the sound.

Reducing the volume of the bottle by adding water for example, speeds up this cycle of pressure changes resulting in a higher pitch. Compare the way the pitch changes with water in the bottle when you strike the edge of the bottle with when you blow...

Timbre is created by the shape of the airflow (embouchure) and the acoustic properties of the inside of the pipe, how much sound is reflected and absorbed. That's why a sax and a clarinet sound different even though the method of creating the sound is the same.

JJV

studiorat wrote: »

There's nothing to explain, it's not the case.

When you blow across the bottle the air begins by going into the bottle. Since there is nowhere for the air to escape pressure builds up. It continues to build until there is so much pressure that a jet of air is pushed out of the bottle. The air is pushed out until the pressure is low enough to suck air back in. This cycle is the vibration which causes the sound.

Reducing the volume of the bottle by adding water for example, speeds up this cycle of pressure changes resulting in a higher pitch. Compare the way the pitch changes with water in the bottle when you strike the edge of the bottle with when you blow...

Timbre is created by the shape of the airflow (embouchure) and the acoustic properties of the inside of the pipe, how much sound is reflected and absorbed. That's why a sax and a clarinet sound different even though the method of creating the sound is the same.

Interesting. So the "sound is caused by an object vibrating in air" statement is a generalisation? Sometimes sound is caused by the disturbance of air molecules as they are forced in & out of containers causing changes in atmospheric pressure?

TroutMask

JJV wrote: »

Here's another one I struggle with. How does one work out the relevant watts & resistances when pairing amplifiers & speakers. For instance if a speaker has a resistance of 8 ohms how will this differ from a speaker with a rating of 4 ohms in terms of the amp's power? How does connecting 2 speakers in series affect the power of the amps etc. If anyone could point me in the direction of a good article on the web I would be most grateful.

First concept to grasp is this: that number stamped on the speaker chassis does not really mean resistance. What it is is an average *representation* of the impedance of the speaker. Impedance is a combination of resistance, inductance and capacitance - so this number is a composite representation of these electrical characteristics. The next thing you need know is that this value changes as the speaker cone moves : nothing is ever at rest in the world of audio. For example, the DC resistance of an 8 ohm speaker can vary between 2 and 32 ohms in real-world operating conditions.

Think of impedance as the amount of inertia that an object presents to an energy source. If you had creels of turf on a cart , you would need a certain amount of force to get it going. If there were 8 creels, and your best effort could send it 10 metres, then halving the number of creels will result in a 20 metre travel for the same force. The nature of the relationship is linear (give or take a bit).
The same relationship holds in the case that you mentioned above. You''ll get roughly a doubling of electrical power each time you half the impedance. This does not mean 'twice as loud', as acoustic power works differently. Also, the amplifier will need: a) to be able to meet the increased power demands and, b) be capable of supporting the reduced impedance without sustaining damage.

frobisher

http://www.gearslutz.com/board/so-much-gear-so-little-time/184297-thread-asking-things-you-should-know-now-but-dont.html

There's a big version

QuadLeo

This one has been troubling me for a while now. I've read a lot about quality headphone preamps that you should use with highend headphones like HD650's. I've read of people buying highend preamps like this one

But what I don't understand is that in order to use this preamp with an iPod or laptop you need to take a line out of the device headphone socket and connect it to the preamp. Therefore you're still using the inferior device preamp in the signal chain. So how will adding an extra preamp improve the quality? Does it? Am I missing something? I know it can make the signal louder but does it actually imrpove the quality?

TroutMask

QuadLeo wrote: »

This one has been troubling me for a while now. I've read a lot about quality headphone preamps that you should use with highend headphones like HD650's. I've read of people buying highend preamps like this one

But what I don't understand is that in order to use this preamp with an iPod or laptop you need to take a line out of the device headphone socket and connect it to the preamp. Therefore you're still using the inferior device preamp in the signal chain. So how will adding an extra preamp improve the quality? Does it? Am I missing something? I know it can make the signal louder but does it actually imrpove the quality?

i think he wants you to use it with your hifi instead of the built-in one. To bypass the headphone amp in a laptop, you would need to use an audio interface of some description. You can get a line out of an iPod with the correct cable.

madtheory

TroutMask wrote: »

i think he wants you to use it with your hifi instead of the built-in one. To bypass the headphone amp in a laptop, you would need to use an audio interface of some description. You can get a line out of an iPod with the correct cable.

And the dock connector on an iPod gives a line level output. Bypasses the eq as well. Cables on eBay.

TroutMask

Dock good! Need dock