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This in fact not a meter specificaly for measuring the
loudness of sound, but it is the basis on which the first meters
measured sound. The meter has a magnetic core and the needle is held at
the '0' position against a hairspring. As the voltage through the meter
raises it pushes against the spring until it settles at a certain point
on the scale.
The needle is incapable of moving very fast, but that doesn't really
matter for measuring a constant voltage.
The volume unit meter was jointly developed by Bell, CBS and NBC and went in to use in 1939. It is a voltage meter calibrated to display an audio signals average level in decibels. Designed to monitor the voltage of sound broadcast across a telephone network, this sort of meter is quite slow. It doesn't manage to show you the highest peaks of a sound, so if you are analysing a short, sharp sound it will not give you an accurate idea of what is going on. Just like the Voltage meter, it has a relatively heavy armature and needle, so much as it can display a voltage, it doesn't have a hope of moving very quickly. After all, it is pushing against a spring. This means that it doesn't react very quickly to changing levels and it is quite easy to over-record fast peaks without even noticing them. It was useful for monitoring the average level of a recording, but if the content of a recording was very dynamic allowances had to be made for the meters inadequacy. These meters can be found on equipment manufactured right up to the 1990's!
This is the most accurate type of analog meter used in professional audio work. It is pre-biased toward the top end of the scale. It takes a little time to react, but unlike the Vu meter which has difficulty in reaching the correct position from the zero point, the PPM reaches its high point very quickly and settles back to '0' slowly. Various standards exist for the PPM. The scale over which the range is set varies in different parts of the world. The version shown here uses the UK scale. Compared to the Vu meter above the '0dB' point on the Vu would read as '5' on the UK PPM. Many engineers still use PPMs, but much as they are great for analog recording they can only realisticly be used in conjunction with the metering required for modern-day digital recording.
The Digital Peak meter is the one which you will look at most
often in the DAW domain. Firstly, because it exisits in in software it
is not subject to any mechanical conditions, so what you see is pretty
much what you get. There is one main difference though. The top of the
scale is marked '0dB full scale' and that is exactly what it means.
0dB represents the highest level before clipping. If you record over
this level, your recording will exhibit digital distortion.
This is where the subject begins to take on some significant meaning. You may have already thought to yourself "So where is the reference point for recording?". Good question!
To answer this we have to have a closer look at analog recording. If you look at the Vu meter above, you will see that it is marked from -20dB to +3dB on one side of the scale and 0-100% on the other. This is because it was intended to have a significant, average level. The scale would be calibrated so that 0dB would represent the required output voltage level of the audio equipment, or 100% of it, which is of course the same thing. In the case of analog recording this was fine because the audio equipment had a certain amount of headroom available above the meter scale. So clipping would not occur as quickly. The reference level was chosen so that this would be the case.
In the case of digital recording, for a long time there was no standard reference point.
Before I carry on there is something that I should make clear:
In the sound domain the decibel is often used to describe the loudness of a sound. In actual fact it describes a ratio. Sound alternates in air rather like ripples on a pond. The pressure stays the same over all, but there are variations in it. The measurements of these changes in air pressure are called Sound Pressure Level (SPL)
The Decibel unit in this context (Example: sound level of
83dB, or 83dB,SPL) refers to the ratio between air pressure with no
sound present and the variation of air pressure with a sound present.
I'm glad to say that this is probably the most complicated statement on
this page.
In the context of audio the dB ratio "dB=10log(P1/P2)" is used to describe a relationship between voltages:
The most important terms for us to consider here are dBFS and dBu.
If we wanted our meters to reflect a reference level of +4dBu, we would calibrate them to register 0dBFS with 1.23V present. Then 0dBFS would represent +4dBu.
If we wanted the meters to reflect the reference level -10dBu,
we would calibrate the meters to register 0dBFS with a voltage of
0.775V.
The 0dBFS would represent -10dBu.
In the digital domain the dB reference is used to describe the ratio between 0dB, the point at which a recording can get no higher, or the maximum level at which a sound can be recorded for a given bit depth. This has slightly different implications. Please note the phrase 'BIT DEPTH' as opposed to 'BIT height'. here lies the clue.
In digital recording the 'loudest' sound that can be recorded will always be 0dB, but the quietist sound will vary according to the bit depth.
It is clear that an average recording level cannot be at 0dB because everything above it would clip. So a standard needed to be recognised whereby digital recordings could be made with an acceptable amount of headroom to accomodate the peaks in the data.
The digital age of recording requires a far better and uniform system of metering than the analog days.
It is now possible to make recordings that have so little background noise that there is very little point in trying to record as loud as possible. Now we have such dynamic range available to us that it would be possible to record the sound of a pistol shot with perfect clarity, play it back at its original level (at 130dBSPL, beyond the threshold of pain) and still be unable to hear the background noise from the recording before the shot was fired.
On the other hand, recording as loudly as possible has become very competetive in the pop world, where material is often played over poor quality media such as night club P.A. systems, In-car systems, FM radio, Myspace, MP3 players and portable equipment. Despite the fact that better quality of sound is available to us, some parts of the recording industry lean toward supplying us with low quality audio equipment and low quality recordings to play.
This does not account for all of the industry, or all of the
public. Personally I feel that I would like to expand my knowlege of
recording so that I can bring my product better to my audience. We can
now record more detail than ever before. In the modern DAW all the
tools exist to create mixes and productions which engineers could only
have dreamed of achieving.
The 'K' system is about achieving these aims.
This site is under on-going development. More pages will be added soon. Thanks for visiting inthepipeline.net