Audio Levels and Readings
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Audio Levels and Readings ”solutions in Audio and Video” Welcome 2 Congratulations on delve further into the world of your purchase of a audio metering, the book Audio DK-Technologies’ Audio Meter Metering by Eddy Bøgh Brixen You are now the owner of one (of which this booklet is an ex- of the most sophisticated audio tract) is available from metering devices available DK-Technologies or from one of today. The Audio Meters from our international distributors. DK-Technologies offer a wealth of features which – once you Good audio requires technolo- have used them in your audio gical as well as musical skills. engineering work – you´ll never To turn these into perfect audio want to be without again. you will also need the best audio metering – and we trust This booklet is an addition to the that your new Audio Meter will regular User´s Manual sup- prove to be just that! And, if this plied with your Audio Meter. It booklet can help you a little bit offers a basic insight into the on the way towards a clearer world of audio levels and me- understanding of audio levels tering and is recommended as and readings, it has served its a quick reference or as a quick purpose. audio ‘brush-up’. If you want to 3 Audio Levels and Readings Why do we measure audio – this acoustic information can interval. With reference to the levels? be converted to an electric sig- waveform as it can be seen on How hard can you drive the nal, i.e. voltage, current, or elec- the oscilloscope, we can descri- audio channel – and how loud is tric power. Mostly we refer to the be the defi nitions used. it? These are the eternal questi- voltage. Another conversion of ons to everyone dealing with the the audio signal can be carried practical work of audio recor- out, if it is stored as magnetic ding, transmission, and sound information on a hard disk or reinforcement. In this case a a magnetic tape or as optical measuring device - the meter - information on a CD/DVD or a can be of great help, if you know fi lm. At the end of the chain we how it works, and if you know can use another transducer, i.e. how to use it. a loudspeaker or a headphone, and turn this electric/magne- These few pages will provide you tic/optical information back to The peak-to-peak value (Upp) is with some basic knowledge con- audible sound. the absolute maximum devia- cerning the use of meters and tion (plus and minus). The peak how to get the right information How do we measure level? value is measured from the zero from the readings. When the volume of an audio line to the maximum amplitude signal has to be determined, it at either side. (Do remember What is an audio signal? is very important that there is an that the peak we are talking When present in the air the agreement on how this is done. about here is not the same peak audio signal – or the sound – is If not, one might end up with as the one we use in the term a modulation of the static pres- numbers and fi gures that are “peak-programme-meter”. We sure. The air is vibrating. If the not comparable. When the audio will come back to that.) level of this vibration is above a signal exists in its electrical certain threshold and within a form, it can be described by the The expression we use the most specifi c frequency range, then amount of voltage or current. Or is the “rms value”. The term the signal can be audible and the signal can be described by rms is short for “root mean we can therefore simply call it the energy contained within the square”. It is a mathematical an audio signal. By the use of signal, i.e. the power delivered way of expressing the energy a transducer – a microphone into a given load in a given time contained within the signal. 4 It makes DC and AC directly comparable if the AC is expres- sed by its rms value. To put it in another way: If we have a 12 volt electrical light bulb, then 12 volt DC and 12 volt AC will make it produce the same light intensity, as long as we are talking about hertz is doubled in each interval, extent perceived as being steps the rms-value of the AC. but the relation is constant. In of equal size. That is why most this case the factor is two. level meters are using the dB- Having a sine wave the rms scale or scales strongly related value is 0.707 times its peak va- Talking about audio levels the to it. lue. If the waveform is a square same phenomenon exists. To wave, then the rms value equals perceive equal steps in level To make the dB-scale absolute the peak value. This tells us that – up or down - the relation we must defi ne a reference two signals with the same peak between each two steps has to level. When the reading on a dB- value may have different rms va- be constant. For instance, if an scale is “0”, it does not mean lues depending on the waveform audio signal of 1 volt is fed into that we have no signal. It means – or two signals with the same a loudspeaker, we will hear a that the level of the signal we rms value may have different certain level. Supplying 2 volts are measuring right now is peak values. will make it sound louder. To exactly that of the reference go one step further and for this signal. A positive value (+xx dB) Why are we using the dB scale? step to be perceived equal to the means that the level is above the The way we perceive sound is to fi rst one we must supply not 3 reference and a negative value some extent logarithmic both volts but 4 volts. Then 8 volts, 16 (-xx dB) tells us that the actual in pitch and loudness. In music volts and so on (if the speaker is level of the audio signal is below we use frequency intervals like not blasted by then). the reference level. the octave. Starting at a specifi c frequency, each octave higher is The dB-scale expresses the The “0 dB” reference is often the double of the frequency. By relation between two values indicated by the use of an ad- and large these intervals sound in a logarithmic manner. Each ditional letter: equal to the ear. The amount of unit on the dB-scale is to some 5 Audio Levels and Readings dBm: The reference is 1 mW This phenomenon exists espe- number of weighting fi lters, that into a 600 ohm load. cially at low levels. are used for the measurement of acoustical noise (for instance dBu: The reference is 0.775 A-weighting) and electrical volt, corresponding to the vol- noise in the equipment (CCIR/ tage across a load of 600 ohm ITU weighting). when 1 mW is delivered into it. dBV: The reference is 1 volt dBFS: The reference is full scale defl ection and the signal level is defi ned individually. In general this is used with digital A-weighting audio. The A-weighting curve that to some degree corrects the mea- Loudness surement of acoustical noise as It must be noted that the dB perceived by the human ear. scale is not telling the full story The equal loudness curves. of how humans perceive audio Furthermore, the direction of levels. The duration of the signal the sound has an infl uence on is also of importance. For in- the perceived level and spec- stance a tone with a duration of trum. This can have an infl uence 10 ms does not sound as loud as These three sine waves (40 Hz, on the production of surround a tone with duration of 1 second 400 Hz, and 4 kHz) have the sound. having the same level. same amplitude and the same duration. However, the loudness Also part of the assessment Concerning frequency, low of these tones is very different. of a signal is the fact that the frequencies are not perceived as perceived loudness will fall loud as frequencies in the The nonlinear frequency respon- during extended exposure due to middle of the frequency range. se of the human ear has led to a the effects of fatigue. Finally the 6 perceived level is infl uenced by nantly related to the perception to obtain stable, reliable, and a “like/dislike” of the program, of noise. International standard comparable readings the elec- a totally subjective factor that is bodies like the ITU as well as tric characteristics are normally very diffi cult to calculate. many private companies have specifi ed in a standard. undertaken much work on how If we want to measure the to measure the loudness of Concerning the standard level level of loudness, many factors program material for cinema, meters, these characteristics therefore have to be taken into broadcast, etc. This has lead to comply with the rms-process, consideration. The measuring – unfortunately – a number of time constants or integration device ideally would perform slightly different methods. Users time, fall time, level range, like the average perception of therefore have to consider the reference level, scaling, and so a sound signal by a group of algorithms/standards they want forth.