Have you ever seen old school sound level meters that have needles in them? Well, if you are already scratching your head trying to figure out what I’m talking about, then I’ll take it as a “NO”. But if you have a real passion for audio technology, then you just need to have enough curiosity in you, to try and find out what is a VU meter all about!
I’m sure that we are all used to seeing flickering LED bulbs when it comes to the measurement of sound levels. This is really not surprising at all, since most modern sound system equipment uses this type of level meter. However, the VU meter still has its place in modern audio practices, and we are here to find out more about its design.
VU Meter – Still Practical?
Also known as volume unit meter, the (VU) meter is designed to display a representation of an audio signal level. In 1942, it was standardized by the Acoustical Society of America, for use in telephone installation and radio broadcast stations. VU meters are also found on consumer audio equipment that are used for recording purposes and also for sound playback.
VU meters have a relatively slow response due to the needle’s mass, resulting in a rise time of 300 ms. “0 VU” is equivalent to “+4 [dBu]”, or “1.228 volts RMS” across a 600 ohm load (or about 2.5 milliWatts). Keep in mind that “0 VU” is often referred to as “0 dB”. A VU meter is designed not to accurately measure the signal, but to actually allow users to aim the signal’s level to “0 VU”.
The VU-meter is intentionally designed to “slow down” measurement, thus averaging out peaks and troughs of short duration. This results in a reflection of the “perceived loudness” of the audio material. Because of this, many audio practitioners up to this day, still prefer using VU meters, despite them not being able to reflect a signal’s peak level, which in most cases, must not pass a defined limit.
Here are the topics that we’ll be looking at:
- Design principles
- Standard characteristics
- Other facts
As a mechanical device, the VU meter is unable to reflect the instantaneous signal peaks of complex audio signals. Hence, designers created the VU meter to not measure peaks, but to simply “infer” them. What this means is that the meter responds to fluctuating audio signals at high speeds, rising from no signal to 99% of “0 VU” when a 1 kHz sine wave tone is applied for 300 milli-seconds.
Any audio system using VU meters, must be calibrated with a sine wave tone at a “reference level” for the system. A VU meter will show “0” for a sine wave tone at reference level, but engineers must know that peak levels for speech and music, are typically between 6dB to 10dB higher than the reference level.
Since most types of audio sources exhibit peaks within this range, a system engineer can thus design a sound system (that uses a VU meter), with better precision. It is standard practice to always allow a little extra “headroom” when mixing, to deal with peculiar conditions where an audio signal might exceed normal peak levels or the sound system operator just fails to properly adjust the levels.
Here are the standard levels considered, when designing systems using a VU meter:
- Reference Level (typically +4dBu, valid with tones only)
- Standard Output Level (10dB above Reference, typical peak levels)
- Clip Level (6dB above Standard Output Level, “headroom” to allow for unusual conditions)
The behaviour of VU meters is defined in ANSI C16.5-1942, British Standard BS 6840, and IEC 60268-17.
A volume indicator’s reading (VU meter) shall be “0 VU” when it is connected to an AC voltage equal to 1.228 Volts RMS across a 600 ohm resistance (equal to +4 [dBu]) at 1000 cycles per second (1000 Hz).
Do take note that the reference stated above, is generally true now and was always true in the recording industry. However, up until the late 20th century, some North American broadcast stations used a reference level of “+8dBm” (0 VU) at large studio installations. This was also another “standard” created in those years and the VU meter’s series resistors were changed to adjust its sensitivity.
The time taken for a VU meter’s needle to reach 99% of the distance to “0 VU” when the VU-meter is submitted to a signal that steps from “0” to a level that reads 0 VU, is known as the rise time. The standard rise time (and also fall time) for VU meters is 300 ms. The overshoot (occurrence of a signal or function exceeding its target) must be within 1 to 1.5%.
The level specification for a VU meter is meant at 1000 Hz. Readings at other frequencies should not deviate from the reading at 1000 Hz by more than 0.2 dB from 35 Hz to 10 kHz or more than 0.5 dB between 25 Hz and 16 kHz.
Take note that the specification mentions only sinusoid waveforms. In general, audio signals do not have a sinusoidal waveform, even if they all fall within the VU-meter bandpass. A VU meter’s reading shows the average of the voltage, and does not provide an indication of the signal’s power, which is proportional to the average of the square of the voltage, or the root-mean-square (RMS) value.
Nonetheless, as a conventional VU reading, it serves its purpose of indicating the overall level and dynamics of the signal and also of the signal’s proximity to the maximum admitted level, to the operators of recording and broadcasting equipment.
The VU-meter was originally developed in 1939 by the combined efforts of Bell Labs and broadcasters CBS and NBC. Furthermore, the consumer audio industry has also produced a number of volume indicators that often do not comply with the “VU meter standard”.
That about sums it up for today’s article. Have you experienced using a VU meter before? If so, do you prefer it over other sound level meters?
Let me know your thoughts down below, and don’t forget to share this article!