Understanding Decibels (part 2)

In the first article we saw that a decibel is not a fixed unit, rather, it is a ratio between two levels.  A decibel reading on its own is meaningless if it isn’t referenced to something. Unless it is spelled out, that reference something is normally 0dB. The mic input on a camera or mixer might say -50dB, that would mean it is 50 dB lower than 0dB. The line out of a mixer or DVD player might be -10dB, meaning it is 10dB lower than 0dB. The stereo out of a mixer might be +4dB, meaning it is 4dB above 0dB.

What is 0dB?

In simple terms, 0dB is the reference level. It is the level that is being referred to. When the measured level is neither above or below the reference level, then the difference in level is obviously zero, hence our reference level is called 0dB. So what is this level? In most situations with audio, 0dB will refer to Line Level. That is, the nominal level that comes from a mixer, keyboard, camera, or DVD player etc.

What level is Line level?

This is the $64,000 question, because there are various levels that are referred to as line level. A good specification table for a camera, amplifier, mixer or recorder will always say what voltage level they are using as 0dB. There are several conventional line levels that can normally be used if no other information is stated.  The secret is to pay close attention to the letter after dB.

  • 0dBV refers to 0dB being one volt (1V)
  • 0dBu refers to 0dB being 0.775 volts
  • 0dBm refers to 0dB also being 0.775 volts, but the power formula for decibel calculations needs to used, meaning 1dBm is close to 2dBu.

Don’t get too bogged down in all this, just make sure you compare apples with apples. That is, if you are looking at two cameras or recorders or mixers and comparing the microphone input sensitivity, make sure both specifications are dBV, or both are dBu or dBm. Before we look at some real world examples, we need to appreciate that 1 volt is a high level for line levels, and not commonly seen in consumer electronics. The “standard” line level in consumer electronics is normally -10dBV, which is 316 millivolts or 0.316 volts.  That is why the tape out of a mixer is often -10dBV. The same level is used for line in on digital recorders, TVs, VCRs and cameras with line in.

Camera Specifications

The popular Panasonic GS500 (similar to most cameras) states that the audio output level (line) is -10dBV (316mV). This is good because this matches the standard consumer line level found in VCRs and TVs. So when we plug the line out from the camera into the line in of a recorder or TV, the levels match and all sounds good.

The specifications also say “Mic input: mic sensitivity -50dB (0dB=1volt)”. Knowing the reference is 1 volt, and that 50dB is a ratio of 316 (see table) we can calculate the mic input is expecting 3.16 millivolts (1 divided by 316). This is about the level from a normal microphone so all will be good when a microphone is plugged in.

When a sensitive or high output microphone (eg Rode Videomic) is used, it is useful to reduce the output on the microphone so as not to overload the camera input. Hence most users switch in the 10dB attenuator on the Rode Videomic.

Mixer Specifications

A common mixer used in churches and schools is the Yamaha MG series. Its specifications are also representative of many mixers. A quick glance at the specifications reveal both dBu and dBV are used. However at the bottom of the page, they do conveniently state that 0dBu=0.775 volts and 0dBV=1 volt. They also helpfully give the equivalent voltages alongside each dB value. Let’s look at some of the specifications that will interest us if we want to connect to a mixer output.

The stereo out has a nominal output of +4dBu (1.23volts) with maximum +24dBu (12.3volts). It should be obvious that we shouldn’t connect this straight into the mic in on our camera! If we need to connect to this output, we would need to reduce the level by at least 50dB.

The better output might be the tape out. The specifications state “Record out: -10dBV (0.316 volts)”. This is closer to -50dBV (3.16mV) but still 40dB higher than nominal level for the mic input . So we need to reduce even this output by 40dB, or by a factor or 100. The other common output often used from a mixer is the headphone output. Some calculations reveal this can have a level between -9dBV to +4.77dBV, depending on the signal level and headphone volume control. This should be used only as a very last resort, and then still using at least a 40dB attenuator.

40dB attenuator

Having a range of attenuators, or a variable attenuator is useful. However a 40dB attenuator will assist in most situations. So what is a 40dB attenuator, and how do you make one? Simply put, a 40dB attenuator reduces (attenuates), or divides the signal by a factor of 100. Most music shops will sell what is often called a DI box. The DI stands for Direct Input, or Direct Injection. Either way, it is simply a box which allows instruments (like keyboards and guitars) to be directly plugged into the microphone input of a mixer. These DI boxes also change the signal to a balanced line to be able to run a longer microphone lead (more on this in a coming article). Many DI boxes have a switch which will enable you to select -40dB, -20dB or 0dB attenuation.

If you need to buy a DI box, here is a link to some of Amazon’s range of DI Boxes.

If you are trying to connect from a mixer (line out) to a video camera or laptop with only a Microphone input socket, then it is easy enough to build a simple 40dB attenuator with two resistors, (a 10K and 100 ohm resistor) and a capacitor. The circuit looks like this:   understanding decibels, 40dB attenuator

This circuit simply divides the input which makes the output 1/101th of the input. It can be made up by anyone who knows a little about electronics and can solder. Similar circuits appear in any number of forums with slight variations. They should all work. The capacitor is used to block any DC voltage coming from the camera or from the mixer, it is not always needed.

Here is a drawing of how you might use this as an attenuator built into a 6.35mm (1/4″) plug (this plugs into the mixer – you’ll need a 3.5mm (1/8″) plug for the mic input). Don’t forget to fit the plug cover on the cable before soldering the resistors and plug on. This doesn’t use the capacitor, but you could fit it in the plug at the other end of the cable if required.

If you want make an attenuator with a different attenuation, use can use my simple decibel calculator for audio.

I hope this has helped in understanding levels and decibels, and the why and how of connecting line level to microphone inputs.

This article is based on one I originally wrote for my friends at CamcorderUser.net, and has been refined by their helpful comments

 

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14 COMMENTS

  1. Geoff thank you for this great resource. I’m still trying to get my head wrapped around the decibel concept but have ran into somewhat of a roadblock. I’ve noticed that a majority of gain/trim controls on a variety of soundboards are labeled with scales such as -10 to +25 or +15 to +60. In others however, like the popular Yamaha MGP series, the gain control is labeled -16 to -60, or with the 26dB switch selected, +10 to -34. Assuming gain knobs are designed to turn the same direction i.e. clockwise to increase and cc to decrease as intuition would have it, why do the MGP gain controls seem to DECREASE the dBs as they are turned clockwise?

    • Hi Greg,

      It seems like you have the basic concepts sorted.

      As you say, on most mixers the gain control indicates how much gain the pre-amp will provide, which increases as you turn the gain clockwise.

      On other mixers turning the gain control clockwise also increases the gain, although the labeling may not indicate this. On these mixers (like the Yamaha you mention) the labeling is indicating the input sensitivity (like microphone sensitivity). It is just a reverse way of describing the same thing: it is saying the input is -60dB, so it will amplify it by 60dB. Very clumsy way doing it. I think they rely on most people not pretending to understand decibels and therefore not worrying about it.

      hope this helps

      Geoff

  2. Hello Geoff,
    Your article has given me inspiration. I am working with a small community theater that wanted a wireless headset back stage tied to their Clear-Com intercom unit. My observation is that the cheapest wireless is a home cell phone on intercom setting. So here is my imagination: the Clear Com ( and competitor RTS-Telex) have a full time duplex telephone style intercom that is operated with a belt pack amplifier at each user’s side, and the user is hearing and speaking through a headset to listen and a microphone to speak. How about I build two of your -40db units and go from the earphone of the headset to the mic input on the home cordless phone – – and go the opposite way from the speaker on the cordless phone into the mic on the comm unit.
    this would keep the speaking and listening separated, and then put the Clear-Comm conversation into the local intercom between hand held home phones in the theater. Yes, this ties up a headset and a phone handset, Yes this is hundreds and thousands cheaper than the better and professional solutions available.
    Your thoughts? this is similar to a old audio modem to set your phone handset on, or putting two phone handsets together so the people on the two phone calls can talk with each other – a move seen in old movies.
    Any thoughts are appreciated, as is your article.
    A donation has been made, as well. Thanks for the inspiration

    • Hi Tim,
      I think your basic concept is fine. Without trying it or thinking too hard, I can only think of a few possible issues:
      1) The levels may need adjusting. You may not need 40dB attenuation. Maybe 20dB might be better. Just try a 40dB att. and if you don’t have enough level, change the 10K resistor to 1K and that will drop the ear piece level by only 20db into the mic
      2) The frequency response may not be as good as on the clear com, as phone frequency response is normally poor.
      3) The normal mic maybe an elect mic, which requires some voltage to make to work. If there is any voltage present across the mic, you will need to make sure the capacitor is included.
      4) You should check that the phone allows full duplex between each handset and the base station at the same time. They might only allow only intercom conversation at the one time
      5) Also check that the intercom function doesn’t automatically turn off after 5 minutes or something like that.

      Apart from that, it looks like it would be worth a try.

      Geoff

  3. Hi Geoff,

    Thank you for this great article, it helps me a lot to understand the basic of decibel.
    One thing that i still don’t understand: what is the function of gain dial in a mixer? Does it work similar with attenuator?

    Thank you very much

    • Hi Johan,
      The gain control on most mixers adjusts how much gain (amplification) of the signal there will be for that channel. For example, if you have a singer shouting into a microphone, you don’t need much gain, so you would turn the gain down. However if someone is speaking softly, you would turn the gain up. It effectively adjusts how much of the signal is coming into the channel, whereas the monitor knobs and the main level control adjusts the output of each channel.

      hope this helps

      Geoff

  4. Thanks Geoff for the reply! I am going to give the 40dB a shot. I do have a couple of questions. When I went to the store today and asked for the above mentioned resistors, he asked me for the wattage! I looked at him like he had 2 heads but apparently it is a valid question. What wattage resistors would I need? Also, I am assuming you are using a stereo (ano not mono) 1/4 inch plug that will go into the mixer?

    • Hi,
      The smaller the resistor the better (to fit into the plug), so that normally means 1/4 watt. The actual wattage going through these resistors if less than 0.0001 watt, so wattage is not a real issue – apart from size, and the smaller the better.

      I usually use a mono plug from the mixer. If you have a stereo one, that will work too, but join the outside sleeve to the middle ring. It is difficult to fit two lots of resistors in a plug, hence why I run in mono. You can do stereo if you like, but you may need to fit the resistors into a little container along the lead, rather than trying to fit them into the plug.

      hope this helps.

      • Hi Geoff,

        Mission Accomplished! The 40dB attenuator like you suggested works great with my camera. Sound quality is immaculate without any distortion. The only issue is that it is a real pain to put resistors in the plug. One of the resistors already broke despite a lot of soldering. So have to get it made again. But works great!

        • Well done! I’m pleased it worked well for you. Yes, it can be a bit tight fitting it in the plug, but it is possible. If it proves too difficult for your first soldering job, then you can put them in a small plastic box/container half way along the lead. That will give you more room for the soldering. But hey, you have done well, you got the result you wanted, made it yourself and learned something along the way. Well done.

  5. Geoff,

    I want to make a 60DB attenuator because my camera the Panasonic MD 9000 has a mic sensitivity of -60dBV. As per the diagram above:

    1. Would I need to replace the the 100ohms resistor by a 1000 ohms resistor?

    Or

    2. Or would I need to replace the 10,000 ohms resistor by a 100,000 Ohms resistor?

    • Hi Pratz,

      Yes, your second idea of using a 100,000 ohm resistor with the 100 ohm resistor would give you a 60dB attenuator. That is a ratio of 1000:1

      However I would be inclined to try the 40dB one first. The reason being, your camera has a maximum gain of 60dB, but if you turn the gain down it will be less. This way you can turn it up if the level is a bit low. You need to remember that you may not be getting full line level from the mixer all the time, you may be getting a lot less. So you will need to add some gain. But if you use a 60dB attenuator, you will have no spare gain in the camera if the level is a bit low.
      Let us know how it works out.

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