Compressor and dBs...

[antennaboy]

Hello Forum,

I have a two-part question about compressors:

a) I have a problem with bass (low frequency sound) from a bar across the street from my apartment. The bar just got a compressor. Can that compressor operate only on the low frequency band and only compress there or does it act on the whole spectrum of frequencies?

I think I have read about multiband compressors where we can select the frequency band....are they pretty standard or are they extremely expensive?

b) When reading about compressors I run into dBs. I know that a dB is a ratio between two numbers. It can be about power (log10 P2/P1) or voltage. I am clear what a dBm is or a dB SPL (the reference is a small pressure, 20 micropascal)....

So, when I read about 10 dB or -5 dB in relation to compressors, what are we talking about? What is the reference? Is it a certain voltage (0.775 volts ?) or a certain amount of power or pressure?

Are they implicitly referring to dBu (dB unloaded) where the reference si 0.775 volts ?

thanks,
Kavan

 

[bouldersoundguy]

The dB changes in a compressor are relative to the input, not to any fixed voltage standard. I'm not sure if it's voltage or power but it would make more sense if it's a power ratio because that's more relevant to how we hear.

Most compressors are broadband. There are multi-band compressors but it's unlikely the bar got one of those. It's also unlikely that the compressor will make your life any better, at least at first. Assuming the compressor is on the whole mix the most likely outcome is that they will drive their signal harder into it resulting in the quieter stuff becoming louder while the loud stuff stays loud, an increase in average power (and SPL). Fortunately for you that means more heat to the voice coils in their woofers and eventually no bass output at all. From your perspective it may be worth encouraging this.

 

[Ethan Winer]

Thanks for posting this here Kavan.

Can that compressor operate only on the low frequency band and only compress there or does it act on the whole spectrum of frequencies?

As boulder explained, there are multi-band compressors, but it's doubtful the bar bought one of those. Most of the multi-band compressors I've seen are plug-ins. A hardware unit would be pretty expensive. Not that audio can't be routed through a computer running software and plug-ins.

b) When reading about compressors I run into dBs. I know that a dB is a ratio between two numbers. It can be about power (log10 P2/P1) or voltage. I am clear what a dBm is or a dB SPL (the reference is a small pressure, 20 micropascal)....

Decibels are always about power, but the reference can be a power as in dBm or a voltage as in dBu and dBv. Either way, adding or subtracting 3 dB doubles or halves the power. And in all cases a dB difference is a ratio.

So, when I read about 10 dB or -5 dB in relation to compressors, what are we talking about?

Compressors reduce the volume by some number of dB based on the input level at the moment and the threshold setting. Is this what you're asking?

--Ethan

 

[antennaboy]

Ok, silly question: so the compressor action cannot be applied to the woofers and subwoofers only....


I am re-reading the following statement: "Assuming the compressor is on the whole mix the most likely outcome is that they will drive their signal harder into it resulting in the quieter stuff becoming louder while the loud stuff stays loud, an increase in average power (and SPL). Fortunately for you that means more heat to the voice coils in their woofers and eventually no bass output at all. From your perspective it may be worth encouraging this."

The purpose of the compressor is to set a threshold. Once sound goes beyond the threshold is it attenuated (based on the ratio).... S the quieter stuff would get louder because they think they have a threshold to protect them and the loud stuff would stay.... Is the bass the loud or quieter part of the sound?

Why would the voice coils of the woofer not output at all if they get warm?

thanks
antennaboy

 

[antennaboy]

I feel like the compressor helped.

One of the bars (there are 2 unfortunatelly) is not as loud anymore. I don't hear the bass as much anymore.

I went inside the bar and the volume was still very loud....but somehow the compressor helped with my bass issue...Why do you think? Just because they reduced the volume enough over the whole spectrum so that the bass does not bother me anymore?

 

[bouldersoundguy]

Ok, silly question: so the compressor action cannot be applied to the woofers and subwoofers only....

It's common practice to apply limiters after the crossover as speaker protection, but at the bar level I'd be surprised if they are that savvy. On the other hand this kind of approach is filtering down to smaller venues so it's possible. But even then they are set up as peak limiters that don't affect the perceived volume too much.

The purpose of the compressor is to set a threshold. Once sound goes beyond the threshold is it attenuated (based on the ratio).... S the quieter stuff would get louder because they think they have a threshold to protect them and the loud stuff would stay....

It really depends on the operator. Compressors also have makeup gain controls to bring up the overall level after the signal has been compressed downward. Sometimes compressing can take away the impact leading operators to turn it all up.

Is the bass the loud or quieter part of the sound?

What frequencies sound louder depends on a number of factors, but bass goes right through many structures we think of as solid so it ends up being the biggest problem for neighbors.

Why would the voice coils of the woofer not output at all if they get warm?

A little bit too much heat increases the impedance of the coil which lowers its output. A bit more heat and the coil former can warp and rub until the coil breaks. If the coil former holds its shape long enough under those conditions the coils may burn off the varnish and short or melt the copper and open. Often that first loss of level fools operators into pushing the system harder and blowing speakers.

 

[witzendoz]

To only compress bass frequencies on a hardware compressor you need a compressor that has a side chain option. You then plug a graphic eq into the side chain, cut all the highs on the eq completely to the point where you want to compressor to start working. The compressor will then only act on the frequencies that have not been cut on the eq. The eq plugged into the side chain does not eq the original sound it only tells the compressor which frequencies to work on.

Side chain compression can also be used to de-ess by cutting all the frequencies on the eq and only boosting the frequencies you want to reduce like the esss.

Alan.

 

[bouldersoundguy]

To only compress bass frequencies on a hardware compressor you need a compressor that has a side chain option. You then plug a graphic eq into the side chain, cut all the highs on the eq completely to the point where you want to compressor to start working. The compressor will then only act on the frequencies that have not been cut on the eq. The eq plugged into the side chain does not eq the original sound it only tells the compressor which frequencies to work on.

Side chain compression can also be used to de-ess by cutting all the frequencies on the eq and only boosting the frequencies you want to reduce like the esss.

Alan.

To be clear, it will react just to what you send through the sidechain (to the detector) but the attenuation will be wideband. A multiband compressor will leave out of band frequencies alone. Eqing the sidechain to de-ess a single vocal track works because sibilance doesn't overlap in time much with other vocal sounds. De-essing a mix really needs a multiband compressor.

 

[witzendoz]

To be clear, it will react just to what you send through the sidechain (to the detector) but the attenuation will be wideband. A multiband compressor will leave out of band frequencies alone. Eqing the sidechain to de-ess a single vocal track works because sibilance doesn't overlap in time much with other vocal sounds. De-essing a mix really needs a multiband compressor.

True, I suppose I was thinking that it could be a way to tighten the sound when the bass hits to help keep the bass under control but it would need to be set up properly so that it was not too noticeable on the highs.

Another thought was that some of the more upmarket crossovers have limiting functions that can be set for each of the frequency splits, but a downtown club would not usually have a classy bit of gear like that. It was suggested above that you could run the low output from the crossover through the compressor, this would work but it depends on the type of PA they have and if they are using active crossovers.

When I ran my touring PA years ago I had a stereo limiter on the mix L/R. While that did work on the whole frequency range I noticed that the PA could actually be run louder without complaints about the volume due to the peaks being under control. I used to have it set so that it acted on the loudest peaks by about 3 to 5 dB and was freewheeling the rest of the time. The mix really tightened up as well. It was also a good protection when the support band brought their own engineer and he was determined to break the world record for volume.

Alan.

 

[Brian Muth]

Fortunately for you that means more heat to the voice coils in their woofers and eventually no bass output at all. From your perspective it may be worth encouraging this.

That's freakin' great!