The Decibel (dB) Scale & Audio Rules 101

[admin]

For those looking to gain a deeper understanding of how audio works, whether to make better-informed decisions or simply for the sake of curiosity, it's useful to lay down some ground rules that govern how audio systems behave relating to loudness and the decibel. One of the most important concepts in audio is the decibel, the unit of measure denoting the ratio of a change in level, whether that level is acoustic Sound Pressure Level (SPL) or electrical signal level. It’s abbreviated dB. As you may or may not be aware, the decibel (dB) scale is a logarithmic system, as opposed to a linear scale. Being aware of the relationships inherent in this scale is important for a variety of reasons, which will hopefully become clear by the time you reach the end of this article. we also discuss how the human hear perceives double loudness differently depending on frequency so +6dB increase in SPL may not seem like such a big deal at 1kHz but at 20-30Hz it certainly is. Read on to find out why.

Discuss "The Decibel (dB) Scale & Audio Rules 101" here. Read the article.

 

[monkish54]

What an excellent article! Articles like these make Audioholics truly special.

 

[psbfan9]

Well written, easy to understand article! Thank you.

 

[Charles Jones]

Hi Steve - in considering the various contours (60, 70, 80, etc) of the equal loudness curves diagram shown above ( some variation of Fletcher Munson, Robinson Dadson, ISO226, etc), I am wondering how you make the assumption that the lines represent “a doubling of perceived volume” at all frequencies?

I knew that approximately 10dB change is often touted as the figure for an untrained listener to conclude the volume has doubled, but as you point out, that is based on the perception of a 1k tone.

Although I have studied these curves for many years, I was never aware that the diagram implied that two consecutive curves represented a doubling in perceived volume.

Having just done a little more reading into the correlation of the dB, phon, and sone scales, is this assumption based on the concept that 10 phon change represents a doubling of the sones value represents a doubling of perceived loudness? And is that true across the spectrum? Could you elaborate on these implications, because honestly after reading about Fletcher Munson / equal loudness contours for years, this very important corollary was never apparent to me.

 

[Steve81]

Having just done a little more reading into the correlation of the dB, phon, and sone scales, is this assumption based on the concept that 10 phon change represents a doubling of the sones value represents a doubling of perceived loudness?

That is the general idea.

https://en.wikipedia.org/wiki/Sone

The sone scale is linear. Doubling the perceived loudness doubles the sone value.

And is that true across the spectrum?

Correct, hence the equal loudness in phons (10, 20, 30, etc) vs dB and frequency.

Could you elaborate on these implications, because honestly after reading about Fletcher Munson / equal loudness contours for years, this very important corollary was never apparent to me.

The main implications are:

1. Obviously it takes a lot more than doubling amplifier output to double perceived loudness. For people fretting about a couple watts here and there shopping for a receiver for example, 110W vs 100W isn't going to make or break anything.

2. At subwoofer range frequencies, our ears are quite insensitive to softer sounds, but beyond the initial threshold, it takes less dB to yield a 10 phon increase vs 1kHz. For people that wonder how much a sub that's 6dB more powerful might give them, it's a more than the usual 10dB = doubling of loudness rule of thumb would lead you to believe. You could also carry that over to how much you might opt to trim / cut subwoofer levels, etc.

 

[Verdinut]

In the Decibel Article, it is mentioned that the lowest musical note of a pipe organ as being 32 Hz. That is inexact.

Thirty-two Hertz is the fundamental resonance of a 16 foot pipe, but most bigger organs feature at least one 32 foot pipe stop with a fundamental lowest frequency of 16.35 Hz. Even a few big instruments have a 64 foot pipe with a fundamental at 8.18 Hz.

One rare organ has a 128 foot pipe for a fundamental resonance of only 4.09 Hertz. You can hear the sound of that organ on Youtube and it's obvious no microphone will pick up neither the 8 Hz or the 4 Hz fundamental resonances, and no subwoofer built to this date will be able to reproduce them. You have to be on the spot to feel them. However, I haven't been able to find where the church containing it is situated.

 

[Bucknekked]

In the Decibel Article, it is mentioned that the lowest musical note of a pipe organ as being 32 Hz. That is inexact.

Thirty-two Hertz is the fundamental resonance of a 16 foot pipe, but most bigger organs feature at least one 32 foot pipe stop with a fundamental lowest frequency of 16.35 Hz. Even a few big instruments have a 64 foot pipe with a fundamental at 8.18 Hz.

One rare organ has a 128 foot pipe for a fundamental resonance of only 4.09 Hertz. You can hear the sound of that organ on Youtube and it's obvious no microphone will pick up neither the 8 Hz or the 4 Hz fundamental resonances, and no subwoofer built to this date will be able to reproduce them. You have to be on the spot to feel them. However, I haven't been able to find where the church containing it is situated.

Verdinut
While I am no organ historian or expert, I can vouch for the bigger organs having longer pipes than 16 feet. I am most familiar with the Tabernacle organ in Salt Lake City and the one across the street in the Conference Center. The original Tabernacle organ began its build in 1867 with about 700 pipes. They kept improving it until we have what's there today at over 11,000 pipes. Its not the largest in the world, but its on the list. It has 6 pipes of 32 feet and probably close to 2 dozen at 16 feet.

To sit and listen to them practice on it during the week is awesome. I have had that privilege just a few times and its a great way to spend an hour or so.

 

[enchamade]

In the Decibel Article, it is mentioned that the lowest musical note of a pipe organ as being 32 Hz. That is inexact.

Thirty-two Hertz is the fundamental resonance of a 16 foot pipe, but most bigger organs feature at least one 32 foot pipe stop with a fundamental lowest frequency of 16.35 Hz. Even a few big instruments have a 64 foot pipe with a fundamental at 8.18 Hz.

One rare organ has a 128 foot pipe for a fundamental resonance of only 4.09 Hertz. You can hear the sound of that organ on Youtube and it's obvious no microphone will pick up neither the 8 Hz or the 4 Hz fundamental resonances, and no subwoofer built to this date will be able to reproduce them. You have to be on the spot to feel them. However, I haven't been able to find where the church containing it is situated.

You are partially correct...up to the YouTube hoax of a 128 foot pipe. You could not hear the sound even if it existed, especially not on YouTube. You could not find where the church containing it is situated because it doesn't exist. There are two full length 64 foot pipes in the world, One is in the Midmer-Losh Opus 5550 in Atlantic City: https://www.boardwalkorgans.org/midmer-losh-info/ and the other is in the Sydney Town Hall organ: https://www.ohta.org.au/confs/Sydney/SYDNEYTOWNHALL.html There are other organs that have 64 foot resultant stops, which is not actually at the pitch specified, but is a combination of stops (usually two) that tricks the brain into thinking it is hearing a stop at the specified pitch. This really only works well on low pitched stops. http://www.organstops.org/r/Resultant.html

Respectfully, since this is my first post on Audioholics, however I have studied pipe organs for almost 50 years.

 

[User5910]

... no microphone will pick up neither the 8 Hz or the 4 Hz fundamental resonances, and no subwoofer built to this date will be able to reproduce them.

Actually the TRW-17 Thigpen Rotary Woofer can reproduce those frequencies:

Frequency Response: 1Hz : 30Hz ± 4dB

I've heard a demo and it is impressive. However once you get down into the teens the effect isn't so much "sound" as separate pulses of pressure change.

 

[User5910]

If you put that single subwoofer so its front radiating panel is flush with the wall (1/2 space or 2π steradians) ...

That's only true if you have no floor. You're already at 1/2 space/2π steradians once you put it on the floor. From Audioholics' Bassaholic Subwoofer Room Size Rating Protocol article:

Half space (ie.groundplane) is how Audioholics and most acoustic professionals measure subwoofers.

This article calls floor/wall space both 1/2 space and 1/4 space in two different places. From the next paragraph:

If you take that subwoofer and place it at the floor/wall intersection, then that’s called a ¼ space or π, and you’ll gain another 6 dB of output reinforcement.

 

[Pogre]

Those rotary subs are cool. It just dawned on me that I've never seen one in action. So I'm looking up videos and saw for the first time that there's a spider behind the fan blades. I never noticed that before and now I think these work much differently than I previously thought.

Yay. Another rabbit hole...

 

[lovinthehd]

Those rotary subs are cool. It just dawned on me that I've never seen one in action. So I'm looking up videos and saw for the first time that there's a spider behind the fan blades. I never noticed that before and now I think these work much differently than I previously thought.

Yay. Another rabbit hole...

Check this out https://www.avsforum.com/threads/interesting-diy-rotary-subwoofer-project.1940241/

 

[Pogre]

Check this out https://www.avsforum.com/threads/interesting-diy-rotary-subwoofer-project.1940241/

I'm scrolling through that thread now and queued up his video with the other 6 or 8 I found. I had it all wrong on how those work. I didn't realize they changed pitch depending on frequency. I thought just the turning of the blades in a special cabinet or something was all it was. It's a little more sciencey than I thought. Pretty cool design.