Amplitude vs SPL vs "loudness"

SloppyJo

SloppyJo

New member
I have a few questions; it's at the point where I am not even sure what to ask, but here goes:

There are several kinds of amplitude, like peak, peak to peak, and RMS... But are any of these amplitudes equivalent to sound pressure level? If so, how are amplitude, SPL, and subjective loudness connected?

I understand what each of these are on their own more or less, but I'm a little fuzzy on how they are related... Can anyone help me out?
 
The following is in the context of a an audio signal being passed by a power amp into a speaker

As the amplifier output signal amplitude increases, so does the excursion of the voice coil and cone of the speaker (ie how much it moves in and out). The excursion of the cone causes air to move move. That is to say, the "up and down" movement of the electric audio signal causes "in-out" movement of the speaker cone, which creates air compression and rarefaction ie sound waves. So, greater signal amplitude translates to higher SPL.

The SPL that will be produced by speakers is specified by their sensitivity, which is often expressed as "X" dB SPL at 1 watt at 1 metre. The higher the sensitivity "X", the greater SPL the speaker is capable for a given power output.

From the sensitivity value, you can calculate the amplifier power required in order to produce a certain SPL at a certain distance, or how loud the speaker will be for a given power input. We Ignore, for this discussion, the influence of environment (venue, temperature and humidity, wind etc), as well as the ratings of equipment for power, distortion, dispersion patterns, etc.

For perception of loudness, this might start you off: Equal-loudness contour - Wikipedia, the free encyclopedia

You are correct that objective (measurable) loudness and subjective (perceived) loudness differ. This is purpose of having various weightings for SPL meters.

Does that help?
 
PRHunt said:
The following is in the context of a an audio signal being passed by a power amp into a speaker

As the amplifier output signal amplitude increases, so does the excursion of the voice coil and cone of the speaker (ie how much it moves in and out). The excursion of the cone causes air to move move. That is to say, the "up and down" movement of the electric audio signal causes "in-out" movement of the speaker cone, which creates air compression and rarefaction ie sound waves. So, greater signal amplitude translates to higher SPL.

The SPL that will be produced by speakers is specified by their sensitivity, which is often expressed as "X" dB SPL at 1 watt at 1 metre. The higher the sensitivity "X", the greater SPL the speaker is capable for a given power output.

From the sensitivity value, you can calculate the amplifier power required in order to produce a certain SPL at a certain distance, or how loud the speaker will be for a given power input. We Ignore, for this discussion, the influence of environment (venue, temperature and humidity, wind etc), as well as the ratings of equipment for power, distortion, dispersion patterns, etc.

For perception of loudness, this might start you off: Equal-loudness contour - Wikipedia, the free encyclopedia

You are correct that objective (measurable) loudness and subjective (perceived) loudness differ. This is purpose of having various weightings for SPL meters.

Does that help?
Click to expand...

Ok; the amount of displacement by the air particles from compression and rarefaction is the amplitude correct? With that said, amplitude by itself is not necessarily measured in dB right? When someone is talking about how a jet engine is around 160 dB or whatever is the "160 dB" are they referring to the amplitude or the SPL?

From what I understand the gist of what you said is that amplitude and SPL are related, but they are not exactly the same, and/or that SPL is a function of amplitude, but they are not synonymous.

Thanks :D
 
db is a logarythmic expression of a ratio, a comparison between one value and another. 160db is essentially a meaningless statement. There must always be a reference value or "scale" attached for it to make any sense. In the case of the jet engine to which you refer it should probably be dbSPL.

IMO Amplitude generally should refer to peak-to-peak levels. It is, literally, the difference between the highest peak and lowest trough in a wave. When people say "peak volume" or whatever, it's shorthand for peak-to-peak.

Loudness really is more about the "average" - or more precisely the RMS - level of a signal.

These can be measured on any number of scales, depending somewhat on the medium in which they exist. SPL, for example, applies to sound waves in air, but not to the voltages representing those waves in wire, nor the bits representing them on a CD. They can be measured as absolute numbers - like Volts or Pascals - or compared to some reference level in a ratio - dB.
 
deciBels (dB) describes the ratio between two physical quantities (not just sound). It was devised as a simpler (hah!) way of expressing such ratios which might otherwise involve large numbers.

To make sense, a dB "statement" must always be accompanied by the physical quantity to which it is referring, for example db SPL (sound pressure). To refer to a sound level as just dB is sloppy, but it seems to be common practice in general conversation.

For sound pressure (SPL), dB expresses the ratio between a reference sound pressure (nominally 0dB SPL, the threshold of human hearing, apparently 0.00002 Pascals) and the measured sound pressure. 120 db SPL represents the level at which hearing damage may occur (20 Pascals) That is a difference of 1,000,000, which expressed as dB as 120.

dB can be used on its own. It make sense, for instance, to boost a signal by 6dB (because it is signifying a ratio). So, if you have a signal of say 2V RMS, boosting it by 6dB will double it. But you can't just say that the sound level is 80dB or a signal level is 4dB.

Best thing is to find a tutorial about dB and just work at it over and over until it makes sense. It does become more intuitive after a while!
 
PRHunt said:
120 db SPL represents the level at which hearing damage may occur (20 Pascals)
Click to expand...

Hearing damage may occur at all kinds of levels. What we need to know is the level of sound and the duration of exposure. Workplace health and safety guidelines have been established to control hearing protection. The ear can defend itself against loud sounds for a while. When the loudness is increased the duration before damage can occur is reduced.

Here's a link with a table showing exposure level vs. time.


Noise Exposure - Permissible Level and Duration
 
Good question, good discussion... may I ask what sparked the curiosity here?

I used to be super paranoid about hearing damage, then I learned to keep the volume at a reasonable level, once your brain gets into it, it really doesn't matter. Volume only makes a big difference if you want to move while you listen (workout, dance, sex) or you are inebriated.
 
snow lizard said:
Hearing damage may occur at all kinds of levels. What we need to know is the level of sound and the duration of exposure.
Click to expand...

That is correct, thanks for pointing that out. The 120 dB SPL was used in the scope of this discussion, to demonstrate a ratio. I should have written "120 dB SPL represents the level at which immediate hearing damage may occur" or similar.

A simple way to remember the duration is 8 hours exposure at 85dB SPL, with a halving of duration for every 3dB increase in SPL.
 
The realtionship between power input (to a loudspeaker say but the early research was probably done on headphones) is such that to double the subjective impression of loudness you have to increase the power tenfold.

This relationship was termed the "Bel" and is split into 10 decibels for general usage. The history of the use and abuse of the decibel is a long and convoluted one. Just learn the ones you need for sound recording and repro!

A handy, ball park formula was not given AFAICS? The SPL of a speaker at one metre is given by...SPL=10log W+S
Where W is the input power in watts* and S is the sensitivity of the speaker as SPL/1W/mtr.

*It is virtually impossible to measure this as actual power so many speaker firms specify 2.828 volts(root 8) rms of pink noise measured with a true rms meter.

Dave.
 
ecc83 said:
The realtionship between power input (to a loudspeaker say but the early research was probably done on headphones) is such that to double the subjective impression of loudness you have to increase the power tenfold.

This relationship was termed the "Bel" and is split into 10 decibels for general usage. The history of the use and abuse of the decibel is a long and convoluted one. Just learn the ones you need for sound recording and repro!

A handy, ball park formula was not given AFAICS? The SPL of a speaker at one metre is given by...SPL=10log W+S
Where W is the input power in watts* and S is the sensitivity of the speaker as SPL/1W/mtr.

*It is virtually impossible to measure this as actual power so many speaker firms specify 2.828 volts(root 8) rms of pink noise measured with a true rms meter.

Dave.
Click to expand...

Wait. Are you implying that dB was devised from subjective loudness? Subjective loudness is measured in sones is it not?
 
SloppyJo said:
Wait. Are you implying that dB was devised from subjective loudness? Subjective loudness is measured in sones is it not?
Click to expand...



"The sone is a unit of perceived loudness proposed by Stanley Smith Stevens in 1936. In acoustics, loudness is the subjective perception of sound pressure. It is a non-SI unit.

According to Stevens' definition,[1] a loudness of 1 sone is equivalent to the loudness of a signal at 40 phons, the loudness level of a 1 kHz tone at 40 dB SPL. But phons scale with level in dB, not with loudness, so the sone and phon scales are not proportional. Rather, the loudness in sones is, at least very nearly, a power law function of the signal intensity, with an exponent of 0.3.[2][3] With this exponent, each 10 phon increase (or 10 dB at 1 kHz) produces almost exactly a doubling of the loudness in sones.[4]

At frequencies other than 1 kHz, the loudness level in phons is calibrated according to the frequency response of human hearing, via a set of equal-loudness contours, and then the loudness level in phons is mapped to loudness in sones via the same power law.

The study of apparent loudness is included in the topic of psychoacoustics and employs methods of psychophysics."

!!!!!!!!!!!!!!!
Which ^ is why I said "learn about JUST the dBs you need for the task in hand"!

In any event the dB (Bel) had to come first. All the frequency/level dependant Fletcher-Munsen foldirol had to be based on that.

Dave.
 
PRHunt said:
That is correct, thanks for pointing that out.
Click to expand...

No problem.

I didn't know the Fletcher-Munson equal loudness contours had been updated with more recent test studies until I looked at the link that you posted earlier, so thanks for that.
 
ecc83 said:
The study of apparent loudness is included in the topic of psychoacoustics and employs methods of psychophysics."

!!!!!!!!!!!!!!!
Which ^ is why I said "learn about JUST the dBs you need for the task in hand"!
Click to expand...

Haha. Well my interest in sound physics doesn't only stop at the metric level! Whether it be metric or psychometric, I'm just trying to absorb all that there is to know! :thumbs up:

As for why I'm curious about all this... besides the fact that it is helpfull in acoustics/engineering I just find it really cool!
 
SloppyJo said:
Haha. Well my interest in sound physics doesn't only stop at the metric level! Whether it be metric or psychometric, I'm just trying to absorb all that there is to know! :thumbs up:

As for why I'm curious about all this... besides the fact that it is helpfull in acoustics/engineering I just find it really cool!
Click to expand...

Err? JUST in case you didn't know! That passage was lifted from Wiki! I did get into all this stuff 30 years ago (before "hi-fi" went sillywalkabout) but the brain is old and fuzzy/medridden these days!

Dave.
 
You must log in or register to reply here.
Back
Top