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Topic: Help a student with a questions! (Read 7835 times) previous topic - next topic
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Help a student with a questions!

I need to figure out this question:

In a sound recorder, dBFS is used to represent the loudness of a sound. The maximum loudness the recorder can handle is 100W. During recording of a human speech, you read on the recorder that the loudness is -8 dB. What is the power of the speech in the unit W?


Ive got no idea how to figure it out ive been sitting here thinking about it for like an hour... Would appreciate if you could explain the problem rather than just answering it. Thank you so much!!!!

Help a student with a questions!

Reply #4 –
Can you ask for a clarification of the question? Because what you are asking doesn't make a lot of sense.

Help a student with a questions!

Reply #5 –
You are mixing up several different things here.

"the recorder can handle 100W" .. W means watts, the unit of power. Did you confuse this with dB SPL?
"you read on the recorder that the loudness is -8 dB" ... loudness is yet another thing. Do you mean the level is displayed at -8 dB? This doesn't necessarily mean -8 dBFS. In fact, the recorder could have several dB headroom...

Normal speech would be about 60 dB SPL at 1m, but this does not tell you anything at what level this will be recorded.
"I hear it when I see it."

Help a student with a questions!

Reply #6 –
The question is to find the power of the recorded speech. "The maximum loudness the recorder can handle is 100W." means that 100W equals 0dB SPL. So how many Watts does -8dB SPL equal? It's pretty easy to find out if you check the wikipedia link saratago gave you.

What specific question do you have?
It's only audiophile if it's inconvenient.

Help a student with a questions!

Reply #7 –
Quote from: Kohlrabi on
The question is to find the power of the recorded speech. "The maximum loudness the recorder can handle is 100W." means that 100W equals 0dB SPL. So how many Watts does -8dB SPL equal? It's pretty easy to find out if you check the wikipedia link saratago gave you.

What specific question do you have?


Ok these are all the questions i have to answer. Ive answered everything except number 4.

1.   Given the reference power to be 2W, the observed power of an audio to be 120W, what is the decibel representation of the audio?
2.   Given the reference amplitude to be 0.1, the observed amplitude of an audio to be 12, what is the decibel representation of the audio?
3.   Using the dBm standard, given an observed power of an audio to be 15W, what is the decibel representation of the audio?
4.   In a sound recorder, dBFS is used to represent the loudness of a sound. The maximum loudness the recorder can handle is 100W. During recording of a human speech, you read on the recorder that the loudness is -8 dB. What is the power of the speech in the unit W?
5.   Given the amplitude of a signal is 10, and the amplitude of the background noise is 2, what is the signal to noise ratio in dB?
6.   What will happen if the sampling frequency is smaller than twice of the frequency of an audio (maximum 20 words)
7.   What is the major difference between audio sampling and audio quantization (maximum 15 words).

Help a student with a questions!

Reply #8 –
Ok, this starts to sound like homework.

Most questions are answered with the link that was already provided. 20*log10(V1/V0) or 10*log10(P1/P0) depending on the quantity.

For the last two questions check out wikipedia articles on the sampling theorem, digital audio quantization and sampling.
"I hear it when I see it."

Help a student with a questions!

Reply #9 –
Quote from: xnor on
Ok, this starts to sound like homework.

Most questions are answered with the link that was already provided. 20*log10(V1/V0) or 10*log10(P1/P0) depending on the quantity.

For the last two questions check out wikipedia articles on the sampling theorem, digital audio quantization and sampling.


ok ive tried 10*log10 (100/-8) but i get an error when i use log saying invalid input

Help a student with a questions!

Reply #10 –
Quote from: apexgamerx on
Quote from: xnor on
Ok, this starts to sound like homework.

Most questions are answered with the link that was already provided. 20*log10(V1/V0) or 10*log10(P1/P0) depending on the quantity.

For the last two questions check out wikipedia articles on the sampling theorem, digital audio quantization and sampling.


ok ive tried 10*log10 (100/-8) but i get an error when i use log saying invalid input


So Power1 = 100 watts and Power2 = -8 watts?  Those aren't the numbers you posted above...

Help a student with a questions!

Reply #11 –
Quote from: saratoga on
Quote from: apexgamerx on
Quote from: xnor on
Ok, this starts to sound like homework.

Most questions are answered with the link that was already provided. 20*log10(V1/V0) or 10*log10(P1/P0) depending on the quantity.

For the last two questions check out wikipedia articles on the sampling theorem, digital audio quantization and sampling.


ok ive tried 10*log10 (100/-8) but i get an error when i use log saying invalid input


So Power1 = 100 watts and Power2 = -8 watts?  Those aren't the numbers you posted above...


no im just using the values in the question... Xnor said they can be answered using that equation so i just used the only two numbers in the question..

Help a student with a questions!

Reply #13 –
Try this:

You have the gain in dB and one power, so you want to solve the equation for the other power....
"I hear it when I see it."

Help a student with a questions!

Reply #14 –
Hi apexgamerx,

Any progress? You said
Quote from: apexgamerx on
Ive got no idea how to figure it out ive been sitting here thinking about it for like an hour... Would appreciate if you could explain the problem rather than just answering it. Thank you so much!!!!
and everyone has given good tips for you to solve it. Everyone is intentionally NOT “just answering it”.

I can’t add more than others already have, the answer is already here… but let’s parse the question (you ask for someone to “explain the problem”).
Quote
In a sound recorder, dBFS is used to represent the loudness of a sound.
So there is a meter or display in this sound recorder, and the scale that it uses is dBFS (decibels, referenced to full scale)
Quote
The maximum loudness the recorder can handle is 100W.
Here, “full scale” is defined. 100W (=100 watts) is a power. Use xnor’s hint for the power relation: dB=10*log10(P1/P0) to see what Kohlrabi told you: dB=10*log10(100W/100W)=0. 100W is full scale and therefore w.r.t. FS, 100W is 0 dB. Note that since the recorder gives dBFS, the reference (FS) (i.e. P0) is always 100W for this recorder.

Now,
Quote
During recording of a human speech, you read on the recorder that the loudness is -8 dB. What is the power of the speech in the unit W?
the human speech gives you a reading of -8dBFS on the recorder’s display (so -8dB w.r.t. FS, see above), and the question asks what is the power in watts.

xnor tells you exactly what to do in post#14, using the correct relation in post#9. Remember, you can’t just stick numbers wherever you want, they have to make sense: (100/-8) is a negative number, and you can’t take the log of a negative number… remember? (And notice that the ratio for the log function is unitless. 100W/-8dB has the units W/dB.)

The problem takes a few seconds to rewrite, then a few seconds to punch in a calculator… I’m looking at the answer right now.
Give it another try. Good luck!

edit: added point about units

Help a student with a questions!

Reply #15 –
Quote from: apexgamerx on
I need to figure out this question:

In a sound recorder, dBFS is used to represent the loudness of a sound.



Yes.

Quote
The maximum loudness the recorder can handle is 100W.


Makes no sense.


Please provide a spec sheet for a sound recorder that even specifies a maximum loudness that it can record.

Help a student with a questions!

Reply #16 –
Quote from: Arnold B. Krueger on
Quote from: apexgamerx on
I need to figure out this question:

In a sound recorder, dBFS is used to represent the loudness of a sound.



Yes.



Pretending this was actually a qustion, the correct answer should have been no.

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Reply #18 –
Quote from: Arnold B. Krueger on
please explain

One specifies amplitude level in a digital system and is an objective measure, the other is a psychological correlate to sound pressure and is a subjective measure.
"I hear it when I see it."

Help a student with a questions!

Reply #19 –
dBFS -> RMS, LUFS -> loudness. LUFS is what the EBU proposed in R128 (https://tech.ebu.ch/docs/r/r128.pdf) IIRC. LUFS values are also presented logarithmically (but not in dBFS).

Chris
If I don't reply to your reply, it means I agree with you.

Help a student with a questions!

Reply #20 –
I don't know why the question used W instead of V. In real life most product specs will say something like 0dBFS = +20dBu or Max level = 8Vrms and so on.
 

Help a student with a questions!

Reply #21 –
Quote from: Arnold B. Krueger on
Quote from: greynol on
Quote from: Arnold B. Krueger on
Quote from: apexgamerx on
I need to figure out this question:

In a sound recorder, dBFS is used to represent the loudness of a sound.



Yes.

 

Pretending this was actually a qustion, the correct answer should have been no.


please explain

Let's assume a playback system/environment that is anechoic, flat*, and held at a constant and reasonable output level**, which will be louder, a 10kHz tone at -15 dB digital FS or a 3.5kHz tone at -20 dB digital FS?

(*) This alone should tip you off, oh great flogger of Fletcher-Munson.

(**) I dunno, say such that a -20dBFS 1 kHz tone would be played back at 80dBSPL (give or take in order to stem nitpicking) at the listening position?  Hopefully you get the idea here.