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Topic: Influence of a cable + Measure of the sound level (Read 3658 times) previous topic - next topic
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Influence of a cable + Measure of the sound level

Hello,

Within the framework of my studies, I wish to know how to quantify the loss which undergoes a sound if we add an extension lead between a usb sound card and a headphones. I found sites which quantifies the attenuation of a network cable but it is different.

On the other hand, do you know how to measure the sound level at the exit of a headphones connected to a usb sound card ?

Thanks in advance

Influence of a cable + Measure of the sound level

Reply #1 –
I would recommend doing an RMAA loopback test, with and without the extension cable. You will need a Y adapter, to allow for keeping the headphones connected as well.

Influence of a cable + Measure of the sound level

Reply #2 –
Get a good digital multimeter and measure the resistance. Most likely the difference will be immeasurable unless the cable is very long or the power very high.

Influence of a cable + Measure of the sound level

Reply #3 –
Quote
I wish to know how to quantify the loss which undergoes a sound if we add an extension lead between a usb sound card and a headphones.
Over any "reasonable distance" you won't have any measurable or audible loss.

At audio frequencies where you aren't getting any transmission line effects, you effectively have a Voltage Divider. 

You might want to double-check all of this, because I did it quickly... 

I just looked-up the resistance of 22 AWG wire.  It's about 16 Ohms per 1000 feet.  If we assume a 50 ft (15 meter) 22 gauge headphone extension, that's a 100 ft "round-trip" for the current, which is about 1.6 Ohms.
 
Now if we assume 32 Ohm headphones, we get a loss of 1.6/(32+1.6) = -4.75%. 

Or in dB, 20log(1.00-.0475) = -2.1dB.

Quote
On the other hand, do you know how to measure the sound level at the exit of a headphones connected to a usb sound card ?
A test tone and a multimeter.

Influence of a cable + Measure of the sound level

Reply #4 –
Quote from: DVDdoug on
Quote
I wish to know how to quantify the loss which undergoes a sound if we add an extension lead between a usb sound card and a headphones.
Over any "reasonable distance" you won't have any measurable or audible loss.

At audio frequencies where you aren't getting any transmission line effects, you effectively have a Voltage Divider. 

You might want to double-check all of this, because I did it quickly... 

I just looked-up the resistance of 22 AWG wire.  It's about 16 Ohms per 1000 feet.  If we assume a 50 ft (15 meter) 22 gauge headphone extension, that's a 100 ft "round-trip" for the current, which is about 1.6 Ohms.
 
Now if we assume 32 Ohm headphones, we get a loss of 1.6/(32+1.6) = -4.75%. 

Or in dB, 20log(1.00-.0475) = -2.1dB.



Actually 32/(32+1.6)  = 0.95238  = -0.4237  dB

Influence of a cable + Measure of the sound level

Reply #5 –
Thanks Arnold!   I KNEW I should have run-through the logic & math one more time!    Like they say, "Never do math in public".  (A loss of 2dB did "feel" a little high, but it didn't raise the red flag in my brain high enough...)

Influence of a cable + Measure of the sound level

Reply #6 –
Thanks for your answers !


Quote
A test tone and a multimeter.


Can you be more precise on the method to follow ?

Influence of a cable + Measure of the sound level

Reply #7 –
Quote
32/(32+1.6)


And could you explain the origin of this formula ? And why the result give us an idea of the loss ? I'm a novice

(Sorry for the double post)

Influence of a cable + Measure of the sound level

Reply #8 –
http://en.m.wikipedia.org/wiki/Voltage_divider

In this case the headphone and the wire are the two resistances in the voltage divider and the voltage is measured across the headphone.

Influence of a cable + Measure of the sound level

Reply #9 –
Thank for you answers but I did not still understand the method to measure the sound level at the exit of a headphones connected to a usb sound card. I could use a sound level meter which would be turned towards the headphones but I have no it.

(By the way, sorry if I make mistakes, my english is not perfect ^^)

Influence of a cable + Measure of the sound level

Reply #10 –
Quote
I did not still understand the method to measure the sound level at the exit of a headphones connected to a usb sound card. I could use a sound level meter which would be turned towards the headphones but I have no it.
A sound level meter is NOT the best way to do it.  It could work, but you'd have to clamp everything down so it doesn't move at all between measurements. 



You should measure the AC voltage with a multimeter.  A very-loud headphone signal will be somewhere in the ballpark of 1 Volt.  I'm sure you don't have a multimeter either, but you asked how to it.    This requires a Y-adapter, and probably another adapter or cable so you can get to the electrical signals while the headphones are connected.  Or, you could cut the Y-adapter to get to the wires and measure the voltage.

Take two measurements - One with the extension and one without.  Use the dB formula to calculate the dB difference between the two voltage measurements (if there's any difference).



Another option would be to "loop back" from headphone-out to line-in and record the signal levels (with a long cable and a short cable), again with a Y-adapter to keep the headphones connected.  Then, you can check and compare the recorded levels with an audio editor. 

Most audio editors have a way to measure the RMS & peak dB levels of a recorded file.  It's best to use RMS or average, because one odd peak can throw everything off.  Subtract to find the dB difference between the two files, and you're done!      (That requires a desktop computer with line-in.  Most laptops only have mic-in and headphone out.)



It's important to use constant test-tones because voice & music varies too much.  Most audio editors can generate test tones.  (If you don't have an audio editor, Audacity is FREE.)

And again, don't expect any measurable meaningful difference unless you have a VERY-long cable! (Or you may get some small differences that are due to normal measurement error & variation.)

Influence of a cable + Measure of the sound level

Reply #11 –
Quote from: fabrich on
Hello,

Within the framework of my studies, I wish to know how to quantify the loss which undergoes a sound if we add an extension lead between a usb sound card and a headphones. I found sites which quantifies the attenuation of a network cable but it is different.

On the other hand, do you know how to measure the sound level at the exit of a headphones connected to a usb sound card ?

Thanks in advance



There are two variables:

(1) The impedance of the load (headphones) which is usually frequency-dependent

(2) The series impedance of the cable, which is also frequency dependent.

One rule of audio measurements is to avoid acoustical measurements wherever possible due to their inherent lack of precision and repeatability, and this is a case where measurements in the electrical domain are most appropriate.

The worst case is a long thin cable and low impedance headphones.

Estimating the headphones at 15 ohms, and the the headphone cable a 25 foot cable made up of 24 gauge wire.

The series impedance of the cable is estimated at its DC resistance, which is 0.025 ohm per foot of wire (round trip) per http://en.wikipedia.org/wiki/American_wire_gauge or 1.25 ohms.

Using the voltage divider equation: attenuation is 15/(1.25+15) =  0.923  or    -0.695 dB.  Barely audible under normal casual, dynamic conditions.

As others have correctly pointed out the way to measure this is to use a digital voltmeter and  a Y-cable so that you can add a tap to the line for connecting the voltmeter.

If you want results in dB this measurement can be accurate even if the voltmeter does not have flat frequency response since it is a relative measurement.

Influence of a cable + Measure of the sound level

Reply #12 –
Hello,

Thanks for your answers, I can now calculate an overestimation (because I do not know exactly the resistance AWG) of the impact of my cable.

I am going to specify my second question: I would like to make a hearing test, the problem is that I have no audiometer which would allow to determine the sound level at the exit of the headphones (40 dB HL for example) and I wanted to know if it was possible to by-pass this difficulty and to find a way to measure the sound level (in db SPL or HL) at the exit of the headphones without audiometer or sound level meter ?