Method to measure SPL of IEM (+-2 or 3 dB?)

[cribeiro]

Hello friends,

I wanted to post this since long time ago, but I wanted to test it myself first... Since it seems I will not manage to check it in the near future, I post my theory to let you try yourselves. I read a lot of posts regarding worries about SPL in IEMs, and I have not read any answer like this... Sorry if somebody came with this idea already!

For this "experiment", you need

1. a cd or other source with a 1000 Hz sine wave.
2. some loudspeakers
3. a loudspeaker amplifier
4. some IEMs (obviously?)
5. An SPL meter
6. Some way to split the signal to feed headphones and speakers at the same time (or a second source)

Methodology:

Plug only one IEM and play the sine wave through it. At the same time, play the sine wave through the loudspeakers and measure their SPL with the meter (at best*, the meter should measure next to your "naked" ear, maybe you can get somebody's help here). Turn up your loudspeakers volume to get some readable output on the SPL meter (65 or 75 dB will be ok), and then turn up the volume of your IEM until it matches what you hear with your "naked" ear. This will give you an idea of the levels you usually listen to, specially since you can compare to your home stereo (provided you have one...).

You could actually make a list with the equivalent levels between your preferred IEM's amplifier and your integrated amp/home cinema amp... With a computer, you could generate several 1000 Hz signals at different attenuation levels, and store them in your X5L (

it "might" work with an ipod too...) "calibrated" such that you can play them any time at "calibration level" to ensure you are listening at a certain level.
A universe of possibilities is open now to you!

I hope it works (I am quite confident about it), and this way no more questions regarding IEM and hearing loss will be asked.

Enjoy

,

Alberto

* At 1000 Hz, the signal intensity will be quite directional, depending strongly on angle respect to the speaker and distance from it.

 

[Vul Kuolun]

Hi Alberto,

i liked the idea first, but did you consider that when you´re matching the levels, the IEM gets an aditional amount of noise from the speakers because it doesnt isolate 100%?

If you put the IEM in your ear, without playing any music through it, you will still hear the speakers a little. This amount of noise adds to the noise played through the IEMs at the moment you think the levels are equal, resulting in a wrong SPL reading. I may be wrong though.

 

[ssingh0]

at least you'll be on the safe side

 

[cribeiro]

Vul Kuolun, that is a very good point I did not consider (let me now include my theoretician's joke... "Man, it is a second order correction!"... Sorry about that

). But I am pretty sure it is negligible (no proof at the moment, let me sleep on it

). At least my experience listening to IEM tells me so...

In any case, the correction for that efect would be to the safe side (that it, the IEM alone will give less SPL).

Regards,

Alberto

PS: I start liking the use of smileys!!

I still did not get a chance to put this

within a good context! But I like it...

EDIT: Vul Kuolun, as experimental proof or disproof for my "feeling" about your point, after matching levels, just mute the loudspeakers and see if the perceived loudness changes. I bet you my first earbuds you won't feel it! Hey, I always play safe...

 

[cribeiro]

I have just plugged my super.fi 5 pro before going to bed and realized about the further steps needed for the "second order correction".... Unfortunately, some maths are involved... And it is too late even for a crazy theoretician like me to start with that... Actually... it is a first order correction, but ok...

Ok, so you just measured the SPL as described above, and want to find out the "Vul Kuolun" correction term? You will need another pair of IEM's, or the possibility to play the sine tone on only one channel.

Leave the speakers playing the sine tone at the "calibrated level", and stop playing it through your IEM. Plug your second IEM (in your second ear

) and play through it (and only it) the sine wave. Set the level on that second IEM to match the level you hear in the other ear (with IEM but only sound from the speakers).
Remove the first IEM. Now the speakers sound louder than your second IEM. Set their level to that of your second IEM and read the SPL meter value. That is your "Vul Kuolun" correction to the "cribeiro" method of SPL measurements for IEM. This almost looks scientific now!

Note that there is a second correction to this... Which is actually in the opposite direction with respect to the first or "Vul Kuolun" one, if I am not yet that sleepy... In fact, there is also a third, fourth, etc... It is an iterative convergent method. But you won't need more than the two steps I described here. In fact, I suspect even the second can be neglected, but for that we need some measurements (we could estimate it using specs data, though)!

One note regarding the math: the dB scale is logaritmic, so you cannot just add the two readings! (actually, substract the second). You must "undo" the logarithm and sum the amplitudes, applying the logarithm again in the end. Am I sleepy? Let's see:

SPL1=20 log (A1/A0) -> A1= A0 Exp(SPL1/20)
SPL2=20 log (A2/A0) -> A2= A0 Exp(SPL2/20)

SPL12=20 log ((A1+A2)/A0) -> SPL12=20 log (Exp(SPL1/20)+Exp(SPL2/20))

There you are!! Could somebody check it, please?

Regards,

Alberto

EDIT: this also shows that a correction will be negligible, since the IEMs have more than 10 dB isolation, you can already see that the correction will be very small (you can enter SPL2=SPL1-isolation. For example, my super.fi 5 pro have 26 dB isolation. This will give you a good measure).

 

[cribeiro]

Hello again,

I have just read one post of somebody saying he has no SPL meter, and no way to get one... For those, I have two options:

1. get a multimeter to measure the speaker output in the amplifier for a given loudness (your calibration loudness), and obtain the approximate dB using the loudspeaker's specified sensitivity (or efficiency?). That is, dB/(W m).

2. Use your common sense. We all have a feeling of how loud is a loudspeaker, so you can set the loudspeakers to your personal's feeling for "low", "mid", "high" and "crazy". Then, you can assign that to your IEM's by doing the matching.

Note: if you don't have any gear, and want to get a feeling of how loud it is, just unplug one IEM and listen to traffic, a conversation, etc... There are tables that tell you how loud are this events (in average), and you can use that to compare its loudness to that of your IEMs.

So, do I leave any possibility open?

 

[cribeiro]

I took this from some website:

The following is a list of DECIBEL INTENSITIES taken from Encarta 2005.

0-Threshold of hearing
10-rustle of leaves, a quiet whisper
20-average whisper
20-50-quiet conversation
40-45-hotel, theater between performances
50-65-loud conversation
65-70-traffic on a busy street
65-90-train
75-80-factory noise( light/medium work)
90-heavy traffic
90-100-thunder
110-140-jet aircraft at takeoff
130-threshold of pain
140-190-space rocket on takeoff

Just to give you an idea...

 

[cribeiro]

I have just made some measurements, so I thought I would share them with you... They are far from accurate, but might give some guidelines. Sorry I was too lazy to estimate the error in the measurements... But it can be good 5 dB!

I used a radio shack-like SPL meter, my hifi (Cambridge Audio 640A and 640C + Castle Richmond 3i) and my portable gear without amp (Iaudio X5L+SF5Pro).

I know I should use test tones, but again, I was too lazy to load them on the player... So I used the song Kayleigh, from Marillion.

Here are the results:

vol dB (weighting C, fast)
9 60
12 75
16-17 83


I did not dare to try 95 or 100, because I have neighbours...
As I said, they are not very accurate, but they serve as reference. If you want, put 5dB on top of the measurement to stay on the safe side...

I was very surprised about the outcome. I expected much higher readings on the SPL meter... I rarely listen above 15, and normally listen at 10 with this gear!!