# iMac Sound Card



## Jeff H (Sep 25, 2010)

I have just received my SPL meter & calibrated the sound card in my iMac & done a sweep for the sub woofer that I built, with the help of G. Perkins

There was an low level warning error from the Room EQ wizard (version 5) .

Can some one please have a look at the attachments & let me know if the sound card in the iMac is ok or if I need to get an external sound card.

I will also include the sweep we did with & without the 1/3rd smoothing.

Your help would be greatly appreciated.


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## gperkins_1973 (Aug 25, 2008)

I received Jeff's two other measurements this morning with two other different locations as marked on the diagram. You can see that there are plus and minuses on both sweeps.

Pics 1 (no smoothing) 2 (with smoothing) is taken at position 2 and 3 (no smoothing) and 6 (with smoothing) are taken at position 3. I have also included a pic to show both traces overlaid.

Without the soundcard calibration done, I am not too sure if the measurements taken would be valid but I wanted Jeff to take them anyway to see how they looked. 

The main reason Jeff needed to do the measurements is because he has had issues with sub 30hz frequencies and also frequencies rolling off above 30hz.

Some advice would be greatly appreciated with regards to the sweeps and his sound card.

cheers

Graham


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## gperkins_1973 (Aug 25, 2008)

And here is a pic to show the sound card cal he did.
cheers

Graham


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## JohnM (Apr 11, 2006)

The soundcard cal is not useable, but the iMac soundcard should be fine. On a PowerPC you will probably need to tick the "Force big-endian" box on the soundcard settings, the way to tell for sure is to play a test tone using the REW signal generator, 1kHz say if it is playing back through the speakers. If it sounds like a pure tone all is good, if it sounds like a buzz saw tick the "Force big-endian" box and bin any measurements made without the box ticked.


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## Jeff H (Sep 25, 2010)

Thanks for your reply John.

I will give it a go & let you know how I go.


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## Jeff H (Sep 25, 2010)

I made the adjustments that JohnM suggested & made a new set of sweeps & sound card calibration.

The sound that came out of the computer was a lot better to.

It was a continuous sound rather than the buzz saw as you described.

Please look at the attached pics & let me know what you think.

Thanks.


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## JohnM (Apr 11, 2006)

That's more like it 

The soundcard cal is fine, the phase trace shows that the input is inverting (that is why phase is around -180/180 degrees), you can compensate for that by ticking the box to invert the input on the REW Soundcard preferences page.


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## Jeff H (Sep 25, 2010)

Thanks for your reply John.

I should of read it before I moved the sub all over the room & did more sweeps.

Any way that's the way it goes.

I have selected the invert option on the soundcard page as you mentioned & did another sweep.

There are two graphs, one with smoothing & one without.

Can you please have a look & let me know what you think.

Thanks.


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## JohnM (Apr 11, 2006)

The comment about phase was just an observation, there are few instances where you need to be concerned with the phase plot. You can use the measurements you have made regardless of whether the invert box was ticked. Turn off the phase trace and focus on the frequency response.


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## gperkins_1973 (Aug 25, 2008)

John,

That last response Jeff posted once EQ'd would be a great response.

cheers

Graham


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## Jeff H (Sep 25, 2010)

Hi John,

I have been working with G.Perkins this morning on doing soundcard calibrations & sub woofer sweeps & we are having a bit of trouble with the sound card clipping.

We are not sure what level the line input should be, so I will include a pic of the system preferences for the line input.

There is also a pic of a warning that came up on REW when we did the spl meter calibration.

The third pic is of the last sweep we did, to show you what is going on.

I hope this makes some sort of sense.

Your help would be greatly appreciated.

Thanks.


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## JohnM (Apr 11, 2006)

Hi Jeff,

Let's try getting the SPL meter cal going as a first step. I'll explain some of the background of what is happening when we calibrate the SPL meter so the steps make more sense - I'll start from the ground up as this may well be helpful to others, so apologies if it gets a bit basic 

REW sees the input signal from the soundcard's line input as a stream of numbers representing the amplitude of the input. The software has no way of knowing what those might represent as a sound pressure because it does not know the sensitivity of the microphone or SPL meter being used (how many volts of output it produces for a given sound pressure at its input) or the sensitivity of the line input (what voltage the input can measure before it reaches the maximum it can handle and starts clipping).

Because we don't know the absolute sensitivity of the input, levels are shown relative to the maximum numeric value, or "full scale". The horizontal bar graph on the REW SPL meter shows the current line input level in units of dB relative to full scale, or "dB FS". We can treat everything as being scaled so that full scale represents a level of 1.0, half full scale would be a level of 0.5, a quarter would be 0.25. To turn those numbers to dB we use 20*log(number), logarithmic scales correspond better to our range of hearing. That turns 0.5 into -6.02 dB FS, or 0.25 into -12.04 dB FS. 0.1 becomes -20 dB FS.

When we calibrate the SPL meter using a signal that is at about the level we want to measure at, we would like the levels at the soundcard input to be high enough that we are well above the noise in the room, but low enough that we have some headroom to measure peaks caused by modal resonances and the like. A good level to see on the horizontal bar would be around -20 dB FS, which means that levels could get 20dB higher (10 times) before they reach the maximum for the input and start to clip. If the level shown on the bar is much lower or higher than -20 dB FS the line input volume or SPL meter range (if feeding the line input from the SPL meter) should be changed to get it closer to - 20 dB FS.

The SPL calibration step tells REW how to relate that input level, let's say -20 dB FS to use a handy number, to an actual SPL level read off your external meter. If your external meter was reading 77dB, and you enter that figure in REW's meter cal dialog box, REW then knows that -20 dB FS on its input is 77 dB in SPL. It can then convert any input level to an SPL figure by just adding the right number of dB (97 in this case). As soon as it has done that REW can also tell you the highest level that can be measured without clipping, the input cannot get higher than 0 dB FS so the maximum level before clipping would be 97 dB SPL. Note that if you change the input volume setting, or the SPL meter range (if the SPL meter is providing the input signal to REW) then the relationship between the numbers REW sees and the actual SPL in the room are changed also and the SP meter calibration needs to be redone.

So that's what's going on behind the scenes. When you did your meter cal you got a message that the maximum level that could be measured was 146 dB. If the SPL in the room when the test signal was playing was 75 dB SPL that would mean your input was at -71 dB FS, way too low to be a proper level, so something has gone wrong there. 

What external SPL meter are you using, and what SPL was it showing during the SPL meter cal?
Is your line input being fed from the external SPL meter, or are you using a mic and preamp?


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## Jeff H (Sep 25, 2010)

Hi John,

Thanks for responding to my question.

I am using a Radio Shack 33-4050 Sound Level Meter.

It is connected directly into the iMac input socket.

The needle was moving between +4 & +6 during the SPL meter cal.


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## JohnM (Apr 11, 2006)

Were you on the meter's 70dB range? It would be better to use the 80dB range, otherwise could clip the meter during measurement. You need to measure with the same range setting as used for calibration.

When you do the SPL meter calibration, what is the level on the REW meter's horizontal bar graph before you start the test signal (when it is quiet in the room), and what does it change to when the test signal starts playing?


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## gperkins_1973 (Aug 25, 2008)

John,

Thanks for your reply this morning. I am just chatting with Jeff now to sort it out. I can't use the 80db range when I run my check levels as my levels come out too low hence why I use the 70db range. It may well be that his soundcard produces a much higher input level than mine. I can never get my levels any higher than -24db when I run check levels so I just go with that. Its always been fine though.

Didn't realised you lived in the UK.

cheers

Graham


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## gperkins_1973 (Aug 25, 2008)

John,

We was playing around yesterday again with REW and didn't manage to get much further. Jeff re-calibrated the sound card again and you can see pic attached to show that the sound card cal measurment is fine. He then set the output level in the 80db range so that the spl meter read 75db and then adjusted the input level so it matched -12db on the VU meter. He then calibrated the spl meter and hit measure. He was clipping when he took the measurement so he reduced the levels so it didn't clip.

You can see on the pics that the soundcard cal is not where it should be and goes off the chart. He originally calibrated using the left input channel and also the right input channel with no great improvement.

I then got him to check levels by aiming for -18db instead off -12db and he managed to get a graph in the correct position but the sound card cal was still off the chart. I think the sound card cal is having a bad effect on the measurement graph and I am wondering if the sound card cal is the issue.

He is doing everything right but I am also wondering if the internal sound card on the Imac is actually any good.
Perhaps you could advise further. Its probably a big ask but I don't know whether you are on skype so you could talk to Jeff as they may help in a big way.

cheers

Graham


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## JohnM (Apr 11, 2006)

The loopback calibration measurement posted is absolutely fine, so no problems there and the iMac soundcard is also more than up to the job. That response (or another successful soundcard calibration measurement) should be saved as a soundcard calibration file using the "Make Cal..." button on the Soundcard preferences, the dotted soundcard cal line should then look exactly like that loopback measurement. Calibrating the soundcard is something that only needs to be done once.

The soundcard cal lines shown in the other plots are NOT valid soundcard calibrations, they look more like room measurements - to have those Jeff would have to have clicked the "Make Cal..." button after making a room measurement. Don't touch that button if you have not just made a soundcard calibration! :nono: The incorrect soundcard cal can be removed from those measurements by clicking the "Change Cal..." button and then clicking "Clear Cal", or browsing to the proper soundcard cal file and loading that. Pics of the steps for that are in the Measurements Panel help.

With a proper soundcard cal file loaded those measurements will probably look more sensible. The only thing that may need changing after making a good soundcard cal and moving on to make room measurements is the input volume, once the input volume have been set sp a room measurement can be made without clipping or excessively low levels, go through the SPL meter cal to get the graph levels correct.


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## gperkins_1973 (Aug 25, 2008)

Hi John,

Jeff did select make cal straight after REW did a measure for the sound card cal and saved it to his desktop. Then he did the usual check levels and spl meter calibration but the dotted line which you can just barely see at the top off the graph came off the chart.

I will get him to redo the soundcard cal again once more time. Is there any preference over left or right input channels to select and I guess that if you use the left input channel to do the sound card cal you must use the same input channel to do the measurement too.

I will let you know how he gets on.

cheers

Graham


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## JohnM (Apr 11, 2006)

Either channel is fine, same channel for cal and for measurement. After making the soundcard calibration (via the Soundcard preferences dialog, following the steps) using "Make Cal''' generates and then loads the soundcard cal file. The best thing to do then, without altering anything, is to make a measurement with the loopback still in place to confirm that the result is now flat.


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## gperkins_1973 (Aug 25, 2008)

Cool will get Jeff to run some new ones and take a measurement with the loopback still in.

cheers

Graham


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## Jeff H (Sep 25, 2010)

Hi All,

Well it looks like we have got the soundcard set up correctly, but the sweeps are showing a drop off after 40hz.

The cross over on the Onkyo is set to 80 Hz, we have even tried it at 100 & 120hz with no change.

Does anyone have any suggestions on how to fix this?

Thanks.


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## JohnM (Apr 11, 2006)

What is the subwoofer being driven by, some sort of plate amp? Looks like the sub itself has a LP filter engaged somewhere which is set to the bottom end of its range, that might be in the amp.


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## gperkins_1973 (Aug 25, 2008)

John,

As you know I have spent a long time with Jeff. He is using a behringer EP2500 which doesn't have a LP filter on it. He is using a Onkyo 577 receiver which gives you LP options but not as low as 40hz. This is why it is so strange. Have you ever seen a response drop off like this?

cheers

Graham


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## JohnM (Apr 11, 2006)

Worth getting some input from the guys on the DIY sub forum. To rule out the Onkyo could drive the EP2500 either directly from the soundcard (reduce sweep level substantially first then increase to a level that suits direct connection) or from one of the main left or right (line level) outputs of the Onkyo, assuming it has them.


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