How do I measure the level at which my sub distorts?

[JDawg]

I've been talking to Tom at SVS about the distortion/clipping issues i've been having. I thought I had fixed it until I watched War of the Worlds. I should be able to get as high as 109-115dB without distortion in my room size but I don't think it gets anywhere near that loud before I run into problems. So i've been asked at what level or how close to reference does the sub start to distort. The problem is i'm not sure how I go about doing this. Do I just play back the DVD scenes where the problems occur and measure the sound level?

 

[MDS]

First, how did you determine that you actually have distortion/clipping issues? Was it by ear, that at some point it starts to sound unpleasant to you?

The only way to truly determine that the sub is the culprit would be to measure the output of the sub and plot an SPL vs THD graph. Somewhere around 10% THD is what is generally considered audible distortion from a subwoofer.

Still, if you calibrated your receiver to reference level using the internal test tones and used 0 as the reference volume level, you could start there and use your SPL meter set to A weighting, fast response to measure the absolute SPL. When you hear something that sounds like distortion, use the reading of the SPL meter as the information to provide back to SVS.

If the distortion you perceive happens before 0 on the volume scale, then the number on the dial (say -10 - 10 dB below reference) would be the information to provide in response to the question 'how close to reference level'.

 

[JDawg]

I can hear it when it happens and it doesn't sound right. During the scene in War of the Worlds when the tripod machine is coming out of the ground there's a continuous low rumble and then i'll hear a pop sound or maybe its called clipping. I'm not sure what the correct terminology is so its hard for me to explain. If I calibrate the sub so that its a few dB's lower compared to the other channels then the problem goes away unless I turn up the main volume some more. I still get clean powerful bass at the lower setting but if I paid this much for a sub I want it to be working as good as its supposed to.

Anyways, when I have time tonight i'll try to measure the level when the problem occurs with the meter set to A weighting.

 

[mtrycrafts]

I've been talking to Tom at SVS about the distortion/clipping issues i've been having. I thought I had fixed it until I watched War of the Worlds. I should be able to get as high as 109-115dB without distortion in my room size but I don't think it gets anywhere near that loud before I run into problems. So i've been asked at what level or how close to reference does the sub start to distort. The problem is i'm not sure how I go about doing this. Do I just play back the DVD scenes where the problems occur and measure the sound level?

A number of issues come into play as was posted by MDS.
But, that subs output will certainly vary by frequency. What is reproduced at 115dB spl at 63Hz most likely will not be reproduced at 20Hz.

Plus, you do need an spl meter to determine spl levels. You cannot do it by guessing. For this use, the RS digital is the best meter for this, even though it is inaccurate at low frequency. Again, it too has a different error with frequency.

As to low frequency distortion audibility, you should read the research at Axiom:

http://www.axiomaudio.com/distortion.html#

Not as easy to detect at low frequency as many think it may be. While 10% is where subs are usually tested, unless it is a Velo servo , that is not the threshold of audibility, by any means.

 

[apatel25314]

the manufacturer may put out theese numbers

 

[Ampdog]

Hi JDawg,

From my side, just to add that firstly, 115dB is a VERY loud level for any driver to handle. One loudness table lists 110 dB as hammer blows on a large steel plate at 2 feet distance, and a large jet engine at 20 feet as 120 dB (threshold of pain on that table is 130dB.) Now your room size comes into it obviously, as do also standing waves at low frequencies. I do not know what equipment is at your disposal; one way to measure audible distortion cheaply is simply with a moderate microphone. (They normally do not distort that much, and you can check that at lower volume close to the sub, then move away until the same output is then shown at high power output.) This will need an oscilloscope.

I also take it that the sub is active, i.e. with built in amplifier (not the right definition, but that is another story). Whatever way, I would also check the driving amplifier's output at maximum supposed output for clipping or even visible distortion, i.e. separate the issues and check one thing at a time. (I often wondered about the rather strict views uttered on hi-fi amplifiers (correctly so), but when sub-woofers are reviewed nobody seems to care much about what goes on with the amplifier itself.)

Please inform us of what you find. Experience is a good teacher, but second-hand is cheaper!

Regards.