Do Enclosures Generate 2nd Harmonic Distortion?

[Patrick Bateman]

Over the weekend I've been working on a box which uses (almost) every trick I know to kill distortion. The woofers are underhung, I am using two of them in push pull, and it's in a horn, which reduces distortion by reducing excursion.

During my distortion testing I noticed a spike at 350hz. I continued my testing, and then listened to the design for a couple hours. While listening I could hear a bit of raspiness in the lower midrange and I attributed it to that spike in distortion that I measured.

On a whim, I took fifty pounds of iron and placed it on top of the enclosure.

The raspiness disappeared.

So here's the question: does the enclosure *itself* generate distortion? And if so, can we see it on a distortion measurement?

John Atkinson at Stereophile measures cabinet sturdiness using a stethoscope. I've always been a bit wary of this measurement, because radiation will vary depending on panel size. (IE, a large panel that moves a fraction of a millimeter is a bigger problem than a small panel that moves a millimeter.)

Is a simple distortion measurement all that we need to measure this?

[Marcus Halberstram]

Of course, if the pressure in the horn throat is high enough, that will generate distortion. But that doesn't seem to be what is happening here.

I'm not clear from your post if the spike you observed is in the frequency response, or in the distortion. From your description what you are observing seems to be directly related to some sort of mechanical enclosure resonance. One method to measure this is to get the impulse response of the enclosure with an accelerometer and an instrumented impact hammer -- the high tech version of John Atkinson's test. This sort of equipment is outside of the reach of most of the folks here, though.

An interesting result -- I've not seen that correlation made before.

[Patrick Bateman]

Originally Posted by Marcus Halberstram

Of course, if the pressure in the horn throat is high enough, that will generate distortion. But that doesn't seem to be what is happening here.

I'm not clear from your post if the spike you observed is in the frequency response, or in the distortion. From your description what you are observing seems to be directly related to some sort of mechanical enclosure resonance. One method to measure this is to get the impulse response of the enclosure with an accelerometer and an instrumented impact hammer -- the high tech version of John Atkinson's test. This sort of equipment is outside of the reach of most of the folks here, though.

An interesting result -- I've not seen that correlation made before.

American Psycho (2000) - Memorable quotes

Paul Allen has mistaken me for this d1ckhead Marcus Halberstram. It seems logical because Marcus also works at P&P and in fact does the same exact thing I do and he also has a penchant for Valentino suits and Oliver Peoples glasses. Marcus and I even go to the same barber, although I have a slightly better haircut.

[littlemike]

Partick Bateman and Marcus Halberstram in the same thread....

What are the odds?

I've seen bare piezo elements used as cabinet resonance detectors - think it was way back in a Speaker Builder. Sort of an electronic stethoscope of sorts.