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Why are test tones not perfect for crossover settings?


TallRedRider

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So I have my stereo all installed, and am just fine tuning and such. I typically will use some test tones to set my crossovers for my inboats, towers and subwoofer and then fine tune from there by ear. In theory, I figured the test tones would work perfectly, but they really do not. There is some fine tuning by ear that still has to happen.

I would like someone to explain the science behind this part of the experience: If I put on a test tone at 110 HZ, and start adjusting my crossover on my inboat speakers, the dropoff is not instantaneous when adjusting the crossover. What happens is that as the test tone is playing, and I turn the crossover up from its lowest setting, there comes a point where the test tone begins to get quieter until it is completely muted out. The problem being that this is over quite a range on the dial. The same is true when coming back down on the crossover. In this example, why isn't there just a very small part of the range on the dial where 110 HZ disappears? Instead, it gradually fades. I was hoping that I could turn the dial a small portion and find the precise spot where each frequency lies. I am instead crammed into the space under the glove box with the radio on trying to listen to when the test tone is at full volume, or just starting to fade, and that is sort of hard when crammed into that space.

On a side note: I did find these free test tones to be pretty cool, and show that you can probably do a lot more with test tones than I understand. But none of them explain the way an amplifier crossover does not just suddenly 'drop' a test tone signal when you get above that frequency.

http://www.audiocheck.net/soundtestsaudiotesttones_index.php

Edited by TallRedRider
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To understand what you are experiencing you have to understand how each crossover filter actually works.

And 'phase' is a big part of it....not just 'frequency'.

Let's look at a second order (12 dB per octave slope) filter for example. The filter begins with a more shallow slope before it graduates into a strict 12 dB per octave slope.

Typically the crossover point on a 12 dB filter is 3 dB down. Different on a 6 dB slope and perhaps different on a 24 dB slope.

With any non-linear change in amplitude there is a corresponding rotation in phase response. Crossover filters, eq circuits, included. The two occur inseparably.

The aim is to have a good phase relationship between the highpass and lowpass filters so that they sum as well as possible through the crossover region, rather than cancel.

Minus 3 dB is considered half power but in acoustic terms as perceived it is a moderate change in amplitude.

A 10 dB change might be considered to be half volume as we perceive it. A second order filter requires a full octave to make a 12 dB attenuation. A full octave related to 110 Hz would be either 55 Hz or 220 Hz.

There are definite reasons why both a steep brick-wall filter and a shallow filter are unwanted.

The idea is to have a coherent phase transition with minimal frequency overlap. That requires both highpass and lowpass filters to intersect at the same - 3 dB frequency. There are exceptions but that is the general objective.

Edited by David
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To understand what you are experiencing you have to understand how each crossover filter actually works.

And 'phase' is a big part of it....not just 'frequency'.

Let's look at a second order (12 dB per octave slope) filter for example. The filter begins with a more shallow slope before it graduates into a strict 12 dB per octave slope.

Typically the crossover point on a 12 dB filter is 3 dB down. Different on a 6 dB slope and perhaps different on a 24 dB slope.

With any non-linear change in amplitude there is a corresponding rotation in phase response. Crossover filters, eq circuits, included. The two occur inseparably.

The aim is to have a good phase relationship between the highpass and lowpass filters so that they sum as well as possible through the crossover region, rather than cancel.

Minus 3 dB is considered half power but in acoustic terms as perceived it is a moderate change in amplitude.

A 10 dB change might be considered to be half volume as we perceive it. A second order filter requires a full octave to make a 12 dB attenuation. A full octave related to 110 Hz would be either 55 Hz or 220 Hz.

There are definite reasons why both a steep brick-wall filter and a shallow filter are unwanted.

The idea is to have a coherent phase transition with minimal frequency overlap. That requires both highpass and lowpass filters to intersect at the same - 3 dB frequency. There are exceptions but that is the general objective.

Thank you David, I was hoping for an answer sort of like that, but maybe dumbed down a hair.

In English, I think you mean to say that a 110 hz test tone will usually disappear when the crossover gets to 55hz? That is what I think I am hearing with my ears. And if on low pass, the same 110 hz disappears at 220 hz. If I understand that little tidbit right, that is super helpful. IIUC, the numbers on the crossover dial are notoriously unreliable.

On a more important note, how come we haven't seen any pics of that new G Red?

I've been sensitive to this being a Malibu site, so really have kept it on the down low.

Maybe I could invade your Meat Talk thread with a 'Hey Malibu guys, here is my Nautique!'?

Edited by TallRedRider
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Thank you David, I was hoping for an answer sort of like that, but maybe dumbed down a hair.

In English, I think you mean to say that a 110 hz test tone will usually disappear when the crossover gets to 55hz? That is what I think I am hearing with my ears. And if on low pass, the same 110 hz disappears at 220 hz. If I understand that little tidbit right, that is super helpful. IIUC, the numbers on the crossover dial are notoriously unreliable.

I've been sensitive to this being a Malibu site, so really have kept it on the down low.

Maybe I could invade your Meat Talk thread with a 'Hey Malibu guys, here is my Nautique!'?

I think most guys here just like good rigs

If you only feel comfortable posting in meat talk thread then please do! I wanna see it!

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It's nice to have an amplifier with a quality front end, like a JL Audio for example, where the detents track accurately and there is a precise click by click guideline. In a few cases I've seen where the nomenclature on the amplifier end cap was a full octave off at given points, which sure makes it harder to accurately tune.

You would also be surprised in the difference in quality of crossover filters brand to brand.

As a tuning method, I probably wouldn't use test tones to locate the crossover but if I did you want to locate the same - 3 dB down point for both HP & LP filters. - 3 dB would be just past the very first sense that you are seeing an attenuation in the signal strength.

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It is an Arc Audio amp, and I think I just used the test tones as a first trial, then my ear after that...Maybe I could pay attention to the numbers on the dial, I didn't even bother with them. It does happen to have a mark on the dial at 110 HZ, which might be a good place to start with the woofer.

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