# How to interpret frequency response data



## npdang (Jul 29, 2005)

1. *Frequency response:*

First off, frequency response measurements are highly dependent on the measurement conditions. For example, how far away you measure, how long the measurement is, the size of the front baffle (the piece of wood you mount the speaker to), diffraction, reflections, whether or not you flush mount the driver, etc.

The technique used here at DiyMobileAudio to measure frequency response is always the same. The speaker is set on some heavy non-resonant surface and secured. There is usually no baffle, unless otherwise stated.

First a measurement is taken right up at the driver's cone. This is called the "nearfield measurement". The nearfield measurement gives an accurate measurement of the driver's low end response, free of room reflections and baffle diffraction. This line is typically represented in yellow. However, the nearfield measurement is only accurate up to 

Frequency max = 4311 / driver diameter in inches. For a 7" driver that's typically 650hz.

Next, we take a series of "gated" measuremends. This just means that the measurement is cut short before any reflections have time to reach the microphone. We do this at 1/2 meter directly on axis with the driver, and both 30 and 60 degrees to the side of the driver. Now regardless of the distance, at DiyMobileAudio the gated measurement SPL will generally be represented at 1 watt, 1 meter.

So even though a driver is measured at 1/2m, the SPL will be adjusted so that it represents the true sensitivity at 1watt of power, at 1 meter distance.

Now some problems with the farfield measurement. First of all, having no baffle means the low end response will drop off rapidly. This is because low frequency wavelengths are long... and they will "wrap" around the speaker. So we have sound radiating both forward and behind the speaker, which causes a loss of spl. Another problem we have is diffraction. Sound waves will "bend" around objects in the room and around the speaker itself, causing dips and peaks in the frequency response.

So let's look at an example:










The yellow line is our nearfield measurement. Since this is a 7" woofer, we can assume it's accurate to about 650hz. 

From there we notice the red line, which is our gated measurement at 1/2 meter. We can see that below 500hz the SPL drops off quickly, but we were expecting that due to lack of a measurement baffle.

Following the red line we notice a bump at 1.5khz, and another one at 5khz. This get's tricky. How do we know if this is diffraction caused by the lack of a baffle, or if it's part of the speaker's frequency response? Well, it helps to take a look at a number of other measurements for speakers of the same size. If they ALL have the same bumps at roughly the same places we can make a safe assumption that it's just diffraction. Also, bumps don't appear out of nowhere for no reason. Look at the distortion graphs. Does the bump coincide with increased distortion or energy storage? Is there a ripple in the impedance curve at those frequencies? If not, it's safe to say it's just diffraction.

Lastly, the 30 degrees and 60 degrees are off-axis measurements represented by the teal and blue line. They just show you the frequency response of the speaker off to it's side. Generally for a car door, you listen at 45-80 degrees off axis.


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## MrLister (Feb 17, 2006)

Great article, very useful. I was looking for something in particular that I had a hard time finding.

What would an ideal mid bass speaker look like on graph? What are the key things to look for in a frequency response graph?


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## jdybnis (Sep 20, 2005)

It doesn't make sense to test a speaker without a baffle. No speaker was designed to be run unbaffled. You can see how it gave erroneous info in the test of the peerless 4" mid http://www.diymobileaudio.com/forum/showthread.php?t=617&page=2&pp=13 

See also http://www.visteon.com/utils/whitepapers/2004_01_1694.pdf


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## npdang (Jul 29, 2005)

jdybnis said:


> It doesn't make sense to test a speaker without a baffle. No speaker was designed to be run unbaffled. You can see how it gave erroneous info in the test of the peerless 4" mid http://www.diymobileaudio.com/forum/showthread.php?t=617&page=2&pp=13
> 
> See also http://www.visteon.com/utils/whitepapers/2004_01_1694.pdf


 For an FR plot or stored energy test, it's debatable and you have to understand the limits of your test. It certainly doesn't effect non-linear distortion performance. I wouldn't go so far as to say "it doesn't make sense".

Think about it from this perspective as well, are you trying to conduct absolute measurements or relative ones?


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## jdybnis (Sep 20, 2005)

I agree about distortion testing. But even for relative comparisons of FR there are problems. Some X" drivers are going to have different interactions with the baffle than others. When you see a peak in the FR of a driver you measure, you can't conclude the resonance is a real issue for that driver. Even if you compare to measurements of similar drivers and the peak isn't on the others, the problem may still be affected by the baffle. Like in the case of the of the peerless the problem might go away. Or the driver might not be designed as well as the peerless and will not improve on a baffle. You have no way of knowing.


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## npdang (Jul 29, 2005)

That's certainly true, although looking at the impedance plot can give some insight.


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## jdybnis (Sep 20, 2005)

That's true too.


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## Bsams (Feb 8, 2007)

BY LOOKING AT SPECS FOR A DRIVER HOW CAN YOU DETERMINE IF IT WILL DO WELL MOUNTED IN A TYPICAL CAR DOOR? FROM WHAT I GATHER A HIGHER QTC IS BETTER FOR FREE AIR OR INFINITE BAFFLE?


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## npdang (Jul 29, 2005)

Bsams said:


> BY LOOKING AT SPECS FOR A DRIVER HOW CAN YOU DETERMINE IF IT WILL DO WELL MOUNTED IN A TYPICAL CAR DOOR? FROM WHAT I GATHER A HIGHER QTC IS BETTER FOR FREE AIR OR INFINITE BAFFLE?


Yes, you most certainly can if you know how to interpret the data.


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## donkeypunch22 (Nov 5, 2008)

Thank you!


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## amungal (Mar 29, 2010)

Bsams said:


> BY LOOKING AT SPECS FOR A DRIVER HOW CAN YOU DETERMINE IF IT WILL DO WELL MOUNTED IN A TYPICAL CAR DOOR? FROM WHAT I GATHER A HIGHER QTC IS BETTER FOR FREE AIR OR INFINITE BAFFLE?


I am very interested in the answers to these questions as well. That is:

1. HOW (not IF) you can determine the performance of a driver mounted in a typical car door based on the specs?

2. Is a higher QTC better for infinite baffle installs?


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## rdlhifi (Sep 8, 2013)

Many thanks for the valuable info!


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