How do I generate a file with the FRD extension?

If you have done frequency response measurements before, learning how to make a FRD file should be a piece of cake. FRD stands for frequency response data file, and it holds information regarding the magnitude and phase of different frequencies. If you have the curiosity to open such a file using a text editor, you will see 3 columns : frequency, magnitude and phase.

To generate these files we are going to use ARTA. First, I considered REW, but the problem is that it doesn’t have box diffraction compensation for near-field measurements. The whole point is to generate files for our particular project. In conclusion, we are measuring each individual speaker, inside the cabinet. Using files from the internet containing half-space measurements, won’t do any good. Maybe only for tweeters.

Before we get down to business. Make sure you read this article where I make a full range frequency response measurement using ARTA. Don’t skip this if you don’t have any experience using ARTA, or you are new to the process, or terminology :

There is no point in describing the same process again. I will make references to this article and point out what to do different.

Device under test

We are going to measure a 2 way bookshelf speaker with a rear firing bass reflex port. So, before we learn how to make a FRD file, we need to understand what is of interest. I purposely chose a more complicated speaker (i.e. bass reflex). If you understand this scenario, you figure out the steps for yourself, in a less complicated one.

2 way speaker front

2 way speaker back

I attached a couple of pictures with the box, so you can get a visual reference. The speakers are the following :

Number one reason of generating a FRD file is to use it in a crossover design software. As a result, we will need to generate 2 files : one for the mid-bass, and one for the tweeter. However, when making the file for the mid-bass, you need to measure the port as well. Measure both the speaker and the port, add them together and then generate the FRD file.

Now you probably understand why internet FRD files are not that useful. When you will learn how to make a FRD file, it will contain all the particular aspects of your enclosure (port, baffle size, diffraction effects etc). Unless you are designing an infinite baffle setup, those files are meaningless, in my opinion.

How to make a FRD file for the tweeter

Generating a FRD file for the tweeter is dead easy. Make a far-field measurement (microphone at 1 meter) only with the tweeter hooked up. Let me describe the steps :

  • Take an impulse response.
  • Place the marker for the window (because we are interested in the gated frequency response).

impulse response

  • Then, click the FR button to reveal the frequency response.

Frequency response tweeter

  • As you can see, the amplitude is around -30 dB. If you use XSim as crossover design software, the default amplitude level is at 70 dB. So if you make a FRD file with a frequency response at -30 dB you will see no response, unless you alter the scale. or center the graph correctly. To avoid this issue, let’s add 100 dB to our response. It doesn’t matter how many dB’s you add or subtract, as long you do it for all of the FRD files you are generating for that particular project. As a result, select “Edit -> Scale level” and enter 100 dB. Then, press the “Fit” button on the top-right corner to center the graph.

frequency response tweeter scaled

  • We have the frequency response, but we are missing one key element : phase. Click “View -> Magn+Phase”.

tweeter magnitude plus phase

  • Now it’s time to generate the FRD file for the tweeter. Click “File -> Export -> ASCII File”. Then it will ask you if you want to export in plain FRD format, and click ok.

how to make a frd file

After you click OK, select the destination location for your newly created FRD, and we’re done with the tweeter side.

How to make a FRD file for the mid-bass

We’re done with the tweeter, now lets see how to make a FRD file for the bass side of the speaker. This is a bit more tedious for a bass reflex speaker. I’m not going to describe the entire process again (check the links at the beginning of the article). I’m just going to list the steps :

  • Make a near-field measurement for the speaker.
  • Click “File -> Save as” to save the response. (We need it later).
  • Do a near-field measurement for the bass reflex port.
  • Scale the port response (according to speaker size).
  • Sum the port response with the speaker response. Click “File -> Load and Sum” and select the near-field measurement of the speaker you saved earlier.
  • Scale the response to far-field.
  • Scale the frequency response chart for an additional 100 dB. (because we did the same for the tweeter).
  • Apply box diffraction. The enclosure is 23 x 33 cm. Click “Edit -> LF box diffraction” and enter the dimensions of the front baffle. Then the response should look like this :

nearfield response scaled

  • Assign this response as an Overlay. Click “Overlay -> Set as overlay”.
  • After that, do a gated far-field measurement for the mid-bass.
  • Then, scale the level for +100 dB. The response should look like this :

stacked responses

  • Now select an appropriate point where to splice the two responses together. Make sure to read the first 2 articles linked at the beginning, to understand how to make this selection. In our case, ideally, we have to search for the point of intersection between 250 Hz and 900 Hz. And that point is precisely at 533 Hz. Place the marker there and click “Edit -> Merge overlay below cursor”.
  • Click “Overlay -> Set as overlay”
  • Go to “Overlay -> Manage overlays”
  • Click the first overlay, the dark yellow one, and then click “Delete sel”. You will be left with the spliced near-field and far-field responses, as an overlay :

finished how to make a frd file

  • Now all you have to do is create the file. Click “File -> Export -> ASCII File”.

Verify in XSim and conclusion

Let’s check if we correctly learned how to make a FRD file. Let’s start Xsim and load the mid-bass FRD file and see if everything works out the way it’s supposed to work.

new frd file

As you can see, our newly generated file works flawlessly. Basically, if you know how to make a quasi-anechoic measurement, you can generate a FRD file with just a few extra clicks. It might look complicated and tedious, but after you made 2-3 measurements, it’ll become second nature.


References

  1. Image source : link.