How To: Measure Hi-Fi & Room Frequency Response

Discussion in 'Member 'How To' Guides' started by Tenson, Mar 28, 2011.

  1. Tenson

    Tenson Moderator

    Joined:
    Nov 12, 2003
    Messages:
    5,947
    Likes Received:
    1
    Location:
    Kent, UK
    To follow this tutorial you will need the following:


    • A computer (near your hi-fi)
    • A sound card (one that can accept phantom powered microphone inputs)
    • A measurement microphone (omni-directional with flat response)
    • Some cables to connect the soundcard output to your hi-fi and the mic to your soundcard
    • ARTA measurement software

    A suitable soundcard must have 'phantom power' which is the ability to power the microphone connected to it. It must also have reasonably low noise and distortion. Such a soundcard can be had for about £150. A few examples:

    Focusrite Saffire 6
    M-Audio Fast Track Mobile
    Tascam US-122
    Presonus AudioBox

    A suitable microphone is usually quite expensive costing a couple of hundred pounds. There is one exception and that is the ubiquitous Behringer ECM8000: Behringer ECM8000

    ARTA is a software package that has full functionality, except saving files, free of charge: ARTA Download

    ARTA will generate a test signal on the soundcards outputs and record whatever appears at the soundcards inputs. You can then analyse the recorded signal. In this case we want to output the test signal to the hi-fi and record the room and speakers via the microphone, then look at the frequency response.

    This is what I am using:

    M-Audio Fast Track Ultra
    Beyer Dynamic MM1 Microphone
    Samsung Laptop
    Jack to RCA cables X2
    XLR to XLR microphone cable

    [​IMG]

    So lets get started. You have your hi-fi set-up (I've setup mine in my work room for this tutorial).
    [​IMG]

    Now you need to connect up your hardware. Plug your soundcard outputs into your hi-fi and select the inputs you are using so that any sound from the computer will go to the speakers.

    [​IMG]

    Now connect your microphone to the left input of your soundcard and make sure you have phantom power enabled.

    [​IMG]

    Place the microphone at your listening position around head height. I have a mic stand so this is quite easy. If you don't, you can either balance it on some cushions or ask a friend to hold it very still.

    The mic doesn't need to be placed directly pointing at the speakers because it is omni-directional. Common practice is to have it pointing upwards. In this case don't be surprised to see the high frequencies rolling down above about 8KHz.

    [​IMG]

    I trust you have already installed ARTA, which is as simple as clicking next, next, next... So run the ARTA main program.

    [​IMG]

    Click the little microphone icon to access Audio Devices Setup. Make sure you have your soundcard selected as the output device in the top drop down list, and if the option is there use ASIO drivers. Make sure the correct input and output channels are selected. Click Okay.

    [​IMG]

    Click the red record icon to access the recoding window. You should set the longest sequence length for the best signal to noise ratio. Sampling rate is okay at 44.1KHz and higher is not really necessary here. Set the correct input channel for your microphone. Disable dual channel measurement mode. Finally set 2 averages, this helps further reduce noise from traffic outside and such.

    [​IMG]

    Click on the generate button to test the levels. You can see the level meters at the bottom of the window, one of which should be unused. Adjust the amp volume until the test noise is about as loud as you can bare comfortably. You want to get the signal level as much above ambient noise as you can for accurate results. Adjust the microphone input level on the soundcard to make sure it is near the top of the level meter, but not clipping.

    [​IMG]

    When the level is correct stop the generate function. Now click on the record button. The system will output the test signal and record the input. The record window should then close and show the recorded 'impulse response'.

    [​IMG]

    In the main window showing the impulse response, click the left mouse button just before the beginning of the waveforms sharp rise. This sets the beginning of the area to be analysed; the beginning of the 'gate'.

    [​IMG]

    Use the zoom buttons on the right-hand side to zoom out, and then use the right mouse button to place the end marker of the area to be analysed. If you look at the bottom of the window you will see some text that says Cursor and Gate. The Gate part should read about 3000-3500.

    [​IMG]

    [​IMG]

    Once these markers are set you can analyse the frequency response by clicking on the FR icon.

    [​IMG]

    [​IMG]

    If you are running the un-licensed version you can't save this data, however you can copy the graph to the clipboard to be pasted into an image editor and saved.

    [​IMG]

    [​IMG]
     
    Tenson, Mar 28, 2011
    #1
    1. Advertisements

  2. Tenson

    Tenson Moderator

    Joined:
    Nov 12, 2003
    Messages:
    5,947
    Likes Received:
    1
    Location:
    Kent, UK
    Averaging Measurements

    If these measurements are for setting up corrective EQ, then it is best to average a number of measurement positions around your listening area. The microphone will only show the sound arriving at that exact point in space and if we apply EQ based that it is easy to go too far, or correct some response blip that changes with just a small movement.

    To make an average measurement we first need to take the initial measurement just as detailed above. We then need to click the 'Overlay' button in the frequency response analysis window.

    [​IMG]

    Now you should move the microphone position. Try a couple of feet to the left. Click the record icon inside the frequency analysis window to open the familiar record dialogue, and press record.

    [​IMG]

    The window will close after taking the measurement and you will now see a new frequency response, as well as the previous one on overlay. Not that the previous maker positions are still being used to select the waveform to be analysed. If you moved the mic a lot closer to the speaker than you first measurement the beginning of the waveform might now be outside the beginning marker and need adjusting. Just close the analysis window and adjust the marker.

    [​IMG]

    Repeat this procedure to set the new response as a second overlay, and record a third measurement with the microphone in another position. You can repeat this as many times as you like to average a larger area.

    [​IMG]

    Once you have all the measurements you want to average, and they have all been set as overlays, we can make the average. Click on 'Edit' in the frequency response analysis window, and select 'Power Average With Overlays'. You will be asked if you want to delete the overlays after averaging, and I'd suggest 'yes' or it looks very messy.

    [​IMG]

    Finally, we have our averaged response! If you are using this for EQ correction, you will want to repeat these steps to confirm your EQ is doing what you want. It can be a tedious task when you have to make lots of averages, but the results of carefully applied EQ based on averaged measurements are well worth the effort.

    [​IMG]
     
    Tenson, Mar 28, 2011
    #2
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Similar Threads
There are no similar threads yet.
Loading...