High Frequency Driver Far Field Response Measurement

Here we measure the acoustical frequency response of a common double chamber 1” dome tweeter unit. This unit has a voice coil resistance of 6 ohms and has a nominal power rating of 100 Watts. Its resonant frequency is 750 Hz making it a good candidate for a two-way system. The driver is flush mounted in a box with front panel dimensions of 11" x 25". The room is semi-reverberant and measures 6.5m(l) x 5.5m(w) x 2.4m(h). 

In the procedure below we will do the following:

• Modify the "32768_MLS_Impedance_Measurement.process" to measure frequency response.
• Wire the measurement circuit for level and frequency calibration.
• Set the level of the sound card and optional amplifier to 2.83V.
• Calibrate the sound card and optional amplifier input level and frequency response.
• Run the process and window out floor and ceiling reflections so that we can observe the relative SPL vs. frequency response of the driver in the spectrum analyzer.
• Apply a microphone calibration curve so we can view the absolute SPL vs. frequency response of the driver in the spectrum analyzer.
• Observe the absolute SPL vs. frequency response as we gradually move the microphone closer to the driver.
• Present the SPL vs. frequency response vs. microphone distance from the DUT in the data-logger.

 Loudspeakers are typically specified in terms of dB SPL for 2.83 volts peak input. The sound pressure level is measured on-axis in anechoic conditions at a distance of 1 metre from the loudspeaker. 2.83 Volts corresponds to the voltage across a standard 8 ohm speaker driven at 1Watt. The current required to supply 1 watt to 8 ohms is 0.353 Amps.

Although +12V DC is available on all computer PCI backplane connectors, the analog output stage of a typical AC97 audio interface is powered from +5VDC. At best these -10du (40-300 ohm) single ended line outputs can produce a full scale output voltage of about 1.25Vrms which falls short of the 2.0Vrms (0.7071 x 2.83Vpeak) required to produce 1 Watt across 8 ohms. Better quality sound cards may have +12V powered, swappable, in common 8-Pin Dip packages (as below) op-amps in a balanced configuration on the line outputs but these at best can deliver 8V into a 600 ohm load (13mA ~100mW).

Figure 1 Eight Pin DIP Op-Amp Packaging

The table below shows the maximum output voltage of two motherboard based audio codecs into 4 load impedances.

Table 1: Maximum Sound Card Output Voltages vs. Load Impedances

In addition to the line level outputs, some older sound cards such as the sound blaster AWE 32 had speaker outputs that could easily drive 2 Watts into an 8 ohm load. If you have such an output this would be the one to use for frequency response measurements.

 The four pin USB 2.0 port found on most computers can output 500mA at +5V or 2.5 Watts. Most USB powered audio interfaces only have balanced international studio line level outputs (+4dBu or 2.19Vpeak nominal). These 300 ohm outputs can produce about +10dBu or 3.49Vpeak maximum but are not suitable for driving 8 ohm loudspeakers. Their headphone outputs are only good for about 100mW into 32 ohms. Although eight pin +12V and +24V powered USB and six or nine pin Fire-wire audio interfaces could easily meet the above standard these types of audio interfaces are usually equipped with the same +4dBu outputs as the USB types.

There are three things you can do to compensate for low audio interface output levels.  

1. Purchase a 1 Watt or greater stereo power amplifier such as the Dayton DTA1 (2x10 Watts -> 4 Ohms @ 0.1% THD @ 1KHz), Pyle PCA1 (2x3 Watts-> 4 Ohms @ 1.0% THD @ 1KHz) or PTA2 (2x8 Watts -> 4 Ohms @ 1.0% THD @ 1KHz), the Sonic Impact TA2024 (2x6 Watts -> 4 Ohms @ 0.1% THD ) or the Nady XA300 (2x120 Watt RMS -> 8 ohms @ 1.0% THD @ 1KHz) ranging in price from $39.00USD (29EUR) to $100.00USD (74EUR).
2. If you already have a consumer HiFi receiver or amplifier you can purchase a 3.5mm TRS to ¼” RCA male adapter cable (as below) and connect the sound cards line-out to the equipments Aux-In input and drive the DUT with the amplifier.
3.  

Figure 2: 3.5mm TRS to ¼” RCA Male Adapter Cable 

3. You only need a single amplified channel to measure loudspeaker on axis frequency response. The +6V split supply powered, +15dB gain class AB amplifier on the left below can be inexpensively constructed and can deliver 4 Watts to a 4 ohm load at less than 0.1% THD at 1 KHz. The +12V single supply amplifier on the right can do the same but is not recommended due to the large output decoupling capacitor required to isolate the DC from the loudspeaker. At 20Hz this 2200uF capacitor would have an impedance of 3.6 ohms. This has the effect of reducing the voltage at the loudspeaker terminals as frequencies are lowered so that 1 Watt is no longer dissipated across the voice coil. Although the IC’s in the circuits below have thermal shut down a 6C/Watt heat sink is required.

Figure 3: Class AB Amplifiers Capable of 1 Watt

Just connect the amplifier in series with the audio interface outputs and adjust the mixer levels to produce an output of exactly 2.83Vpeak (2.00VRMS) into the loudspeaker.

If you are measuring impedance or relative responses (such as you would when you are adjusting cross-over components) you do not need to calibrate levels. If you are measuring absolute responses like we are here you should calibrate levels as you probably would like to refer your measurement to some manufacturers data sheet. In order to calibrate a sound card line input you need to compare it to a reference standard. An inexpensive way to do this is to purchase a pocket multimeter such as the 4 1/2 digit EXTECH DM110. It costs about $34.99 USD or 27.41 EUR. Its AC RMS voltage accuracy from 40-400Hz on its 4.000V range is +1.0% + 10 digits. This translates to about 0.05V. More accurate 6 1/2 digit meters such as the Agilent 34401A with VAC accuracies of better than +0.06% are available. You must ensure that your meter measures AC voltage at the frequency (~200Hz) that sonic beacon outputs during level calibration. For instance, during voltage calibration for a sample rate of 96000S/S at an FFT Size of 1024 a 187.5Hz waveform is output.

We will modify "32768_MLS_Impedance_Measurement.process" to perform the frequency response measurement. This process ships with the release version of this product. When we are done it will consist of six modules. The first is the signal generator, which generates an 8192 length MLS stimulus to excite the DUT. Second is the SoundIO module, which plays the stimulus and records the response of the driver. Third is the Arbitrary Filter, which will invoke our microphone compensation curve. Fourth is a Spectrum Analyzer, which will perform an FHT on the MLS time domain data in order to convert it to an impulse response. Fifth is the Oscilloscope module, which will allow us to window the impulse response in order to remove room reflections. Finally another Spectrum Analyzer, which performs an FFT on the impulse and allows us to view amplitude vs. frequency and phase vs. frequency graphs.

1.       Install the driver in the desired enclosure or baffle. Bypass any driver crossover unit. Place the enclosure on a stand about 1 meter from the floor.

2.       Open “C:\Users\Public\Documents\Sonic Beacon\Sonic Beacon\32768_MLS_Impedance_Measurement.process” from the applications File…Open… menu. Press OK if the “No Compatible Calibration File Present” message box appears.

3.       Select Options…Process from the applications menu. The Process Select dialog will open. Highlight Oscilloscope in the Module List box. Highlight Arbitrary Filter in the Available Modules list box. Press the Insert button. Highlight Oscilloscope in the Module List box. Highlight Spectrum Analyzer in the Available Modules list box. Press the Insert button again. The Process Select dialog should appear as in Figure 2.

Figure 4: Driver Measurement Process Select Dialog

4.       Press the Ok button in the Process Select dialog. The modified process will open as in Figure 3. Press OK when the “No Compatible Calibration File Present” message box appears.

Figure 5: Driver Measurement Process

5.       Open the FFT Options dialog from the applications Options…FFT… menu and change the FFT Size to 8192. Press OK in the FFT Options dialog box. Press OK when the “No Compatible Calibration File Present” message box appears.

6.       Select 4 from the SoundIO Repeat Sequence: combo box.

7.       Select the Step: 4 Spectrum Analyzer module. Select FHT from the Type combo box and check the Apply Freq. Cal. checkbox in the Options group box.

Figure 6: Step 4: Spectrum Analyzer Settings

8.       Select the Step: 6 Spectrum Analyzer module. Select Log100 from the Xaxis Sel: combo box. Select dBRel from the Yaxis Sel: combo box. Select 5dB/div scale from the Yaxis Sel: combo box. Enter 1000 into the Ref1: Edit control and press the enter key. Select FFT from the Type combo box and check the Apply Freq. Cal. checkbox in the Options group box.

Figure 7: Step 6: Spectrum Analyzer Settings

9.       Wire the circuit for calibration as shown in Figure 1. Use short, low resistance or shielded wiring. Note that external amplifiers should be in the calibration loop. These devices will produce more output swing than is tolerated by the sound cards line inputs so keep levels low.

Figure 8: Driver Measurement Process Calibration Wiring

10.   You need to adjust the level of your selected sound card recording path. If you are running Windows 7 or Vista right click the sound icon on the windows task bar and select Recording devices from the popup menu that appears. Double click the selected sound card in the Sound dialog box Recording tab. Select the Levels tab and adjust the slider to its one-quarter setting. Press the OK button. If you are running XP or below; select the Levels tab Press the Open Mixer button the SoundIO modules Options group. Select Options… Properties… Choose your sound card from the Mixer Device and press the Recording radio button in the Adjust Volume for group. Press the OK button. Deselect all Record Control mixer paths except the Line In. Adjust the Line In mixer slider to its one-quarter setting and equalize its balance slider.

11.   You need to adjust the output level of your selected sound cards playback path. If you are running Windows 7 or Vista select the Playback tab of the Sound dialog box and double click the selected sound card. Select the Levels tab and adjust the Line In to its 25% setting. Otherwise if you are using XP or lower select Options… Properties… Press the Playback radio button in the Adjust Volume for group. Press the OK button. Mute all Playback mixer gain settings except the Volume Control and the Wave Out. Equalize the Volume Control and the Wave Out mixer balance sliders. Adjust the Volume Control and the Wave mixer sliders to their one-quarter settings.

11.   You now need to calibrate the sound card if you do not already have a valid calibration file loaded for the input and output devices selected in the SoundIO module. Press the Calibration button in the SoundIO module. The Calibration dialog box will open.

12.   Select Input from the Calibration Type Select combo box and press the Run button to output the applications AC internal reference to Line-Out. A sine wave of the closest harmonic of 200Hz possible with the processes SoundIO module sample rate and FFT size will be directed to Line-out. Plug in one end of  the TRS cable into Line-Out (green) and measure the signal amplitude with an Oscilloscope or AC voltmeter. Then connect the other end of the TRS cable to the Line-in input (blue) and enter the measured amplitude in peak (oscilloscope) or RMS (AC voltmeter) volts in the Reference Level Edit controls. Select either Vpeak or Vrms in the Reference Levels Combo Boxes depending on the measurement instrument used and press the Stop Button to update the gain that is applied to the all the processes modules that display amplitudes.The dialog boxes Reference Level edit, Sound Card Input and Input Level static text controls will contain the user entered reference or measured Line-out level (must not be 1.0), the selected input of the processes SoundIO module and the mixer gain setting of the input channel (0-65535) respectively as below.

Figure 9: Level Calibration Group Box Controls after Pressing Stop Button

13.   Select Auto from the Calibration Type Select: combo box in the Calibration Status group box. Select MLS from the Signal Type combo box in the Frequency Calibration group box. Press the Run button and wait for the Frequency Calibration Complete status message to appear. If a “No data in record buffer” message box appears Press the Open Mixer button and increase the applications output level slider. The SoundIO modules input device level may also be increased by right clicking on the Windows Task Bar Sound icon and selecting the Recording and Levels and adjusting the devices slider but Levels Calibration will have to be repeated. Frequency calibration may be restarted by pressing the Run button. If successful, the calibration dialog should look as in Figure 3. The Calibration Progress bar may not update in certain versions of Windows.

Figure 10: SoundIO Module Calibration Controls After Calibration, Input Level Settings May Vary Depending On Sound Card Capabilities.

12.   Press the Save button to save the calibration file to disk. Enter a file name when the Save Calibration File dialog appears. Press the OK button in the calibration dialog and select Yes when the “Calibration Parameter Has Changed. Save To Process File?” message box appears. Do not change the Recording Control Line-In gain settings after this point or re-calibration will be required. Changes to the Playback mixer gain settings may be made and will not affect signal level measurements.

13.   Now rewire the circuit as shown in Figure 9. Place the driver enclosure on a stand about 1 meter from the floor. Place the mike about 1 meter away so that it is in line with the dust cap of the driver.

Figure 11: Driver Measurement Process Test Wiring

14.   Press the Run button. Four bursts of MLS sequence will be sent to the driver. Now observe the resulting trace in the oscilloscope. You should see a single large aberration at the beginning of the trace followed by a group of aberrations about 3 mSec later. This trace represents the impulse response of the driver and the room. The first aberration is the driver’s impulse response and the later aberrations are room reflections.  They will distort the response curve and must be excluded from the measurement. Place the mouse at the beginning of the oscilloscope trace and press the left button. Now sweep the mouse to the first room reflection and release the left mouse button. Only the trace highlighted in inverse video will be included in the measurement. See Figure 10. The distance (d2) for any height (h) and microphone placement (d1) may be calculated as follows.

The time to first reflection can be calculated as follows.

where: c = speed of sound (344.5 m/s at 20°C @ sea level)

Figure 12: Windowing the Drivers Impulse Response

15.   Now press Run again and observe the spectrum analyzers channel 1 trace as in Figure 11. This is relative response of the driver with respect to a 1KHz reference frequency.

Figure 13: Drivers Relative SPL Response

15.   Now we will enter microphone sensitivity in order to make true sound pressure level measurements. In the second spectrum analyzer module select dBVSPL from the YAxis: Sel: combo box. Find the microphone sensitivity in its manufacturers data sheet. It is usually expressed in terms of output voltage (mV) per Pascal (Pa). One pascal is equal to 94dB SPL. The spectrum analyzer dBVSPL scale is referenced to 94dB SPL. If the microphone sensitivity is expressed in some other terms, conversion is necessary (see the conversion factors below). Our microphone sensitivity is 0.031V/Pa. Now enter 0.031 into the Spectrum Analyzers YAxis: VRef: edit box and press the enter key. Now press the Run button and observe channel one of the second spectrum analyzer. Figure 12 is the absolute sound pressure level of the driver at 1 meter.  

0dB SPL = 20uN/m² = 20uPa = 0.2nBar = 200udyne/cm² = 2.9015 X 10^-9 lb/in² = hearing threshold:

74dB SPL = 0.1 N/m² = 0.1 Pa = 1uBar = 1dyne/cm² = 1.454 X 10^-5 lb/in² = average factory noise

94dB SPL = 1N/m² = 1Pa = 10uBar = 10dyne/cm² = 1.454 X 10^-4 lb/in² = air compressed riveter

194.1dB SPL =101,330 N/m² = 101,330Pa = 1Bar =10133 X 10⁺⁶ dyne/cm² = 14.693lb/in² = 1atmosphere.

Figure 14: Drivers Absolute SPL Response

16.   Now we introduce the microphone frequency and phase correction data to the process. If your microphone does not come with an ASCII format correction file you may enter one manually in the Arbitrary Filter edit control using the manufacturers supplied frequency and phase response curves. See “To enter an arbitrary response envelope manually via the keyboard” in the arbitrary filter section of the user manual. Our microphone comes with an ASCII file so we simply open the file in the arbitrary filter module. The arbitrary filters edit control can translate almost any correction file that has the form shown in Figure 13.

Figure 15: Required Correction File Format

A note of caution; most correction files come with the actual response curve of the microphone element. In this case the response curve must be inverted. You must press the Invert button in the arbitrary filter dialog bar for these files to give the correct response output. The arbitrary filter multiplies the input signal amplitude response by its amplitude response and adds its phase response to the input signals phase response. Highlight the arbitrary filter module and press the Edit button on the dialog bar. The edit control should open on the right side of the module. Press the Open button on the dialog bar. Select the All Files(*.*) option from the Files of type combo box. Select the correction file from the Open dialog and press the OK button. Press the Update button on the dialog bar. If a Frequency Element out of Response Range message appears, go to the given line number and place a semicolon in front of it. This will cause the line not to be parsed. You may also delete it altogether. This application has a maximum response range of 0 Hz to 22050 Hz. Now look up the microphone manufacturers frequency response curve. If the plot in the graph window is equivalent to the curve press the Invert button. Now press the Save button and enter a file name in the Save As dialog File Name edit control. Press the Save button.

17.   Now we can measure the response verses the microphone distance from the driver. Each time the distance is halved the response should increase by 6dB. We will create a new process consisting of only the DataLogger module to illustrate this point. Press the New button from the applications File menu. Open the FFT Options dialog from the applications Options…FFT… menu and change the FFT Size to 8192 and Sample Rate to 44100. Press OK in the FFT Options dialog. From the Options menu select Process... The Process Select dialog box will open. From the Available Modules list box select DataLogger and press the Insert button. The DataLogger will appear in the Modules List box. Press the OK button. When the DataLogger opens change the Xaxis scale selection to F1Log100. Change the YAxis1 scale selection to dbVSPL. Enter the microphone sensitivity in the YAxis1 Ref: edit control. Change the YAxis2 scale selection to None. Press the Title button and enter a plot title in the Title edit control.  

Figure 16: Data Logger Settings

18.   We begin by taking a measurement at 1 meter. Highlight the “8192_MLS_Response_Measurement.process” and press the Run button. When the process is done sweep out a time window in oscilloscope module as shown in the Table 2 below.

Table 2: Loudspeaker to Microphone Distance Verses Floor Bounce and Impulse Windowing Time

18.   Press the Run button again. Now highlight the oscilloscope module and press the left mouse button. Select Sel Ch1 from the popup menu that appears. Press the left mouse button again and select Copy from the popup menu. Now highlight the DataLogger process and select Plot1 from the Plot Adj: Sel: combo box. Press the left mouse button and select Paste from the popup menu. Repeat for the remaining microphone distances each time sweeping out the new oscilloscope time window and incrementing the DataLogger plot number. You should end up with four plots in the DataLogger each separated by about 6 dB as shown Figure 15.

Figure 17: Driver Absolute SPL Response Verses Mic Distance

What We Do

Sonic beacon produces electrical and acoustical data acquisition and analysis software for the Windows operating system.

About Us

Sonic beacon is a Canadian organization that provides a free set of virtual instruments that are useful for measuring the time and frequency domain responses of audio components using a personal computer. It is located in Pakenham Ontario which is near Ottawa, Canada.

News and Events

March 23, 2014: 32 and 64 Bit Versions of sonic beacon (1.1.0.9) released. Tested on Microsoft Windows 8 (64 Bit), Microsoft Window 7 Home Premium (64 Bit) and Windows XP 32 Bit Professional.