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Sonic beacon Loudspeaker Test Software

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Two Driver Sealed Box Acoustical Measurement Techniques

To Calibrate or Not to Calibrate

The alignment of a closed box system drivers and their crossover usually consists of taking an acoustical measurement then changing a crossover component or adding stuffing or bracing to the box. This is then repeated until the systems response is flat. The calibration of the sound card or the microphone may not be necessary for this tedious work as long as all acoustic measurements are always done at the same drive level. Once the relative response is flat then the system can be calibrated for a single absolute response measurement.

Look at the response of the sound card below. It is flat to within +1dB from 30 to 19000Hz when using an FFT size of 8192. This is usually adequate for initial crossover adjustment. To determine the response of the sound card connect the line out to the line in and send an MLS signal through a SoundIO module in Record/Play mode to a Spectrum Analyzer in FHT/FFT mode. Using the SoundIO Repeat feature pre-stimulates the sound card greatly increasing stability.   

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Figure 1: Typical Sound Card Frequency Response

 

Look at the measurement microphone response (green) and phase (yellow) curves below. The response only deviates by 1dB at frequencies above 12kHz. Unless there are large deviations in the tweeter response above 10KHz, calibration is not necessary as most of the work on response adjustment for two way systems is usually done in the crossover (1 to 4 kHz) region.  

 

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Figure 2: Typical Measurement Microphone Frequency/Phase Response

Obtaining the Response Curve for Two-Way System Adjustment

For most two way crossover adjustments only the combined far-field woofer and tweeter response is required. It is obtained by placing the microphone at tweeter level, 1 meter from the tweeter dome. It is usually valid down to 250Hz which is usually well below the crossover frequency. An MLS from a Signal Generator module is sent to a SoundIO module in Record/Play mode. This is followed by a Spectrum Analyzer in FHT mode to create a time domain impulse response. An Oscilloscope module is used to gate the resulting impulse to mask room reflections. It is gated at the point where the first reflection arrives at the microphone, usually 4 to 5 mSec for a typical size room. The gated impulse is sent to a Spectrum Analyzer module in FFT mode for generation of the frequency response curve.

 

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Figure 3: Sonic Beacon Process Document to Evaluate Loudspeaker Far Field Frequency/Phase Response

Crossover Adjustment

Two drivers with the responses below were installed in a 44 liter sealed enclosure. The crossover board was suspended outside the enclosure so that components could be easily changed.

 

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Figure 4: Free Air Woofer and Tweeter Impedance and Frequency Response Curves

 

Initially a 2nd order Linkwitz-Riley crossover was designed for this system but the woofers response (green) at the 2100Hz crossover frequency caused phase problems that left a 4dB dip in the combined response (yellow) between 800 and 3000Hz. Here woofer (green), tweeter (blue) as well as the combined far-field measurements were separately taken and pasted into the Datalogger.

 

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Figure 5: Four dB Dip in the Sealed Box System Response Using a 2nd Order Linkwitz-Reily Crossover

This was replaced with second order Butterworth network, with a 3dB peak at the crossover, in the hopes that the dip would be eliminated. A 4dB artifact remained between 2 and 3 KHz. The purple trace is the response with the tweeter polarity inverted.

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Figure 6: Minus Four dB Artifact in the Sealed  Box System Response Using a Butterworth Crossover with 3dB Peak

A 2nd order Chebyshev, with a 6dB peak at the crossover frequency, replaced the Butterworth network. Although this did not remove the majority of the dip it brought it more in line with the rest of the tweeters response

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Figure 7: Sealed  Box System Response Using a 2nd order Chebyshev Network with 6dB Peak

Once the tweeter attenuation was adjusted the following far-field +2.85dB response resulted.

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Figure 8: Combined Far Field Sealed Box System Response Using a 2nd order Chebyshev Network with 6dB Peak

Combining the Acoustical Response Curves

After the response in the crossover region is flattened the system is then calibrated so that the system sensitivity is measured correctly. In order to create a frequency response graph for the dual driver sealed box system the two measurements must be combined.

First the near-field woofer response is obtained. This is done by placing the microphone at 0.25 from the woofers dust cap with the Signal Generator, SoundIO, Spectrum Analyzer, Oscilloscope, Spectrum Analyzer setup from above. Impulse gating is usually done at about 50 mSec to get a response down to 20Hz.   

The near-field response is pasted into plot1 of a separate instance of a Datalogger module. Note the very high SPL due to the close proximity of the microphone.

 

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Figure 9: Near Field Woofer Sealed Box Response Pasted into Datalogger (Plot1)

The combined far-field woofer and tweeter response is obtained (gating at 4 to 5 mSec) and pasted into plot 2 of the datalogger.

 

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Figure 10 Near Field Woofer Sealed Box Response (Plot 1) and Combined Far-field Woofer and Tweeter Response (Plot2) in Datalogger

The level of the near-field response is brought into coincidence with that of the far-field by reducing the gain of plot 1. Select Plot1 in the Sel: Combo box of the Plot Adj: group and press the Gain: Dn button until the levels of the two plots are equal at the proposed merge point. In this case the near-field data is valid up to about 718Hz (fmax = 4311/ effective cone diameter) and the far-field data is valid down to 250Hz. Both curves are reasonably flat between 200 and 300Hz so 250Hz might be a good merge point.

 

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Figure 11: Determining the Merge Points of the Near and Far Field Plots

 Then the near-field response is merged with the far-field response. Enter the merge frequency in the Freq: edit control in the Mode group box and select Merge in the Sel: combo box. Note the response dip below 500Hz due to spreading loss caused by the enclosures front baffle. Also note that the system sensitivity is that of the tweeters original free air graph.

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Figure 12 Merged Near and Far Field Plots In DataLogger

 

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.

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