SpeakerDraft gains Universal status

Faber Acoustical is pleased to announce that SpeakerDraft 1.2 is now available for download on the iTunes App Store. Version 1.2 brings a few important enhancements, as well as compatibility with the iPhone and iPod touch.

New in version 1.2:

  • SpeakerDraft now runs on all iPhone OS devices, including the iPhone and iPad
  • Driver properties now include nominal diameter (in inches), nominal impedance, and mounting depth.
  • Box dimensions are constrained by the size of the driver (you can’t make a box that’s too small for the selected driver).
  • Port dimensions are constrained by the size of the box.
  • Some cosmetic issues have been corrected in the iPad version.

SpeakerDraft 1.2 also brings with it a new, lower price.

Happy subwoofer drafting!

Speaker modeling comes to the iPad

We are please to announce SpeakerDraft, a new loudspeaker modeling app for iPad. SpeakerDraft makes it easy and fun to visually optimize the bass response of a simple loudspeaker design, such as a subwoofer.

SpeakerDraft takes advantage of the iPad’s large screen to plot the speaker’s bass response, which updates in real time as the user drags virtual slider controls corresponding to the speaker’s physical dimensions. Users can zero in on the right box and port dimensions for their next car or home theater subwoofer project, with minimal effort. Even selecting a specific loudspeaker driver for a given speaker box becomes a simple matter of tapping on different drivers listed in the app–the speaker’s bass response plot is updated immediately with each tap.

SpeakerDraft supports the design of sealed and ported loudspeaker boxes. In either case, the width, height, and depth of the box are fully adjustable. Port diameter and length can also be specified for ported box designs. Users can create an arbitrary number of speaker designs, or add drivers to their driver list. The app also includes a driver database.

SpeakerDraft costs $19.99 and is available now for download on the iTunes app store.

SignalScope 2 for iPhone offers upgrades via in-app purchase

Faber Acoustical, LLC announced today that SignalScope 2.0 for iPhone and iPod touch devices is available for immediate download on the iTunes App Store. With version 2, SignalScope users can now enjoy the full features of the Pro version without having to pay full price for SignalScope Pro. Faber Acoustical’s SignalScope 2.0 includes full and partial upgrades, via in-app purchase, that unlock additional tools and functionality, which previously were only available in the Pro version.

In addition to enabling users to upgrade to the full functionality of SignalScope Pro for iPhone all at once, SignalScope 2.0 offers incremental upgrades. Available upgrades include individual tools, such as the level meter (functionally equivalent to the sound level meter tool in the popular SoundMeter app), octave analyzer, and signal generator. Another upgrade can be purchased to enable data acquisition features, such as saving data to text or MAT files, which can then be downloaded to any Mac or PC via a standard web browser.

Available in-app purchase upgrades include:

  • a full upgrade to the functionality of SignalScope Pro for $59.99 (includes all other upgrades);
  • a Signal Generator tool for $7.99;
  • an Octave and 1/3-octave Analyzer tool (RTA) for $19.99;
  • a Level Meter tool for $14.99;
  • and a Data acquisition upgrade (text and MAT file data capture) with downloading to Mac or PC via a standard web browser for $19.99.

IMPORTANT NOTE: iPod touch devices require additional external hardware to get audio signals into SignalScope. For audio signals, the 1st gen iPod touch requires a dock connector input device (such as the MacAlly iVoice Pro) to be used with SignalScope. All other devices can perform measurements with the headset connector, but using a stereo dock connector input device is recommended for the highest quality measurements.

Minimum Requirements:
* iPhone or iPod touch, with iPhone OS 3.0 or later

Pricing and Availability:
SignalScope is now available for download on the iPhone App Store for $24.99 (USD). External hardware may be required for analog input capability. More information on SignalScope can be found at Faber Acoustical online. More information regarding suitable input devices for specific iPhone or iPod touch models can be found on the Faber Acoustical blog.

SignalScope App Store URL
SignalScope Web Page

How about an iPhone-ProTrack SLM

Alesis finally got around to making shields for securing an iPhone or iPod touch to the ProTrack stereo iPod recording device. This means that you can insert your iPhone (any model) or iPod touch (2G or later), plug in a phantom-powered measurement microphone, run SoundMeter, and turn the unit upside down for a handheld (albeit somewhat large) sound level meter. Of course, if you use SignalScope Pro, you’ll also have access to 1/3-octave and narrowband spectral analysis.

SignalScope Pro on iPhone with a ProTrack

One downside is that the headphone output is completely blocked for the iPod touch. The shield itself blocks the headphone output of the iPhone, although a drill could solve that problem fairly easily.

iPod Touch 3G Headset IO Frequency Response

To add to the previously published results, we decided to measure the frequency response of the iPod Touch 3G. Electroacoustics Toolbox 2.1.7 was used with a MOTU UltraLite mk3 audio interface to measure the frequency response. These measurements include the response of the headset output as well as the headset input. The audio was routed through the iPod Touch using SignalScope Pro, and the measurements were made on a 3rd generation 32 GB iPod Touch.

With each new iteration of the iPhone, the low-end roll off of the headset input has been steeper and steeper.  The iPod Touch seems to be following a similar trend. The iPod Touch 2G had a 3 dB frequency around 30 Hz while the iPod Touch 3G has a 3 dB frequency around 150 Hz. If you plan on using the headset input for any type of low frequency measurements, you should definitely take this into consideration. Hopefully this trend will not continue with future iterations of the iPhone/iPod Touch.

Headset Input Frequency Response

As with the previous measurements, the dock connector input was very flat, and has not changed significantly from the older hardware. The 3 dB frequency is still below 10 Hz, with a small amount of ripple at low frequencies, and it stays flat until above 20 kHz.

Frequency Response Measurement of Logitec LIC-iREC03P

There haven been a few requests regarding a recently released line input device called the Logitec LIC-iREC03P. To add to the frequency response measurements done previously, we decided to add this new device to the group.

This measurement was made using an iPod Touch 2G. As before, the audio was routed through the phone using SignalScope Pro, and the measurement includes the response of the iPod’s headphone output.

When compared to the previous measurements,  the Logitec has the lowest 3dB point of all the devices tested (around 5 Hz). One problem with this device is the availability. Currently, it appears to only be available from Japan. I was able to order it fairly painlessly from geekstuff4you.com, but your mileage may vary. Another possible issue is the fact that the dock connector is pretty loose, and the device itself doesn’t sit flush against the iPod, so it can wiggle around quite a bit while plugged in. While this didn’t affect these measurements negatively, if durability is a big priority for you, you might want to stay away.

If you’re looking for a simple line input device with a flat response, and you don’t mind slightly flimsy construction, this may be the way to go.

SignalScope Pro includes SoundMeter tool

SignalScope Pro 1.2 for iPhone is now available for download on the iTunes App Store

SignalScope Pro turns an iPhone or iPod touch device into a hand-held dynamic signal analysis tool, with support for 1/3-octave real-time spectral analysis. SignalScope Pro’s new Meter tool builds on what SoundMeter has to offer and extends it beyond measuring acoustic signal levels to support time-weighted and equivalent levels for any supported signal type (including voltage, current, acceleration, and velocity, in addition to acoustic pressure).

Within the new Meter tool, SignalScope Pro gains SoundMeter’s ability to post measured levels directly to a Twitter account or to send them via email. Levels can also be overlaid on an image from the photo library, or taken with the camera on an iPhone.

Download SignalScope Pro

 

SignalScope 1.4 for iPhone is now available for download on the iTunes App Store

SignalScope 1.4 now allows frequency resolution to be specified directly for the FFT analyzer, rather than selecting the number of spectral lines. Help documents can now be accessed within the app without requiring an internet connection and several cosmetic issues and bugs have been corrected in the new version.

Download SignalScope

 

SoundMeter 1.6.1 is now available for download on the iTunes App Store

SoundMeter corrects an issue that prevented custom sensitivities for the iPhone’s built-in microphone from being properly applied or remembered.

Download SoundMeter

Measuring Loudspeaker Impedance with IOScope

Today, a new video, Measuring Loudspeaker Impedance with IOScope, was published on this site, as well as on the Faber Acoustical YouTube channel. The video is both a demonstration of IOScope, as well as a simple tutorial on measuring loudspeaker impedance. Although the video is largely self-explanatory, I thought it would be beneficial to include some further explanation and tips for those who are interested. The movie is essentially broken into four chapters and a similar format will be followed here.

What is impedance? How is it measured?

By a generalized version of Ohm’s law, we understand that voltage is equal to the product of electrical current and impedance. This means that electrical impedance is equal to the ratio of voltage and current, or:

Z = V/I

where Z represents impedance, V represents voltage, and I represents current. This means that we can calculate impedance as long as we can measure voltage and current. Voltage is easily measured from the iPhone’s audio inputs, but how do we measure current? By returning to Ohm’s law, we see that if we measure the voltage across a resistor (a known impedance), we can compute the current by I=V/Z. So, as long as we have a known resistance and a means to measure the voltage on both sides of it, we can effectively measure current.

Voltage calibration

Although audio signals come into the iPhone as voltages, those voltages get converted to digital values. In order to measure voltage correctly within IOScope, we need to perform some type of calibration that will define the relationship between the digital values and the actual voltage reaching the iPhone audio input jack.

As is demonstrated in the video, voltage calibration in IOScope can be easily achieved by measuring the voltage going into the iPhone with an rms voltmeter. Once the rms voltage has been measured with the voltmeter, that measured quantity should be entered into the Ref Input Level text box of IOScope’s calibration screen. All that’s left is to press the Calibrate button and confirm the action–IOScope will automatically compute the audio input device’s voltage sensitivity.

  • The accuracy of the calibration will largely depend on the accuracy of the voltmeter.
  • In the video, IOScope’s built-in signal generator is used to produce a single tone with a frequency of 1 kHz. As long as the tone is turned on in the Excitation tab of IOScope’s main screen, the signal will be output when the calibration screen appears.
  • When entering values into a text box in IOScope, you can enter unit magnitude prefixes, like m for milli, or u for micro. For example, in the video, “618m” is entered into the text box for 618 millivolts. Alternatively, “.618” could have been entered for the same value.
  • When performing a voltage calibration, you need to be sure that the input device or input channel units are set to V (for volts).

Current calibration

When working with a stereo input device, like the Belkin TuneTalk Stereo that was used in the video, IOScope adds a special third channel to the device. That channel is the difference between input channel 2 and input channel 1. Such a channel enables convenient measurement of current by measuring the voltage difference across a resistor. Since the previous voltage calibration was performed at the device level, which means that the calibrated voltage sensitivity applies to all input channels, calibrating the difference channel for accurate current measurement simply requires you to enter the value of the resistor you will be using. In this case, the resistor was measured to have a resistance of 999 ohms. Instead of using the Calibrate button on the channel calibration screen, the known sensitivity of 999 Volts per Amp can be entered directly into the Input Sensitivity text box.

How do we know the sensitivity of the Ch2-Ch1 difference channel should be 999 V/A? Again, Ohm’s law gives us the answer. Since V=IZ (or, in this case V=IR, where R is resistance), if we put 1 amp of current through a 999 ohm resistor, we should read 999 volts, or 999 volts per amp.

  • When performing a current calibration, you need to be sure that the input channel units are set to A (for amps).

Setting up the loudspeaker measurement

As I indicated before, in order to measure the impedance of a loudspeaker, we need to measure the voltage across the loudspeaker’s terminals and the current flowing through them. Measuring the voltage across them is as simple as connecting one of our input channels to the terminals. However, we need a little help from Kirchoff’s current law to understand that if we put a resistor in series with the loudspeaker, the electrical current through the resistor and the loudspeaker will be identical. Since we’re already prepared to measure current across a resistor, we’re good to go.

  • IOScope’s Measurement Configuration screen provides a simple graphic representation of the measurement, which makes it easy to identify which input channel should be which. Since we’re measuring loudspeaker impedance, which is equal to V/I, we set the Y signal to Input Channel 1 and the X signal to Input 2 – Input 1.
  • The proper connections of the signals to the loudspeaker are shown in the video (at 3 minutes).
  • Again, the built-in excitation signal is employed, although this time it is in the form of a logarithmic frequency sweep. The length of the sweep corresponds to the length of the FFT used by IOScope, which allows excellent results to be obtained with little or no averaging (which is also evident in the video).

Making the measurement

Once things are configured, making the measurement just takes a tap of the start button in the toolbar of IOScope’s Frequency tab.

  • A double tap in the vertical axis label region of the display will toggle auto scaling on and off (it’s on by default).
  • Notice that the magnitude measurement is displayed in units of ohm’s (IOScope is smart enough to know that volts divided by amps means ohms).
  • The video also demonstrates saving the frequency domain data into a .mat file, which can be downloaded from IOScope onto a Mac or PC, via a web browser. Data contained in .mat files can be loaded in to MATLAB, Gnu Octave or FreeMat.

iPhone Microphone Frequency Response Comparison

With the advent of sound level meter apps for the iPhone OS (of which SoundMeter was the first) people began to ask, “How flat is the frequency response of the iPhone’s microphone?” Early testing indicated that the built-in microphone of the original iPhone was not a good candidate for sound level measurements, but that the iPhone’s headset microphone enjoyed a fairly flat response. Since then, additional iPhone models have arrived on the scene, each with its own set of weaknesses with respect to microphone frequency response. Additional Apple and third party headset microphones have also been introduced.

At long last, some relevant frequency response measurements are presented here for the benefit of those who would really like to “see” how flat a particular microphone is. These results have implications on the use of certain microphones for making sound level measurements, as well as on the use of these microphones for spectral analysis in which relative amplitudes need to be determined with some degree of accuracy.

The following measurements were made relative to a Type 1 precision microphone in a fairly quiet room. These measurements were not made in an anechoic chamber and although the coherence was very good across the audio band, the measurement error is non-negligible at high frequencies, because of diffraction effects.

Built-in Microphones

Built-in iPhone Microphone Frequency Response

Built-in iPhone Microphone Frequency Response Comparison

As I have often said, “The built-in microphone of the original iPhone is not recommended for sound level measurements.” Now, you can really see what I mean. Interestingly, the built-in microphone of the iPhone 3GS isn’t recommended, either, unless you don’t care about frequency content below 200 Hz. This behavior is consistent with the headset input frequency response of the iPhone 3GS (I suspect that the built-in microphone signal goes through the same high-pass filter that gets applied to the headset input). The iPhone 3G microphone’s response is clearly the best of the bunch, but its low end rolls off by 15 dB or more at 20 Hz. Not surprisingly, none of the iPhone models rivals a lab-grade sound level meter with its built-in microphone, but either of the 3G models can potentially give you a decent ball-park estimate of the current sound level, although the low frequencies will be de-emphasized.

Headset Microphones

The goal, here was not to measure every headset microphone on the market, but to take a look at some of the more common options. These measurements were made of each microphone’s electrical output, so they do not include the response of any iPhone input or output circuitry. The microphones included in these measurements are:

iPhone Headset Microphone Frequency Response Comparison

iPhone Headset Microphone Frequency Response Comparison

In the world of headset microphones (at least those that are presented, here), the official iPhone headset microphone and the SwitchEasy ThumbTacks microphone win the day. The USBFever microphone also exhibits a flat response between 20 Hz and 2 kHz, although its response appears to break up more severely by the time it gets up to 10 kHz. In light of recent headset input frequency response measurements, the best case scenario for inexpensive sound level measurement might be to use the ThumbTacks microphone with the original iPhone.

These results are also interesting, in that they strongly suggest that the newer Apple headsets, which are designed primarily for iPods, shouldn’t be used for sound level measurements, either. Their response certainly seems to follow an apparent trend with Apple’s microphone-related circuitry to de-emphasize low frequencies.

It may be important to keep in mind that the goal, here, is to see what makes sense in terms of using iPhone OS devices as inexpensive, portable sound level and spectrum analysis tools. Obviously, there was never an expectation that the iPhone’s inexpensive microphones would perform in a manner consistent with precision measurement mics that are (justafiably) much more expensive. It is possible to connect such high-end microphones to an iPhone, though (via the dock connector)–more on that, later…

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