Loudspeaker/Room Loudspeaker/Room Measurement & AnalysisMeasurement & Analysis

AXPONA 2014AXPONA 2014

On The Plate

● Why take measurement?● DIY Measurement Gear● Take some demonstration measurements● Initial interpretation● High frequency & speaker correction● Frequency & Wavelength● Room Modes & Room Mode Modeler● Room Treatment

Why Take DIY Measurements?

● The loudspeaker and room should be considered one system● Loudspeaker specifications are vague (at best)● Specs usually have nothing to do with real-world room/speaker performance

● You ears are subjective● Hearing system is amazing, but it's attached to an emotional brain● Mics don't lie, cheat, or change over time

● Information is power!● Seeing what you're hearing allows you to make informed acoustic decisions

Today's Goals

● Demonstrate how easy and inexpensive it can be to take measurements

● Gain an appreciation for what the speaker and room are doing● Start to think about sound as frequency/wavelength

● What won't happen is to become a pro overnight● Taking measurements is easy● Interpreting the results meaningfully is pretty tricky● Automated systems are getting better and on the way!

Measurement Hardware

● Measurement Mic (USB or XLR)● Dayton EMM-6 $50● Dayton UMM-6 $70● Audix TM-1 $300● Earthworks M30 $650

● External Pro Audio USB Soundcard ● Focusrite Scarlett 2i2 $150● Anything with phantom power for mic (unless using USB mic)

USB Mic (no need for external card)

Measurement Software● Room EQ Wizard

● Free!● Works best with Windows. Mac/Linux OK, but sort of weird

● Fuzzmeasure Pro● Free to try, $150 to buy● Mac only

● ARTA● $100● Window only

Speaker

USB Soundcard

Computer

Mic

Amplifier

Basic measurement setup with pro external soundcard

Speaker

USB DAC

Computer

USB Mic

Amplifier

Basic measurement setup with USB Mic and DAC

Taking Measurements● Impulse measurements

● Include both time and frequency information

● Log Sweep● Effectively move the entire sweep to t=0● Very, very good SNR

● 16 bit at 44.1 or 48 is plenty sufficient for most observations● Slower and multiple sweeps are better, but fairly unnecessary

Stuff to Look At● Frequency Response● Phase

● Group Delay

● Reverberation Time (RT60)● Distortion● Waterfall/Spectrogram

● Shows frequency, time, and amplitude

● Gating analysis window before first reflection (around 5 ms) we can look at anechoic speaker response down to around 300 Hz

● Taking incremental off-axis response we can start to see diffraction and directivity information

Room vs. Speaker Response

Room RoomSpeaker

● Generally for small rooms (not halls/auditoriums) we'll see most room effects in the low frequencies

● Reverberant HF energy is there, but doesn't affect frequency response much (you can see it in the waterfall/spectrogram)

Speaker

Smaller Room Larger Room

Speaker Correction● Can be done internally in some speakers, outboard in others● EQ is the easiest

● DSP and/or analog● Won't correct diffraction issues (usually make them worse!)

● DSP● Can use sophisticated algorithms

Low Frequency Info

● Often times we see huge frequency and phase swings from room modes and reflections

Standing Waves● Resonant frequencies of the space

● Constructively and destructively interfere● Create pressure and velocity nodes and antinodes (we care about

pressure nodes and antinodes)

● Standing waves are a function of the dimensions of the room● The larger the room dimension the lower the fundamental mode● Harmonics are integer multiples of the fundamental

Pressure Velocity½ λ ½ λ

λ λ

3/2λ 3/2λ

1st ModeFundamental

2nd Mode

3rd Mode

wall

1st Mode

2nd Mode

Length L

3rd Mode

Drawing Room Modes Like an Acoustician

Pressure waves are drawn rectified

v=fλv = 343 m/s (speed of sound at 20v = 343 m/s (speed of sound at 2000C)C)f = frequency in Hzf = frequency in Hzλλ = wavelength in meters = wavelength in meters

λ=v/fv = 343 m/s (speed of sound at 20v = 343 m/s (speed of sound at 2000C)C)f = frequency in Hzf = frequency in Hzλλ = wavelength in meters = wavelength in meters

Some Wavelengths

● 20 Khz = 1.72 cm● 2 kHz = 17.2 cm● 200 Hz = 1.72 m● 20 Hz = 17 m

f=v/λf=v/λf=v/2Lf=v/2L

v = 343 m/s (speed of sound at 20v = 343 m/s (speed of sound at 2000C)C)f = frequency in Hzf = frequency in Hzλλ = wavelength in meters = wavelength in metersL = Length of the room in metersL = Length of the room in meters

Fundamental Room Mode Calculation

Low Frequency Treatment● Passive Systems

● Convert pressure waves (potential energy) into kinetic energy, which is relatively easily absorbed

● Must be custom tuned and built for specific space● Panel/diaphragmatic, Helmholtz resonator

● Active Systems● Multiple subwoofers● DSP

Everest, F. Alton: Pohlman, Ken. Master Handbook of Acoustics. 5th Edition. McGraw Hill, 2009. 

Toole, Floyde E. Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms . Focal Press, 2008.

Master Handbook of Acoustics, 5th Ed.

Sound Reproduction

Suggested Reading

Huge thanks to Madisound for letting us use their awesome speaker for the demo!

Check out the demo gear in room 338

Jeff Merkeljeff@merkelacoustics.com

Skype: jeff8houses

303-359-0737

MERKELAcoustic Research & Design

Web: merkelacoustics.com Twitter: jeff8houses