Directional Response

For this experiment, my group (Harrison and Alec) ran sound through a Roland Jazz Chorus amp, with flattened EQ. I must say that this amp is not meant to reproduce sounds with the same accuracy as a studio monitor or a loudspeaker, and the sound was probably very colored. We figured that we should use the amp instead of the Mackie monitors that are in the tech closet because they (the monitors) cannot reproduce frequencies less than 150 or so Hz with accuracy.

We placed the amp about five feet from the front wall of Studio F next to a table from which we ran max using cycle~ noise~ and pink~ to produce the desired tones for the experiment. Placing the amp close to the walls would create a flutter effect, where high frequencies reflect off the wall and distort your perception of the sound. Alec recorded data and timed the experiment, Harrison took SPL measurements using the Decibel 10th iPhone app and I used Max to generate the sine waves and noise. We moved a chair with Harrison’s phone sat on it around the amp in order to record data.

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POLAR GRAPH FOR WHITE & PINK NOISE:

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POLAR GRAPH FOR SINE WAVES @ 9600Hz and 100Hz

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When we recorded data, it was clear that SPL changed depending on the angle at which the SPL meter was placed in relation to the amp. It is clear that each tone or noise generated by the amp has a higher SPL on axis or off 45o off axis from the amp. This particular amplifier is designed to project sound forward - therefore it makes sense that sounds would have a higher SPL in front of the amp.

Off-axis sounds have lower SPL than on-axis sounds, because of the directionality of the amp. However low frequencies, with long wavelength are omnidirectional and tend to bend around smaller objects such as an amplifier. In this case the 100Hz tone had an SPL from 50-55dBA at 1.5 ft and the 250Hz tone had a range of 53-65dBA. This 250Hz tone has a shorter wavelength than the 100Hz tone and therefore is less omnidirectional.

High frequencies are very directional because of their short wavelength. A high frequency noise will reflect off a surface that it comes into contact with instead of bending around it. This explains why the 9.6kHz tone had higher SPL readings in the front and back than on the sides.

Hold up…. In the back? Why is this? I thought that high frequency noises were very directional. Off-axis high frequency sounds tend to cancel because of their similar wavelength. This explains why they have lower SPL than in the front or in the back. This still does not explain why the SPL is as high in the back as it is in the front. Because the amp was placed fairly close to the wall the increase in SPL could be attributed to rear-reflected sounds escaping from the back of the amp that are reflected off the wall.

When we measured SPL for pink noise and white noise there were several distinct differences in SPL. Firstly it would make sense to create a distinction between white noise and pink noise. White noise has energy per interval of frequency and pink noise has equal energy per octave. Therefore, white noise has more energy in the high frequency range and pink noise has more energy in the low frequency range. As shown by the polar chart and the data gathered, pink noise is more omnidirectional than white noise, although it has a lower average SPL than white noise does. White noise is more directional because it has more energy in higher frequencies, and is more directional. The noise signals differ from the sine waves because they are not simple sounds. The sine waves only vibrate at one frequency, whereas the noise signals are combinations of many sine waves. Therefore sine waves and noise will have different polar patterns.

If this experiment were conducted in an anechoic chamber, there would be no noise floor, unlike the noise floor of more than 30dBA in Studio F. The noises that the experimenters would make, such as rustling of clothes, footsteps or clicking of keyboard keys would be factored into the SPL reading. Furthermore, there would be no reflections from surrounding walls, providing a more accurate reading of the directionality of the speaker used in the experiment.

The Roland Jazz Chorus amp is designed to project on axis sound so that it may be heard far from the amp. This amp succeeds at its purpose, but does so with fairly large amounts of rear and side-radiated sound, which is to be expected from any amplifier or speaker. This experiment has also provided tangible proof of the directionality of high frequencies and the omnidirectionality of low frequencies, and shows off-axis sound cancelation in action.

When mixing music it makes sense to place monitors at head level and make sure that you are from 0 to 45 degrees on axis of the monitors. If you fail to do this, it could result in uneven frequency response and significant loss of high frequencies, which will negatively impact your mix. Furthermore, it is ideal to mix in an environment with a very low noise threshold. When we performed this experiment, it was nearly impossible to hear the 100Hz at low volume due to the background noise. This could cause you to boost LF and HF and negatively impact your mix.

DiStAnCe VS iNtEnSiTy

We ran audio through the leftmost monitor in Studio E, an Alesis Monitor 2, which is hung from the ceiling. Although I doubt it made much of a difference in our data, we had Andrew stand on chairs in line with the cone of the speaker, first at 3ft away from the cone, then 6ft then 12ft, and measure the SPL (sound pressure level) with the iPhone app “Decibel 10th.” We used Max MSP to generate a sine wave first at 1000hz then 100hz then 420hz. We played each sine wave for 6 seconds and recorded the SPL afterwards.

I must concede that our data may be inconsistent because of the inaccuracy of our measurements. We roughly estimated the distances using our shoe size (for lack of a ruler or measuring tape) and this could have led to any inconsistency in our data.

Initially when we measured the data for 100hz, we thought we had done something wrong. We took multiple measurements and to our dismay saw that none of our figures changed. Perhaps the speaker was messed up or something.

Upon later thought I realize that low frequencies are relatively omnidirectional due to their large wavelength. The wavelength of 100hz sine wave is approximately 11.25ft. At 3ft away from the speaker, the wave has not peaked yet. At 6ft the wave is just over its peak, and at 12ft it is in between its peak and its trough.

A sine wave of 420hz (lololol get it?) clearly has a wavelength in between that of 100hz and 1kHz. Its wavelength is about 2.7ft, which is close to the distance we stepped back from the speaker for every measurement. This explains the fact why our three measurements were fairly similar for this frequency.

At 1000hz a sine wave has a wavelength of about 1ft. I attribute the decrease in SPL with increase in distance to the proximity of the sound to the speaker. The farther away from the speaker, the lower the SPL must be.

This experiment showed that SPL is not as simple of a concept as one might initially think. Sound does not travel linearly in a room - rather the SPL fluctuates with the peaks and troughs of a wave. I think it was interesting how 420hz had a wavelength of the increment where we took measurements from the speaker (~3ft), even though we picked this value as a joke. Science!

"LISTEN RECORD LISTEN"

For this experiment, I used my Zoom H2n in its omnidirectional stereo setting to record sound in each different environment. 

First, I recorded the outside environment:

It isn’t every day that I have the opportunity to sit and try to individually listen to each different soundscape that is happening around me, and to be conscious of each different layer of sound present in the environment in which we live. For this experiment, I sat on the steps of an apartment building on 12th St, off of 2nd Ave, and recorded for five minutes holding the recorder in my hand.

The first thing I noticed when listening was that New York City has a distinct roar, that sits behind all of the other sounds closer to your immediate vicinity. It sounds quite like the ocean, however it has much more low frequencies and has an unnatural overwhelming quality to it. The roar crescendos and decrescendos and occasionally, high frequency “wooshing“ seems to peak out over top of the dense mass of sound.

While I sat on the steps of the apartment building, many people walked past me, their shoes tapping on the concrete and their clothing and personal affects jingling and clinking together as they walk. Many people were talking to each other, or to their cellphones as they walked down the sidewalk. The human voice is a predominantly mid frequency sound, although it has both high and low frequencies.

Cars also passed me, their engines a constant low frequency rumble in the distance on 2nd Ave. As they accelerate in front of me, their engines become higher pitched as they crescendo. Bikes also passed me; their gears make a high-pitched clicking sound as they move by. As a sound moves closer to me they seem to get higher pitched in frequency.

In the background, I could hear the hum of AC units and generators, however they seem to blur with the roar of the city in the background.

As I was sitting on the steps, two women passed me to go into their apartments. The door closing behind them created a very loud low-mid frequency sound as it slammed, in part due to its proximity to me, and due to their mild discomfort with my presence on their doorstep.

After listening to my recording of 12th Street, I notice that the sounds further away from me were more difficult to hear. I attribute this to the gain of the microphone, as I set its gain fairly low when I started recording. However, the sounds very close to me, like the door, and the people and cars directly in front of me appeared to be much louder than they did when I listened to them before having recorded.

Also the wind created a low frequency sound, as it hit the grill of my microphone, although I can’t hear the wind normally, it’s presence and motion were picked up by the microphone.

Recording in Frederick Lowe Theatre and my bathroom:          

Whenever I hear recordings of my voice, I notice that I sound very different than I do inside my head. Besides that, the size of the rooms in which I recorded was immediately made clear in the recordings. It was easy to hear which room was the larger room, because of the reverb tails of each word that I spoke. The large room had more high frequency noise, and my voice sounded clearer and less muddy than it did in the bathroom. In the bathroom, my voice sounded very damp and muffled, I assume because of the more frequent reflections of the sounds because of the small size of the room.

In the theatre, I could hear the sounds of people walking through the lobby, which I did not notice when I was doing the recording. In the bathroom, I could hear the high frequency buzzing of the light above me, and the tapping of my finger on the bottom of my microphone.

            Overall, it is clear to see that microphones do not pick up sound the same way that human hearing does, and there are many other factors that may influence the way a recording sounds, such as environment, gain and microphone placement.

https://soundcloud.com/carlstevensssounds/12th-st-and-2nd-ave

https://soundcloud.com/carlstevensssounds/frederick-lowe-theatrewav

https://soundcloud.com/carlstevensssounds/my-bathroom-wav-1